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

TL;DR

This paper develops multidimensional non-LTE radiative transfer models for hydrogen and helium emission lines in classical T Tauri stars, incorporating complex flow geometries and testing against observations to understand stellar winds and accretion processes.

Contribution

It introduces an improved radiative transfer code that models hydrogen and helium lines in 2D geometries of T Tauri stars, including He I and He II levels, and applies it to complex flow scenarios.

Findings

Models reproduce observed line profiles qualitatively.

Disc wind causes narrow blueshifted He I (10830) absorption.

Stellar wind causes wider P-Cygni absorption features.

Abstract

We present multidimensional non-LTE radiative transfer models of hydrogen and helium line profiles formed in the accretion flows and the outflows near the star-disk 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, has 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 I (10830) and the accretion diagnostic lines such as Pa-beta, Br-gamma and He I (5876). The code has been tested in 1 and 2-D 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 I (10830) 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 temperatures of the disc wind, stellar wind and the magnetosphere cannot be much higher than ~10,000 K, on the basis of the strengths of hydrogen lines. With these low temperatures, we find that the photoionzation by high energy photons (e.g. X-rays) is necessary to produce He I (10830) in emission and to produce the blueshifted absorption components.