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

¹ Zentrum für Astronomie Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
² Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: viki@mpia.de
³ Princeton University, Dept. of Astrophysics, 4 Ivy Lane, Princeton, NJ 08544, USA

Received: 5 October 2011
Accepted: 24 October 2011

Abstract

The pre-main sequence star CHXR 74 (M4.25) in Chamaeleon I was found a few years ago to be a very low-mass spectroscopic binary. A determination of its mass would provide a valuable dynamical mass measurement at young ages in the poorly constrained mass regime of  <0.3   M_⊙. We carried out follow-up radial velocity monitoring with UVES/VLT between 2008 and 2011 and high-resolution adaptive-optic-assisted imaging with NACO/VLT in 2008 with the aim of constraining the binary orbit. We present an orbital solution of the system based on the combined radial velocity data set, which spans more than eleven years of UVES monitoring for CHXR 74. The best-fit Kepler model has an orbital period of 13.1 years, zero eccentricity, and a radial velocity semi-amplitude of 2.2 km s^-1. A companion mass M₂sini (which is a lower limit due to the unknown orbital inclination i) of 0.08 M_⊙ is derived by using a model-dependent mass estimate for the primary of 0.24 M_⊙. The binary separation (a₁sini + a₂) for an inclination of 90° is 3.8 AU, which corresponds to 23 mas. Complementary NACO/VLT images of CHXR 74 were taken with the aim to directly resolve the binary. While there are marginal signs of an extended point spread function (PSF), we have detected no convincing companion to CHXR 74 in the NACO 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 the two binary components of 0.5. This allows us to estimate an upper limit of the companion mass of 0.14 M_⊙ by applying evolutionary models. Thus, we confirm that CHXR 74 is a very low-mass spectroscopic binary and constrain the secondary mass to lie within the range of about 0.08 and 0.14 M_⊙. 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 radial velocity data determine an absolute companion mass.