Standard stars for the ZIMPOL polarimeter in the SPHERE/VLT planet finder instrument

Standard stars for the ZIMPOL polarimeter in the SPHERE/VLT planet finder instrument

1´´

108

log I

103

log I

107

log I

Example: Sun – Jupiter system at 5 pc

X-AO

tiny polarized planetary signal in bumpy and variable PSF halo

SPHERE / ZIMPOL Science goal: detection of reflected=polarized light from extra-solar planets

coro

SPHERE-Design

Grenoble, Dec. 2011

synchronization (kHz)

modulator

polarizer

demodulating CCD detector

S(t) I(t)Spolarization signal

modulatedpolarization signal

modulatedintensitysignal

ZIMPOL: basic polarimetric principle (fast modulation)

Advantages: • images of two opposite polarization modes are created almost simultaneously modulation faster than seeing variations• both images are recorded with same pixel (buffers are different)• both images are subject to almost exactly the same aberrations• integration over many modulation cycles without readout (low RON)

SPHERESPHERE•DerotatorDerotator•40x40 X-AO system 40x40 X-AO system •vis. Wavefront sensorvis. Wavefront sensor•IR and vis. CoronagraphIR and vis. Coronagraph•2 IR and 1 vis. Focal plane inst.2 IR and 1 vis. Focal plane inst.

CPICPI

IRDISIRDIS IFSIFS

ZIMPOLZIMPOL

ITTMITTMPTTMPTTM

DMDM

DTTDTTPP

DTTDTTSS

WFWFSS

De-rotatorDe-rotator

VIS ADCVIS ADCNIR ADCNIR ADC

Focus 1Focus 1

Focus 2Focus 2

Focus 3Focus 3

Focus 4Focus 4

NIR NIR coronocorono

VIS VIS coronocorono

HWP2HWP2

HWP1HWP1

Polar CalPolar Cal

ZIMPOLZIMPOL•fast mod. Imaging fast mod. Imaging pol.pol.•2 arms2 arms

ImplementatiImplementationon

Polarimetry with VLT / SPHERE

ZIMPOL (Zurich Imaging Polarimeter)• FoV (detector): 3.5 x 3.5 arcsec; res. 15 mas at 600 nm• wavelength range 550-890 nm• filters: broad R,I, …; narrow CH4, KI…; line, Hα, OI…. • Polarimetric sensitivity 10-5

SPHERE • Extreme AO, (9mag star), Strehl ~50% for 600-900 nm• coronagraphy (Lyot coronagraphs, 4QPM)• IRDIS: polarimetry in the 1 – 2.2 µm range

Goals:• polarization contrast limit 10-8 for bright stars• detect planets around nearby stars d < 5pc• characterize scattered light from circumstellar disks

On-sky polarimetric calibration and testing needed:

Nasmyth focus plus polarization compensation

polarization depends on pointing direction and filter needs polarimetric calibration with 0-pol. and high-pol. standard stars (each night)

Complicated instrument: derotator, AO, beamsplitter, coronagraph Instrument polarization depends on system configuration Internal lamps and polarization components available Needs on sky tests with 0-pol. and high pol standards and twilight sky polarimetry,

for checks of polarimetric calibration, polarimetric structure of PSF, field dependent effects (initial characterization, yearly updates, instrument polarization checks for each run)

Fast modulation imaging polarimeter pol. efficiency depends on wavelength, mod. frequency, frame transfer and

detector X-Y position Internal lamps and polarization components available needs on sky checks with high pol. stars and twilight sky polarimetry (initial

characterization, check pol. efficiency for each run)

Science requirementsExtra-solar planets (S/N ~ 5-10):

Measure tiny signal p< 0.01% on top of coronagraphic PSF of bright star (= 0-pol.reference)

• rough rel. pol. calibration ΔQ/Q < ± 0.05 (same for U)• rough pos. angle calibration: Δθ < 5 degr

Bright targets:

binaries, circumstellar disks, resolved objects (red giants, asteroids), etc. as accurately as possible:

• accurate calibration at the ΔQ/I ~ ± 0.1% (same for U)• accurate pos. angle calibration Δθ ~ 1 degr • and much higher on a relative scale within FoV

ZIMPOL polarization filters (and Bessell V, R, I)

0-pol. calibration stars• calibrate/check telescope + instrument pol. for setup

(each run)– polarization offset due to instrument / field dependence?– polarized diffraction/ghost features?– polarized structure in coronagraphic PSF?

Requirements:

• brightness between mR = 3 – 8 mag (like targets)

• low pol. p<0.03%, if possible p<0.01%• similar colors because of broad-band filters• no narrow-band polarization in H-alpha

Selection of 0-pol. calibrators• Serkowski (1974): mv<5mag (includes binaries like 36 Oph AB• Serkowski (1974): nearby 7mag G-stars (UKIRT) only few in the south • Tinbergen (1979): mv<5mag, mixed list incl. e.g. Sirius, • Hsu and Breger (1982), few good, well established objects• Turnshek et al. (1990), few good, well established objects• Other list provide not much more• Bailey et al. (2010) mv<5 mag, 10-5 accuracy, limited sky coverage

Selection for ZIMPOL / SPHERE:• 30 stars: nearby <20 pc, 4<mv<8, late F to early K, single, ``normal’’ MS

(most have measurements, but not homogeneous)

evenly distributed calibrators with similar color

• 7 stars from Bailey (2010) – high precision calibrators

high-pol. calibration stars• calibrate/check telescope + instrument pol. position

angle and pol. efficiency (each run / regularly)– position angle offset for this system configuration?– polarization efficiency for this instrument mode?

Requirements:

• brightness mR < 8 mag (AO compatible)

• position angle accurate to Δθ < ± 1 degr • polarization known to ΔQ/I < ± 0.1%, ΔQ/Q ~ ± 0.01• polarization constant in H-alpha

Selection of high-pol. calibrators• Serkowski (1974): ~ 15 high polarization stars – good start• Serkowski (1975): ~300 stars (large reservoir) mostly UBV only• Hsu and Breger (1982), few good, well established objects UBVR[750]• Bailey & Hough (1982), few well established objects UBVRI + IR• Whittet et al. (1992) mostly mv>8mag (too faint for SPHERE AO)

• FORS list the classical objects, and mV>8mag objects (too faint)

Selection for ZIMPOL / SPHERE:• 17 stars, mostly selected from the above sources• no Cepheids

Still many problems• Many are variables (0.1 mag level)• Supergiants, e.g. A2Iab have line emission• only few well established: o Sco, ζ Oph, HD154445, HD 183143• miserable sky distribution

z~45 degr

Distribution of ZIMPOL/SPHERE polarization standard stars

HD 23512

p

wl

Problem:For each standard star we needpolarization value for each ZIMPOL filter

Not available:- accurate spectropolarimetry- investigation of pol. variability

For high precision polarimetry: quality of standard stars muststill be established

Special issues for polarimetry with AO systems

Is the instrumental polarization of off-axis point sources different fromcentral bright source are all faint sources potential polarized planets

Figure 1. Trapezium cluster with rough outline of the ZIMPOL field of view for θ1 Ori A,B,C,D, and E (from Petr-Gotzens et al. (2008), Jour. Phys. Conf. Ser. 131

There is hardly a field on the sky with several bright stars for the testing of FoV effects

Wish list and things to do for SPHERE/ZIMPOL

0-pol standards:- 0-pol. standard stars: study individual object to reject critical objects (var., binarity, dust) observe more than one 0-pol standards per run

High polarization standards:- produce from existing photomety and polarimety synthetic absolute spectropolarimetry - high-pol. standard stars: investigate the existence of spectropolarimetric data in the archives.- study polarimetric variability limits

Test targets:- investigate extended/multiple high pol. sources on-sky unpolarized bright star in front of rich field with high interstellar polarization