Mapping ices in protostellar environments on 1000 AU scales: Methanol-rich ice in the envelope of Serpens SMM 4

We present VLT-ISAAC L-band spectroscopy toward 10 stars in SVS 4, a 30"x45" 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 H2 throughout SVS 4, but drops by at least an order of magnitude only 75" 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+/-0.2) * 10^-4 relative to H2 at a distance of 5000 AU to the center of SMM 4. The water ice abundance outside the jump remains constant at (9+/-1) * 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 CO2 and CH3OH in the ice. Therefore, only 1/3 of the CO originally frozen out will return to the gas phase as CO upon warmup.