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Serb. Astron. J. } 183 (2011), 1 - 35 UDC 520.36–14DOI:
10.2298/SAJ1183001J Invited review
ASTRONOMICAL OPTICAL INTERFEROMETRY.II. ASTROPHYSICAL
RESULTS
S. Jankov
Astronomical Observatory, Volgina 7, 11060 Belgrade 38,
SerbiaE–mail: [email protected]
(Received: November 24, 2011; Accepted: November 24, 2011)
SUMMARY: Optical interferometry is entering a new age with
several ground-based long-baseline observatories now making
observations of unprecedented spatialresolution. Based on a great
leap forward in the quality and quantity of interfer-ometric data,
the astrophysical applications are not limited anymore to
classicalsubjects, such as determination of fundamental properties
of stars; namely, theireffective temperatures, radii, luminosities
and masses, but the present rapid devel-opment in this field
allowed to move to a situation where optical interferometry is
ageneral tool in studies of many astrophysical phenomena.
Particularly, the adventof long-baseline interferometers making use
of very large pupils has opened the wayto faint objects science and
first results on extragalactic objects have made it areality. The
first decade of XXI century is also remarkable for aperture
synthesis inthe visual and near-infrared wavelength regimes, which
provided image reconstruc-tions from stellar surfaces to Active
Galactic Nuclei. Here I review the numerousastrophysical results
obtained up to date, except for binary and multiple stars
milli-arcsecond astrometry, which should be a subject of an
independent detailed review,taking into account its importance and
expected results at micro-arcsecond precisionlevel. To the results
obtained with currently available interferometers, I associatethe
adopted instrumental settings in order to provide a guide for
potential usersconcerning the appropriate instruments which can be
used to obtain the desiredastrophysical information.
Key words. instrumentation: interferometers – methods:
observational – tech-niques: high angular resolution
1. INTRODUCTION
The new generation of ground-based instru-ments for high angular
resolution from visual andinfrared interferometry provided a
qualitatively newinformation for improving our understanding of
var-ious astrophysical objects through the comparison
ofobservational results with the predictions of theoret-ical
models. Althoug the optical interferometry hasstill some
limitations in application to various prob-lems (mainly due to the
lack of sufficient signal-to-
noise ratio for faint objects), it’s range of applicationshows a
significant growth, particularly in the pastdecade.
In Section 2 of the present paper I review sev-eral results that
represent important contribution tothe study of the Sun and Solar
system, while the re-sults concerning the determination of stellar
funda-mental parameters are presented in Section 3, andrelated
asteroseismological results concerning NonRadially Pulsating stars
are described in Section 4.The past decade has seen some
astonishing resultson stellar rotation, which I present in Section
5. The
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S. JANKOV
related Section 6 concerns Be stars and their cir-cumstellar
environements while Section 7 describesachievements in the study of
Red giants and Super-giants, considering the case of Betelgeuse as
the mostrepresentative one. Section 8 deals with the Asymp-totic
Giant Branch stars, particularly Cepheids andMira type stars. Some
remarkable results obtained inthe study of Planetary nebulae and
their central starsare presented in Section 9, while Section 10
describesthe results concerning Luminous Blue Variables andfocusing
to η Carinae, probably the most studied starafter our Sun. The
phenomenon of Novae has beenonly recently studied by means of
optical interfer-ometry and this new results are presented in
Section11. Section 12 is dedicated to Young Stellar Ob-jects,
namely Herbig Ae/Be stars, T Tauri stars, FUOrionis stars and
Massive Young Stellar Objects, allof them being very important for
our understand-ing of how accretion discs evolve into
protoplanetarydiscs and finally to debris discs and planets.
Theresults concerning debris discs and planet formationare
presented in Section 13, while Section 14 treatscontributions of
optical interferometry to the verypoupular search for exoplanets.
Section 15 describesthe results obtained in the study of Central
regions ofour Galaxy and Section 16 presents the compilationof
contributions dedicated to Active Galactic Nuclei.Since the image
reconstruction became one of themost valuable tools which generally
contributes toall astrophysical fields treated here, I review some
ofoutstanding results in Section 17. Finally, Section18 presents
the conclusion on this work based on allresults obtained up to
date.
2. SUN AND SOLAR SYSTEM
The Sun and Solar system have not been theprime candidates for
application of astronomical op-tical interferometry due to to the
fact that the spa-tial resolution is provided by the proximity of
thisscientific targets. In the past, the single
apertureinterferometry has been contributing to study of So-lar
system and Sun; for example, the solar featureswith sizes of the
order of 100 km or smaller werefound by means of speckle imaging
(Harvey 1972,Harvey and Breckinridge 1973). From observationsof
photospheric granulation from the disc center tothe limb, Wilken et
al. (1997) found a decreaseof the relative contrast of the
center-to-limb gran-ular intensity. A time series of high spatial
resolu-tion images revealed the highly dynamical evolutionof the
sunspot fine structure, namely, umbral dots,penumbral filaments,
facular points (Denker 1998).Small-scale brightening in the
vicinity of sunspots,was also observed in the wings of strong
chromo-spheric absorption lines. These structures which
areconcomitant with strong magnetic fields show bright-ness
variations close to the diffraction-limit (∼0.16arcsec at λ =
550nm), of the Vacuum Tower Tele-scope, Observatorio del Teide
(Tenerife). With thespeckle method, Seldin et al. (1996) found the
pho-
tosphere to be highly dynamic at scales below 0.3arcsec. Speckle
imaging has been successful in re-solving the Pluto-Charon system
(Bonneau and Foy1980), as well as in determining shapes of
asteroids(Drummond et al. 1988).
However, the investigation of Sun and So-lar system by the means
of optical interferometryhas stalled, mainly due to the fact that
satelliteobservatories and Solar system probes (which arenot
limited by atmospheric turbulence) provided avery high spatial
resolution. But, the application ofinterferometry is not excluded
as shown by Delboet al. (2009) who obtained the first successful
in-terferometric measurements of asteroid sizes andshapes by means
of the Very Large Telescope In-terferometer (VLTI) at European
Southern Observa-tory (ESO), and it’s MID-infrared Interferometric
in-strument (MIDI). They observed, as a typical benchmark, the
asteroid (951) Gaspra, visited in the pastby the Galileo space
probe, and they derived a shapein good agreement with the results
coming from thein situ measurements by the Galileo mission (see
Fig.1). They have also observed the asteroid (234) Bar-bara, known
to exhibit unusual polarimetric proper-ties, and they found
evidence of a potential binarynature. In particular, their data
were best fitted bya system of two bodies of 37 and 21 km in
diameter,separated by a center-to-center distance of ∼ 24
km(projected along the direction of the baseline at theepoch of
their observations).
Fig. 1. Interferometricaly deduced shape and imageof (951)
Gaspra taken by the Galileo mission. Com-parison of an image of
(951) Gaspra taken by theGalileo mission on 29/10/1991 at 22:26 UT
from adistance of 5300 km and the shape model of Delboet al. 2009
(dashed line) observed under the samecircumstances.
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OPTICAL INTERFEROMETRY II
3. STELLAR FUNDAMENTALPARAMETERS
Interferometry has long been a valuable toolin the study of
stellar physics, primarily in obtain-ing a broad sample of
measurements of fundamentalstellar parameters: radius, temperature,
luminosity,mass, and chemical composition. However, they arestill
not well known for a broad range of stellar types.Some properties
can be measured by traditional as-tronomical means. For example,
the chemical com-position at the surface can be determined by
study-ing spectral lines. A star’s temperature can be de-termined
from its luminosity and physical size. Todetermine the size and the
mass, astronomers requireextremely high-resolution data that are
not easily ob-tained except by interferometry.
A compilation of direct measurements by highangular resolution
methods, as well as indirect es-timates of stellar diameters can be
found in theCatalog of High Angular Resolution Measurements(CHARM,
Richichi and Percheron 2002) which in-cludes most of the
measurements obtained by thetechniques of lunar occultations and
long-baseline in-terferometry at visual and infrared wavelengths,
andwhich have appeared in the literature or were other-wise made
public until mid-2001.
Star’s mass is a crucial parameter in stellar as-trophysics,
however this valuable information cannotbe easily obtained directly
for a single star. Interfer-ometers bring a new level of resolution
to bear onspectroscopic binaries, enabling the full extractionof
physical parameters for the component stars withhigh accuracy. In
the case of double-lined systems,a geometrically determined orbital
parallax becomesavailable as presented in the CHARA (Center forHigh
Angular Resolution Astronomy) Catalog of Or-bital Elements of
Spectroscopic Binary Stars (Tayloret al. 2003).
Using the Mark III Stellar Interferometer onMount Wilson,
Mozurkewich et al. (2003) obtainedobservations of 85 stars at
wavelengths between 451and 800 nm. Angular diameters were
determinedby fitting a uniform-disc model to the visibility
am-plitude versus projected baseline length. Half theangular
diameters determined at 800 nm have for-mal errors smaller than 1
%. Further, the VLTIinterferometer and its VINCI and AMBER
near-infrared recombiners were used, together with lit-erature
measurements, to examine the luminosity-radius and mass-radius
relations for K and M dwarfs,Demory et al. (2009) obtained the
precision of inter-ferometric radii which competes with what can
beobtained for double-lined eclipsing binaries.
Following this work, an updated Cata-log of High Angular
Resolution Measurements(CHARM2), which includes results available
untilJuly 2004, has been presented by Richichi et al.(2005) and
complemented by the catalog of the an-gular sizes of dwarf stars
and subgiants (Kervella etal. 2008) who compiled the existing
long-baseline in-terferometric observations of nearby dwarf, and
sub-giant stars and the corresponding broadband pho-tometry in the
Johnson and Cousins bands. For the
Hyades giants, γ , δ1, ², and θ1Tau, from interfero-metric
measurements with the CHARA Array, Boya-jian et al. (2009) deduced
the limb-darkened angulardiameters with errors less than 2 %. In
combina-tion with additional observable quantities, they
de-termined the effective temperatures, linear radii, andabsolute
luminosities for each of these stars, provid-ing a new calibration
of effective temperatures witherrors well under 100 K.
At this precision level, the study of stellar at-mospheres
became possible as shown by Wittkowskiet al. (2004a) who presented
K-band interferometricmeasurements of the limb-darkened intensity
profileof the M4 giant star ψ Phoenicis obtained with theVLTI/
VINCI instrument. High-precision squaredvisibility amplitudes in
the second lobe of the visibil-ity function were obtained employing
two 8.2 m UnitTelescopes. In addition, they sampled the
visibilityfunction at small spatial frequencies using the 40cmtest
siderostats (see Fig. 2). Their measurement con-strained the
diameter of the star and its center-to-limb intensity variation, as
well as (together with theHipparcos parallax and evolutionary
models) stellarmass and surface gravity.
Fig. 2. ψ Phe diameter and limb darkening de-termination. The
left panel shows the full range ofthe visibility function while the
right panel is an en-largement of the low squared visibility
amplitudes inthe second lobe. Measured squared visibility
ampli-tudes of ψ Phe are shown (symbols with error bars)together
with the (solid black line) spherical radia-tive transfer model
prediction with model parametersTeff log g and mass as derived from
spectrophotometryand model evolutionary tracks, and best fitting
angu-lar diameter value. Shown are also the (dotted
anddashed-dotted line) two plane-parallel radiative trans-fer
models, all with corresponding model parametersand best fitted
angular diameter. As a reference forthe strength of the
limb-darkening, the gray lines de-note the corresponding uniform
disc (upper line) andfull darkened disc (lower line) model
visibility func-tions.
Van Belle et al. (2007) performed the directangular size
measurements of the G0 IV subgiant ηBoo from the Palomar Testbed
Interferometer (PTI),deducing a limb-darkened angular size of a
bolo-metric flux which provided an effective temperatureand
luminosity for this object. In conjunction withthe mass estimate
from the MOST asteroseismologysatellite investigation, a surface
gravity is establishedfor this star. Using the Infrared Optical
TelescopeArray (IOTA) and it’s IONIC3 recombiner, Lacour
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S. JANKOV
et al. (2008) performed spectro-interferometric ob-servations (R
= λ/∆λ ≈ 35) of Arcturus. Imagereconstruction was performed using
two software al-gorithms: Wisard and Mira (e.g. Thiebaut and
Gio-vannelli 2010). No companion was detected from theclosure
phases with an upper limit on the brightnessratio of 8x10−4 at 1
AU. The upper limit was also de-rived for the level of brightness
asymmetries presentin the photosphere.
The high precision of direct measurements ofstellar angular
diameter by the VINCI instrumenton VLTI stimulated studies which
allow the applica-tions in the asteroseismology: Kervella et al.
(2003a)compared the first direct angular diameter measure-ments
obtained on α Centauri to recent model diam-eters constrained by
asteroseismic observations. Theangular diameters of the two main
components of thesystem (α Cen A and B), were measured with a
rel-ative precision of 0.2% and 0.6% respectively whileBigot et al.
(2006) improved the angular diameter ofα Cen B using limb-darkening
predictions from a 3Dhydrodynamical radiative transfer model of its
atmo-sphere to fit the interferometric visibility measure-ments.
Pijpers et al. (2003) have studied the radiusof the nearby star τ
Cet and the limb-darkened discdiameter is determined with an
internal precision of0.5%. Kervella et al. (2003b) reported a
direct mea-surement of the angular diameter of the bright
starSirius A. They obtain a uniform disc angular diame-ter and (in
combination with the Hipparcos parallax)a linear diameter. Using
the published properties ofSirius A, they derived internal
structure models cor-responding to ages between 200 and 250 ± 12
Myr.
The prospects for using the asteroseismologyof stars were
hampered by the large uncertaintyin fundamental stellar parameters.
Particularly forrapidly oscillating Ap (roAp) results in the
litera-ture for the effective temperature (Teff) often span arange
of 1000 K. In order to reduce systematic errorsand improve the Teff
calibration of Ap stars Brunttet al. (2008) performed the first
detailed interfer-ometric study of a rapidly oscillating roAp star,
αCir. They used the Sydney University Stellar Inter-ferometer
(SUSI) to measure the angular diameterand accurate Hipparcos
parallax to determine the ra-dius constraining the bolometric flux
from calibratedspectra which determine an effective temperature,and
thus provided the first direct determination ofthe temperature of
an roAp star. Further, Brunttet al. (2010) obtained long-baseline
interferometricobservations of β CrB using the
CHARA/FLUORinstrument. By combining the flux of the A compo-nent
with its measured angular diameter, they de-termined the effective
temperature Teff(A) = 7980 ±180 K (2.3 %), with uncertainty in
effective temper-ature similar to that of α Cir (± 170 K).
Observing with the CHARA/VEGA spectro-interferometer, Bigot et
al. (2011) measured theangular diameter of the CoRoT satellite
targetHD49933. A three-dimensional radiative hydrody-namical
modeling was used to compute the limbdarkening and to derive a
reliable diameter. Theother fundamental stellar parameters (mass,
age, and
Teff) were found by fitting the large and small p-mode frequency
separations using a stellar evolutionmodel.
4. NON RADIALLY PULSATING STARS
The important observational constraints arenecessary for
detailed studies of the atmosphericstructure and pulsation as shown
by Kervella et al.(2004a) who reported the angular diameter
measure-ment obtained with the VLTI/VINCI instrument onthe nearby
star Procyon A (α CMi A, F5IV-V), at arelative precision of ± 0.9
%. They used deduced lin-ear diameter (with the Hipparcos parallax)
in com-bination with the spectroscopic, photometric and
as-teroseismic data to constrain the model of this star:age, an
initial helium mass fraction Yi, and an initialmass ratio of heavy
elements to the hydrogen. Theyalso computed the adiabatic
oscillation spectrum ofProcyon A giving a mean large frequency
separationin agreement with the seismic observations by Martićet
al. (2004). The interferometric diameter and theasteroseismic large
frequency spacing together sug-gest a mass closer to 1.4 M¯ rather
than to 1.5 M¯.From this analysis, they conclude that Procyon
iscurrently ending its life on the main sequence, as itsluminosity
class indicates. Thévenin et al. (2005)used VLTI/VINCI angular
diameter measurementsand constrained the evolutionary status of
three as-teroseismic targets: δ Eri, ξ Hya, η Boo. The mainstellar
modeling parameters: mass, age and metallic-ity were adjusted to
fit observational data (effectivetemperature Teff , luminosity L
and surface metallic-ity [Z/X]surface). In Fig. 3 representing the
zoom
Fig. 3. δ Eri evolutionary status and asteroseis-mology. Zoom of
the evolutionary tracks in the H-Rdiagram for δEri from label ”a”
(6140 Myr) to label”j” (6230 Myr), shown by lower case letters and
tri-angles with time steps of 10 Myr (except label ”G” at6200 Myr
labeled by an upper case letter). The rect-angular error boxes are
derived from the values andaccuracies of the adopted stellar
parameters while themeasured radius and its confidence interval
appear asdiagonal lines. The best model is close to label ”f”
at6194 Myr. The mean large frequency splitting foundfor the best
model is 45.27 µHz.
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OPTICAL INTERFEROMETRY II
of evolutionary tracks on HR diagram for δ Eri, therectangular
error boxes are derived from the val-ues and accuracies of the
adopted stellar parameterswhile the measured radius and its
confidence inter-val (appearing as diagonal lines) allowed the
accuratedetermination of stellar evolutionary status.
Measuring the visual orbit with the Mark IIIoptical
interferometer and the Navy Prototype Op-tical Interferometer
(NPOI), combining it with theHipparcos proper-motion-based parallax
and deter-mining the masses and magnitude difference of
thecomponents of the Hyades spectroscopic binary θ2Tauri, Armstrong
et al. (2006) found that both com-ponents appear to be less massive
and/or brighterthan predicted from some recent evolutionary
mod-els. The small scale of the λ Vir orbit (∼ 20 mas)was resolved
by Zhao et al. (2007) using the IOTAinterferometer which, together
with spectroscopicdata, allowed to determine its elements, as well
asthe physical properties of the components. The ac-curately
determined properties (0.7% and 1.5% er-rors, for A and B component
respectively) allowedcomparisons between observations and current
stel-lar evolution models, and reasonable matches arefound. The
best-fit stellar model gives λ Vir a sub-solar metallicity of
Z=0.0097 and an age of 935 Myr.The orbital and physical parameters
of the star alsoallowed to study its tidal evolution timescales
andstatus.
North et al. (2007) used the Sydney Univer-sity Stellar
Interferometer (SUSI) to measure the an-gular diameter of β Hydri.
This star is a nearbyG2 subgiant whose mean density was measured
withhigh precision using asteroseismology. They deter-mined the
radius and effective temperature of thestar and, by combining the
radius with the meandensity, made a direct estimate of the stellar
massfinding a value of 1.07 ± 0.03M¯(2.8 %). Thisvalue agrees with
published estimates based on fit-ting in the Hertzsprung-Russell
diagram but hasmuch higher precision, which places valuable
con-straints on theoretical models of β Hyi and its oscil-lation
frequencies. Mazumdar et al. (2009) studiedthe GIII red giant star
² Oph which has been foundto exhibit several modes of oscillation
by the MOSTsatellite. They interpret the observed frequencies
ofoscillation in terms of theoretical radial p-mode fre-quencies of
stellar models, and evolutionary modelsof this star, in both the
shell H-burning and core He-burning phases of evolution, are
constructed. Theyalso obtained an independent estimate of the
photo-spheric radius with highly accurate
interferometricobservations in the infrared K’ band (1.9-2.3 µm)by
using the CHARA/FLUOR instrument togetherwith the Hipparcos
parallax. The radius obtainedfrom the asteroseismic analysis
matched the inter-ferometric value quite closely even though the
radiuswas not constrained during the modeling.
However, with classical interferometric arraysit is possible to
resolve spatially only a limited num-ber of nearby bright stars and
even more difficultto measure the radii with sufficient precision
for as-teroseismology. On the other hand, the high resolu-tion
spectroscopy allows for (through Doppler Map-ping) indirect
observational information on atmo-
spheric structures of an unresolved star by model-ing the
observed flux distribution across the spec-tral lines. For
instance, the differential interferome-try (Beckers 1982, Petrov
1988) makes it possible tomeasure the shift of the stellar
photometric barycen-ter (photocenter) of an unresolved star as a
functionof wavelength, providing the first order moment ofthe
spatial brightness distribution in addition to thezero order moment
spectroscopic information, andallowing better spatial resolution of
non-radial stel-lar pulsations when compared to the classical
inter-ferometric and Doppler imaging methods (Jankov etal. 2001,
2002).
Despite of a remarkable progress in under-standing stellar
interiors, we know surprisingly littleabout the internal structure
of stars spinning neartheir critical limit. New interferometric
imaging ofthese rapid rotators (see Section 5.) combined
withadvances in asteroseismology promises to lift thisveil and
probe the strongly latitude-dependent pho-tospheric characteristics
and even reveal the internalangular momentum distribution of these
objects. Us-ing the physical properties of α Oph, recently
deter-mined from long-baseline interferometry data fromthe CHARA
Array, and the high-precision photome-try (based on 30 continuous
days of monitoring usingthe MOST satellite) of this rapidly
rotating δ Scutivariable star, Monnier et al. (2010) have
identified57 ± 1 distinct pulsation modes. Remarkably, theyhave
also discovered that the fast rotation modulatesthe low-frequency
modes identified as a rich familyof g-modes (|m| up to 7). They
found that the spac-ing of the g-modes is surprisingly linear
consideringCoriolis forces which are expected to strongly
distortthe mode spectrum suggesting prograde ”equatorialKelvin”
waves (modes l = m).
5. RAPIDLY ROTATING STARS
Optical long baseline interferometry is a pow-erful tool to
study detailed stellar shapes. In par-ticular, a rapid rotation
induces interferometric sig-natures requiring a detailed modeling
to correctlyinterpret high angular resolution data. Domicianode
Souza et al. (2002) studied the effects of uniformstellar rotation
on interferometric observables using aphysically coherent model
that includes gravity dark-ening and geometrical deformation, as
well as a ra-diation transfer code. They investigated the use
ofmulti-baseline and/or multi-wavelength-channel ob-servations,
both in the continuum and spectral lines,in order to obtain the
unique solutions for relevantmodel parameters. They showed that
this is possi-ble and provided a guide for observers to performthis
task. Soon after, Domiciano de Souza et al.(2003) reported the
first observations of a rapidly ro-tating Be star Achernar (α
Eridani), using the Earth-rotation synthesis on the VLTI (Fig. 4).
Their mea-surements correspond to an a/b = 1.56 ± 0.05 appar-ent
oblate star (Fig. 5), a and b being the equivalentuniform disc
angular radii in the equatorial and polardirection respectively.
Considering the presence
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S. JANKOV
Fig. 4. Achernar’s observations with VLTI/VINCI.VLTI ground
baselines for Achernar observationsand their corresponding
projections onto the skyat different observing times. Left: Aerial
view ofVLTI ground baselines for the two pairs of 40 cmsiderostats
used for Achernar observations. Right:Corresponding baseline
projections onto the sky asseen from the star.
Fig. 5. Achernar’s shape reconstruction. Fit ofan ellipse over
the observed squared visibilities ofAchernar translated to
equivalent uniform disc an-gular diameters. Each value is plotted
together withits symmetrical azimuthal value. The fitted
ellipsereveals an extremely oblate shape with an equatorialto polar
ratio a/b = 1.56± 0.05.
of a circumstellar envelope they argued that theirmeasurement
corresponds to a truly distorted star,since α Eridani exhibited a
negligible Hα emis-sion during their interferometric observations.
Inthis framework, they conclude that the commonlyadopted Roche
approximation (uniform rotation andcentrally condensed mass) should
not apply to this
star. This result opened new perspectives to basicastrophysical
problems such as the rotationally en-hanced mass loss and internal
angular momentumdistribution. In addition to its intimate relation
withmagnetism and pulsation, a rapid rotation thus pro-vides a key
to the Be phenomenon (see Section 6):one of the outstanding
non-resolved problems in stel-lar physics.
This astonishing result has been confirmedtheoretically (Jackson
et al. 2004) and observation-ally: McAlister et al. (2005) reported
K-band inter-ferometric observations of the bright, rapidly
rotat-ing star Regulus (type B7 V) made with the
CHARAinterferometer, deducing a high stellar obleteness of1.32±
0.06 (Fig. 6).
Fig. 6. K-band image of Regulus. The fitted ellipserevealed an
extremely oblate shape with an equatorialto polar ratio 1.32±
0.06.
Infrared interferometric angular size measure-ments using
narrowband channels in the spectrom-eter at PTI (Palomar Testbed
Interferometer, MtPalomar), indicated a non-circular projected
discbrightness distribution for the A7IV-V star Altair(val Belle et
al. 2001). Given the known rapid ro-tation of this star, they
modeled the data as arisingfrom an elongated rigid atmosphere. To
the first or-der, an ellipse with an axial ratio of a/b=1.140
±0.029 could be fitted to their interferometric diame-ter
measurements. In addition, Ohishi et al. (2004)reported an
asymmetric surface brightness distribu-tion of this rapidly
rotating star measured by theNPOI instrument. The outstanding
characteristicsof these observations were the high resolution
withthe minimum fringe spacing of 1.7 mas, easily resolv-ing the 3
mas stellar disc, and the measurement ofthe closure phase, which is
a sensitive indicator ofthe asymmetry of the brightness
distribution of thesource. They fitted the measured observables to
amodel with a bright spot on a limb-darkened disc
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OPTICAL INTERFEROMETRY II
and found that the observations are well reproducedby a bright
spot which has relative intensity of 4.7%, on the limb-darkened
stellar disc. Rapid rotationof Altair indicated that this bright
region is a pole,which is brighter than other part of the star
owingto gravity darkening. Including new data (squaredvisibilities
in the H and K bands from VLTI/VINCI)as well as previously
published data (squared visibil-ities in the K band from PTI and
squared visibili-ties, triple amplitudes, and closure phases in the
vis-ible from NPOI), Domiciano de Souza et al. (2005)showed that
the observations can only be explainedif Altair has a
gravity-darkening compatible with theexpected value for hot stars,
i.e., the von Zeipel effect(Teff ∼ g0.25).
Van Belle et al. (2006) presented observationsof the A7 IV-V
star Alderamin (α Cep) from theCHARA Array. The infrared
interferometric angu-lar size measurements indicated a non-circular
pro-jected disc brightness distribution for this rapid ro-tator.
The interferometric observations were mod-eled as arising from an
elongated rigid atmospherewith an axial ratio of 1.298 ± 0.051. The
inclinationof Alderamin to the line of sight indicated by
thismodeling is (i = 88.2◦+1.8−13.3) and the star has a rota-tional
velocity which is ∼ 83 % of breakup velocity.A remarkable aspect of
this modeling was a deter-mination of the gravity-darkening
coefficient which,at a value of β = 0.084+0.026−0.049, is
consistent with aconvective photosphere as expected for an A7
IV-Vstar.
Peterson et al. (2006a) reported the success-ful fitting of a
Roche model with a surface tempera-ture gradient following the von
Zeipel gravity dark-ening law to observations of Altair made with
theNavy Prototype Optical Interferometer. They con-firm the claim
by Ohishi et al. (2004) and Domicianode Souza et al. (2005) that
Altair displays an asym-metric intensity distribution due to
rotation. Themodeling indicated that Altair is rotating at 0.90
±0.02 of its breakup (angular) velocity. Their resultsare
consistent with the apparent oblateness found byvan Belle et al.
and show that the true oblatenessis significantly larger owing to
an inclination of therotational axis of ∼ 64◦. Of particular
interest, theyconclude that instead of being substantially
evolvedas indicated by its classification A7 IV-V, Altair isonly
barely off the zero-age main sequence and rep-resents a good
example of difficulties rotation can in-troduce in the
interpretation of this part of the HRdiagram.
Peterson et al. (2006b) reported a closurephase optical
interferometric observations at theNPOI that show that Vega has the
asymmetricbrightness distribution of the bright, slightly off-set
polar axis of the star rotating at 93% of itsbreakup velocity. In
addition to explaining the un-usual brightness and line shape
peculiarities, thisresult leads to the prediction of an excess of
near-infrared emission compared to the visible, in agree-ment with
observations. The large temperature dif-ferences predicted across
its surface call into ques-tion composition determinations, adding
uncertaintyto Vega’s age and opening the possibility that its
debris disc could be substantially older than previ-ously
thought. Yoon et al. (2008) reported a reanal-ysis of Vega’s
composition. A full spectral synthesisbased on the Roche model
derived earlier from NPOIinterferometry is used. They find the line
shapesin Vega’s spectrum to be more complex than justflat-bottomed,
which have been previously reported.They investigate the effects of
rotation on the de-duced abundances and show that the dominant
ion-ization states are only slightly affected compared toanalyses
using nonrotating models. They argue thatthe rapid rotation
requires the star to be fully mixed.This composition leads to
masses and particularlyages that are quite different compared to
what wereusually assumed.
Zhao et al. (2009) presented sub-milli-arcsecond resolution
imaging and modeling of twonearby rapid rotators, α Cephei and α
Ophiuchi,obtained with the CHARA array. Incorporating
agravity-darkening model, they determine the incli-nation, the
polar and equatorial radius and temper-ature, as well as the
fractional rotation speed of thetwo stars with unprecedented
precision. The polarand equatorial regions of the two stars have ∼
2000K temperature gradient causing their apparent tem-peratures and
luminosities to be dependent on theirviewing angles. Their modeling
(Fig. 7) allowed todetermine the true effective temperatures and
lumi-nosities of α Cep and α Oph permitting to investigatetheir
true locations on the H-R diagram. These prop-erties in turn give
estimates of the masses and ages ofthe two stars within a few
percent of error using stel-lar evolution models. Also, based on
their gravity-darkening modeling, they propose a new method
toestimate the masses of single stars in a more directway through
Ve sin i measurements and precise geo-metrical constraint.
Fig. 7. Best-fit standard gravity-darkening model ofα Oph. The
contours in the left panel indicate the lo-cal brightness
temperatures on the surface of the star.The right panel shows the
latitude and longitude ofα Oph to help visualize its geometry.
An alternative approach to study the rapidlyrotating stars has
been proposed by Jankov et al.(2003a and 2003b) who considered the
case of stellaractivity, showing the potential of new methods
whichcombine the classical spectroscopy (TomographicImaging) and
Long Baseline Interferometry, provid-ing informations that cannot
be obtained otherwiseby each of these techniques taken separately.
By
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S. JANKOV
means of numerical simulations, Rousselet-Perrautet al. (2004)
investigated the ability of optical inter-ferometry via the fringe
phase observable to addressstellar activity and magnetism. To
derive abundancemaps and stellar rotation axes they use the
colordifferential interferometry which couples high angu-lar
resolution to high spectral resolution while, toconstrain magnetic
field topologies, they add a po-larimetric mode. They emphasize the
crucial needfor developing and validating inversion algorithms
sothat future instruments on optical aperture synthesisarrays can
be optimally used.
6. Be STARS
Be stars show evidence of mass loss and cir-cumstellar envelopes
(CSE) from UV resonance lines,near-IR excesses, and the presence of
episodic hydro-gen emission lines. The geometry of these
envelopesis still uncertain, although it is often assumed thatthey
are formed by a disc around the stellar equatorand a hot polar
wind. This envelope emission is moreextended, and thus more easily
resolvable, than thetiny photosphere itself. The envelope of γ Cas
wasfirst resolved by Thom et al. (1986) using the
I2Tinterferometer, and Mourard et al. (1989) using theGI2T
interferometer. The high spectral resolutionof GI2T later also
uncovered asymmetric emission inthese Be star envelopes in Hα (Stee
et al. 1995), andobserved in other lines too (Stee et al. 1998).
Vakiliet al. (1998) proposed that the emission line regionis very
one-sided and time-variable.
With a good range of baselines, the Mark IIIinterferometer was
able to detect definite asymme-tries in γ Cas and ζ Tau
(Quirrenbach et al. 1993,1994) while Tycner et al. (2004) presented
opticalinterferometric observations of ζ Tau obtained usingthe
NPOI. The observations suggested a strong de-parture from circular
symmetry which has been de-scribed by an elliptical model. Tycner
et al. (2005)presented the long-baseline interferometric
observa-tions, obtained with the NPOI, of the Hα-emittingenvelopes
of the Be stars η Tau and β CMi, demon-strating a clear dependence
of the net Hα emission onthe linear size of the emitting region.
These resultsare consistent with an optically thick line
emissionthat is directly proportional to the effective area ofthe
emitting disc. Chesneau et al. (2005a) presentedthe first VLTI/MIDI
observations of the Be star αAra showing a nearly unresolved
circumstellar discin the N band. These measurements put an
upperlimit on the envelope size corresponding to 14 R∗assuming
R∗=4.8 R¯ and the Hipparcos distance of74 pc. These observations
also placed complemen-tary constraints on the density and geometry
of theα Ara circumstellar disc.
Tycner et al. (2006) presented interferometricobservations of
two well-known Be stars, γ Cas andϕ Per, collected and analyzed to
determine the spa-tial characteristics of their circumstellar
regions. Theobservations were obtained using the NPOI equippedwith
narrowband filters which isolate the Hα emis-sion line from the
nearby continuum radiation, re-sulting in an increased contrast
between the inter-
ferometric signature due to the Hα-emitting circum-stellar
region and the central star, thus allowing theinterferometric
signal in the Hα channel to be cali-brated with respect to the
continuum channels. Theobservations used in this study represent
the highestspatial resolution measurements of the
Hα-emittingregions of Be stars obtained to date. These
observa-tions allowed them to demonstrate for the first timethat
the intensity distribution in the circumstellarregion of a Be star
cannot be represented by a uni-form disc or ring-like structures
whereas a Gaussianintensity distribution appeared to be fully
consistentwith their observations. Kervella and Domiciano deSouza
(2006a) presented long-baseline interferomet-ric observations of
Achernar with the VLTI/VINCIbeam recombiner in the H and K bands by
usingvarious telescope configurations and baseline lengthswith a
wide azimuthal coverage. They clearly de-tected a CSE elongated
along the polar axis of thestar as well as rotational flattening of
the stellar pho-tosphere. They conclude that this CSE could
belinked to free-free emission from the radiative pres-sure driven
wind originating from the hot polar capsof the star.
Gies et al. (2007) presented the first K’-band,long-baseline
interferometric observations of the Bestars γ Cas, ϕ Per, ζ Tau,
and κ Dra. The measure-ments were made with multiple telescope
pairs ofthe CHARA interferometer resolving the circumstel-lar discs
of the targets. The disc resulting densitiesare in broad agreement
with prior studies of the IRexcess flux, and the resulting
orientations generallyagree with those from interferometric Hα and
contin-uum polarimetric observations. They find that theangular
size of the K’ disc emission is smaller thanthat determined for the
Hα emission, and they arguethat the difference is the result of a
larger Hα opac-ity and the relatively larger neutral hydrogen
frac-tion with increasing disc radius. Domiciano de Souzaet al.
(2007) presented the first high spatial andspectral resolution
observations of the circumstellarenvelope of a B[e] supergiant
(CPD-57◦2874), per-formed with the VLTI. Spectra, visibilities and
clo-sure phase were obtained using the beam-recombinerinstruments
AMBER, near-IR interferometry withthree 8.3 m Unit Telescopes (UTs)
and MIDI withtwo UTs. The interferometric observations of theCSE
are well fitted by an elliptical Gaussian modelwith FWHM diameters
varying linearly with wave-length. The major-axis position angle of
the elon-gated CSE in the mid-IR agrees well with
previouspolarimetric data, hinting that the hot-dust
emissionoriginates in a disc-like structure and supporting
thenon-spherical CSE paradigm for B[e] supergiants.
Using the VLTI/AMBER instrument operat-ing in the K-band,
Meilland et al. (2007a) studiedthe geometry and kinematics of the
disc around theBe star α Arae as a function of wavelength,
espe-cially across the Brγ (λ 2.1657 µm) emission line,which
provided a gain by a factor of 5 in spatial res-olution compared to
previous VLTI/MIDI observa-tions. Consequently, it was possible to
combine thehigh angular resolution provided with the medium(R∼
1500) spectral resolution of AMBER to studythe kinematics of the
inner part of the disc and to in-
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OPTICAL INTERFEROMETRY II
fer its rotation law. For the first time, they obtaineddirect
evidence that the disc is in the Keplerian ro-tation, answering the
question that has existed sincethe discovery of the first Be star
(γ Cas). They foundthat the disc around α Arae is compatible with
adense equatorial matter confined to the central re-gion whereas a
polar wind is contributing along therotational axis of the central
star (Fig. 8).
Fig. 8. Models of α Arae. The four upper panelsare a cut of the
circumstellar disc in a plane definedby the observer line of sight
and the stellar rotationalaxis (the observer is on the right in
each picture);the corresponding projections onto the sky-plane
withthe interferometric data points from MIDI and AM-BER
over-plotted are the central pictures, whereas a3Dview’ is plotted
into the lower row for each model.
Meilland et al. (2007b) observed withVLTI/AMBER the
circumstellar environment of theBe star κ CMa in the Brγ emission
line and its nearbycontinuum to study the kinematics within the
discand to infer its rotation law. Using differential vis-ibilities
and differential phases across the Brγ linethey detected an
asymmetry in the disc (see Fig. 9).Moreover, they found that κ CMa
seems difficult tofit within the classical scenario for Be stars,
illus-trated by previous α Arae observations, i.e. a fast ro-tating
B star close to its breakup velocity surroundedby a Keplerian
circumstellar disc with an enhancedpolar wind. They discuss the
possibility that κ CMais a critical rotator with a Keplerian
rotating discand examine whether the detected asymmetry canbe
interpreted within the ”one-armed” viscous discframework.
Meilland et al. (2008) studied the Be star δCen circumstellar
disc in the H and K band usinglow (R=35) and medium (R=1500)
spectral resolu-tion observations. They detected an oscillation in
thevisibility curve plotted as a function of the spatialfrequency
which is a clear signature of a companionaround the star. They also
report an envelope fluxaround the Be primary that contributes up to
about50 % of the total flux, in agreement with SpectralEnergy
Distribution (SED). The envelope size wasestimated but no departure
from spherical symme-try was detected.
Fig. 9. κ CMa intensity map in the contin-uum at 2.15µm. The
inclination angle is 60◦, thecentral black dot represents the κ CMa
photosphere(0.25mas).
Using the VLTI/VINCI instrument, Carciofiand Domiciano de Souza
(2008) performed a new in-terferometric study of Achernar,
identifying two dif-ferent disc models that simultaneously fit the
spec-troscopic, polarimetric, and interferometric observa-tional
constraints: a tenuous disc in hydrostatic equi-librium (i.e. with
small scale height) and a smaller,scale height enhanced disc. They
concluded thatthese relatively small discs correspond to the
tran-sition region between the photosphere and the cir-cumstellar
environment and that they are probablyperturbed by some
photospheric mechanism.
Kervella et al. (2009) searched for the signa-ture of
circumstellar emission at distances of a fewstellar radii from
Achernar, in the thermal IR do-main. They obtained interferometric
observationson three VLTI baselines in the N band (8-13 µm),using
the MIDI instrument. From the measured vis-ibilities, they derive
the angular extension and fluxcontribution of the N band
circumstellar emission inthe polar direction of Achernar. This flux
contribu-tion is in good agreement with the photometric IRexcess
measured by fitting the spectral energy dis-tribution. They
concluded that this polar envelope,already detected at 2.2 µm, is
most probably an ob-servational signature of the fast wind ejected
by thehot polar caps of the star.
Meilland et al. (2009) obtained calibratedvisibility
measurements for stars: p Car, ζ Tau,κ CMa, α Col, δ Cen and β CMi,
α Ara, usingthe VLTI/MIDI instrument in the N band. Theycompared
their results with previous K band mea-surements obtained with the
VLTI/AMBER instru-ment and/or the CHARA interferometer,
concludingthat the size of the circumstellar envelopes for
theseclassical Be stars does not seem to vary stronglyon the
observed wavelength between 8 and 12 µm.Millan-Gabet et al. (2010)
presented near-infraredH and K-band spectro-interferometric
observationsof the gaseous disc around the primary Be star inthe δ
Sco binary system. Using observations at theCHARA/MIRC instrument
in the H band they re-solved an elongated disc, while using the
Keck Inter-
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S. JANKOV
ferometer (KI) the source of the K-band continuumemission was
only marginally spatially resolved. Onthe other hand, the line
emission in He I λ 2.0583µm and Brγ, was clearly detected with ∼ 10
% lowervisibilities than those of the continuum. When tak-ing into
account the continuum/line flux ratio thistranslates into much
larger sizes for the line emis-sion regions. Their KI data also
reveal a relativelyflat spectral differential phase response,
ruling out asignificant off-center emission.
Meilland et al. (accepted 2011) carried outobservations of the
sample composed of eight brightclassical Be stars: α Col, κ CMa, ω
Car, pCar, δ Cen, µ Cen, α Ara, and o Aqr, with theVLTI/AMBER
instrument combining high spectral(R=12000) and high spatial
(θmin=4mas) resolu-tions. They determined the disc extension in
theline and the nearby continuum for most targets, con-straining
the disc kinematics and showing that it isdominated by rotation
with a rotation law close tothe Keplerian one. The survey also
suggests thatthese stars are rotating below their critical
velocities(Vc) with a mean rotational rate of 0.82±0.08 Vc,leading
to the conclusion that their Be stars samplesuggests that the
rotation alone cannot explain theorigin of the Be phenomenon and
that other mecha-nisms are playing a non-negligible role in the
ejectionof matter.
7. RED GIANTS ANDSUPERGIANTS: Betelgeuse
Historically, red giants and supergiants wereone of the favorite
targets for stellar interferometry,and there are many of them for
which the funda-mental parameters are precisely derived.
However,the most famous case in the quest for surface
inho-mogenities in giants stars is the M2 supergiant Betel-geuse (α
Ori). The interferometry research to studythe inhomogenities of
stellar surfaces began by us-ing (visible-light) aperture masking
on the WilliamHerschel telescope in the Canary Islands (Baldwinet
al. 1986, Haniff et al. 1987). Interferometricnon-redundant masking
imaging was performed andearly results showed bright features
(strong depar-ture from circular symmetry) on the surface of
theBetelgeuse (Buscher et al. 1990), confirming someprevious
reports (e.g. Roddier and Roddier 1983).No long baseline
(separate-element) interferometerwould be able to investigate the
nature of these fea-tures for years, and the Cambridge masking
grouphas spent more than a decade thoroughly investigat-ing
”hotspots” on red supergiants and giants. Overthe two last decades,
it was shown that asymmetriesare common (although not omnipresent)
around redsupergiants and giants at visible wavelengths (Wil-son et
al. 1992, Tuthill et al. 1997, 1999), thatthese hotspots vary on a
timescale of months (Wil-son et al. 1997), and that the asymmetries
be-come less-pronounced (even disappearing) into theIR (Young et al
2000). However, the first image ofa stellar photosphere using
optical aperture synthe-
sis with the Cambridge Optical Aperture SynthesisTelescope
(COAST) interferometer showed a feature-less (uniform and
circularly symmetric disc) Betel-geuse (Burns et al. 1997).
Ohnaka et al. (2009) presented spatially re-solved,
high-spectral resolution K-band observationsof the Betelgeuse using
VLTI/AMBER in order toprobe inhomogeneous structures in the
dynamicalatmosphere of the star. Betelgeuse was observed inthe
wavelength range between 2.28 and 2.31 µm withspectral resolutions
of 4800-12000 allowing to studyinhomogeneities seen in the
individual CO first over-tone lines. The data could roughly be
explained by asimple model in which a patch of CO gas is
movingoutward or inward with velocities of 10-15 km s−1,while the
CO gas in the remaining region in the at-mosphere is moving in the
opposite direction at thesame velocities. Also, the data are
consistent withthe presence of warm molecular layers extending
to1.4-1.5 R∗. Haubois et al. (2009) reported on
H-bandinterferometric observations of Betelgeuse made bythe
three-telescope interferometer IOTA. They im-aged the star and its
asymmetries to deduce the spa-tial variation of the photosphere,
including its di-ameter, limb darkening, effective temperature,
sur-rounding brightness, and star spots. Resolved im-ages of
Betelgeuse in the H band are asymmetricat the level of a few
percent while the amount ofmeasured limb-darkening is in good
agreement withmodel predictions. The two spots imaged at the
sur-face of the star are potential signatures of convectivecells
(Fig. 10).
Fig. 10. Image reconstruction of Betelgeuse. Upperpanel: contour
image reconstruction of Betelgeusefrom MIRA. Lower panel: contour
image reconstruc-tion from WISARD of Betelgeuse in a 60 mas
field.Both images were reconstructed with the same apri-ori object
and the same type of regularization.
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OPTICAL INTERFEROMETRY II
Ravi et al. (2011) presented an interferomet-ric study of the
continuum surface of Betelgeuseat 11µm wavelength, using data
obtained with theBerkeley Infrared Spatial Interferometer (ISI)
eachyear between 2006 and 2010. These data allowed aninvestigation
of an optically thick layer within 1.4R∗, which has an optical
depth of ∼ 1 at 11 µm,and varies in temperature between 1900 K and
2800K and in outer radius between 1.16 and 1.36 stellarradii. The
layer has a non-uniform intensity distribu-tion that changes
between observing epochs. Theseresults indicate that large-scale
surface convective ac-tivity strongly influences the dynamics of
the inneratmosphere of Betelgeuse and mass-loss processes.
8. ASYMPTOTIC GIANTBRANCH STARS
The apparent interferometric sizes of variablered giants vary
dramatically up to a factor of 3 withwavelength and pulsation phase
(Tuthill et al. 1995,Weiner et al. 2000, Mennesson et al. 2002,
Perrin etal. 2004a, Weiner 2004). These large variations chal-lenge
the hydrodynamical and line opacity modelsof these stars indicating
that their extended atmo-spheres are extremely complex. This
recently stim-ulated a large sample of stars to be observed
usingbenefits of high spatial resolution provided by newgeneration
of interferometers. Infrared interferome-try of Asymptotic Giant
Branch (AGB) stars has re-cently been reinterpreted as revealing
the presence ofdeep molecular layers. Empirical models for a
photo-sphere surrounded by a simple molecular layer or en-velope
have led to a consistent interpretation of pre-viously inconsistent
data. The stellar photospheresare found to be smaller than
previously understoodand the molecular layer is much higher and
denserthan predicted by the hydrostatic equilibrium.
The size of the continuum photospheres of αOri, α Her, R Leo,
and χ Cyg have been measuredat 11 µm by Weiner et al. (2003), using
heterodyneinterferometry (obtained from ISI) to accuracies ashigh
as 1%. The resulting apparent diameters forα Ori and α Her are ∼ 30
% larger than measurednear-infrared sizes, whereas the Mira
variables R Leoand χ Cyg, have 11 µm apparent diameters,
roughlytwice their reported near-infrared sizes. Mennessonet al.
(2005) reported interferometric observationsof the semiregular
variable star RS CrB, a red gi-ant with strong silicate emission
features. The datawere among the first long baseline mid-infrared
stel-lar fringes obtained between the Keck telescopes, us-ing parts
of the new nulling beam recombiner. Thelight was dispersed by a
low-resolution spectrome-ter allowing simultaneous measurement of
the sourcevisibility and intensity spectra from 8 to 12 µm, andthe
observations provided a non-ambiguous deter-mination of the dust
shell spatial scale and relativeflux contribution. Ragland et al.
(2006) have mea-sured non-zero closure phases for the sample of
56nearby AGB stars using the three-telescope IOTA in-terferometer
at near-infrared wavelengths (H band)and with angular resolutions
in the range 5-10 mas.Reporting the measured stellar angular sizes,
they
hypothesize that most Mira stars would show de-tectable
asymmetry if observed with adequate an-gular resolution since the
detected non-zero closurephases can only be generated by asymmetric
bright-ness distributions of the target stars or their
sur-roundings. Deroo et al. (2006) presented the firstmid-IR long
baseline VLTI/MIDI interferometric ob-servations of the
circumstellar matter around binarypost-AGB stars, SX Cen and HD
52961, confirmingthe disc interpretation of the spectral energy
distri-bution of both stars.
8.1. Cepheids
Although phenomenologically related to mea-surements of
pulsating AGB stars, observations ofCepheids are quite distinct in
their scientific goals.As has been discussed (e.g. Davis 1976),
optical in-terferometry should play an important role in
inde-pendent calibration of the Cepheid distance scale,a crucial
element of the cosmic distance ladder.By measuring the changing
diameter of a nearbyCepheid and the radial velocity curve through a
pul-sation cycle, the distance can be directly inferred viathe
Baade-Wesselink method, and unbiased angulardiameter measurements
are required for accurate dis-tances to Cepheids.
The initial results have appeared from GI2T(Mourard et al.
1997), NPOI (Armstrong et al.2001), IOTA (Kervella et al. 2001),
and PTI (thefirst direct detection of Cepheid pulsation; Lane etal.
2000). The field is rapidly developing, both ob-servationally and
theoretically. Lane et al. (2002)presented observations of the
Galactic Cepheids ηAql and ζ Gem, and their observations were able
toresolve the diameter changes associated with pulsa-tion. This
allows the distance to the Cepheids tobe determined independently
of photometric obser-vations, providing calibration of surface
brightnessrelations for use in extragalactic distance
determi-nation. They also provided a measurement of themean
diameter of these Cepheids which is crucialfor constructing
reliable structural models of thisstellar class. Nordgren et al.
(2002) used directdiameter observations of Cepheid variables to
cali-brate the Barnes-Evans Cepheid surface brightnessrelation.
Fifty-nine separate Cepheid diameter mea-surements from four
different optical interferometerswere used to calculate surface
brightnesses as a func-tion of magnitude and color. The linear
relation toCepheid surface brightness versus color was in
ex-cellent agreement with functions found using inter-ferometric
observations of nonvariable giant and su-pergiant stars. Using
these relations, they deduceddistance of δ Cephei, and compared it
to an inde-pendent distance which is known from
trigonometricparallax.
Kervella et al. (2004b) reported the angulardiameter
measurements of seven classical Cepheids,X Sgr, η Aql, W Sgr, ζ
Gem, β Dor, Y Oph andl Car, obtained with the VLTI/VINCI
instrument.They also used reprocessed archive data obtainedwith the
IOTA/FLUOR instrument on ζ Gem, inorder to improve the phase
coverage of their obser-vations. They obtained average limb
darkened angu-
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S. JANKOV
lar diameters for four of these stars, η Aql, W Sgr,β Dor, and l
Car, and they detected the pulsationalvariation of their angular
diameters. This allowedto compute directly their distances, using a
modifiedversion of the Baade-Wesselink method. Kervella etal.
(2004c) derived new calibrations of the Cepheidperiod-radius (P-R)
and period-luminosity (P-L) re-lations using the interferometric
angular diametermeasurements of seven classical Cepheids reportedin
Kervella et al. (2004b), and complemented bypreviously existing
measurements. Taking advan-tage of the significantly larger color
range covered bythese observations, they derived high precision
cali-brations of the surface brightness-color relations us-ing
exclusively Cepheid observations, making it pos-sible to improve
the distance to Cepheids througha Baade-Wesselink type technique.
However, usingthis method, the distance is known to a
multiplica-tive factor called the projection factor. This factorhas
been measured directly, for the first time usinginterferometry at
the CHARA Array by Mérand etal. (2005) who deduced a geometrical
measurementof the projection factor of δ Cep.
VLTI observations of the brightest and an-gularly largest
classical Cepheid, l Carinae have re-solved with high precision the
variation of its angulardiameter with phase. Kervella et al.
(2004d) com-pared the measured angular diameter curve to theone
that they derive by an application of the Baade-Wesselink-type
infrared surface brightness techniqueand found an almost perfect
agreement between thetwo curves. The mean angular diameters of l
Carfrom the two techniques agree very well within theirtotal error
bars (1.5 %) as do the derived distances(4 %) indicating that the
calibration of the surfacebrightness relations used in the distance
determina-tion of far-away Cepheids is not affected by large
bi-ases. Kervella et al. (2006b) presented the results
oflong-baseline interferometric observations of l Cari-nae in the
infrared N (8-13 µm) and K (2.0-2.4 µm)bands, using the MIDI and
VINCI instruments of theVLT Interferometer. In the N band they
resolved alarge circumstellar envelope. The signature of
thisenvelope was also detected in the K band data asa deviation
from a single limb darkened disc visibil-ity function. Considering
a possibility that this CSEis linked to the relatively large mass
loss rate of lCar, they discuss an analogy with the molecular
en-velopes of Red supergiants, and Miras. Mérand etal. (2006)
presented the results of long-baseline in-terferometric
observations of the classical CepheidsPolaris (α UMi) and δ Cep in
the near infrared K’band, using the CHARA/FLOUR Array.
Followingtheir previous detection of a circumstellar envelopearound
l Car (Kervella et al. 2006b), they reportsimilar detections around
Polaris and δ Cep.
Mérand et al. (2007) observed a Cepheid YOph for which the
pulsation has been well resolvedusing the long-baseline
near-infrared interferometrywith CHARA/FLOUR. They found that the
obser-vations suggest the star surrounded by a circumstel-lar
envelope with characteristics similar to that foundpreviously
around other Cepheids, pointing towardthe conclusion that most
Cepheids are surrounded
by faint circumstellar envelopes. Observations of thesouthern
Cepheid l Car to yield the mean angular di-ameter and angular
pulsation amplitude, have beenmade with the SUSI array at a
wavelength of 696nm by Davis et al. (2009). The interferometric
re-sults have been combined with radial displacementsof the stellar
atmosphere derived from selected radialvelocity data to determine
the distance and mean di-ameter of l Car, showing excellent
agreement withpreviously published values. However, no evidencewas
found for a circumstellar envelope at 696 nm.
8.2. Miras
Mira variables are low- to intermediate-massasymptotic giant
branch (AGB) stars that pulsatewith a period of about one year.
They have a cool(Teff≤ 3000 K) and extended (R > 100R∗)
photo-sphere. The circumstellar environment of Mira vari-able stars
is characterized by cool temperatures andrelatively high densities
leading to a richer chemistrythan that found in hotter stars and to
formation ofsolid-state dust grains.
The long-term monitoring of Mira variablesat the PTI performed
by Thompson et al. (2002a)provided high-resolution narrowband
angular sizes ofthe oxygen-rich Mira S Lac and the carbon-rich
MiraRZ Peg in the near-infrared. Their data set spannedthree
pulsation cycles for S Lac and two pulsation cy-cles for RZ Peg and
represents the first study to cor-relate multi-epoch narrowband
interferometric dataof Mira variables. As a part of a long-term
observa-tional program using the IOTA Array, van Belle etal. (2002)
presented angular size measurements of 22oxygen-rich Mira variable
stars in order to character-ize the observable behavior of these
stars. A simpleexamination of the resultant sizes of these stars
inthe context of pulsation mode was consistent with atleast some of
these objects pulsating in the funda-mental mode.
Thompson et al. (2002b) reported high-resolution (< 0.05 mas)
angular size measurementsof the Mira variable star R Tri using the
PTI in the Kband (2.0-2.4 µm), and modeling with simple
geome-tries. For the axially symmetric models, the positionangles
are roughly perpendicular to visual polariza-tion position angles,
which supports an axially sym-metric source of light scattering.
For the elliptical ge-ometry, the axial ratio of 1.33 has been
determined,similar to that previously determined for other Miraand
semiregular variable stars.
Mennesson et al. (2002) observed nine brightO-rich Mira stars
and five semiregular variable coolM giants with the IOTA
interferometer in both K’ (∼2.15 µm) and L’ (∼ 3.8 µm) broadband
filters. Toexplain large apparent diameter increases betweenthe K’
and L’ bands they propose a simple two-component model consisting
of a warm (1500-2000K), extended (up to ∼ 3 stellar radii),
optically thin(τ ∼ 0.5) layer located above the classical
photo-sphere which could explain the observed variationof Mira
uniform disc diameters. This interpretationwas consistent with the
extended molecular gas lay-ers (H2O , CO, etc.) inferred around
some of theseobjects from previous IOTA K’-band interferometric
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OPTICAL INTERFEROMETRY II
observations. The two-component model had imme-diate
implications: the Mira photosphere diametersare smaller than
previously recognized which implieshigher effective temperatures,
favoring fundamentalmode pulsation. Using the same instrument,
Hof-mann et al. (2002) presented K’-band observationsof five Mira
stars: M-type Miras: X Oph, R Aql, RUHer, R Ser, and the C-type
Mira V CrB. The derivedangular Rosseland radii and the bolometric
fluxesallowed the determination of effective temperatureswhich,
together with deduced linear radii, confirmedfundamental mode
pulsation interpretation.
Perrin et al. (2004b) have observed Mira starson the IOTA
interferometer in narrow bands around2.2 µm wavelength, finding
systematically larger di-ameters in bands contaminated by water
vapor andCO. Their visibility measurements could be inter-preted by
a model comprising a photosphere sur-rounded by a thin spherical
molecular layer while thededuced photospheric diameters were found
smallerthan in previous studies by several tens of
percent,indicating that all Mira stars are fundamental
modepulsators; the previous studies leading to the conclu-sion of
the first-overtone mode were biased by toolarge diameter
estimates.
Woodruff et al. (2004) presented K-band ob-servations of the
Mira star prototype o Cet obtainedby the VLTI/VINCI instrument and
two siderostats.Their comparison of deduced Rosseland radii,
effec-tive temperatures, and the shape of the observedvisibility
functions with model predictions confirmedthat o Cet is a
fundamental mode pulsator. Ohnakaet al. (2005) presented the
results of the firstmid-infrared interferometric observations of
the Miravariable RR Sco with the VLTI/MIDI, togetherwith K-band
observations using VLTI/VINCI. Theirmodel calculations show that
optically thick emis-sion from a warm molecular envelope consisting
ofH2O and SiO can cause the apparent mid-infrareddiameter to be
much larger than the continuum di-ameter. The observed increase of
the uniform-discdiameter longward of 10 µm could be explained byan
optically thin dust shell consisting of silicate andcorundum
grains. Millan-Gabet et al. (2005) pre-sented the first spatially
resolved observations of asample of 23 Mira stars simultaneously
measured inthe near-infrared J, H, and K’ bands, using the
IOTAinterferometer. For each star they present visibil-ity
amplitude measurements as a function of wave-length, revealing the
general relation: J diameter <H diameter < K’ diameter.
Ireland et al. (2005) performed optical inter-ferometric
polarimetry measurements of the Mira-like variables R Car and RR
Sco, using the SUSI ar-ray. By making visibility measurements in
two per-pendicular polarizations, the relatively
low-surfacebrightness light scattered by atmospheric dust couldbe
spatially separated from the bright Mira photo-spheric flux. This
was the first reported successfuluse of long-baseline optical
interferometric polarime-try. Observations were able to place
constraints onthe distribution of circumstellar material; the
innerradius of dust formation for both stars was foundto be less
than 3 stellar radii: much closer thanthe expected innermost stable
location for commonly
assumed astrophysical ”dirty silicate” dust (silicatedust with a
significant iron content) in these systems.Fedele et al. (2005)
presented near-infrared K-bandinterferometric measurements of the
Mira star RLeonis obtained with the VLTI/VINCI. These mea-surements
indicate a center-to-limb intensity varia-tion that is clearly
different from a uniform disc in-tensity profile. Also, they showed
that these mea-sured visibility values are consistent with
predic-tions from recent self-excited dynamic Mira modelatmospheres
that include molecular shells close tocontinuum-forming layers.
Perrin et al. (2005) re-ported IOTA/FLUOR interferometry with
narrowspectral bands, isolating the near-continuum andstrong
molecular features, obtained for the super-giant µ Cep. Their model
shows that a stellar pho-tosphere of angular diameter 14.11 ± 0.60
mas issurrounded by a molecular layer of diameter 18.56 ±0.26
mas
Boboltz et al. (2005) presented the first co-ordinated Very Long
Baseline Array (VLBA)/VeryLarge Telescope Interferometer (VLTI)
measure-ments of the stellar diameter and circumstellar atmo-sphere
of a Mira variable star. Observations of theSiO maser emission
toward the Mira variable S Oriwere conducted using the VLBA, while
near-infraredK-band measurements of the stellar diameter
wereperformed using VLTI/VINCI and closely spaced intime to the
VLBA observations. Their measure-ments show that the masers lie
relatively close to thestellar photosphere at a distance of ∼2
photosphericradii, consistent with model estimates. Weiner etal.
(2006) performed mid-infrared observations ofIK Tau at 11.15 µm
with the three-telescope ISI,and also using individual segments of
the Keck tele-scope for multiple-aperture interferometry at 10.7µm.
Both experiments provided closure phase andshow temporal variations
and asymmetries in thesurrounding dust, with a difference of about
15 % inintensity between two sides of the star. Comparisonwith
earlier interferometric measurements showed asubstantial reduction
in dust surrounding the starover one decade. Interferometric
observations of sixMira-type stars: R Aqr, CIT 3, χ Cyg, W Aql,
RLeo, and U Ori have been performed by Tatebe et al.(2006) at
Infrared Spatial Interferometer (ISI) whichwas comprised of three
telescopes. The deduced one-dimensional images show significant
changes in thestars and surrounding dust between the two
consec-utive years, indicating a non-constant gas emission.
Wittkowski et al. (2007) presented the firstmulti-epoch study
that includes mid-infrared and ra-dio interferometry of the
oxygen-rich Mira star SOri obtained with VLTI/MIDI at four epochs
andmaser emission obtained with the VLBA at threeepochs. They
concluded that S Ori shows significantphase-dependences of
photospheric radii and dustshell parameters and that the Al2O3 dust
grains andSiO maser spots form at relatively small ∼
1.8-2.4photospheric radii. Their results suggest increasedmass loss
and dust formation close to the surfacenear the minimum visual
phase when Al2O3 dustgrains are co-located with the molecular gas
andthe SiO maser shells, and a more expanded dustshell after visual
maximum. Ohnaka et al. (2007)
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S. JANKOV
carried out VLTI/MIDI observations of carbon-richMira variable V
Oph at three different phases withthree different baselines using
four 8.2 m Unit Tele-scopes. Their observations and modeling
indicatethat carbon-rich Miras also have extended layersof
polyatomic molecules as previously confirmed inoxygen-rich Miras.
The temporal variation of theN-band angular size has been found to
be largelygoverned by the variations of the opacity and the
ge-ometrical extension of the C2H2 layers and the dustshell, which
masks the size variation of the photo-sphere.
Using a grism at the Keck Interferometer, Eis-ner et al. (2007a)
obtained spectrally dispersed (R ∼230) interferometric measurements
of the Mira starR Vir. Their data showed that the measured radiusof
the emission varies substantially from 2.0 to 2.4µm and simple
models could reproduce these varia-tions using extended molecular
layers which absorbstellar radiation and reemit it at longer
wavelengths.As they observed spectral regions with and
withoutsubstantial molecular opacity, they could determinethe
stellar photospheric radius, uncontaminated bymolecular emission.
They infer that most of themolecular opacity arises at
approximately twice theradius of the stellar photosphere.
Asymmetries and motions in the dust shellsurrounding o Ceti have
been reported by Chan-dler et al. (2007). The measurements were
takenwith the ISI, a three-element interferometer operat-ing at
11.15 µm and three years of data permiteddetection of the movement
of dust shells in time.Wittkowski et al. (2008) presented J, H, K
spec-trally dispersed interferometry of S Ori with a spec-tral
resolution of 35 for the Mira variable S Ori-onis, with the
VLTI/AMBER instrument between1.29 µm and 2.32 µm. The measured
visibility anduniform disc diameter variations with wavelength
re-semble and generally confirm the predictions by pre-vious
dynamic model atmospheres. These size vari-ations with wavelength
could be understood as theeffects arising from water vapor and CO
layers lyingabove the continuum-forming photosphere.
The sizes and shapes of the stars o Ceti andR Leonis have been
measured in the mid-infrared byTatebe et al. (2008) using the ISI
instrument. Thestar o Ceti appeared to be rather symmetric whilethe
shape of the R Leonis appeared more consistentwith a uniform disc
plus a point source. Pluzhniket al. (2009) presented the results of
long baselineinterferometric observations of the Mira-type starU
Ori at 1.51 µm (H2O band), 1.64 µm (pseudo-continuum), and 1.78 µm
(H2O band), using thethree-element IOTA interferometer. They
performedmodel-independent image reconstruction of the enve-lope
around the star using measured visibilities andclosure phases. The
images show asymmetric struc-ture of the U Ori envelope (Fig. 11),
and they dis-cussed the geometric and kinematic structure of
theenvelope based on a model of an almost face-on ex-panding and
rotating disc around the star.
Le Bouquin et al. (2009a) obtained single-epoch interferometric
observations of T Lep with a
continuous dataset in the spectral domain (λ =1.5-2.4 µm) and in
the spatial domain (interferometricbaselines ranging from 11 to 96
m), and they per-formed a model independent image reconstructionfor
each spectral bin. Reconstructed images (Fig.12) confirm the
general picture of a central star par-tially obscured by the
surrounding molecular shell ofchanging opacity. At 1.7 µm, the
shell becomes opti-cally thin, with corresponding emission
appearing asa ring circling the star. This was the first direct
ev-idence of the spherical morphology of the molecularshell in Mira
stars while model fitting confirmed aspherical layer of constant
size and changing opacityover the wavelengths.
Fig. 11. Reconstructed images of U Ori. Imagesare reconstructed
at 1.51 µm, 1.64 µm, and 1.78 µm(top, from left to right). The
corresponding imagesof the envelope, obtained by removing the model
ofthe central source are shown in the second row. Thebright spots
A, B, and C correspond to H2O maserfeatures. The synthesized beams
of the interferome-ter for all wavelengths are shown at the
bottom.
Fig. 12. Reconstructed images of T Lep. Imagesare shown for
several AMBER spectral bins acrossthe H and K bands. The
interferometric beam sizeis displayed at the bottom-right part of
each image.Spatial scale is in mas.
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OPTICAL INTERFEROMETRY II
Lacour et al. (2009) presented infrared inter-ferometric imaging
of the S-type Mira star χ Cygni.The star was observed at four
different epochs withthe IOTA interferometer (H band) using the
inte-grated optics recombiner IONIC. Images show up to40 %
variation in the stellar diameter, as well assignificant changes in
the limb darkening and stel-lar inhomogeneities. The model fitting
gave precisetime-dependent values of the stellar diameter,
andreveals presence and displacement of a warm molec-ular layer.
The constant acceleration of the COmolecules during 80 % of the
pulsation cycle leadto argument for a free-falling layer.
In order to explore the photosphere of thevery cool late-type
star VX Sgr and, in particu-lar, the characterization of molecular
layers abovethe continuum forming photosphere, Chiavassa etal.
(2010) obtained interferometric observations withthe VLTI/AMBER
instrument. Reconstructed im-ages and visibilities showed a strong
wavelengthdependence and the H-band images displayed twobright
spots whose positions were confirmed by thegeometrical model.
9. PLANETARY NEBULAE
After having been extensively studied in thevisible, the
Planetary Nebulae and among them theyoungest and more dusty ones
are now studied in thenear and mid-infrared. It is commonly
accepted thatdiscs surrounding the central star can be an
essentialingredient to the shaping of planetary nebulae, butthe
spatial resolution of single aperture astronomicalinstruments is
usually not sufficient for detecting andstudying these discs. The
geometry of the disc andthe mass stored are the key parameters for
constrain-ing the models of nebula formation, and for tracingback
the evolution of the central star. The disc in-ner edge, as seen
from the star, could be thick anddense enough to collimate stellar
winds into lobesand knowing its geometry it enables us to better
un-derstand the distribution of illuminated and shad-owed regions
in the extended nebula. The significantprogress for these studies
has been achieved with theVLT Interferometer and its two
instruments: AM-BER operating in the near-infrared and MIDI in
themid-infrared providing a typical spatial resolution of2 and 10
mas, respectively, which is well suited forthe study of many
aspects of the late evolution ofstars and in particular, to deal
with the asymmetryin the Planetary Nebula, back to the AGB
stars.
Chesneau et al. (2006) presented high spatialresolution
observations of the dusty core of CPD-56 8032, taken with the
mid-infrared interferometerVLTI/MIDI. The infrared core was almost
fully re-solved with the three baselines and the signal is
in-terpreted in terms of a ring structure which woulddefine the
bright inner rim of the equatorial disc.Geometric models allowed
derivation of the main ge-ometrical parameters of the disc, showing
that thedisc is mostly optically thin in the N band and
highlyflared. Lagadec et al. (2006) reported on
infraredobservations of the planetary nebula Hen 2-113 ob-
tained with the same instrument. No clear core at8.7 µm and no
fringes through the N band couldbe detected and a qualitative
interpretation of theobject structure is proposed by using a
diabolo-likegeometrical model. In order to obtain a high spa-tial
resolution information on the dusty core of bipo-lar planetary
nebulae and to directly constrain theshaping process, Chesneau et
al. (2007a) obtainedobservations of the dusty core of the extreme
bipo-lar planetary Ant nebula (Mz 3, Hen 2-154) takenwith the
VLTI/MIDI. The core was clearly resolvedand they suggest an edge-on
disc whose major axisis perpendicular to the axis of the bipolar
lobes forwhich they deduce inclination and position angle.
With the purpose of studying the structure ofthe nebula around
the post AGB, the binary star 89Her, Bujarrabal et al. (2007)
obtained N-band inter-ferometric data on the extent of the hot dust
emissionobtained with the VLTI/MIDI instrument and theIRAM Plateau
de Bure Interferometer (PdBI) ob-servations of the 12CO(1-0) and
12CO(2-1) lines andcontinuum emission. They presented
high-resolutionPdBI maps, and derived the properties of the neb-ula
from model fitting. Two nebular components arefound: an extended
hour-glass-like structure and anunresolved very compact component,
smaller than0.4 arcsec. Assuming that the compact componentis a
Keplerian disc, they derive disc properties thatare compatible with
expectations for such a struc-ture; in particular, the size of the
rotating gas discthat is very similar to the extent of the hot dust
com-ponent from VLTI data. Assuming that the equatorof the extended
nebula coincides with the binary or-bital plane, they provide new
results on the compan-ion star mass and orbit. In order to probe
the in-nermost region of the circumstellar dust shell of thedeeply
embedded Wolf Rayet star WR 118, Millouret al. (2009a) carried out
spectro-interferometric ob-servations using the VLTI/AMBER
instrument inthe low-spectral resolution mode (R=35). The K-band
observations were obtained with three 1.8 mtelescopes spanning
projected baselines between upto 40 m which allowed them to
conclude that WR118 possibly harbors a pinwheel nebula, suggestinga
binary nature of the system.
10. LUMINOUS BLUEVARIABLES: η Carinae
The bright star η Carinae is the most mas-sive and luminous star
in our region of the MilkyWay. Though it has been extensively
studied usingmany different techniques, its physical nature andthe
mechanism that led to the creation of the Ho-munculus nebula are
still debated. Van Boekel etal. (2003) presented new high angular
resolution ob-servations at near-IR wavelengths of the core of
thenebula by using the VLTI/VINCI instrument. Theobservations
provided spatial information on a scaleof 11 AU at the distance of
η Carinae and the datashow that the object is elongated with a
de-projectedaxis ratio of approximately 1.5 and that the majoraxis
is aligned with that of the nebula. The most
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S. JANKOV
likely explanation for this observation was a model inwhich
stellar rotation near the critical velocity causesenhanced mass
loss along the rotation axis resultingfrom the large temperature
difference between thepole and equator in rapidly rotating stars.
They con-clude that η Carinae must rotate in excess of 90% ofits
critical velocity to account for the observed shapeand that the
large outburst may have been shapedin a similar way.
To constrain spatially and spectrally the warmdusty environment
and the central object, Ches-neau et al. (2005b) observed the core
of the neb-ula surrounding η Carinae with the interferom-eter
VLTI/MIDI. In particular, narrow-band im-ages at 3.74 µm and 4.05
µm reveal the butterflyshaped dusty environment close to the
central starwith unprecedented (sub-arcsecond) spatial resolu-tion
(Fig. 13). A void whose radius corresponds tothe expected
sublimation radius has been discoveredaround the central
source.
Fig. 13. The butterfly shaped dusty environmentclose to η Car.
Zoom into the deconvolved imagesfrom the 3.74 µm (top) and 4.05 µm
(bottom) fil-ters. The raw images are shown on the left side,
thedeconvolved images at iteration 10 and 40 are shownon the middle
and on the right side.
To study the wavelength dependence of theη Carinae optically
thick wind region with a highspatial resolution of 5 mas (11 AU),
Weigelt et al.(2007) presented the first NIR
spectro-interferometryof the star. The analysis has been performed
on theVLTI/AMBER spectrally dispersed interferograms(spectral
resolutions of 1500 in the medium spec-tral resolution mode and
12000 in the high resolu-tion mode) which allowed the investigation
of thewavelength dependence of the visibility, differentialphase,
and closure phase. For the interpretation ofthe non-zero
differential and closure phases measuredwithin the Brγ line, they
present a simple geometricmodel of an inclined, latitude-dependent
wind zone.Their observations supported theoretical models
ofanisotropic winds from fast-rotating, luminous hotstars with
enhanced high-velocity mass loss near thepolar regions.
With the purpose of resolving the central en-gine of the η
Carinae complex in the near-infraredon angular scales of a few
milli-arcseconds, Kervella
(2007) used the VLTI/VINCI data in the K bandobtained with
either two 0.35 m siderostats or two8-m Unit Telescopes. He
reported visibility measure-ments in satisfactory agreement with
the previousresults obtained with VLTI/AMBER.
Aiming to constrain the rotational velocity ofthe primary star
and to probe the influence of thecompanion, Groh et al. (2010)
analyzed K-bandcontinuum visibilities from VLTI/VINCI and clo-sure
phase measurements from VLTI/AMBER. Theyconclude that the density
structure of the primarywind can be sufficiently disturbed by the
compan-ion, thus mimicking the effects of fast rotation,
andtherefore the fast rotation may not be the only ex-planation for
the interferometric observations.
11. NOVAE
The high spatial resolution studies of Novaein the optical
domain has been recently stimulatedby emergence of large optical
interferometers. Par-ticularly, the outburst that occurred on
February 12,2006 on recurrent nova RS Oph has been
extensivelyobserved by different interferometers. Following
theoutburst, Monnier et al. (2006a) measured its near-infrared size
using the IOTA, Keck, and PTI inter-ferometers at multiple epochs.
They concluded thatthe characteristic size of∼ 3 mas hardly changed
overthe first 60 days of the outburst, ruling out popularmodels
where the near-infrared emission arises fromhot gas in the
expanding shock. The emission wasalso found to be significantly
asymmetric, evidencedby nonzero closure phases detected by IOTA.
Usingthe PTI interferometer, Lane et al. (2007)
performedobservations of RS Oph, resolving the emission fromthe
nova for several weeks after the outburst. Theyreported that the
near-IR source which initially ex-panded to a size of ∼ 5 mas,
began to shrink aroundday 10, and reached ∼ 2 mas by day 100. They
ar-gue that the fact that the emission region appears toshrink does
not necessarily imply that the material isfalling; what one can see
is the effective photosphericdiameter, which is a function of the
density of thematerial, and if the mass-loss rate from the
centralwhite dwarf drops, the apparent photospheric radiuswould be
expected to shrink even as the materialcontinues flowing
outward.
Chesneau et al. (2007b) reported on spec-trally dispersed
VLTI/AMBER observations of RSOph five days after the discovery of
the outburst.Using three baselines and a spectral resolution
ofR=1500, they measured the extension of the milli-arcsecond scale
emission in the K band continuumand in the Brγ and He I 2.06 µm
lines, which al-lowed to get an insight into the kinematics of
theline forming regions. Their results confirm the basicfireball
model (Fig. 14), contrary to the conclusionsof other
interferometric observations conducted byMonnier et al. (2006a).
Barry et al. (2008) reportedN-band (8 to 12.5 µ) observations of RS
Oph usingthe Keck Interferometer 3.8 days following the out-burst.
The data show evidence of enhanced neutralatomic hydrogen emission
and atomic metals includ-
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OPTICAL INTERFEROMETRY II
ing silicon located in the inner spatial regime near thewhite
dwarf. They report also nebular emission linesand evidence of hot
silicate dust in the outer spatialregion, centered at ∼ 17 AU from
the white dwarf,which were not found in the inner regime.
Theirresults support a model in which the dust appearsto be present
between outbursts and is not createdduring the outburst event.
Fig. 14. RS Oph outburst. Sketch of the near-IRellipses
extensions compared with the radio structureobserved at t= 13.8d
(thick extended ring, O’Brienet al. 2006). The continuum ellipse is
delimited bythe solid line, the ellipse that corresponds to the
coreof Brγ by the dotted line and the one correspondingto the core
of HeI by the dashed line. The small dot-ted line delimits the Brγ
ellipse scaled at t= 13.8 d.North is up, east left.
Lane et al. (2007) have resolved the classicalnova V1663 Aql
using a long-baseline near-IR inter-ferometry from the PTI
interferometer covering theperiod from ∼ 5 to 18 days after the
peak bright-ness. They directly measured the shape and size ofthe
fireball, (which they found to be asymmetric)and the apparent
expansion rate. Assuming a linearexpansion model, the time of the
initial outburst ap-proximately 4 days prior to the peak brightness
wasinfered. In order to improve the distance determina-tion and to
constrain the mechanisms leading to veryefficient dust formation
under the physical conditionsencountered in novae ejecta, Chesneau
et al. (2008)presented the first high spatial-resolution
monitoringof the dust-forming nova V1280 Sco. Spectra and
vis-ibilities were regularly acquired between the onset ofdust
formation, 23 days after the discovery (11 daysafter the maximum),
using the beam-recombiner in-struments AMBER (near-IR) and MIDI
(mid-IR).These observations allowed determination of the ap-parent
linear expansion rate for the dust shell 0.35± 0.03 mas day, and
the approximate ejection time
of the matter in which dust formed (tejec = 10.5± 7d), i.e.
close to the maximum brightness.
Chesneau et al. (2011) reported on near-IRinterferometric
observations of the recent 2011 out-burst of the recurrent nova T
Pyx. They obtainednear-IR observations of T Pyx at dates ranging
from2.4 to 48.2 days after the outburst with the CLASSICrecombiner
located at the CHARA array and withthe PIONIER and AMBER
recombiners located atthe VLTI array. Comparing expansion of the H
andK band continua as well as the Brγ emission line,and infering
information on the kinematics and mor-phology of the early ejecta,
they concluded that itcould be most simply interpreted within the
frameof a bipolar model oriented nearly face-on.
12. YOUNG STELLAR OBJECTS
The very close environments of young stars arethe hosts of
fundamental physical processes such asstar-disc interactions, mass
accretion, and ejection.The extraordinary interest in the inner
region of acircumstellar disc results from the assumption thatthe
formation of planets is favored there, focusing onhow accretion
discs evolve into protoplanetary discsand, finally, to debris discs
and planets. Optical in-terferometry is playing an important role
in eluci-dating the earliest stages of planetary formation
byprobing the density and temperature structure of thediscs
presumably before the planet formation. Thefirst successful
observation of a Young Stellar Ob-jects (YSOs) with stellar
interferometry was reportedby Malbet et al. (1998) who resolved the
young out-bursting star FU Ori using a 100-m long baseline inthe K
band with the PTI. Using the IOTA interfer-ometer Millan-Gabet et
al. (1999) resolved the Her-big Ae/Be star AB Aur, finding the
near-IR emissionto be much larger than expected.
Radically improved infrared detectors have re-cently allowed
optical interferometers to investigatethe inner accretion discs
around young stellar objects(Millan-Gabet et al. 2001, Akeson et
al. 2000, 2002,Tuthill et al. 2001, Danchi et al. 2001). In
mostcases, the near-IR sizes were found to be significantlylarger
than expected from the favored disc models ofthe time (Malbet and
Bertout 1991). The complexmorphological structure of these
environments hasbeen confirmed by the now quite rich data sets
ob-tained by near-infrared long-baseline interferometry.While
observations in millimeter range probe coolerouter disc regions and
layers close to the midplane ofcircumstellar discs, observations in
the mid-infraredwavelength regime are more sensitive to warmer
discarreas, such as the surface of the inner regions wherethe dust
is directly irradiated by the central star.
12.1. Herbig Ae/Be stars
Pre-main sequence stars in the intermediatemass range (1.5-3
M¯), called Herbig Ae and Bestars, are observed to be surrounded by
circumstel-lar material which reveals itself by discrete
emission
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S. JANKOV
lines and by continuous excess emission in spectralenergy
distribution (SED). The spatial distributionof this material
however has been subject to de-bate: both geometrically flat disc
models and spher-ically symmetric envelope mo