H2CO and CH3OH abundances in the envelopes around low-mass protostars

This paper presents the third in a series of single- dish studies of molecular abundances in the envelopes around a large sample of 18 low- mass pre- and protostellar objects. It focuses on typical grain mantle products and organic molecules, including H2CO, CH3OH and CH3CN. With a few exceptions, all H2CO lines can be fit by constant abundances of 7 x 10(-11)- 8 x 10(-9) throughout the envelopes if ortho- and para lines are considered independently. The current observational dataset does not require a large H2CO abundance enhancement in the inner warm regions, but this can also not be ruled out. Through comparison of the H2CO abundances of the entire sample, the H2CO ortho- para ratio is constrained to be 1.6 +/- 0.3 consistent with thermalization on grains at temperatures of 10- 15 K. The H2CO abundances can be related to the empirical chemical network established on the basis of our previously reported survey of other species and is found to be closely correlated with that of the nitrogen- bearing molecules. These correlations reflect the freeze- out of molecules at low temperatures and high densities, with the constant H2CO abundance being a measure of the size of the freeze- out zone. An improved fit to the data is obtained with a drop abundance structure in which the abundance is typically a few x 10(-10) when the temperature is lower than the evaporation temperature and the density high enough so that the timescale for depletion is less than the lifetime of the core. The location of the freeze- out zone is constrained from CO observations. Outside the freeze- out zone, the H2CO abundance is typically a few x 10(-9)- 10(-8). The observations show that the CH3OH lines are significantly broader than the H2CO lines, indicating that they probe kinematically distinct regions. CH3OH is moreover only detected toward a handful of sources and CH3CN toward only one, NGC 1333- IRAS2. For NGC 1333- IRAS2, CH3OH and CH3CN abundance enhancements of two- three orders of magnitude at temperatures higher than 90 K are derived. In contrast, the NGC 1333- IRAS4A and IRAS4B CH3OH data are fitted with a constant abundance and an abundance enhancement at a lower temperature of 30 K, respectively. This is consistent with a scenario where CH3OH probes the action of compact outflows on the envelopes, which is further supported by comparison to high frequency, high excitation CS J = 10- 9 and HDO line profiles which uniquely probe warm, dense gas. The extent to which the outflow dominates the abundance enhancements compared with the passively heated inner envelope depends on the filling factors of the two components in the observing beam.