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

We present the discovery of a brown-dwarf companion to the 130-400 Myr-old G8V star HD 203030. 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) solar masses. TheK-band spectral type of L7.5+/-0.5 places HD 203030B near the critical L/Ttransition in brown dwarfs, which is characterized by the rapid disappearanceof dust in sub-stellar photospheres. From a comparative analysis withwell-characterized field L/T transition dwarfs, we find that, despite its youngage, HD 203030B has a bolometric luminosity similar to the >1 Gyr-old fielddwarfs. Adopting a radius from current models of sub-stellar evolution, wehence obtain that the effective temperature of HD 203030B is only 1206(+74;-116) K, markedly lower than the ~1440 K effective temperatures of fieldL/T transition dwarfs. The temperature discrepancy can be resolved if either:(1) the ages of field brown dwarfs have been over-estimated by a factor of\~1.5, leading to under-estimated radii, or (2) the lower effective temperatureof HD 203030B is related to its young age, implying that the effectivetemperature at the L/T transition is gravity-dependent.Comment: 16 pages, 4 figures, 2 tables. To appear in ApJ November 10, 2006, v651 issu