Issue |
A&A
Volume 479, Number 3, March I 2008
|
|
---|---|---|
Page(s) | 751 - 760 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20066499 | |
Published online | 04 December 2007 |
The circumstellar environment of high-mass protostellar objects*
IV. C17O observations and depletion
Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, Manchester M13 9PL, UK e-mail: H.Thomas@postgrad.manchester.ac.uk
Received:
3
October
2006
Accepted:
20
November
2007
Aims.The presence of depletion (freeze-out) of CO around low-mass protostars is well established. Here we observe 84 candidate young high-mass sources in the rare isotopologues C17O and C18O to investigate whether there is evidence for depletion towards these objects.
Methods.Observations of the J = 1 2 transitions of C18O and
C17O are used to derive the column densities of gas
towards the sources and these are compared with those derived from
submillimetre continuum observations. The derived fractional
abundance suggests that the CO species show a
range of degrees of depletion towards the objects. We then use the
radiative transfer code RATRAN to model a selection of the sources
to confirm that the spread of abundances is not a result of assumptions
made when calculating the column densities.
Results.We find a range of abundances of C17O that cannot be accounted for
by global variations in either the temperature or dust properties and so
must reflect source to source variations. The most likely explanation is
that different sources show different degrees of depletion of the CO.
Comparison of the C17O linewidths of our sources with those of CS
presented by other authors reveal a division of the sources into two
groups. Sources with a CS linewidth >3 km s-1 have low abundances of
C17O while sources with narrower CS lines have typically higher
C17O abundances. We suggest that this represents an
evolutionary trend. Depletion towards these objects shows that the gas
remains cold and dense for long enough for the trace species to deplete.
The range of depletion measured suggests that these objects have lifetimes
of 105 years.
Key words: ISM: molecules / line: profiles / stars: abundances / stars: evolution / stars: formation
© ESO, 2008
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