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

Abstract

Abstract. We present a survey of the formaldehyde emission in a sample of eight Class 0 protostars obtained with the IRAM and JCMT millimeter telescopes. The range of energies of the observed transitions allows us to probe the physical and chemical conditions across the protostellar envelopes. The data have been analyzed with three different methods with increasing level of sophistication. We first analyze the observed emission in the LTE approximation, and derive rotational temperatures between 11 and 40 K, and column densities between 1 and 20 × 10 13 cm −2 . Second, we use a LVG code and derive higher kinetic temperatures, between 30 and 90 K, consistent with subthermally populated levels and densities from 1 to 6 × 10 5 cm −3 . The column densities from the LVG modeling are within a factor of 10 with respect to those derived in the LTE approximation. Finally, we analyze the observations based upon detailed models for the envelopes surrounding the protostars, using temperature and density profiles previously derived from continuum observations. We approximate the formaldehyde abundance across the envelope with a jump function, the jump occurring when the dust temperature reaches 100 K, the evaporation temperature of the grain mantles. The observed formaldehyde emission is well reproduced only if there is a jump of more than two orders of magnitude, in four sources. In the remaining four sources the data are consistent with a formaldehyde abundance jump, but the evidence is more marginal (≤2 σ). The inferred inner H 2 CO abundance varies between 1 × 10 −8 and 6 × 10 −6 . The absolute values of the jump in the H 2 CO abundance are uncertain by about one order of magnitude, because of the uncertainties in the density, ortho to para ratio, temperature and velocity profiles of the inner region, as well as the evaporation temperature of the ices. We discuss the implications of these jumps for our understanding of the origin and evolution of ices in low mass star forming regions. Finally, we give predictions for the submillimeter H 2 CO lines, which are particularly sensitive to the abundance jumps.

On the origin of H$_\mathsf{2}$CO abundance enhancements in low-mass protostars

“…The 3 22 → 2 21 /3 03 → 2 02 line ratio is sensitive to temperature (e.g., van Dishoeck et al 1993;Mangum & Wootten 1993), especially in the regime of 50−200 K. For L1448-C, the interferometer data give a ratio of 0.68 ± 0.39 indicating the presence of hot gas with T > ∼ 70 K. For comparison, the single-dish line ratio is 0.12 ± 0.04 (Maret et al 2004), corresponding to T ≈ 20−30 K. Optical depth effects and abundance variations with radius (i.e., temperature) can affect this ratio, however, so that more detailed radiative transfer modeling is needed for a proper interpretation.…”

“…Detailed modeling of multi-transition single-dish lines toward the deeply embedded low-mass protostar IRAS 16293-2422 has demonstrated that similar enhancements of molecules like H 2 CO and CH 3 OH can occur for low-mass objects Ceccarelli et al 2000b;Schöier et al 2002). Recently, Maret et al (2004) have suggested that this is a common phenomenon in low-mass protostars. The location at which this enhancement occurs is consistent with the radius at which ices are expected to thermally evaporate off the grains (T > ∼ 90 K).…”

The desorption of H2CO from interstellar grains analogues

“…Its abundance with respect to water ice varies from 1% to 6% in high- (Keane et al 2001;Dartois 2005) or low- (Boogert et al 2008) mass protostars, or hot corinos (Maret et al 2004). It is believed that CO is hydrogenated on the surface of amorphous solid water (ASW) or in bulk ASW ice to form H 2 CO, which is further hydrogenated to form CH 3 OH (Hiraoka et al 1994;Watanabe & Kouchi 2002;Fuchs et al 2009).…”

Experimental studies of the dissociative recombination processes for the dimethyl ether ions CD3OCD$_{2}^{+}$ and (CD3)2OD+

“…The unhydronated dimethyl ether (DME) is found in objects known to host complex chemistry such as hot cores and corinos (Sutton et al 1995;Cazaux et al 2003;Maret et al 2004;Groner et al 1998). On the other hand, it has neither been detected in cold molecular clouds (Peeters et al 2006;Friberg et al 1988) nor cometary comae (Crovisier et al 2004).…”