Doppler probe of accretion onto a T Tauri star^⋆

¹ Crimean Astrophysical Observatory, Taras Shevchenko National University of Kiev, 98409 Nauchny, Crimea, Ukraine
e-mail: petrov@crao.crimea.ua
² Stockholm Observatory, AlbaNova University Centre, Stockholm University, 106 91 Stockholm, Sweden
³ The Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, HaiDian Qu, 100871 Beijing, PR China
⁴ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁵ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA

Received: 11 June 2014
Accepted: 31 July 2014

Abstract

Context. The YY Ori stars are T Tauri stars with prominent time-variable redshifted absorption components that flank certain emission lines. S CrA, one of the brightest of these stars, affords the rare opportunity of directly probing the accretion processes on the line of sight to one of the components of this wide visual pair.

Aims. We followed the spectral changes in S CrA to derive the physical structure of the accreting gas.

Methods. A series of high-resolution spectra of the two components of S CrA was obtained during four nights with the UVES spectrograph at the Very Large Telescope.

Results. We found that both stars are very similar with regard to surface temperature, radius, and mass. Variable redshifted absorption components are particularly prominent in the SE component. During one night, this star developed a spectrum unique among the T Tauri stars: extremely strong and broad redshifted absorption components appeared in many lines of neutral and ionized metals, in addition to those of hydrogen and helium. The absorption depths of cooler, low-ionization lines peak at low velocities – while more highly ionized lines have peak absorption depths at high velocities. The different line profiles indicate that the temperature and density of the accretion stream increase as material approaches the star. We derive the physical conditions of the flow at several points along the accretion funnel directly from the spectrum of the infalling gas. We estimated mass accretion rates of about 10^-7 M_⊙/yr, which is similar to that derived from the relation based on the strength of Hα emission line.

Conclusions. This is the first time the density and temperature distributions in accretion flows around a T Tauri star have been inferred from observations. Compared with predictions from standard models of accretion in T Tauri stars, which assume a dipole stellar magnetic field, we obtained higher densities and a steeper temperature rise toward the star.