Observations of the Onset of Complex Organic Molecule Formation in
  Interstellar Ices
  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 2-5 $\mu$m
with NASA's Infrared Telescope Facility (IRTF) simultaneously display
signatures from the cores of H$_2$O (3.0 $\mu$m), CH$_3$OH (C-H stretching
mode, 3.53 $\mu$m) and CO (4.67 $\mu$m) 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 CH$_3$OH. Two of these sight lines also show
independent detections of CH$_3$OH. For these we find the ratio of the
CH$_3$OH:CO$_r$ is 0.55$\pm$0.06 and 0.73$\pm$0.07 from L483 and L694-2,
respectively. The detections of both CO and CH$_3$OH for the first time through
lines of sight toward background stars observationally constrains the
conversion of CO into CH$_3$OH ice. Along the lines of sight most of the CO
exists in the gas phase and $\leq$15% of the CO is frozen out. However,
CH$_3$OH ice is abundant with respect to CO (~50%) and exists mainly as a
CH$_3$OH-rich CO ice layer. Only a small fraction of the lines of sight
contains CH$_3$OH ice, presumably that with the highest density. The high
conversion of CO to CH$_3$OH can explain the abundances of CH$_3$OH ice found
in later stage Class 1 low mass YSO envelopes (CH$_3$OH:CO$_r$ ~ 0.5-0.6). For
high mass YSOs and one Class 0 YSO this ratio varies significantly implying
local variations can affect the ice formation. The large CH$_3$OH ice abundance
indicates that the formation of complex organic molecules is likely during the
pre-stellar phase in cold environments without higher energy particle
interactions (e.g. cosmic rays).
