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

¹ Universidade Federal de Minas Gerais, 31270-901 Pampulha, Belo Horizonte – MG, Brasil
e-mail: palmeida@fisica.ufmg.br
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ ESO, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁵ Crimean Astrophysical Observatory, Russian Academy of Sciences, 298409 Nauchny, Crimea

Received: 19 September 2016
Accepted: 16 December 2016

Abstract

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 (≈K5) of ~1 M_⊙ that belongs to a triple system. Independent photometric surveys discovered that AS 205A has two distinct periods (P₁ = 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 ~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 ≈ 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 ≈ 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≃ 19.25 M_Jup 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.