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Precision Imaging Polarimetry with ACS

Precision Imaging Polarimetry with ACS. Dean C. Hines S pace Telescope Science Institute. Polarimetry. Polarization can provide vital and otherwise unobtainable constraints on the origin of light from astronomical sources the nature of particles involved in emission and scattering

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Precision Imaging Polarimetry with ACS

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  1. Precision Imaging Polarimetry with ACS Dean C. Hines Space Telescope Science Institute

  2. Polarimetry • Polarization can provide vital and otherwise unobtainable constraints on • the origin of light from astronomical sources • the nature of particles involved in emission and scattering • the geometry of the regions producing polarized light • Polarization probes asymmetry • Provides a “scattering mirror” to “see” around obscuration • Science Examples • Synchrotron emission in M87 jet • Unified AGN Model • Aspherical supernovae explosions • Detailed structure, composition, shape/size, or orientation of scattering dust particles STScI

  3. Scattering & Polarization Polarization induced by scattering off small particles STScI

  4. Polarimetry 101 Sk = AkI + ek (BkQ + CkU),Ak = ½ tk (1+lk),Bk = Ak cos (2fk), Ck = sin (2fk) Hines et al. (2000: see also Sparks & Axon 1999) I = 2/3(S0 + S60 + S120), Q = 2/3(2S0 + S60 + S120), U = 2(S60 – S120)/sqrt(3) ACS Instrument Handbook (see also Jackson!) p = 100% (Q2 + U2 )1/2/Iθinstr = 0.5 tan−1 (U/Q) Every Polarimetry Paper…Ever! Hines et al. (2000) STScI & SSI

  5. Calibration of a Polarimeter • Ground-based TV testing with polarized and unpolarized flat field • measure throughputs • polarizing efficiencies • FOV variations • instrumental polarization • fiducial polarization “axis” of the instrument • On-orbit using polarized and unpolarized ‘standards” • Multiple “rolls” for separating throughput vs. polarizing efficiency • Measure know extended polarized source to assess field distortions STScI & SSI

  6. Updated Calibration • ACS internal flat mirrors • ACS Cal programs • Previous calibration • Cracraft, M., & Sparks, W. B. 2007, ISR ACS 2007–10 • Biretta, J., et al. 2004, ISR ACS 2004–09 • New calibrations • Repeatability ~ 0.003 fractional polarization • Absolute ~ 0.003 fractional polarization • Position angle ~ 2-3˚ IcorPOL∗ V = CPOL∗V x Iobs x POL*V , CPOL0V = 1.2960, CPOL60V = 1.3238, CPOL120V = 1.2781 STScI & SSI

  7. Field Distortion NICMOS Cryogenic Optical Bench KPNO Hines et al. (2000) STScI & SSI

  8. Field Distortion Polarization Position Angles Perpendiculars Weintraub et al. (2000) STScI & SSI

  9. Field Distortion I p Theta p*I STScI & SSI

  10. Field Distortion I p Theta p*I STScI & SSI

  11. Field Distortion I p Theta p*I STScI & SSI

  12. HST/ACS Observations of Comet ISON • Observed with Wide Field Camera (WFC) module of the Advanced Camera for Surveys (ACS) • Two Orbits DD time May 8, 2013 • F606W (Broad-V) • Weak or no emission lines • rh = 3.81 AU from Sun, D = 4.34 AU from Earth • Phase angle was α ≈ 12.25º (“bottom” of negative polarization branch) • Three Orbits GO time October 26/27, 2013 • F606W (Broad-V) & F775W (SDSSi) • Strong C2 in F606W • Minimal CN in F775W • rh = 1.12 AU, D = 1.37 AU, α ≈ 45.60˚ STScI

  13. ACS/WFC F606W Imaging Polarimetry To Sun 5″ Hines et al. (2014, ApJ, 780, 32L) STScI

  14. ACS/WFC F606W Imaging Polarimetry • Halo p ~ -1.8%, suggest abundance of abundant absorbing particles • Central region p ~ + 2.5%, suggests abundant ice-coated grains. Characteristic of cometaryjets. STScI

  15. ACS/WFC F606W Imaging Polarimetry • Halo p ~ -1.8%, suggest abundance of abundant absorbing particles • Central region p ~ + 2.5%, suggests abundant ice-coated grains. Characteristic of cometaryjets. Hines et al. (2014, ApJ, 780, 32L) STScI

  16. Polarimetry Observations of Comet 67/P • Rosetta will measure the properties of the coma of 67P, including the inner coma during Philae lander release by 10 km altitude. • 21 Orbit, Cycle 22 HST/ACS F606W (PI: Hines) • Observations bracket the period when Rosetta operates closest to 67P, and will deploy the Philae lander. • 2014-Aug-19: rh = 3.52 AU, D = 2.76 AU, α ≈ 12.0˚ • 2014-Nov-17: rh = 2.96 AU, D = 3.43 AU, α ≈ 15.7˚ • Planning to propose Post-Perihelion follow-up • Contemporaneous observations with remote sensing assets from more distant vantage points will enable linkage of the properties and distribution of dust observed from Earth to the material measured in-situ near the nucleus. STScI

  17. Fin STScI & SSI

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