HD 203030B: An Unusually Cool Young Substellar Companion near the L/T Transition

We present the discovery of a brown dwarf companion to the star HD 203030: a G8 V Solar analog with an estimated age between 130 and 400 Myr old G8 V. Separated by 11.9" (487 AU in projection) from its host star, HD 203030B has an estimated mass of 0.023^+0.008_-0.011 M_solar. The K-band spectral type of L7.5+/-0.5 places HD 203030B near the critical L/T transition in brown dwarfs, which is characterized by the rapid disappearance of dust in substellar photospheres. From a comparative analysis with well-characterized field L/T transition dwarfs, we find that, despite its young age, HD 203030B has a bolometric luminosity similar to the >1 Gyr old field dwarfs. Adopting a radius from current models of substellar evolution, we hence obtain that the effective temperature of HD 203030B is only 1206⁺⁷⁴_-116 K, markedly lower than the ~1440 K effective temperatures of field L/T transition dwarfs. The temperature discrepancy can be resolved if either (1) the ages of field brown dwarfs have been overestimated by a factor of ~1.5, leading to underestimated radii, or (2) the lower effective temperature of HD 203030B is related to its young age, implying that the effective temperature at the L/T transition is gravity dependent.