HIP 38939B: A New Benchmark T Dwarf in the Galactic Plane Discovered with Pan-STARRS1

TL;DR

This paper reports the discovery of HIP 38939B, a benchmark T4.5 brown dwarf companion to a metal-poor star, providing valuable data for understanding T dwarf atmospheres and evolution.

Contribution

It presents the discovery and characterization of a rare early/mid-T dwarf companion to a main sequence star, expanding the sample of benchmark T dwarfs for atmospheric studies.

Findings

HIP 38939B has a projected separation of 1630 AU.

Its mass is estimated at 38±20 Mjup.

Atmospheric models with recent updates better match evolutionary predictions.

Abstract

We report the discovery of a wide brown dwarf companion to the mildly metal-poor ([Fe/H]=-0.24), low galactic latitude (b = 1.88 deg) K4V star HIP 38939. The companion was discovered by its common proper motion with the primary and its red optical (Pan-STARRS1) and blue infrared (2MASS) colors. It has a projected separation of 1630 AU and a near-infrared spectral type of T4.5. As such it is one of only three known companions to a main sequence star which have early/mid-T spectral types (the others being HN Peg B and eps Indi B). Using chromospheric activity we estimate an age for the primary of 900{+1900,-600} Myr. This value is also in agreement with the age derived from the star's weak ROSAT detection. Comparison with evolutionary models for this age range indicates that HIP 38939B falls in the mass range 38+/-20 Mjup with an effective temperature range of 1090+/-60 K. Fitting our spectrum with atmospheric models gives a best fitting temperature of 1100 K. We include our object in an analysis of the population of benchmark T dwarfs and find that while older atmospheric models appeared to over-predict the temperature of the coolest objects compared to evolutionary models, more recent atmospheric models provide better agreement.