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

SPHERE / ZIMPOL Science goal: detection of reflected=polarized light from extra-solar planets 108 log I Example: Sun – Jupiter system at 5 pc 107 log I 1´´ 103 log I coro X-AO tiny polarized planetary signal in bumpy and variable PSF halo

SPHERE-Design Grenoble, Dec. 2011

ZIMPOL: basic polarimetric principle (fast modulation) synchronization (kHz) modulator polarizer demodulating CCD detector S(t) I(t) S polarization signal modulated intensity 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)

Implementation SPHERE ZIMPOL CPI ZIMPOL IFS IRDIS ITTM PTTM DM DTTP DTTS WFS De-rotator VIS ADC NIR ADC Focus 1 Focus 2 Focus 3 Focus 4 NIR corono VIS corono HWP2 HWP1 Polar Cal Implementation SPHERE Derotator 40x40 X-AO system vis. Wavefront sensor IR and vis. Coronagraph 2 IR and 1 vis. Focal plane inst. ZIMPOL fast mod. Imaging pol. 2 arms

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 needs on sky checks with high pol. stars and twilight sky polarimetry (initial characterization, check pol. efficiency for each run)

Science requirements Extra-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

Distribution of ZIMPOL/SPHERE polarization standard stars HD 23512 z~45 degr

For high precision polarimetry: Problem: For each standard star we need polarization value for each ZIMPOL filter Not available: accurate spectropolarimetry investigation of pol. variability For high precision polarimetry: quality of standard stars must still be established p wl

Special issues for polarimetry with AO systems Is the instrumental polarization of off-axis point sources different from central bright source  are all faint sources potential polarized planets

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

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