Point Source Polarimetry with the Gemini Planet Imager: Sensitivity Characterization with T5.5 Dwarf Companion HD 19467 B

TL;DR

This study uses the Gemini Planet Imager to search for polarized light from the T5.5 dwarf companion HD 19467 B, setting an upper limit on polarization and discussing implications for cloud models.

Contribution

First polarimetric observations of a T dwarf companion with GPI, establishing polarization upper limits and informing cloud and variability models.

Findings

No polarization detected from HD 19467 B

Upper limit on polarization degree is 2.4%

Results inform T dwarf cloud and variability models

Abstract

Detecting polarized light from self-luminous exoplanets has the potential to provide key information about rotation, surface gravity, cloud grain size, and cloud coverage. While field brown dwarfs with detected polarized emission are common, no exoplanet or substellar companion has yet been detected in polarized light. With the advent of high contrast imaging spectro-polarimeters such as GPI and SPHERE, such a detection may now be possible with careful treatment of instrumental polarization. In this paper, we present 28 minutes of $H$-band GPI polarimetric observations of the benchmark T5.5 companion HD 19467 B. We detect no polarization signal from the target, and place an upper limit on the degree of linear polarization of $p_{\text{CL}99.73\%} \leq 2.4\%$. We discuss our results in the context of T dwarf cloud models and photometric variability.