2008ApJ...678.1032O

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. 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 CH₄. The solid CH₄/H₂ 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 H₂ O column densities above 2x10¹⁸/cm2, 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 H₂ O reveal a closer relationship of solid CH₄with CO₂ and H₂ O than with solid CO and CH₃OH. The inferred solid CH₄abundances 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 CH₃ OH and formation in gas phase with subsequent freezeout.