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

Kavli Affiliate: Moritz Guenther

| First 5 Authors: Hans Moritz Guenther, , , ,

| Summary:

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.

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