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 M ☉ . 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 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.