The c2dSpitzerSpectroscopic Survey of Ices around Low‐Mass Young Stellar Objects. III. CH₄

CH 4 is proposed to be the starting point of a rich organic chemistry. Solid CH 4 abundances have previously been determined mostly toward high-mass star-forming regions. Spitzer IRS now provides a unique opportunity to probe solid CH 4 toward low-mass star-forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH 4 abundance toward a large sample of low-mass young stellar objects. A total of 25 out of 52 ice sources in the "Cores to Disks" (c2d) Legacy program have an absorption feature at 7.7 μm, attributed to the bending mode of solid CH 4 . The solid CH 4 /H 2 O abundances are 2%-8%, except for three sources with abundances as high as 11%-13%. The latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H 2 O column densities above 2 × 10 18 cm −2 , the CH 4 abundances (20 out of 25) are nearly constant at 4.7% ± 1.6% . Correlation plots with solid H 2 O, CH 3 OH, CO 2 , and CO column densities and abundances relative to H 2 O reveal a closer relationship of solid CH 4 with CO 2 and H 2 O than with solid CO and CH 3 OH. The inferred solid CH 4 abundances are consistent with models where CH 4 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 CH 3 OH and formation in gas phase with subsequent freezeout.