Solid CH4 toward low-mass protostars: How much is there to build complex organics?

Abstract

AbstractWe use Spitzer IRS spectra to determine the solid CH4 abundance toward a large sample (52 sources) of low mass protostars. 50% of the sources have an absorption feature at 7.7 μm, attributed to solid CH4. The solid CH4/H2O abundances are 2–13%, but toward sources with H2O column densities above 2 × 1018 cm−2, the CH4 abundances (20 out of 25) are nearly constant at 4.7 ± 1.6%. Correlations with CO2 and H2O together with the inferred abundances are consistent with CH4 formation through sequential hydrogenation of C on grain surfaces, but not with formation from CH3OH and formation in gas phase with subsequent freeze-out.