The structure and spectrum of the accretion shock in the atmospheres of young stars

TL;DR

This study models the accretion shock in young stars, revealing how radiative cooling, non-equilibrium effects, and pre-shock gas influence the spectral features and structure of the accretion region in Classical T Tauri Stars.

Contribution

It provides a self-consistent simulation of the accretion shock structure and spectrum, incorporating radiative cooling, non-equilibrium effects, and the transition to the stellar atmosphere.

Findings

HeII narrow component forms in the irradiated atmosphere

Pre-shock gas affects UV spectrum with emission and absorption lines

High-density accretion region spectrum resembles a star with temperature from energy flux

Abstract

The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters typical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into account the self-absorption and non-equilibrium (time-dependent) effects in the level populations. These effects modify the standard cooling curve for an optically thin plasma in coronal equilibrium, however the shape of high-temperature ($T>3\times10^5$ K) part of the curve remains unchanged. The applied methods allow us to smoothly describe the transition from the cooling flow to the hydrostatic stellar atmosphere. Thanks to this approach, it has been found that the narrow component of HeII lines is formed predominantly in the irradiated stationary atmosphere (hotspot), i.e. at velocities of the settling gas $<2$ km s$^{-1}$. The structure of the pre-shock region is calculated simultaneously with the heated atmosphere. The simulation shows that the pre-shock gas produces a noticeable emission component in HeII lines and practically does not manifest itself in HeI lines ($\lambda\lambda$ 5876, 10830 {\AA}). The UV spectrum of the hotspot is distorted by the pre-shock gas, namely numerous red-shifted emission and absorption lines overlap each other forming a pseudo-continuum. The spectrum of the accretion region at high pre-shock densities $\sim10^{14}$ cm$^{-3}$ is fully formed in the in-falling gas and can be qualitatively described as a spectrum of a star with an effective temperature derived from the Stefan-Boltzmann law via the full energy flux.