Heating and emission in the accreting material onto young stars. The study: “Effects of radiation in accretion regions of classical T Tauri stars. Pre-heating of accretion column in non-LTE regime” of S. Colombo (UNIPA/OAPA/LERMA) recently appeared on A&A

Pre-Main Sequence stars are young (few million years) stars whose nuclei are not powered yet by the thermonuclear reactions, and that may still accrete gas from a surrounding disk (called protoplanetary or accretion disks). Even if the accretion disks are typically extended more than 100 Astronomical Units (AU, the mean distance between Earth and the Sun, equal to 150 million of km), accretion is dictated by the processes occurring in the very inner region of the disks (e.g. less than 0.1 AU). In this region, the gas is heated up to more than 1000 degrees by the stellar radiation, and its dynamic is affected by the stellar magnetic field. Accretion occurs thus along accretion funnels created by the magnetic field, where the gas is extracted from the disk and it falls onto the star at the free-fall velocity of a few hundreds km/sec. The process is thus determined by the interaction between the hot gas and the magnetic field, and it must be analyzed adopting a complex magnetohydrodynamic approach.

 

In the region where the accretion column hits the stellar surface a shock develops, where the gas is stops and it heats up to 105, 106 degrees. At these high temperature and densities, the post-shoc gas emits X-ray and UV radiation. This is the signature of ongoing accretion, and it allows astronomers to study the accretion process. However, sometime the results obtained by analyzing X-ray and UV radiation are discrepant. For instance, the rate at which stars accrete gas determined from the X-rays are typically lower than those predicted from UV radiation.

 

A reason for this discrepancy is suggested by the simulations presented in the study: “Effects of radiation in accretion regions of classical T Tauri stars. Pre-heating of accretion column in non-LTE regime” of S. Colombo (University of Palermo, Sorbonne Université, INAF-Observatory of Palermo), recently appeared on Astronomy & Astrophysics. The author, also thanks to the collaboration of astronomers of the Sorbonne University and the INAF – Astronomical Observatory of Palermo, developed a new radiative transport module to include in the magnetohydrodynamic model used for the simulations. These simulations reproduce an accretion column onto a young star, with the aims at finding the thermal structure along the column, accounting for the radiation emitted by the post-shock region and absorbed by accreting gas. Accounting for this self-absorption, the authors have demonstrated that the pre-shock gas is heated up to several 105 degrees, becoming source of UV radiation. This contribution to the UV radiation emitted by the accreting gas, suggested for the first time thanks to the module developed by the authors, can explain the discrepancy between the accretion rates observed in X-rays and UV. The astronomers S. Orlando e R. Bonito of INAF-Astronomical Observatory of Palermo and C. Argiroffi and G. Peres of the University of Palermo have participated to this study.

 

The figure (link) shows a cartoon of an accreting column in a pre-Main Sequence star, with marked the shock and post-shock regions, the magnetic field lines, the temperature of the gas in the pre- and post-shock regions, and the gas free-fall velocity

 

by Mario Giuseppe Guarcello  ( follow mguarce)