Theoretical H2CO emission from protostellar envelopes

We present theoretical predictions of the formaldehyde emission from the envelopes of low mass protostars. The model accounts for the density profile across the envelope, assuming the Shu ([CITE]) solution for the collapse, as well as for the gas temperature profile. The gas temperature is derived from the thermal balance according to the model previously developed by Ceccarelli et al. ([CITE]). The formaldehyde abundance profile is approximated by a step function: in the outer envelope it is similar to that of molecular clouds, whereas in the inner envelope, where the dust temperature reaches the ice mantle evaporation temperature, it jumps to larger values. The results of the modeling for a 30 $L_\odot$ source are reported for a large range of values of the formaldehyde abundance, both in the inner and outer envelope. Additional results for sources with different luminosities can be found in the web site www-laog.obs.ujf-grenoble.fr/~ceccarel/mepew/mepew.html. They are meant to be directly used by the interested reader to estimate the two main parameters of the model: the inner and outer formaldehyde abundance. The model is applied to the observations of the well known low mass protostar IRAS 16293-2422 to illustrate the practical use of the diagnostic tools. Furthermore, we discuss how the observation of the H$_2^{12}$CO and H$_2^{13}$CO lines can in principle be used to discriminate between infalling and outflowing gas. Finally, the same web site hosts simpler non-LTE LVG predictions for a large range of densities, temperatures and column densities of several molecules. Again the goal of the web site is to provide users with easy to use theoretical predictions for a first assessement of expected and/or observed signals.