Multidimensional models of hydrogen and helium emission line profiles for classical T Tauri stars: method, tests and examples

Abstract

We present multidimensional non‐local thermodynamic equilibrium radiative transfer models of hydrogen and helium line profiles formed in the accretion flows and the outflows near the star–disc interaction regions of classical T Tauri stars (CTTSs). The statistical equilibrium calculations, performed under the assumption of the Sobolev approximation using the radiative transfer code torus, have been improved to include He i and He ii energy levels. This allows us to probe the physical conditions of the inner wind of CTTSs by simultaneously modelling the robust wind diagnostic line He iλ10830 and the accretion diagnostic lines such as Paβ, Brγ and He iλ5876. The code has been tested in 1D and 2D problems, and we have shown that the results are in agreement with established codes. We apply the model to the complex flow geometries of CTTSs. Example model profiles are computed using the combinations of (1) magnetospheric accretion and disc wind, and (2) magnetospheric accretion and the stellar wind. In both cases, the model produces line profiles which are qualitatively similar to those found in observations. Our models are consistent with the scenario in which the narrow blueshifted absorption component of He iλ10830 seen in observations is caused by a disc wind, and the wider blueshifted absorption component (the P‐Cygni profile) is caused by a bipolar stellar wind. However, we do not have a strong constraint on the relative importance of the wind and the magnetosphere for the ‘emission’ component. Our preliminary calculations suggest that the temperature of the disc wind and stellar winds cannot be much higher than ∼10 000 K, on the basis of the strengths of hydrogen lines. Similarly the temperature of the magnetospheric accretion cannot be much higher than ∼10 000 K. With these low temperatures, we find that the photoionization by high‐energy photons (e.g. X‐rays) is necessary to produce He iλ10830 in emission and to produce the blueshifted absorption components.

AA Tauri’s sudden and long-lasting deepening: enhanced extinction by its circumstellar disk

“…The late recovery of the 8.19 d period at the end of the 2012/13 season suggests that the scattered/direct light fraction has changed by then. Similarily, the deep central absorption component of the Balmer line profiles, always present in the bright state, presumably arises from the photons being absorbed either very close to the star by the slow-moving material at the top of the accretion funnel flows or farther away at the base of a dense disk wind nearly perpendicular to the line of sight (Kurosawa et al 2006(Kurosawa et al , 2011. Scattered photons that have escaped the central system along trajectories located well above the disk midplane do not encounter either of these absorbing media and therefore do not exhibit central absorption signatures in the line profiles (cf.…”

Accretion dynamics in the classical T Tauri star V2129 Ophiuchi

“…Progress has been made in the determination of the surface magnetic fields of CTTSs (Donati et al 2007;Hussain et al 2009;Gregory & Donati 2011) and the development of 3D MHD simulations that include non-axisymmetric multipolar components of the stellar magnetic field (Long et al 2007(Long et al , 2008Romanova et al 2008). It is now possible to compute emission line profiles from the funnel flow structure determined by 3D MHD simulations that are based on observed magnetic field configurations of CTTSs (Kurosawa et al 2008(Kurosawa et al , 2011.…”

“…These distributions were mapped on to the radiative transfer grid of the TORUS code (Harries 2000;Kurosawa et al 2006Kurosawa et al , 2011, and the corresponding line source functions were calculated. The theoretical profiles of Hα and Hβ were computed using the Sobolev escape probability method, as described in Klein & Castor (1978) and Hartmann et al (1994).…”

Observational clues to the physics at the magnetosphere in young stellar objects

“…Their observed characteristics have been successfully described over the years by magnetospheric accretion models [12,28,30,32,33,36,41]. In theses models, the stellar magnetic field is strong enough to truncate the circumstellar disk at a few stellar radii from the star, where the magnetic stress equals the stress of the accreting matter, forming a magnetospheric cavity.…”

“…Due to the presence of a major cold spot, the photospheric lines get distorted as the cold spot comes in and out of view and their radial velocity is therefore related to the cold 08001-p.2 We were subsequently very confident that the proposed magnetic intensity and topology at the stellar surface was close to reality. We decided to try to reproduce the observed Hα and Hβ line variability with radiative transfer calculations [32] based on the 3D MHD structure of the proposed magnetic configuration [39]. These lines were expected to come primarily from the accretion columns and were not used in the calculation of the surface magnetic field.…”