Abundance profiles of CH3OH and H2CO toward massive young stars as evolutionary tracers

Abstract

The chemical evolution of thirteen regions of massive star formation is studied through single-dish and interferometer line observations at submillimeter wavelengths. 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 lines. 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 location of the jump at T~100 K 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 few x 10^-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.