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 ^ { \circ } ) 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 \epsilon Indi B ) .
Using chromospheric activity we estimate an age for the primary of 900 \pm ^ { 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 \pm 20 M _ { Jup } with an effective temperature range of 1090 \pm 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 .