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