Abstract
The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modelling locally the impact of the infalling gas onto the chromosphere. We find that the structure and dynamics of the post-shock region is strongly dependent on the plasma-beta (thermal over magnetic pressure), different values of which may give distinguishable emission signatures, relevant for observations. In particular, a strong magnetic field effectively confines the plasma inside its flux tubes and leads to the formation of quasi-independent fibrils. The fibrils may oscillate out of phase and hence the sum of their contributions in the emission results in a smooth overall profile. On the contrary, a weak magnetic field is not found to have any significant effect on the shocked plasma and the turbulent hot slab that forms is found to retain its periodic signature.
Highlights
The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field
We find that the structure and dynamics of the post-shock region is strongly dependent on the plasma-beta, different values of which may give distinguishable emission signatures, relevant for observations
We perform numerical simulations for three models: a) B0 = 30 G, b) B0 = 100 G, and c) B0 = 300 G. These magnetic field values are chosen in order to explore plasma-β values around unity, a variable that plays a critical role in the dynamics of accretion shocks
Summary
The hot slab is susceptible to optically thin radiation losses with a typical cooling time of 10 to 1 000 s. This leads to a pressure drop and in turn the collapse of the reverse shock, because the infalling gas can no longer be supported. The formation-collapse cycle of the hot slab is repeated, i.e. the reverse shock follows a quasi-periodic oscillation with a frequency that depends on the cooling time. We do not consider such possibilities, but we explore the evolution of an accretion shock in the simple regime of optically thin radiation losses and a uniform magnetic field. We simulate the system in more dimensions, introducing perturbations in the system in order to study their effects on the structure and evolution of the post-shock region
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