Simulations of protostellar collapse using multigroup radiation hydrodynamics - I. The first collapse (original) (raw)

A&A 543, A60 (2012)

I. The first collapse

1 École Normale Supérieure de Lyon, CRAL (UMR CNRS 5574), 69364 Lyon Cedex 07, France
e-mail: neil.vaytet@ens-lyon.fr
2 Maison de la Simulation, USR 3441, CEA – CNRS – INRIA – Université Paris-Sud – Université de Versailles, 91191 Gif-sur-Yvette, France
3 CEA/DSM/IRFU, Service d’Astrophysique, Laboratoire AIM, CNRS, Université Paris Diderot, 91191 Gif-sur-Yvette, France
4 School of Physics, University of Exeter, Exeter, EX4 4QL, UK
5 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
6 Laboratoire de radioastronomie, UMR 8112 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France

Received: 17 April 2012
Accepted: 23 May 2012

Abstract

Context. Radiative transfer plays a major role in the process of star formation, the details of which are still not entirely understood.

Aims. Many previous simulations of gravitational collapse of a cold gas cloud followed by the formation of a protostellar core have used a grey treatment of radiative transfer coupled to the hydrodynamics. However, the dust opacities that dominate circumstellar extinction show strong variations as a function of frequency. In this paper, we used a frequency-dependent formalism for the radiative transfer to investigate the influence of the opacity variations on the properties of Larson’s first core.

Methods. We used a multigroup _M_1 moment model for the radiative transfer in a 1D Lagrangean Godunov radiation hydrodynamics code to simulate the spherically symmetric collapse of a 1 _M_⊙ cold cloud core. Monochromatic dust opacities for five different temperature ranges were used to compute Planck and Rosseland means inside each frequency group.

Results. The results are very consistent with previous studies and only small differences were observed between the grey and multigroup simulations. For a same central density, the multigroup simulations tend to produce first cores with a slightly higher radius and central temperature. We also performed simulations of the collapse of a 10 and 0.1 _M_⊙ cloud, which showed that the properties of the first core (size, mass, entropy, etc.) are independent of the initial cloud mass, with again no major differences between grey and multigroup models.

Conclusions. For Larson’s first collapse, where temperatures remain below 2000 K, the vast majority of the radiation energy lies in the infrared regime and the system is optically thick. In this regime, the grey approximation satisfactorily reproduces the correct opacities, as long as there are no strong opacity variations on scales much smaller than the width of the Planck function. However, the multigroup method is expected to yield additional more important differences in the later stages of the collapse when high-energy (UV and X-ray) radiation is present and matter and radiation are strongly decoupled.

Key words: stars: formation / methods: numerical / hydrodynamics / radiative transfer