Trigonometric parallaxes of two T dwarfs with Keck and ShaneAO astrometry

TL;DR

This paper reports precise trigonometric parallax and proper motion measurements for two T-type brown dwarfs using adaptive optics over five years, improving astrometric accuracy and providing valuable calibration data for stellar formation models.

Contribution

It presents the first precise parallax measurements of these two T dwarfs using Keck and ShaneAO adaptive optics, with improved distortion correction and reference frame transformation.

Findings

Parallaxes of 73.5±9.2 mas and 70.1±6.7 mas for the two T dwarfs.

Measurements agree within 2σ of previous data, validating the methods.

First results from an ongoing T dwarf survey at Lick and Keck observatories.

Abstract

We present trigonometric parallax and proper motion measurements for two T-type brown dwarfs. We derive our measurements from infrared laser guide star adaptive optics observations spanning five years from the ShaneAO/SHARCS and NIRC2/medium-cam instruments on the Shane and Keck telescopes, respectively. To improve our astrometric precision, we measure and apply a distortion correction to our fields for both instruments. We also transform the Keck and ShaneAO astrometric reference frames onto the ICRS using five-parameter parallax and proper motion solutions for background reference stars from Gaia DR2. Fitting for parallax and proper motion, we measure parallaxes of $73.5\pm9.2$ mas and $70.1\pm6.7$ mas for WISEJ19010703+47181688 (WISE1901) and WISEJ21543294+59421370 (WISE2154), respectively. We utilize Monte Carlo methods to estimate the error in our sparse field methods, taking into account overfitting and differential atmospheric refraction. Comparing to previous measurements in the literature, all of our parallax and proper motion values fall within $2\sigma$ of the published measurements, and 4 of 6 measurements are within $1\sigma$. These data are among the first parallax measurements of these T dwarfs and serve as precise measurements for calibrating stellar formation models. These two objects are the first results of an ongoing survey of T dwarfs with Keck/NIRC2 and the Shane Adaptive Optics system at Lick Observatory.