2020ApJ...904...86C

Isolated dense molecular cores are investigated to study the onset of complex organic molecule formation in interstellar ice. Sampling three cores with ongoing formation of low-mass stars (B59, B335, and L483) and one starless core (L694-2), we sample lines of sight to nine background stars and five young stellar objects (YSOs; A_K ∼ 0.5-4.7). Spectra of these stars from 2 to 5 µm with NASA's Infrared Telescope Facility (IRTF) simultaneously display signatures from the cores of H₂O (3.0 µm), CH₃OH (C-H stretching mode, 3.53 µm), and CO (4.67 µm) ices. The CO ice is traced by nine stars, in which five show a long-wavelength wing due to a mixture of CO with polar ice (CO_r), presumably CH₃OH. Two of these sightlines also show independent detections of CH₃OH. For these we find the ratio of the CH₃OH:CO_r is 0.55 ± 0.06 and 0.73 ± 0.07 from L483 and L694-2, respectively. The detections of both CO and CH₃OH for the first time through lines of sight toward background stars observationally constrains the conversion of CO into CH₃OH ice. Along the lines of sight, most of the CO exists in the gas phase and <=15% of the CO is frozen out. However, CH₃OH ice is abundant with respect to CO (∼50%) and exists mainly as a CH₃OH-rich CO ice layer. Only a small fraction of the lines of sight contains CH₃OH ice, presumably those with the highest density. The high conversion of CO to CH₃OH can explain the abundances of CH₃OH ice found in later stage Class 1 low-mass YSO envelopes (CH₃OH:CO_r ∼ 0.5-0.6). For high-mass YSOs and one Class 0 YSO, this ratio varies significantly, implying local variations can affect ice formation. The large CH₃OH ice abundance indicates that the formation of complex organic molecules is likely during the prestellar phase in cold environments without higher energy particle interactions (e.g., cosmic rays).