EDP Sciences
Free access
Issue
A&A
Volume 426, Number 3, November II 2004
Page(s) 925 - 940
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20041276


A&A 426, 925-940 (2004)
DOI: 10.1051/0004-6361:20041276

Mapping ices in protostellar environments on 1000 AU scales

Methanol-rich ice in the envelope of Serpens SMM 4
K. M. Pontoppidan1, E. F. van Dishoeck1 and E. Dartois2

1  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
    e-mail: pontoppi@strw.leidenuniv.nl
2  Institut d'Astrophysique Spatiale, Bât. 121, Université Paris XI, 91405 Orsay Cedex, France

(Received 12 May 2004 / Accepted 9 July 2004)

Abstract
We present VLT-ISAAC L-band spectroscopy toward 10 stars in SVS 4, a $30\arcsec\times45\arcsec$ dense cluster of pre-main sequence stars deeply embedded in the Serpens star forming cloud. The ISAAC spectra are combined with archival imaging from UKIRT and ISOCAM to derive accurate extinctions toward the SVS 4 stars. The data are then used to construct a spatial map of the distribution of ice in front of the cluster stars with an average angular resolution of 6´´or 1500 AU, three orders of magnitude better than previous maps. We show that water ice is present throughout the region and confirm the presence of methanol ice with an abundance of up to 25% relative to water. It is shown that methanol ice maintains a very high abundance relative to H 2 throughout SVS 4, but drops by at least an order of magnitude only  $75\arcsec$ away from SVS 4. The maps indicate that some of the lines of sight toward the SVS 4 stars pass through the outer envelope of the class 0 protostar SMM 4. The abundance of water ice relative to the refractory dust component shows a sudden increase by 90% to $(1.7\pm0.2)\times 10^{-4}$ relative to H 2 at a distance of 5000 AU to the center of SMM 4. The water ice abundance outside the jump remains constant at $(9\pm1) \times 10^{-5}$. We suggest that this is an indication of a significantly enhanced ice formation efficiency in the envelopes of protostars. The depletion of volatile molecules in the envelope of SMM 4 is discussed. In particular, it is found that up to 2/3 of the depleted CO is converted into CO 2 and CH 3OH in the ice. Therefore, only 1/3 of the CO originally frozen out will return to the gas phase as CO upon warmup.


Key words: astrochemistry -- stars: formation -- stars: circumstellar matter -- infrared: ISM -- ISM: molecules -- ISM: dust, extinction

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