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We are carrying out a physical and chemical study of the protostellarenvelopes in a representative sample of IM Class 0 protostars. In our firstpaper (Crimier et al. 2010), we determined the physical structure(density-temperature radial profiles) of the protostellar envelopes. Here, westudy the CO depletion and N2H+ deuteration. We observed the millimeter linesof C18O, C17O, N2H+ and N2D+ toward the protostars using the IRAM 30mtelescope. Based on these observations, we derived the C18O, N2H+ and N2D+radial abundance profiles across their envelopes using a radiative transfercode. In addition, we modeled the chemistry of the protostellar envelopes. Allthe C18O 1-0 maps are well fit assuming that the C18O abundance decreasesinwards within the protostellar envelope until the gas and dust reach the COevaporation temperature, 20-25K, where the CO is released back to the gasphase. The N2H+ deuterium fractionation in Class 0 IMs is[N2D+]/[N2H+]=0.005-0.014, two orders of magnitude higher than the elemental[D/H] value in the interstellar medium, but a factor of 10 lower than inpre-stellar clumps. Chemical models account for the C18O and N2H+ observationsif we assume the CO abundance is 2 times lower than the canonical value in theinner envelope. This could be the consequence of the CO being converted intoCH3OH on the grain surfaces prior to the evaporation and/or thephotodissociation of CO by the stellar UV radiation. The deuteriumfractionation is not fitted by chemical models. This discrepancy is very likelycaused by the simplicity of our model that assumes spherical geometry andneglects important phenomena like the effect of bipolar outflows and UVradiation from the star. More important, the deuterium fractionation isdependent on the ortho-to-para H2 ratio, which is not likely to reach thesteady-state value in the dynamical time scales of these protostars.

Chemical study of intermediate-mass (IM) Class 0 protostars. CO depletion and N2H+ deuteration