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

P. Viana Almeida, J. F. Gameiro, P. P. Petrov, C. Melo, N. C. Santos, P. Figueira, S. Alencar

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 m2sin i≃ 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.

Keywords
infrared: planetary systems, stars: pre-main sequence, techniques: radial velocities, brown dwarfs, protoplanetary disks

Notes
Based on observations collected with the CRIRES spectrograph at the VLT/UT1 8.2-m Antu Telescope (ESO runs ID 385.C-0706(A) and 093.C-0400(A)) at the Paranal Observatory, Chile.

Astronomy and Astrophysics
Volume 600, Article Number A84, Number of pages 6
2017 April

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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