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[71.06] Vibrationally Excited Molecular Lines as a Tracer for Accretion Disk Structure

C. Groppi, C. Walker, C. Kulesa (Steward Observatory)

The vast majority of molecular lines observed at (sub)millimeter wavelengths are associated with rotational excitation of the lowest vibrational state (v=0). However, several vibrationally-excited molecules, such as CS, H₂, SiO, HC₃N, CH₃CN, and HCN have already been detected in the interstellar medium. Since these vibrational bands lie typically 1000K or more above the ground state, significant vibrational excitation requires either regions of hot, dense gas (perhaps associated with a shock) and/or nonthermal vibrational excitation by radiative pumping. Since these excitation requirements are only expected to be fulfilled in a very compact region close to the embedded source, they make vibrationally excited molecules with (sub)millimeter-wave rotational transitions excellent probes of the physical conditions very near embedded infrared sources. We have observed a sample of young stellar objects, searching for the (0,1,0) and (0,2,0) vibrational transitions of HCN(J=4-3) with the Heinrich Hertz Telescope. We have detected these lines toward four sources, and have placed strong upper limits on the other five sources. These observations allow the determination of the excitation mechanism, critical to characterizing the line-emitting environment. This new survey highlights the utility of vibrational HCN emission as a sensitive probe of the structure of circumstellar and protoplanetary disks, even when they are spatially unresolved by single dish measurements.

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The author(s) of this abstract have provided an email address for comments about the abstract: cgroppi@as.arizona.edu

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.