ar X iv : a st ro - p h / 06 09 13 5 v 1 5 Se p 20 06 Gas - phase CO 2 emission toward Cepheus A East : the result of shock activity ? 1

We report the first detection of gas-phase CO2 emission in the star-forming region Cepheus A East, obtained by spectral line mapping of the ν2 bending mode at 14.98 μm with the Infrared Spectrograph (IRS) instrument onboard the Spitzer Space Telescope. The gaseous CO2 emission covers a region about 35 ′′ × 25 in extent, and results from radiative pumping by 15 μm continuum photons emanating predominantly from the HW2 protostellar region. The gaseous CO2 exhibits a temperature distribution ranging from 50 K to 200 K. A correlation between the gas-phase CO2 distribution and that of H2 S(2), a tracer of shock activity, indicates that the CO2 molecules originate in a cool post-shock gas component associated with the outflow powered by HW2. The presence of CO2 ice absorption features at 15.20 μm toward this region and the lack of correlation between the IR continuum emission and the CO2 gas emission distribution further suggest that the gaseous CO2 molecules are mainly sputtered off grain mantles – by the passage of slow non-dissociative shocks with velocities of 15-30 km s – rather than sublimated through grain heating. Subject headings: ISM: Molecules — ISM: Clouds, molecular processes — Starforming region: individual (Cepheus A East) Based on observations with the Spitzer Space Telescope. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. Universidad de Alcalá de Henares, Departamento de F́isica, Campus Universitario, E-28871 Alcalá de Henares, Madrid, Spain. Department of Astronomy, University of Michigan, 825 Dennison Building, 500 Church Street, Ann Arbor, MI 48109. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627.