TY - JOUR
T1 - On the role of the H2 ortho
T2 - Para ratio in gravitational collapse during star formation
AU - Vaytet, Neil
AU - Tomida, Kengo
AU - Chabrier, Gilles
N1 - Funding Information:
The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/20072013 Grant Agreement No. 247060). K.T. is supported by Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Research Abroad. The authors would also like to thank the anonymous referee for useful comments.
PY - 2014/4
Y1 - 2014/4
N2 - 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.
AB - 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.
KW - Equation of state
KW - Hydrodynamics
KW - Molecular processes
KW - Radiative transfer
KW - Stars: formation
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U2 - 10.1051/0004-6361/201322855
DO - 10.1051/0004-6361/201322855
M3 - Article
AN - SCOPUS:84898007098
VL - 563
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
SN - 0004-6361
M1 - A85
ER -