Sulfur chemistry: 1D modeling in massive dense cores

¹ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
² CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
e-mail: wakelam@obs.u-bordeaux1.fr

Received: 18 November 2010
Accepted: 18 February 2011

Abstract

Aims. The main sulfur-bearing molecules OCS, H₂S, SO, SO₂, and CS have been observed in four high mass dense cores (W43-MM1, IRAS 18264, IRAS 05358, and IRAS 18162). Our goal is to put some constraints on the relative evolutionary stage of these sources by comparing these observations with time-dependent chemical modeling.

Methods. We used the chemical model Nahoon, which computes the gas-phase chemistry and gas-grain interactions of depletion and evaporation. Mixing of the different chemical compositions shells in a 1D structure through protostellar envelope has been included since observed lines suggest nonthermal supersonic broadening. Observed radial profiles of the temperature and density are used to compute the chemistry as a function of time.

Results. With our model, we underproduce CS by several orders of magnitude compared to the other S-bearing molecules, which seems to contradict observations, although some uncertainties in the CS abundance observed at high temperature remain. The OCS/SO₂, SO/SO₂, and H₂S/SO₂ abundance ratios could in theory be used to trace the age of these massive protostars since they show a strong dependence with time, but the sources are too close in age compared to the accuracy of chemical models and observations. Our comparison between observations and modeling may, however, indicate that W43-MM1 could be chemically younger than the three other sources. Turbulent diffusivity through the protostellar envelopes has to be less efficient than 2 × 10¹⁴ cm² s^-1. Otherwise, it would have smoothed out the abundance profiles, and this would have been observed.

Conclusions. The sulfur chemistry depends strongly on the 1D physical conditions. Any observed set of abundances should be compared with a chemical model computed with the same temperature and density traced by the observations. In our case, no conclusion can be given on the relative age of IRAS 18264, IRAS 18162 and IRAS 05358 except that they are very close. W43-MM1 seems younger than the other sources. Turbulent mixing could occur in young high-mass protostars on a geometric scale that is too small to affect the chemical abundance profiles through the envelope.