Detection of Acetylene toward Cepheus A East with Spitzer

The first map of interstellar acetylene (C 2 H 2 ) has been obtained with the Infrared Spectrograph on board the Spitzer Space Telescope . A spectral line map of the ν 5 vibration-rotation band at 13.7 μm, carried out toward the star-forming region Cepheus A East, shows that the C 2 H 2 emission peaks in a few localized clumps where gas-phase CO 2 emission was previously detected with Spitzer . The distribution of excitation temperatures, derived from fits to the C 2 H 2 line profiles, ranges from 50 to 200 K, which is consistent with that derived for gaseous CO 2 —suggesting that both molecules probe the same warm gas component. The C 2 H 2 molecules are excited via radiative pumping by 13.7 μm continuum photons emanating from the HW2 protostellar region. We derive column densities ranging from a few × 10 13 to ~ 7 × 10 14 cm -2 , corresponding to C 2 H 2 abundances of 1 × 10 -9 to 4 × 10 -8 with respect to H 2 . The spatial distribution of the C 2 H 2 emission and a roughly constant N (C 2 H 2 )/ N (CO 2 ) strongly suggest an association with shock activity, most likely the result of the sputtering of acetylene in icy grain mantles.