Origins of the H, He i and Ca ii line emission in classical T Tauri stars

Abstract

We perform local excitation calculations to obtain line opacities and emissivity ratios, and compare them with observed properties of H, He i, O i, Ca ii and Na i lines to determine the requisite conditions of density, temperature and photon ionization rate. We find that ultraviolet photoionization is the most probable excitation mechanism for generating the He iλ10830 opacities that produce all the associated absorption features. We also calculate the specific line flux at an observed velocity of vobs=±150 km s−1 for both radial wind and infall models. All the model results, together with observed correlations between absorption and emission features and between narrow and broad emission components, are used to deduce the origins of the strong H, He i and Ca ii broad line emission. We conclude that the first two arise primarily in a radial outflow that is highly clumpy. The bulk of the wind volume is filled by gas at a density ∼109 cm−3 and optically thick to He iλ10830 and Hα, but optically thin to He iλ5876, Paγ and the Ca ii infrared triplet. The optically thick He iλ5876 emission occurs mostly in regions of density ≥1011 cm−3 and temperature ≥1.5 × 104 K, while the optically thick Hα and Paγ emission occurs mostly in regions of density around 1011 cm−3 and temperature between 8750 and 1.25 × 104 K. In producing the observed line fluxes at a given vobs, the covering factor of these emission clumps is sufficiently small to not incur significant absorption of the stellar and veiling continua in either He i or H lines. The strong Ca ii broad line emission likely arises in both the magnetospheric accretion flow and the disc boundary layer where the gases dissipate part of their rotational energies before infalling along magnetic field lines. The needed density and temperature are ∼1012 cm−3 and ≤7500 K, respectively.