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Discovery of a Young Substellar Companion in Chamaeleon.pdf

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Discovery of a Young Substellar Companion in Chamaeleon

a r X i v : a s t r o - p h / 0 6 0 9 1 8 7 v 1 7 S e p 2 0 0 6 Draft version February 5, 2008 Preprint typeset using LATEX style emulateapj v. 6/22/04 DISCOVERY OF A YOUNG SUBSTELLAR COMPANION IN CHAMAELEON K. L. Luhman1, J. C. Wilson2, W. Brandner3, M. F. Skrutskie2, M. J. Nelson2, J. D. Smith4, D. E. Peterson2, M. C. Cushing4, and E. Young4 Draft version February 5, 2008 ABSTRACT During an imaging survey of the Chamaeleon I star-forming region with the Advanced Camera for Surveys aboard the Hubble Space Telescope, we have discovered a candidate substellar companion to the young low-mass star CHXR 73 (τ ～ 2 Myr, M ～ 0.35 M⊙). We measure a projected separation of 1.′′30 ± 0.′′03 for the companion, CHXR 73 B, which corresponds to 210 AU at the distance of the cluster. A near-infrared spectrum of this source obtained with the Cornell Massachusetts Slit Spectrograph at the Magellan II telescope exhibits strong steam absorption that confirms its late- type nature (M9.5). In addition, the gravity-sensitive shapes of the H- and K-band continua demonstrate that CHXR 73 B is a young, pre-main-sequence object rather than a field star. The probability that CHXR 73 A and B are unrelated members of Chamaeleon I is ～ 0.001. We estimate the masses of CHXR 73 B and other known substellar companions in young clusters with a method that is consistent with the dynamical measurements of the eclipsing binary brown dwarf 2M 0535- 0546, which consists of a comparison of the bolometric luminosities of the companions to the values predicted by the evolutionary models of Chabrier coworkers and Burrows coworkers. We arrive at mass estimates of 0.003-0.004, 0.024± 0.012, 0.011+0.01 ?0.003, and 0.012 +0.008 ?0.005 M⊙ for 2M 1207-3932 B, GQ Lup B, DH Tau B, and CHXR 73 B, respectively. Thus, DH Tau B and CHXR 73 B appear to be the least massive companions to stars outside the solar system that have been detected in direct images, and may have masses that are within the range observed fo