LHS 2803B: A very wide mid-T dwarf companion to an old M dwarf identified from Pan-STARRS1

TL;DR

This paper reports the discovery of a wide T5.5 dwarf companion to an M4.5 star, using Pan-STARRS1 and 2MASS data, with detailed spectroscopic analysis and age estimation.

Contribution

First identification of a very wide mid-T dwarf companion to an old M dwarf using large survey data and spectroscopic follow-up.

Findings

Companion classified as T5.5 spectral type.

System estimated to be between 3.5 and 10 Gyr old.

Secondary's physical properties include ~1120 K temperature and ~72 Jupiter masses.

Abstract

We report the discovery of a wide (approximately 400 AU projected separation), common proper motion companion to the nearby M dwarf LHS 2803 (PSO J207.0300-13.7422). This object was discovered during our census of the local T dwarf population using Pan-STARRS1 and 2MASS data. Using IRTF/SpeX near-infrared spectroscopy, we classify the secondary to be spectral type T5.5. University of Hawai`i 2.2m/SNIFS optical spectroscopy indicates the primary has a spectral type of M4.5, with approximately solar metallicity and no measurable H_alpha emission. We use this lack of activity to set a lower age limit for the system of 3.5 Gyr. Using a comparison with chance alignments of brown dwarfs and nearby stars, we conclude that the two objects are unlikely to be a chance association. The primary's photometric distance of 21 pc and its proper motion implies thin disk kinematics. Based on these kinematics and its metallicity, we set an upper age limit for the system of 10 Gyr. Evolutionary model calculations suggest the secondary has a mass of 72^+4_-7 M_Jup, temperature of 1120+-80 K, and log g=5.4+-0.1 dex. Model atmosphere fitting to the near-IR spectrum gives similar physical parameters of 1100 K and log g=5.0.