Accretion, jets and winds: High-energy emission from young stellar objects

TL;DR

This paper reviews high-energy emission mechanisms in young stellar objects, focusing on accretion shocks, coronae, and jets, supported by X-ray and UV spectroscopy and modeling, highlighting the role of shocks and magnetic fields.

Contribution

It introduces models for accretion shocks and jet-related X-ray emission in young stars, explaining observed spectral features and line ratios with detailed physical processes.

Findings

Accretion shocks and coronae contribute to X-ray emission in CTTS.

Models explain peculiar line ratios in He-like triplets.

Jet shocks can produce X-ray emission similar to coronal sources.

Abstract

This article summarizes the processes of high-energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high-resolution X-ray and UV spectroscopy and modeling. Three mechanisms contribute to the high-energy emission from CTTS: 1) CTTS have active coronae similar to main-sequence stars, 2) the accreted material passes through an accretion shock at the stellar surface, which heats it to a few MK, and 3) some CTTS drive powerful outflows. Shocks within these jets can heat the plasma to X-ray emitting temperatures. Coronae are already well characterized in the literature; for the latter two scenarios models are shown. The magnetic field suppresses motion perpendicular to the field lines in the accretion shock, thus justifying a 1D geometry. The radiative loss is calculated as optically thin emission. A mixture of shocked and coronal gas is fitted to X-ray observations of accreting CTTS. Specifically, the model explains the peculiar line-ratios in the He-like triplets of Ne IX and O VII. All stars require only small mass accretion rates to power the X-ray emission. In contrast, the HAeBe HD 163296 has line ratios similar to coronal sources, indicating that neither a high density nor a strong UV-field is present in the region of the X-ray emission. This could be caused by a shock in its jet. Similar emission is found in the deeply absorbed CTTS DG Tau. Shock velocities between 400 and 500 km/s are required to explain the observed spectrum.