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Published November 10, 2006 | Published
Journal Article Open

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_☉. 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.

Additional Information

© 2006 American Astronomical Society. Received 2006 April 21; accepted 2006 July 20. The authors acknowledge many insightful discussions with Travis Barman, J. Davy Kirkpatrick, and Ian McLean during the preparation of the manuscript. We are grateful to our support astronomers and engineers Randy Campbell, Rick Burress, and Jeff Hickey for their guidance in using Keck and Palomar AO, to Keith Matthews and Dave Thompson for advice with NIRC2 spectroscopy, and to our telescope operators at the Palomar and Keck II telescopes, Jean Mueller, Karl Dunscombe, and Cynthia Wilburn. We also wish to extend special thanks to those of Hawaiian ancestry, on whose sacred mountain of Mauna Kea we are privileged to be guests. Without their generous hospitality, the spectroscopic observations presented herein would not have been possible. Support for S.A.M. was provided by NASA through the Spitzer Fellowship Program, under award 1273192. This work was supported by NASA grant NNG05GJ37G and NASA/JPL contract 1224566.

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