Evidence of a substellar companion around a very young T Tauri star

We present results from a near-infrared multi-epoch spectroscopic campaign to detect a young low-mass companion to a T Tauri star. AS 205A is a late-type dwarf (approximate to K5) of similar to 1 M circle dot that belongs to a triple system. Independent photometric surveys discovered that AS 205A has two distinct periods (P-1 = 6 : 78 and P2 = 24 : 78 days) detected in the light curve that persist over several years. Period P1 seems to be linked to the axial-rotation of the star and is caused by the presence of cool surface spots. Period P2 is correlated with the modulation in AS 205A brightness (V) and red color (V-R), consistent with a gravitating object within the accretion disk. We here derive precise near-infrared radial velocities to investigate the origin of period P2 which is predicted to correspond to a cool source in a Keplerian orbit with a semi-major axis of similar to 0.17 AU positioned close to the inner disk radius of 0.14 AU. The radial velocity variations of AS 205A were found to have a period of P approximate to 24 : 84 days and a semi-amplitude of 1.529 km s 1. This result closely resembles the P2 period in past photometric observations (P approximate to 24 : 78 days). The analysis of the cross-correlation function bisector has shown no correlation with the radial velocity modulations, strongly suggesting that the period is not controlled by stellar rotation. Additional activity indicators should however be explored in future surveys. Taking this into account we found that the presence of a substellar companion is the explanation that best fits the results. We derived an orbital solution for AS 205A and found evidence of a m2 sin i similar or equal to 19 : 25 MJup object in an orbit with moderate eccentricity of e ' 0 : 34. If confirmed with future observations, preferably using a multiwavelength survey approach, this companion could provide interesting constraints on brown dwarf and planetary formation models.