EDP Sciences
Free access
Volume 389, Number 3, July III 2002
Page(s) 908 - 930
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020681

A&A 389, 908-930 (2002)
DOI: 10.1051/0004-6361:20020681

Physical structure and CO abundance of low-mass protostellar envelopes

J. K. Jørgensen, F. L. Schöier and E. F. van Dishoeck

Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

(Received 14 December 2001 / Accepted 30 April 2002 )

We present 1D radiative transfer modelling of the envelopes of a sample of 18 low-mass protostars and pre-stellar cores with the aim of setting up realistic physical models, for use in a chemical description of the sources. The density and temperature profiles of the envelopes are constrained from their radial profiles obtained from SCUBA maps at 450 and 850  $\mu$m and from measurements of the source fluxes ranging from 60 $\mu$m to 1.3 mm. The densities of the envelopes within ~10 000 AU can be described by single power-laws $\rho\propto r^{-\alpha}$ for the class 0 and I sources with $\alpha$ ranging from 1.3 to 1.9, with typical uncertainties of $\pm$0.2. Four sources have flatter profiles, either due to asymmetries or to the presence of an outer constant density region. No significant difference is found between class 0 and I sources. The power-law fits fail for the pre-stellar cores, supporting recent results that such cores do not have a central source of heating. The derived physical models are used as input for Monte Carlo modelling of submillimeter C 18O and C 17O emission. It is found that class I objects typically show CO abundances close to those found in local molecular clouds, but that class 0 sources and pre-stellar cores show lower abundances by almost an order of magnitude implying that significant depletion occurs for the early phases of star formation. While the 2-1 and 3-2 isotopic lines can be fitted using a constant fractional CO abundance throughout the envelope, the 1-0 lines are significantly underestimated, possibly due to contribution of ambient molecular cloud material to the observed emission. The difference between the class 0 and I objects may be related to the properties of the CO ices.

Key words: stars: formation -- ISM: molecules -- ISM: abundances -- stars: circumstellar matter -- radiative transfer  -- astrochemistry

Offprint request: J. K. Jørgensen, joergensen@strw.leidenuniv.nl

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