STAR FORMING REGIONS AND YOUNG STELLAR OBJECTS Antonio Maggio Istituto Nazionale di Astrofisica Osservatorio Astronomico di Palermo Messaggeri della Conoscenza Struttura, origine e caratterizzazione dei pianeti nel Sistema Solare e in sistemi esterni
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STAR FORMING REGIONS AND YOUNG STELLAR OBJECTS
Antonio Maggio Istituto Nazionale di Astrofisica
Osservatorio Astronomico di Palermo
Messaggeri della Conoscenza Struttura, origine e caratterizzazione
dei pianeti nel Sistema Solare e in sistemi esterni
Relevance for exoplanets • Stellar (and planetary) formation
₋ Understanding physical conditions and mechanisms
⇒ Do stars and planets form at the same time? ⇒ Are planets hosted preferentially by stars with
specific characteristics? • Planetary growth and dynamical evolution
• What is the role of central stars in early (proto-planetary) phases?
• Planet ageing and life development • How star-planet interactions influence planetary
environment and habitability conditions?
Some historic milestones • 1755, Kant: The Nebular Hypothesis • 1796, Laplace: Nebular break-up hypothesis • XVIII – XIX: observations of dark clouds • 1904, Hartmann: Discovery of insterstellar gas • 1930, Trumpler: Discovery of interstellar dust • 1939, Spitzer: Theory of Turbulent Viscosity • 1940s, Joy: Identification of T Tauri Stars • 1950s: Radio astronomy and the study of ISM • 1961, Hayashi: Theory of PMS evolution • 1962, Herbig: T Tauri star formation in clouds • 1977, Shu: Theory of cloud collapse • 1980s – 1990s, IR observations of YSOs from space
Relatore
Note di presentazione
Vedi PSPF_Lec7 (Mayer) and amanda.pdf
Early-type stars
and Star Forming Regions are mainly found along the spiral arms of the Galaxy
However, detailed observations of SFRs are possible only within few kpc from the Sun
Relatore
Note di presentazione
Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations. H II regions are marked as dots colored in the same color as their spiral arm. They come in three sizes, measured by the excitation parameter U: small - U > 200 pc cm-2 medium - 200 > U > 110 pc cm-2 large - 110 > U > 70 pc cm-2
Mass distribution in our Galaxy
Exercise: estimate the total mass within 8 kpc from Galactic center, assuming a solar velocity of 220 km/s along the Galactic orbit; compare your result with the values reported above (Sofue Y. 2012, PASJ 64, 75).
Mass Scale radius Bulge 1.7 1010 M⊙ 0.5 kpc
Disk 3.4 1010 M⊙ 3.2 kpc
Dark Halo (< 8 kpc) 2.7 1010 M⊙ R < Sun distance from Galactic center
Dark Halo (< 12.5 kpc) 5.1 1010 M⊙ 12.5 kpc
Dark Halo (< 20 kpc) 8.9 1010 M⊙ 20 kpc
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Luminous (massive) stars mostly in arms of spiral galaxies
Spiral arms traced by Bright H II regions,
associated to early-type (OB) stars
Dark (dusty) lanes, associated to Giant Molecular Clouds
Location of Star Forming Regions
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Interstellar Medium in Galaxy Arms • Hot gas
₋ Ionized hydrogen (H II regions), very bright in Hα emission (656 nm)
₋ Other ionized low-Z elements, identified by their forbidden-line emission (⇔ low-density gas)
• Cold gas ₋ Neutral hydrogen (H I regions), mainly observed in radio
(21-cm emission) ₋ Molecular hydrogen (H2) and several other molecules
including the most abundant elements (in particular C, N, and O). Best observations at mm wavelength from CO.
• Dust ₋ Dark clouds (⇒ high extintion of background stars) ₋ Reflection Nebulas (⇐ Mie scattering)
Relatore
Note di presentazione
Halpha (Balmer series transition n=3->2)
Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)
regions are complex environments Expanding SNRs OB stars and associated wind
shocks Stars in different evolutionary
stages (protostars, T Tauri stars, ecc.)
Several hydro-dynamical and radiative effects may influence the early phases of stellar formation
About 40 Star Forming Regions (SFRs) within 3 kpc already studied with multi-wavelength observations Best known SFRs, up to date:
Orion, Taurus-Auriga, Ophiucus
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
• H II regions carved by O/B star radiation inside a giant molecular cloud in the LMC
Sequential star formation
Relatore
Note di presentazione
The star-forming region, catalogued as N11B, lies in the Large Magellanic Cloud (LMC), located only 160,000 light-years from Earth. The image illustrates a perfect case of sequential star formation in a nearby galaxy where new star birth is being triggered by previous-generation massive stars. A collection of blue- and white-colored stars near the left of the image are among the most massive stars known anywhere in the universe. The region around the cluster of hot stars in the image is relatively clear of gas, because the stellar winds and radiation from the stars have pushed the gas away. When this gas collides with and compresses surrounding dense clouds, the clouds can collapse under their own gravity and start to form new stars. The cluster of new stars in N11B may have been formed this way, as it is located on the rim of the large, central interstellar bubble of the N11 complex. The stars in N11B are now beginning to clear away their natal cloud, and are carving new bubbles in turn. Yet another new generation of stars is now being born in N11B, inside the dark dust clouds in the center and right-hand side of the Hubble image. This chain of consecutive star birth episodes has been seen in more distant galaxies, but it is shown very clearly in this new Hubble image. Farther to the right of the image, along the top edge, are several smaller dark clouds of interstellar dust with odd and intriguing shapes. They are seen silhouetted against the glowing interstellar gas. Several of these dark clouds are bright-rimmed because they are illuminated and are being evaporated by radiation from neighboring hot stars.
J. Hester, P. Scowen (ASU), HST, NASA
Evaporating Gaseous Globules (EGGs) in
the Eagle Nebula (M16)
Relatore
Note di presentazione
These pictures from NASA's Hubble Space Telescope show newborn stars emerging from rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs" in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens. Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space — a process called "photoevaporation. The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.
Spectroscopy at millimeter wavelengths provides diagnostics for temperature, density, and abundance of cold melecular gas
Stability condition (Virial theorem) General equation that relates the time-average of the total kinetic energy, 𝑲 , of a stable system consisting of N particles, bound by potential forces, with the time-average of the total potential energy, 𝑼 :
𝟐 𝑲 + 𝑼 = 𝟎 On the other hand, during the evolution of the system, the total energy is
𝑬 = 𝑲 + 𝑼 Any decrease of the total internal energy of the system goes into radiation
𝑳 = − 𝒅𝑬𝒅𝒅
= − 𝒅𝑲𝒅𝒅
+ 𝒅𝑼𝒅𝒅
= −𝟏𝟐𝒅𝑼𝒅𝒅
and 𝒅𝑲𝒅𝒅
= −𝟏𝟐𝒅𝑼𝒅𝒅
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Jeans criterion for collapse
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Properties of Molecular Clouds
Exercise: verify if the above astrophysical environments are gravitationally stable according to the Jean criterion.
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
But, nature is complex… • Molecular Clouds cannot be treated as an isolated
system: pressure of the gas in the external ISM must be taken into account
⇒ Equilibrium solution is a «Bonnor-Ebert’s Sphere» • Molecular clouds may start with a non-null angular
momentum (Exercise: compute angular momentum of clumps in a molecular cloud, assuming a characteristic velocity of 3×104 cm/s, and compare it with that of the Sun, Earth, and Jupiter)
• Molecular Clouds are permeated with magnetic fields (10-100 µG measured in Cloud Cores) ⇒ If the medium is ionized, the field is «frozen», and the magnetic flux, ∫𝐵 ∙ 𝑑𝑑, must be conserved
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
The largest, most complex hydrodynamical star formation calculation ever performed (2009) (http://www.astro.ex.ac.uk/people/mbate/Animations/)
Initial Mass Function (IMF) • The IMF, ξ(M), introduced by Edwin Salpeter in
1955, gives the number of stars per unit mass range
• The IMF can be deduced from observational data (stellar counts in SFRs)
• Caveat: Most of the known IMFs are relative to the local Galactic neighborhood, hence they may not be valid in very far away, physically different astropysical environments
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
According to Salpeter… • The IMF follows a power law
d NS / d log10 M ~ M-1.35
or, in linear form:
ξ(M) = d NS / d M ~ M-2.35 where ξ(M) is the number of stars with mass
between M and M+dM.
• Low-mass stars dominate the stellar population!
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Other IMF determinations
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Marchi et al. (2010); Bastian, Covey, & Meyer (2010, ARAA)
Young SFRs, open
clusters, stellar associations, and field stars show very similar slope at high masses
Some difference in the low-mass branch
Young SFRs and associations tend to have stars with lower masses than old open clusters and globular clusters (dependence on metallicity)
Observed IMFs
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Simplified Scheme for the IMF
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Possible physical mechanisms behind the IMF
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
30
Universality of the IMF
• Open questions • How can the IMF be independent from
initial conditions, such as rotation of the protostellar cloud, presence of magnetic fields, and chemical composition (metallicity) of the environment?
• Can the physical mechanisms responsible
for the stellar IMF determine also the planetary population? (see next lecture by C. Argiroffi)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
From cloud collapse to main sequence
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
HR diagram of the Orion Nebula Cluster
In practice, determination of stellar ages is difficult!
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
IR images of Protostars Color code: Blue 3.6 µm Green 4.5 µm Red 8.0 µm
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Relatore
Note di presentazione
New evidence from NASA's Spitzer Space Telescope is showing that tight-knit twin stars might be triggered to form by asymmetrical envelopes like the ones shown in this image. All stars, even single ones like our sun, are known to form from collapsing clumps of gas and dust, called envelopes, which are seen here around six forming star systems as dark blobs, or shadows, against a dusty background. The greenish color shows jets coming away from the envelopes. The envelopes are all roughly 100 times the size of our solar system. Two of the six star systems are known to have already formed twin, or binary stars (Spitzer can see the envelopes but not the stars themselves). Astronomers believe that the irregular shapes of the envelopes, revealed in detail by Spitzer, might trigger binary stars to form, or might have already triggered them to form. From top left, moving clockwise, the stars are: IRAS 03282+3035, CB230, IRAS 16253-2429, L1152, L483, HH270 VLA1. IRAS 03282+3035 and CB230 are the two known to have already formed binary stars. Infrared light with a wavelength of 3.6 microns has been color-coded blue; 4.5-micron light is green; and 8.0-micron light is red.
IR images of Protostars
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Relatore
Note di presentazione
The "Cores to Disks" Spitzer Legacy team is using the two infrared cameras on NASA's Spitzer Space Telescope to search dense regions of interstellar molecular clouds (known as "cores") for evidence of star formation. Part of the study targeted a group of objects with no known stars to study the properties of such regions before any stars have formed. The first of these "starless cores" to be examined held a surprise: a source of infrared light appeared where none was expected. The core is known as L1014, the 1,014th object in a list of dark, dusty "clouds" compiled by astronomer Beverly Lynds over 40 years ago. These have proved to be homes to a rich variety of molecules and are the birthplaces of stars and planets. The Spitzer image is a 3.6 micron (blue), 8.0 micron (green) and 24.0 micron (red) composite image. The light seen in the infrared image originates from very different sources. The bright yellow object at the center of the image is the object detected in the "starless core". The red ring surrounding the object is an artifact of the reduced spatial resolution of the telescope at 24 microns. At 3.6 microns the light comes mainly from the object at the heart of the core. At longer wavelengths, the light from the object becomes stronger, a signature that it is not a background star. Also in the longer wavelengths (8.0 to 24.0 microns), astronomers saw the glow from interstellar dust, glowing green to red in the Spitzer composite image. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. The red color traces a cooler dust component. No previous observations showed any hint of a source in L1014. For example, the visible light image is from the Digital Sky Survey and is a B-, R-, and I-band composite image (wavelengths ranging from 0.4 to 0.7 microns). The dark cloud in the center of the image is the core, completely opaque in the visible due to obscuration by dust. The L1014 core lies in the direction of Cygnus. It is thought to be about 600 light years away, but the distance is somewhat uncertain. The results from this study are published by C. Young and the "Cores to Disks" team in the Astrophysical Journal.
IR images of YSOs with disks
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Model of stellar collapse including magnetic fields
• Problem: how can protostars get rid of excess angular momentum and magnetic flux?
• Possible solution: Magnetized disks and jets (see lecture by S. Orlando)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Shu, Adams & Lizano 1987, ARAA
Magnetospheric Accretion Disk Model
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Relative lifetimes Class 0: 0.2 Myr Class I : 0.5 Myr Class II: 3 Myr Class III: 10 Myr
Lada 1987, Andrè et al. 1993
Young Stellar Objects classification
STAR FORMING REGIONS AND YOUNG STELLAR OBJECTS
Part II Antonio Maggio
Istituto Nazionale di Astrofisica Osservatorio Astronomico di Palermo
Messaggeri della Conoscenza Struttura, origine e caratterizzazione
dei pianeti nel Sistema Solare e in sistemi esterni
X-ray emission of YSOs
Feigelson & Montmerle, 1999
• X-ray emission signals the onset of stellar magnetic activity (but when?) • Since then, stellar activity starts affecting the evolution of the environment • Feedback on stellar evolution, disk longevity, planetary formation and primordial “space climate”
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
X-ray view of the Orion Nebula Cluster
About 1600 sources detected in 800 ks Chandra
observattion, mostly Young Stellar Objects with ages 105 −106 yr
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
BN/KL region in the Orion Molecular Cloud: Several new sources discovered in X-rays, deeply embedded 22.2 < log NH < 23.6 (the hydrogen column density, 𝑁𝐻 = ∫𝑛𝐻 𝑑𝑑 , is a measure of the absorbing material along the line of sight)
Heavily obscured protostellar cores
Red 0.5–1.7 keV Green 1.7–2.8 keV Blue 2.8–8.0 keV
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Grosso et al. (2005)
Relatore
Note di presentazione
43 sources found in 40''×50'' region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL), 18 newly discovered. Colors indicate photon energies.
X-ray luminosity of YSOs in Orion
Mean X-ray luminosity Lx ≈ 10-3.5 Lbol (saturated level)
for non-accreting (class III) stars Depressed X-ray emission and large scatter for
accreting (class I–II) stars
Preibisch et al. (2005)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Orion Nebula Cluster: the COUP movie Continuous flaring activity for essentially all sources
Duration from minutes to days
Peak X-ray luminosities Lx ≈ 1032 erg/s
Total radiated energy up to E ≈ 1036.5 erg [0.5-8 keV]
Frequency of largest flares: every few days per object
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
X-ray Flare Characteristics
Fast rise and slower exponential decay Explained by hydrodynamical models of
Analysis of broad-band X-ray spectroscopy data allows to estimate the size of magnetic structures where the flaring plasma is confined
Favata et al. (2005)
Sizes of flaring regions and peak plasma temperatures
• Scale lengths from 5 x 1011 to 5 x 1012 cm, i.e. up to 10 − 20 R*
• Plasma peak temperatures up to 7 x 108 K (but extreme values are poorly constrained)
SYNCHROTRON
NON-THERMAL Bremsstrahlung
Active stars
Orion bright flares
Super-hot
flares
Getman et al. (2008a)
Magnetospheres of accreting YSOs
In accreting T Tauri stars, coronal extent limited by the disk
(truncated at the co-rotation radius) Closed field ⇒ hot plasma ⇒ X-ray emission, flares Open field ⇒ stellar wind ⇒ mass & angular momentum losses Star-disk field ⇒ accretion ⇒ shocks ⇒ soft X-ray excess ⇒ giant flares? Open issue...
Getman et al. (2008b)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Role of Magnetic Fields In accreting T Tauri stars,
magnetic fields may affect ⇒ hot plasma confinement
and heating ⇒ steady X-ray emission and
flares ⇒ mass inflow and outflow ⇒ angular momentum balance ⇒ star-disk locking and
magnetic torques ⇒ disk disruption ⇒inner edge for planet
migration?
Matt & Pudritz (2005)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Magnetospheres in non-accreting stars
Coronal extent limited by gas/B-field pressure ratio Fast rotation ⇒ efficient magnetic dynamo ⇒ enhanced
activity and X-ray emission Intense high-energy irradiation of young planetary bodies Coronal Mass Ejections and Stellar Energetic Particle flows Possible star-planet magnetospheric interaction (see next
lecture by A. Maggio)
Getman et al. (2008b) Sun 4x
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Stellar activity effects on Circumstellar/Protoplanetary Disks
Circumstellar disks are subject to high-energy radiation,
winds, and energetic particles originating from the central star ⇒ heating, ionization, evaporation
How is planetary formation affected? A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
X-ray Diagnostics of X-rayed Disks
Reflection of X-rays off the
disk ⇒ Fluorescent 6.4 keV iron emission line following photo-ionization of a K-shell electron
Absorption of X-ray emission from central stars by gas in disks with edge-on orientation (called proplyds)
Imanishi et al. (2001) Kastner et al. (2005)
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
IR Diagnostics of X-rayed Disks Detection of
[Ne II] 12.81 µm line emission
Pascucci et al. (2007) Hot H2O and CO
molecular layer observed is some YSOs
Carr et al. (2004)
H2O
CO
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
XUV Irradiation of Circumstellar Disks Heating and ionization of
gas in disk outer layers out to several AU B-field freezing ⇒ disk truncation MRI turbulence ⇒ angular momentum
transport ⇒ mass accretion ⇒ planetary migration?
Out of equilibrium molecular chemistry in the disk interior
Ilgner & Nelson (2006)
Temperature
Ionization Fraction
Verti
cal H
eigh
t [A
U]
Radial distance [AU]
Verti
cal H
eigh
t [A
U]
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Disk ionization models Ingredients: X-ray photo-
ionization, viscous heating, radiative cooling, and turbulent mixing of gas, dust and grain phases, with given chemical composition
Boundary between active (= turbulent) and dead (= laminar) zone occurs at very low ionization fraction, log Xe ~ -12
The size of the dead zone likely depends on the frequency, energetics, and hardness of stellar flares
Ilgner & Nelson (2006)
Col
umn
Den
sity
Radial distance
dead zone
X-ray active zone
Thermal active zone
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Effects on planets Traditional theory of growth
from interstellar grains to larger bodies requires calm dynamics and gravitational settling towards the disk midplane
X-ray induced MRI turbulence produces inhomogeneities and gravitational torques ⇒ inhibited sedimentation of solids ⇒ planetary formation possibly occurring in dead zones only
When formed, planetesimals undergo random walks, rather than simple migration
Close-in gaseous planets have magnetosphere which interact with the stellar one ⇒ enhanced activity
Planet atmospheres are subject to high-energy irradiation and stellar winds ⇒ heating, evaporation
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Relatore
Note di presentazione
Johansen et al. 2007, Nature During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies1. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem2: boulders are expected to stick together poorly3, and to spiral into the protostar in a few hundred orbits owing to a 'headwind' from the slower rotating gas4. Gravitational collapse of the solid component has been suggested to overcome this barrier1, 5, 6. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer2, 7, and turbulence must be present to explain observed gas accretion in protostellar disks8. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas9, and these concentrations are augmented a further order of magnitude by a streaming instability10, 11, 12 driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.
Stellar X-ray source variability • Stellar coronal sources are known to vary
on several time scales ₋ Short-term (from minutes to a few days)
variability due to flares ₋ Medium-term variability (from a few hours to
tens of days): rotational modulation ₋ Long-term variability (years) due to magnetic
cycles • X-ray emission from YSOs may vary, at
least in principle, also due to • Variable accretion rate • Absorption by the circumstellar disk
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Sun
Ribas et al. (2005)
X-ray emission at later epochs X-ray luminosity
decreases with age Wind-driven loss of
stellar angular momentum causes less effective dynamo action
Stellar coronae become cooler Softening of the
spectrum, i.e. less high-energy photons
Decreasing flaring activity Smaller increase of
X-ray emission, less frequent high-energy events
Oce
ans
and
atm
osph
ere
on E
arth
Firs
t bac
teria
Euc
ario
tes
Now
Preibisch & Feigelson (2005)
Med
ian
log(
L x) [
erg/
s]
log(age) [yr]
1.0 M
0.7 M
0.3 M
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Sun
Ribas et al. (2005)
X-ray emission at later epochs Stellar coronae
become cooler Softening of the
spectrum, i.e. less high-energy photons
Decreasing flaring activity Smaller increase of
X-ray emission, less frequent high-energy events
Oce
ans
and
atm
osph
ere
on E
arth
Firs
t bac
teria
Euc
ario
tes
Now
1.0 M
0.7 M
0.3 M
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Sun compared with Active Stars Sun Young Active Stars
X-ray luminosities
Lx/Lbol ~ 10-6 (quiescent)
Lx/Lbol ~ 10-5 (large flares)
Lx/Lbol ~ 10-3 (quiescent)
Lx/Lbol ~ 10-1 (large flares)
Occurrence of large flares
1 every 10 days (at max of solar cycle)
A few per day (no magnetic cycle?)
Flare time scales up to a few hours up to a few days
Coronal plasma temperatures
≈ 106 K (quiescent)
≈ 107 K (flaring)
≈ 107 K (quiescent)
≈ 108 K (flaring) !!!
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Flare energy distributions
A. Maggio, 2014, Messaggeri della conoscenza, Dip. Fisica e Chimica, Uni. Palermo
Relatore
Note di presentazione
Normalized cumulative distributions of flare total energies observed in X-rays in the Orion Nebula Cluster (ONC) with the Chandra satellite (COUP project) and in the Taurus-Auriga Molecular Cloud (TMC) with the XMM-Newton satellite (XEST project).
BIBLIOGRAPHY • M. Zeilik, Astronomy (9th edition) Chapter 11: The Origin and Evolution of the Solar System Chapter 14: Starbirth and Interstellar Matter (Elementary level) • M. Harwit, Astrophysical Concepts (2nd edition) Chapter 1 and Chapter 9 (Cosmic Gas and Dust) (Intermediate level) • M. Meyer (ETH/IfA on-line course) Physics of Star and Planet Formation http://www.astro.ethz.ch/education/courses/Physics_of_Star_a
nd_Planet_Formation/pspf_program (Advanced level) • E.D. Feigelson, T. Montmerle High-Energy Processes in Young Stellar Objects 1999, ARAA, Vol. 37, p. 363 (Advanced level)