3D numerical modeling of YSO accretion shocks

¹ CEA, IRAMIS, Service Photons, Atomes et Molécules, 91191 Gif-sur-Yvette, France
² Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
³ LERMA, Observatoire de Paris, Université Pierre et Marie Curie and CNRS, 5 Place J. Janssen, 92195 Meudon, France
⁴ Université Paris Diderot, Sorbonne Paris Cité, AIM, UMR 7158, CEA, CNRS, 91191 Gif-sur-Yvette, France
⁵ Laboratoire Lagrange, Université de Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06304 Nice cedex 4, France
⁶ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

^a e-mail: titos.matsakos@cea.fr

Published online: 8 January 2014

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.