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Orbit of the young very low-mass spectroscopic binary CHXR 74

The pre-main sequence star CHXR74 (M4.25) in ChaI was detected a few years ago to be a very low-mass spectroscopic binary. Determination of its mass would provide a valuable dynamical mass measurement at young ages in the poorly constrained mass regime of <0.3 Msun. We carried out follow-up radial velocity (RV) monitoring with UVES/VLT between 2008 and 2011 and high-resolution adaptive optic assisted imaging with NACO/VLT in 2008 with the aim to constrain the binary orbit. We present an orbital solution of the system based on the combined RV data set which spans more than 11 years of UVES monitoring for CHXR74. The best-fit Kepler model has an orbital period of 13.1 yrs, zero eccentricity, and a RV semi-amplitude of 2.2 km/s. A companion mass M2sini of 0.08 Msun is derived by using a model-dependent mass estimate for the primary of 0.24 Msun. The binary separation for i=90deg is 3.8 AU (23 mas). Complementary NACO images of CHXR74 were taken with the aim to directly resolve the binary. While there are marginal signs of an extended PSF, we have no convincing companion detected to CHXR74 in these images. From the non-detection of the companion together with a prediction of the binary separation at the time of the NACO observations, we derive an upper limit for the K-band brightness ratio of 0.5. This allows us to estimate an upper limit of the companion mass of 0.14 Msun by applying evolutionary models. Thus, we have confirmed that CHXR74 is a very low-mass spectroscopic binary and constrained the secondary mass to lie within the range of about 0.08 and 0.14 Msun. We predict an astrometric signal of the primary between 0.2 and 0.4 mas when taking into account the luminosity of the companion. The GAIA astrometric mission might well be able to solve the astrometric orbit of the primary and in combination with the presented RV data to determine an absolute companion mass.