CO Observations of the W51B H II Region Complex

Details CO Observations of the W51B H II Region Complex CO Observations of the W51B H II Region Complex

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...518..760k&link_type=abstract

The Astrophysical Journal, Volume 518, Issue 2, pp. 760-768.

Astronomy and Astrophysics

Astronomy

20

H Ii Regions, Ism: Clouds, Ism: Individual (W51), Ism: Molecules, Radio Lines: Ism, Stars: Formation

Scientific paper

We have carried out ^12CO J=1-0 and J=2-1 line observations of the W51B H II region complex. CO J=1-0 observations show that the molecular gas associated with W51B is organized into a long (~30' or ~40 pc at a distance of 5 kpc) filamentary structure approximately parallel to the Galactic plane (b~=-18^'). The filamentary structure appears to be composed of two separate clouds, clouds NE and SW. We derive their parameters and determined their masses to be 3.9x10^4 M_solar and 8.2x10^4 M_solar, respectively. The compact H II region G49.2-0.3 is associated with cloud NE and all the other compact H II regions in W51B are associated with cloud SW. We derive CO J=2-1/J=1-0 luminosity ratios of 1.2 and 1.4 for these clouds, which are much higher than typical ratios (0.5-0.8) for nearby molecular clouds. Based on the large observed ratios and existing ^13CO results, we propose that the W51B clouds are externally heated by far-ultraviolet radiation and that the ^12CO lines originate from the photodissociation regions on the surfaces of dense (>~10^4 cm^-3) clumps. We estimate the filling factors of these clumps to be small (<0.1). Our results further suggest that the clouds do not have low-density envelopes, which could have been destroyed by UV photons and stellar winds from nearby OB stars. We estimate that the star formation efficiency (SFE) of the W51B region is very high, i.e., 7% and 15% for clouds NE and SW, respectively. The very high SFE could be due to enhanced star formation by the spiral density wave.

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