Observational constraints on methanol production in interstellar and preplanetary ices

Abstract

Methanol (CH3OH) is thought to be an important link in the chain of chemical evolution that leads from simple diatomic interstellar molecules to complex organic species in protoplanetary disks that may be delivered to the surfaces of Earthlike planets. Previous research has shown that CH3OH forms in the interstellar medium predominantly on the surfaces of dust grains. To enhance our understanding of the conditions that lead to its efficient production, we assemble a homogenized catalog of published detections and limiting values in interstellar and preplanetary ices for both CH3OH and the other commonly observed C- and O-bearing species, H2O, CO, and CO2. We use this catalog to investigate the abundance of ice-phase CH3OH in environments ranging from dense molecular clouds to circumstellar envelopes around newly born stars of low and high mass. Results show that CH3OH production arises during the CO freezeout phase of ice-mantle growth in the clouds, after an ice layer rich in H2O and CO2 is already in place on the dust, in agreement with current astrochemical models. The abundance of solid-phase CH3OH in this environment is sufficient to account for observed gas-phase abundances when the ices are subsequently desorbed in the vicinity of embedded stars. CH 3OH concentrations in the ices toward embedded stars show order-of-magnitude object-to-object variations, even in a sample restricted to stars of low mass associated with ices lacking evidence of thermal processing. We hypothesize that the efficiency of CH3OH production in dense cores and protostellar envelopes is mediated by the degree of prior CO depletion. © 2011. The American Astronomical Society. All rights reserved.