# New insight on Young Stellar Objects accretion shocks -- a claim for NLTE opacities

## TL;DR

This study investigates how including NLTE opacities and radiation-hydrodynamics coupling affects the structure, dynamics, and X-ray emission of accretion shocks in young stellar objects, revealing significant impacts on their observable properties.

## Contribution

It introduces a novel simulation approach that incorporates NLTE ionization and radiation-hydrodynamics coupling to better model accretion shocks in CTTSs.

## Key findings

- Chromospheric acoustic heating influences the shock cycle duration.
- Radiation-hydrodynamics coupling heats the accretion flow and chromosphere.
- X-ray luminosity varies significantly with model parameters, between 30% and 94% of mechanical energy.

## Abstract

Context. Accreted material onto CTTSs is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match with observations. They all rely on a static model for the chromosphere model and on the assumption that radiation and matter are decoupled. Aims. We explore the effects on the structure and on the dynamics of the accretion flow of both a shock-heated chromosphere and of the coupling between radiation and hydrodynamics. Methods. We simulate accretion columns falling onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the two first momenta equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ions densities from collisional-radiative NLTE ionisation equilibrium. Results. The chromospheric acoustic heating has an impact on the duration of the cycle and on the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and the chromosphere, inducing a possible unburial of the whole column. These two last conclusions are in agreement with the computed monochromatic intensity. Both effects (acoustic heating and radiation coupling) have an influence on the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on the model considered.