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Hot H2CO: Observations of Formaldehyde at 650GHz

Alwyn Wootten (NRAO), Jeffrey Mangum (NRAO), and Mary Barsony (UC-Riverside)


  1. We determine the physical conditions in warm dense cores in molecular clouds via observations of the submillimeter wavelength transitions of formaldehyde (H2CO). H2CO is the ideal molecule for this purpose, since its numerous transitions in the submillimeter spectrum allow independent temperature and density determination of the emitting gas. The transitions in the 650 GHz atmospheric window arise from gas which is too warm for depletion onto grains to have hidden it from view. The lowest energy transitions lie at levels 160 K above ground, while the highest energy transitions in the window are the K=8 transitions arising from levels more than 1000 K above the ground state. Since formaldehyde is destroyed at temperatures above about 300K, little emission is expected from the most highly excited transitions. However, Schilke (private communication) reports the detection of the K=7 transitions at 654 GHz toward IRc2 in OMC1. These lines, which arise from levels 800 K above ground may occur in a transient chemistry, and provide information on conditions in shocked or ablated material quite close to the young star(s) buried in the dense core (Wootten, Loren and Bally 1984 Ap. J. 277, 189). We report observations of several highly excited lines of formaldehyde towards warm HII regions made with the Caltech Submillimeter Telescope (CSO). Our attempt to understand the conditions under which its emission is excited in star-forming molecular clouds confirms that denser, hotter regions give rise to more highly excited submillimeter transitions than heretofore found. In DR21(OH)MM1, for example, densities a factor of four higher than suggested by Mangum and Wootten (1993, Ap. J. Supp. 89, 123) are required to reproduce the strength of the 9(19)->8(18) transition observed at 632 GHz.

    The CSO is operated by the California Institute of Technology under funding from the National Science Foundation, Contract No. AST-93-13929.

    Integrated intensity in the 218.2 GHz K=0 line of formaldehyde observed with the IRAM 30m telescope and its 12" beam. Sources identified in interferometric ammonia images are shown.

    Formaldehyde profiles towards DR21OH-MM1.

    Formaldehyde profiles towards W51M.

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    Al Wootten
    Wed Oct 30 10:35:59 EST 1996