Origins of the H, He I, and Ca II Line Emission in Classical T Tauri Stars

TL;DR

This study investigates the origins of emission lines in classical T Tauri stars, revealing that He I and H lines mainly originate from a clumpy, radial outflow, while Ca II lines form in accretion flows and boundary layers, using excitation calculations and observed data.

Contribution

It provides a detailed analysis of line formation mechanisms in T Tauri stars, combining excitation calculations with observational data to identify the physical regions responsible for specific emission features.

Findings

He I 10830 absorption features are mainly caused by UV photoionization.

H and He I broad emissions originate from a clumpy radial outflow.

Ca II emission arises in magnetospheric accretion and disk boundary layers.

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 density, temperature, and photon ionization rate. We find that UV photoionization is the most probable excitation mechanism for generating the He I 10830 opacities that produce all the associated absorption features. We also calculate the specific line flux at an observed velocity of v_obs = +/- 150 km/s 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 ~10^9 cm^-3 and optically thick to He I 10830 and H alpha, but optically thin to He I 5876, Pa gamma, and the Ca II infrared triplet. The optically thick He I 5876 emission occurs mostly in regions of density greater than or equal to 10^11 cm^-3 and temperature greater than or equal to 1.5x10^4 K, while the optically thick H alpha and Pa gamma emission occur mostly in regions of density around 10^11 cm^-3 and temperature between 8750 and 1.25x10^4 K. In producing the observed line fluxes at a given v_obs, the covering factor of these emission clumps is sufficiently small not to 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 disk boundary layer, where the gases dissipate part of their rotational energies before infalling along magnetic field lines. The needed density and temperature are ~10^12 cm^-3 and less than or equal to 7500 K.