Effects of radiation in accretion regions of classical T Tauri stars

Abstract

Context. Models and observations indicate that the impact of matter accreting onto the surface of young stars produces regions at the base of accretion columns, in which optically thin and thick plasma components coexist. Thus an accurate description of these impacts requires to account for the effects of absorption and emission of radiation. Aims. We study the effects of radiation emerging from shock-heated plasma in impact regions on the structure of the pre-shock downfalling material. We investigate if a significant absorption of radiation occurs and if it leads to a pre-shock heating of the accreting gas. Methods. We developed a radiation hydrodynamics model describing an accretion column impacting onto the surface of a Classical T Tauri Star. The model takes into account the stellar gravity, the thermal conduction, and the effects of both radiative losses and absorption of radiation by matter in the non local thermodynamic equilibrium regime. Results. After the impact, a hot slab of post-shock plasma develops at the base of the accretion column. Part of radiation emerging from the slab is absorbed by the pre-shock accreting material. As a result, the pre-shock accretion column gradually heats up to temperatures of 10 5 K, forming a radiative precursor of the shock. The precursor has a thermal structure with the hottest part at T ≈ 10 5 K, with size comparable to that of the hot slab, above the post-shock region. At larger distances the temperature gradually decreases to T ≈ 10 4 K. Conclusions. Our model predicts that ≈ 70% of radiation emitted by the post-shock plasma is absorbed by the pre-shock accretion column immediately above the slab and re-emitted in the UV band. This may explain why accretion rates derived from UV observations are systematically larger than rates inferred from X-ray observations.

Laboratory evidence for an asymmetric accretion structure upon slanted matter impact in young stars

“…At this time, a shock develops at the base of the accretion column and propagates upward through the stream, heating the accreting material up to temperatures of a few million degrees (thus contributing to emission mainly in the X-ray band). This newly formed hot slab of plasma is partially rooted in the chromosphere, so that the X-ray-emitting plasma is buried under a column of optically thick material and is partially absorbed (e.g., Reale et al 2013;Bonito et al 2014;Revet et al 2017;Costa et al 2017;Colombo et al 2019b). We note that the evolution of the simulated accretion process is shown here over tens of minutes, whereas the scaling discussed in Sect.…”

Accreting protoplanets: Spectral signatures and magnitude of gas and dust extinction at Hα

“…This contrasts to the stellar case, in which the precursor region is thin and located close to the star (see e.g. Calvet & Gullbring 1998;Colombo et al 2019;de Sá et al 2019) and is due to the absorption of photoionising radiation. This infinite precursor implies that the temperature profile is given by…”

Study of radiative shocks using 2D interferometry and XUV spectroscopy

“…This leads to the development of the radiative precursor preceding the density discontinuity. Such precursors are visible in various circumstances, for instance, the top of high velocity (several 100 km/s) stellar jets propagating in the interstellar medium, 17 in the case of accretion shocks, [18][19][20] or when the shocks emerge from the atmosphere of exploding supernovae. 21,22 Radiative shocks have been the objects of many theoretical works.…”