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Precision Imaging Polarimetry with ACS Dean C. Hines Space 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 – the geometry of the regions producing polarized light • Polarization probes asymmetry • Provides a “scattering mirror” to “see” around obscuration • Science Examples – – – – 5/24/2017 Synchrotron emission in M87 jet Unified AGN Model Aspherical supernovae explosions Detailed structure, composition, shape/size, or orientation of scattering dust particles STScI DCH - 2 Scattering & Polarization Polarization induced by scattering off small particles 5/24/2017 STScI DCH - 3 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/I θinstr = 0.5 tan−1 (U/Q) Every Polarimetry Paper…Ever! Hines et al. (2000) 5/24/2017 STScI & SSI DCH - 4 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 5/24/2017 STScI & SSI DCH - 5 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 IcorPOL∗ V = CPOL∗V x Iobs x POL*V , CPOL0V = 1.2960, CPOL60V = 1.3238, CPOL120V = 1.2781 – Repeatability ~ 0.003 fractional polarization – Absolute ~ 0.003 fractional polarization – Position angle ~ 2-3˚ 5/24/2017 STScI & SSI DCH - 6 Field Distortion NICMOS Cryogenic Optical Bench KPNO Hines et al. (2000) 5/24/2017 STScI & SSI DCH - 7 Field Distortion Polarization Position Angles Perpendiculars Weintraub et al. (2000) 5/24/2017 STScI & SSI DCH - 8 Field Distortion I p Theta 5/24/2017 p*I STScI & SSI DCH - 9 Field Distortion I p Theta 5/24/2017 p*I STScI & SSI DCH - 10 Field Distortion I p Theta 5/24/2017 p*I STScI & SSI DCH - 11 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˚ 5/24/2017 STScI DCH - 12 ACS/WFC F606W Imaging Polarimetry To Sun 5″ Hines et al. (2014, ApJ, 780, 32L) 5/24/2017 STScI DCH - 13 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 cometary jets. 5/24/2017 STScI DCH - 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 cometary jets. Hines et al. (2014, ApJ, 780, 32L) 5/24/2017 STScI DCH - 15 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. 5/24/2017 STScI DCH - 16 Fin 5/24/2017 STScI & SSI DCH - 18