Resolution and calibration effects in high contrast polarimetric imaging of circumstellar scattering regions

Abstract

Many circumstellar dust scattering regions have been detected and investigated with polarimetric imaging. However, the quantitative determination of the intrinsic polarization and of dust properties is difficult because of complex observational effects. This work investigates instrumental convolution and polarimetric calibration effect for high contrast imaging polarimetry with the aim to define procedures for accurate measurements of the circumstellar polarization. For this we simulate the instrumental convolution and polarimetric cancellation effects for a Gaussian PSF and an extended PSF_{AO} typical for a modern adaptive optics system. Further, polarimetric zero-point corrections (zp-corrections) are simulated for different cases like coronagraphic observations or systems with barely resolved circumstellar scattering regions. We find that the PSF convolution reduces the integrated azimuthal polarization Q_phi for the scattering region while the net Stokes signals Q and U are not changed. For non-axisymmetric systems a spurious U_phi-signal is introduced. These effects are strong for compact systems but scattering regions can still be detected down to small separations while unresolved scattering regions can be constrained by the central Stokes Q,U signal. The smearing by PSF_{AO} produces an extended, low surface brightness polarization signal changing the angular distribution of the polarization, but the initial signal can be recovered partly from the Stokes Q and U quadrant pattern. A polarimetric zp-correction applied for the removal of offsets from instrumental or interstellar polarization depends on the selected reference region and can also introduce strong bias effects for the azimuthal distribution of the polarization signal. Strategies for the zp-correction are described for coronagraphic data or observations of partly unresolved systems.

29 pages, 25 figures, accepted for publication in Astronomy & Astrophysics

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