The multiplicity of planet host stars - New low-mass companions to planet host stars

We present new results from our ongoing multiplicity study of exoplanet host stars, carried out with the infrared camera SofI at ESO-NTT. We have identified new low mass companions to the planet host stars HD101930 and HD65216. HD101930AB is a wide binary systems composed of the planet host star HD101930A and its companion HD101930B which is a M0 to M1 dwarf with a mass of about 0.7Msun separated from the primary by ~73arcsec (2200AU projected separation). HD65216 forms a hierarchical triple system, with a projected separation of 253AU (angular separation of about 7arcsec) between the planet host star HD65216A and its close binary companion HD65216BC, whose two components are separated by only ~0.17arcsec (6AU of projected separation). Two VLT-NACO images separated by 3 years confirm that this system is co-moving to the planet host star. The infrared photometry of HD65216B and C is consistent with a M7 to M8 (0.089Msun), and a L2 to L3 dwarf (0.078Msun), respectively, both close to the sub-stellar limit. An infrared spectrum with VLT-ISAAC of the pair HD65216BC, even though not resolved spatially, confirms this late spectral type. Furthermore, we present H- and K-band ISAAC infrared spectra of HD16141B, the recently detected co-moving companion of the planet host star HD16141A. The infrared spectroscopy as well as the apparent infrared photometry of HD16141B are both fully consistent with a M2 to M3 dwarf located at the distance of the planet host star.Comment: MNRAS accepted, 8 pages, 6 figures, and 1 tabl

HD3651B: the first directly imaged brown dwarf companion of an exoplanet host star

In the course of our ongoing multiplicity study of exoplanet host stars we detected a faint companion located at ~43arcsec (480AU physical projected separation) north-west of its primary -- the exoplanet host star HD3651 at 11pc. The companion, HD3651B, clearly shares the proper motion of the exoplanet host star in our four images, obtained with ESO/NTT and UKIRT, spanning three years in epoch difference. The magnitude of the companion is H=16.75+-0.16mag, the faintest co-moving companion of an exoplanet host star imaged directly. HD3651B is not detected in the POSS-II B-, R- and I-band images, indicating that this object is fainter than ~20mag in the B- and R-band and fainter than \~19mag in the I-band. With the Hipparcos distance of HD3651 of 11pc, the absolute magnitude of HD3651B is about 16.5mag in the H band. Our H-band photometry and the Baraffe et al. (2003) evolutionary models yield a mass of HD3651B to be 20 to 60MJup for assumed ages between 1 and 10Gyr. The effective temperature ranges between 800 and 900K, consistent with a spectral type of T7 to T8. We conclude that HD3651B is a brown-dwarf companion, the first of its kind directly imaged as a companion of an exoplanet host star, and one of the faintest T dwarfs found in the solar vicinity (within 11pc).Comment: 5 pages, 3 figures, 2 tables, accepted for publication in MNRAS LETTER

X-ray emission from young stars in the Tucanae association

We report on X-ray emission from members of the recently discovered Tucanae association, a group of stars with youth signatures and similar space motion. The Tucanae association is the nearest known region of recent star formation (at 45 pc) far from molecular clouds. We have made use of the ROSAT Data Archive, and searched for X-rays from Tucanae stars in both ROSAT All-Sky Survey (RASS) and pointed observations. The RASS detection rate for the Tucanae members is 59%. We compare the X-ray luminosity function of the Tucanae association to that of other star forming regions and young star clusters. Both the X-ray luminosity function and the strong variability observed in the X-ray lightcurves suggest that the examined stars are young, on the order of 10-30 Myrs.Comment: 13 pages, accepted by A&A main journa