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We present a survey of the formaldehyde emission in a sample of eight Class 0protostars obtained with the IRAM and JCMT telescopes. The data have beenanalyzed with three different methods with increasing level of sophistication.We first analyze the observed emission in the LTE approximation, and deriverotational temperatures between 11 and 40 K, and column densities between 1 and20 x 10^13 cm^-2. Second, we use a LVG code and derive larger kinetictemperatures, between 30 and 90 K, consistent with subthermally populatedlevels and densities from 1 to 6 x 10^5 cm^-3. The column densities from theLVG modeling are within a factor of 10 with respect to those derived in the LTEapproximation. Finally, we analyze the observations based upon detailed modelsfor the envelopes surrounding the protostars, using temperature and densityprofiles previously derived from continuum observations. We approximate theformaldehyde abundance across the envelope with a jump function, the jumpoccurring when the dust temperature reaches 100 K, the evaporation temperatureof the grain mantles. The observed formaldehyde emission is well reproducedonly if there is a jump, more than two orders of magnitude, in four sources. Inthe remaining four sources the data are consistent with a formaldehydeabundance jump, but the evidence is more marginal (~2 sigma). The inferredinner H2CO abundance varies between 1 x 10^-8 and 6 x 10^-6. We discuss theimplications of these jumps for our understanding of the origin and evolutionof ices in low mass star forming regions. Finally, we give predictions for thesubmillimeter H2CO lines, which are particularly sensitive to the abundancejumps.Comment: Accepted at A&

The H2CO abundance in the inner warm regions of low mass protostellar envelopes

The H₂CO abundance in the inner warm regions of low mass protostellar envelopes