  CH4 is proposed to be the starting point of a rich organic chemistry. Solid
CH4 abundances have previously been determined mostly toward high mass star
forming regions. Spitzer/IRS now provides a unique opportunity to probe solid
CH4 toward low mass star forming regions as well. Infrared spectra from the
Spitzer Space Telescope are presented to determine the solid CH4 abundance
toward a large sample of low mass young stellar objects. 25 out of 52 ice
sources in the $c2d$ (cores to disks) legacy have an absorption feature at 7.7
um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances
are 2-8%, except for three sources with abundances as high as 11-13%. These
latter sources have relatively large uncertainties due to small total ice
column densities. Toward sources with H2O column densities above 2E18 cm-2, the
CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation
plots with solid H2O, CH3OH, CO2 and CO column densities and abundances
relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than
with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with
models where CH4 is formed through sequential hydrogenation of C on grain
surfaces. Finally the equal or higher abundances toward low mass young stellar
objects compared with high mass objects and the correlation studies support
this formation pathway as well, but not the two competing theories: formation
from CH3OH and formation in gas phase with subsequent freeze-out.

Astrophysics