Accretion, jets and winds: High‐energy emission from young stellar objects – Doctoral Thesis Award Lecture 2010

Abstract

AbstractThis 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 highenergy 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–1 are required to explain the observed spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)