A Multi-transition CO and (13) CO Survey of the Galactic Plane

Details A Multi-transition CO and (13) CO Survey of the Galactic Plane A Multi-transition CO and (13) CO Survey of the Galactic Plane

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.8604s&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #86.04; Bulletin of the American Astronomical Society, Vol. 24, p.1261

Astronomy and Astrophysics

Astronomy

Scientific paper

A survey of the Galactic Plane in the (2->1) and (3->2) rotational transitions of CO and () 13CO has been carried out using the 10m telescope of the Caltech Submillimeter Observatory (CSO). These data, plus previous observations of (1->0) emission from CO and () 13CO obtained at the Five College Radio Astronomy Observatory (FCRAO), have been used to determine the excitation properties of the molecular gas (T_kin, n(H_2)) throughout the disk of the Milky Way. The bulk of the molecular gas outside the galactic center (R > 0.5 kpc) appears to be slightly colder, T_kin = 10-15 (deg) K, and denser, n(H_2) = 10(3) -10(4) cm() -3, than previously assumed. There are no obvious gradients in temperature and cloud H_2 density with galactocentric radius, although the molecular clouds in the galactic center and near the peak of the galactic ring at R/R_sun = 0.4-0.7 are on average warmer (T_kin ~ 15-20 (deg) K) than the molecular material at other radii. The large H_2 volume density derived from the CO data implies that a general internal property of molecular clouds is a low volume filling factor ( ~ 1-10%) for the molecular gas, implying a larger mean-free-path for ionizing photons within molecular clouds than previously assumed. A low volume filling factor would imply that high excitation lines (e.g. C(+) ) are not simply emitted from the outer cloud surface, but may originate throughout the cloud volume.

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