Astronomy and Astrophysics, volume 447, 1035-1039 (2006/3-1)
Radius and mass of a transiting M dwarf near the hydrogen-burning limit. OGLE-TR-123.
PONT F., MOUTOU C., BOUCHY F., BEHREND R., MAYOR M., UDRY S., QUELOZ D., SANTOS N. and MELO C.
Abstract (from CDS):
The system OGLE-TR-123, detected by the OGLE planetary transit survey in Carina, was shown by subsequent spectroscopic follow-up to be an eclipsing binary with a very low mass companion (M2∼0.07M☉). Neither the light curve nor the radial velocity data were sufficient to constrain the parameters more precisely and to exclude alternative explanations in terms of a triple system. In this paper, new HARPS radial velocity data and a precise photometric monitoring of the transit with VLT/FORS2 are presented, resulting in an improved mass and radius determination for OGLE-TR-123b and the rejection of alternative scenarios. We find M2=0.085±0.011M☉ and R2=0.133±0.009R☉ for the transiting companion, confirming its nature as a small M dwarf near the hydrogen-burning limit. Its mass-radius relation corresponds to expectations for a relatively young object. The primary is an F dwarf that does not rotate synchronously with the orbital motion. Asynchronous rotation is unexpected in such a close binary (P=1.8-days) but could be explained by the small mass of the secondary, the young age for the system, and the absence of convective envelope in the primary. OGLE-TR-123b is one of only two known objects below 0.1M☉ with directly determined radii, after a similar object was characterised around OGLE-TR-122. The characterisation of OGLE-TR-123 provides another example of the type of system that can closely mimic planetary transits and contaminate transit surveys.
binaries: eclipsing - stars: low-mass, brown dwarfs
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