The polarisation of HD 189733

TL;DR

This study presents high-precision linear polarization measurements of the HD 189733 exoplanet system, challenging previous reports of large polarization variations and providing refined constraints on polarization amplitude.

Contribution

The paper provides the most precise polarization observations of HD 189733 to date, refuting earlier claims of large polarization variations and offering new quantitative polarization estimates.

Findings

No large polarization variations detected.

Measured polarization amplitude of 29.4 +/- 15.6 ppm.

Observed background polarization slightly higher than interstellar expectations.

Abstract

We present linear polarization observations of the exoplanet system HD 189733 made with the HIgh Precision Polarimetric Instrument (HIPPI) on the Anglo-Australian Telescope (AAT). The observations have higher precision than any previously reported for this object. They do not show the large amplitude polarization variations reported by Berdyugina et al. 2008 and Berdyugina et al. 2011. Our results are consistent with polarization data presented by Wiktorowicz et al. 2015. A formal least squares fit of a Rayleigh-Lambert model yields a polarization amplitude of 29.4 +/- 15.6 parts-per-million. We observe a background constant level of polarization of ~ 55-70 ppm, which is a little higher than expected for interstellar polarization at the distance of HD 189733.