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

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 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 −1 are required to explain the observed spectrum.

Interpreting Near-Infrared Hydrogen Line Ratios in T Tauri Stars

“…Moreover, although these pre-shock densities are in line with those from magnetospheric accretion models, the inferred mass accretion rates based on x-ray line ratios are 10 −9 M yr −1 for BP Tau and 10 −11 M yr −1 for TW Hya (Günther 2011), an order of magnitude or more lower than would be inferred from Figure 1 based on Paβ luminosities or from the accretion shock models of Calvet & Gullbring (1998) based on optical/UV continuum excess. The tendency for mass accretion rates inferred from X-ray line ratios to be consistently lower than those based on accretion shock models of optical/UV emission excess is recognized by Günther (2011), who suggest that this might arise from inhomogeneous spots, partial absorption in buried shocks, or the presence of accretion streams that impact at velocities considerably below freefall speeds (see also Ingleby et al 2013).…”

“…These line ratios in CTTS cannot be explained by coronal emission and are attributed to formation in the accretion shock (Güdel & Nazé 2009). A study of these line ratios in several low accretion rate CTTS indicate pre-shock densities ∼ 10 13 cm −3 for two stars in our current study, BP Tau and TW Hya (Günther 2011), again an order of magnitude higher than we would infer. Moreover, although these pre-shock densities are in line with those from magnetospheric accretion models, the inferred mass accretion rates based on x-ray line ratios are 10 −9 M yr −1 for BP Tau and 10 −11 M yr −1 for TW Hya (Günther 2011), an order of magnitude or more lower than would be inferred from Figure 1 based on Paβ luminosities or from the accretion shock models of Calvet & Gullbring (1998) based on optical/UV continuum excess.…”

MN Lup: X-RAYS FROM A WEAKLY ACCRETING T TAURI STAR

“…In comparison with the sample of CTTS of Günther (2011), which all have X-ray grating spectroscopy, MN Lup shows similar abundances, coronal temperatures and it has a comparable shock speed. However, the density of the accreting plasma is at least an order of magnitude lower than in TW Hya, BP Tau, V4046 Sgr and MP Mus.…”

“…With sufficiently high quality X-ray data CTTS can be singled out from other X-ray sources by, first, their strong soft X-ray excess Günther 2011) and, second, by unusually high densities in the He-like triplets first seen in the CTTS TW Hya (Kastner et al 2002). These densities were confirmed later in several consecutively deeper observations (Stelzer & Schmitt 2004;Raassen 2009;Brickhouse et al 2010).…”

TYC 8241 2652 1 and the case of the disappearing disk: No smoking gun yet

“…Such disks thus are often discovered through surveys in the mid-IR. Some of the mass is accreted onto the central star, causing a veiling of the stellar photospheric spectrum, bright hydrogen emission lines, and an excess of soft X-ray emission (for a review see Günther 2011). Furthermore, mass is lost from the disk through disk winds or photoevaporation and dust in the disk can coagulate to form larger grains, pebbles, and eventually planetesimals and planets, which may be massive enough to accrete gas from the disk as well.…”