On the role of the H₂ ortho:para ratio in gravitational collapse during star formation

¹ École Normale Supérieure de Lyon, CRAL, UMR CNRS 5574, Université Lyon I, 46 allée d’Italie, 69364 Lyon Cedex 07, France
e-mail: neil.vaytet@ens-lyon.fr
² Department of Astrophysical Sciences, Princeton University, Princeton NJ 08544, USA
³ School of Physics, University of Exeter, Exeter EX4 4QL, UK

Received: 16 October 2013
Accepted: 14 January 2014

Abstract

Context. Hydrogen molecules (H₂) come in two forms, ortho- and para-hydrogen, corresponding to the two different spin configurations of the two hydrogen atoms. The relative abundances of the two flavours in the interstellar medium are still very uncertain, and this abundance ratio has a significant impact on the thermal properties of the gas. In the context of star formation, theoretical studies have recently adopted two different strategies when considering the ortho:para ratio (OPR) of H₂ molecules. The first considers the OPR to be frozen at 3:1, while the second assumes that the species are in thermal equilibrium at all temperatures.

Aims. As the OPR potentially affects the protostellar cores that form as a result of the gravitational collapse of a dense molecular cloud, the aim of this paper is to quantify precisely what role the choice of OPR plays in the properties and evolution of the cores.

Methods. We used two different ideal gas equations of state for a hydrogen and helium mix in a radiation hydrodynamics code to simulate the collapse of a dense cloud and the formation of the first and second Larson cores. The first equation of state uses a fixed OPR of 3:1, and the second assumes thermal equilibrium.

Results. The OPR was found to markedly affect the evolution of the first core. Systems in simulations using an equilibrium ratio collapse faster at early times and show noticeable oscillations around hydrostatic equilibrium, to the point where the core expands for a short time right after its formation, before resuming its contraction. In the case of a fixed 3:1 OPR, the core’s evolution is a lot smoother. The OPR was, however, found to have little impact on the size, mass, and radius of the two Larson cores.

Conclusions. It is not clear from observational or theoretical studies of OPR in molecular clouds which OPR should be used in the context of star formation. Our simulations show that if one is solely interested in the final properties of the cores when they are formed, it does not matter which OPR is used. On the other hand, if one’s focus lies primarily on the evolution of the first core, the choice of OPR becomes important.