2000A&A...361..327V

The chemistry of CH₃OH and H₂CO in thirteen regions of massive star formation is studied through single-dish and interferometer line observations at submillimeter wavelengths. Single-dish spectra at 241 and 338 GHz indicate that T_rot=30-200K for CH₃OH, but only 60-90 K for H₂CO. The tight correlation between T_rot(CH₃OH) and T_ex(C₂H₂) from infrared absorption suggests a common origin of these species, presumably outgassing of icy grain mantles. The CH₃OH line widths are 3-5km/s, consistent with those found earlier for C¹⁷O and C³⁴S, except in GL 7009S and IRAS 20126, whose line shapes reveal CH₃OH in the outflows. This difference suggests that for low-luminosity objects, desorption of CH₃OH-rich ice mantles is dominated by shocks, while radiation is more important around massive stars. The wealth of CH₃OH and H₂CO lines covering a large range of excitation conditions allows us to calculate radial abundance profiles, using the physical structures of the sources derived earlier from submillimeter continuum and CS line data. The data indicate three types of abundance profiles: flat profiles at CH₃OH/H₂∼10^–9 for the coldest sources, profiles with a jump in its abundance from ∼10^–9 to ∼10^–7 for the warmer sources, and flat profiles at CH₃OH/H₂ ∼ few 10^–8 for the hot cores. The models are consistent with the ≃3'' size of the CH₃OH 107 GHz emission measured interferometrically. The location of the jump at T≃100K suggests that it is due to evaporation of grain mantles, followed by destruction in gas-phase reactions in the hot core stage. In contrast, the H₂CO data can be well fit with a constant abundance of a fewx10^–9 throughout the envelope, providing limits on its grain surface formation. These results indicate that T_rot (CH₃OH) can be used as evolutionary indicator during the embedded phase of massive star formation, independent of source optical depth or orientation. Model calculations of gas-grain chemistry show that CO is primarily reduced (into CH₃OH) at densities n_H≲10⁴cm^–3, and primarily oxidized (into CO₂) at higher densities. A temperature of ≃15K is required to keep sufficient CO and H on the grain surface, but reactions may continue at higher temperatures if H and O atoms can be trapped inside the ice layer. Assuming grain surface chemistry running at the accretion rate of CO, the observed abundances of solid CO, CO₂ and CH₃OH constrain the density in the pre-protostellar phase to be n_H > a few 10⁴cm^–3, and the time spent in this phase to be ≲10⁵yr. Ultraviolet photolysis and radiolysis by cosmic rays appear less efficient ice processing mechanisms in embedded regions; radiolysis also overproduces HCOOH and CH₄.