* Astronomy

Title: HIP 38939B: A New Benchmark T Dwarf in the Galactic Plane Discovered with Pan-STARRS1
Authors: Niall R. Deacon (1), Michael C. Liu (1), Eugene A. Magnier (1), Brendan P. Bowler (1), Joshua Redstone (2), Bertrand Goldman (3), W. S. Burgett (1), K. C. Chambers (1), H. Flewelling (1), N. Kaiser (1), J.S. Morgan (1), P.A. Price (4), W.E. Sweeney (1), J.L. Tonry (1), R.J. Wainscoat (1), C. Waters (1) ((1) IfA/Hawaii (2) Facebook, (3) Max Planck Institute for Astronomy, (4) Princeton)

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) K4V star HIP 38939 using Pan-STARRS1 data. The companion was discovered due to its common proper motion with the primary and its red optical (Pan-STARRS1) and blue infrared (2MASS) colours. It has a projected separation of 1630 AU, and is spectrally typed as T4.5. As a companion to HIP 38939A, it is only the third object of its spectral subclass to have a measured trigonometric parallax. 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 implies 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.

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Position (J2000):     R.A. 07 58 04.37013  |  Dec. -25° 37' 35.8638''