Infrared spectroscopy of a brown dwarf companion candidate near the young star GSC 08047-00232 in Horologium

Abstract

We present infrared H- and K-band spectra of a companion candidate 3" north of the young star GSC 08047-00232, a probable member of the nearby young Horologium association. From previously obtained JHK-band colors and the magnitude difference between primary and companion candidate, the latter could well be substellar (Neuh"auser et al. 2003) with the spectral type being roughly M7-L9 from the JHK colors (Chauvin et al. 2003). With the H- and K-band spectra now obtained with ISAAC at the VLT, the spectral type of the companion candidate is found to be M6-9.5. Assuming the same age and distance as for the primary star (~35 Myrs, 50 to 85 pc), this yields a mass of ~25 Jupiter masses for the companion, hence indeed substellar. After TWA-5 B and HR 7329 B, this is the third brown dwarf companion around a nearby (up to 100 pc) young (up to 100 Myrs) star. A total of three confirmed brown dwarf companions (any mass, separation above 50 AU) around 79 stars surveyed in three young nearby associations corresponds to a frequency of 6 \pm 4 % (with a correction for missing companions which are almost on the same line-of-sight as the primary star instead of being separated well), consistent with the expectation, if binaries have the same mass function as field stars. Hence, it seems that there is no brown dwarf desert at wide separations.