Optical and near-infrared linear polarization of low and intermediate-gravity ultra-cool dwarfs

TL;DR

This study investigates the optical and near-infrared linear polarization of young and old ultra-cool dwarfs, revealing similar polarization levels across ages and spectral types, with evidence of short-term and long-term variability.

Contribution

First comprehensive polarimetric analysis comparing young low- and intermediate-gravity ultra-cool dwarfs with older counterparts, highlighting variability and atmospheric effects.

Findings

Young dwarfs show similar polarization to older ones.

Intense polarization likely due to surrounding disks in Taurus sources.

Both short-term and long-term polarimetric variability observed.

Abstract

We aim to study the optical and near-infrared linear polarimetric properties of a sample of young M7-L7 dwarfs ($\approx$1-500 Myr) with spectroscopic signatures of low- and intermediate-gravity atmospheres. We collected optical ($RIZ$) and near-infrared ($YJHK_{\rm s}$) linear polarimetry images on various time scales from $\sim$0.2 h to months. Linear polarization degrees in the interval 0-1.5\% ($I$- and $J$-bands) were measured with accuracies ranging from $\pm$0.1 $\%$ to $\pm$0.9 $\%$ depending on the observing filter and the target brightness. We found that the young field dwarfs in our sample show similar polarimetric degrees at both $I$- and $J$-bands, and that there is no obvious trend with the spectral type. The two Taurus sources in our sample show intense levels of $J$-band linear polarization probably due to surrounding disks. By compiling data from the literature for high-gravity M7-L7 dwarfs with likely ages $\ge$500 Myr, we did not observe any apparent difference in the linear polarimetry intensity between the young and old samples that could be ascribed to differing atmospheric gravities. Polarimetric variability with peak to peak amplitudes up to 1.5 $\%$ is detected on scales of about a rotation in two out of four targets that were monitored over several hours. Long-term polarimetric variability is also detected in nearly all dwarfs of the sample with data spanning months to years.