Star Formation in the Outer Galaxy

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Session 39 -- Interferometry II
Oral presentation, Tuesday, 2:00-6:30, Zellerbach Playhouse Room

[39.07] Star Formation in the Outer Galaxy

Alexander L. Rudolph (NASA/Ames Research Center), Eugene J. de Geus (University of Maryland)

We have made high-resolution ($\sim 5''$ resolution) interferometric observations with the BIMA array and the VLA of star-forming regions in the outer Galaxy, in order to study molecular cloud properties and the star formation process in a physical and chemical environment different from the Solar neighborhood. These clouds are close enough to allow us to resolve structures as small as individual 0.25 pc star-forming cores.

Eleven sources have been mapped at 6, 3.6, and 2 cm with the VLA in order to probe the ionized gas in the regions. Of these 11 sources, three have also been imaged with the BIMA array in CO (J=1-0) and CS (J=2-1) emission, to probe the molecular gas content of the clouds, and to determine their structure. The BIMA array observations have been combined with single-dish maps of the sources, in order to recover the total flux from the clouds.

Using the radio fluxes and sizes observed, we have calculated various H~II region parameters, such as emission measure, excitation parameter, electron density, and N$_{\rm L}$, the number of Lyman-alpha photons implied by the total emission. From N$_{\rm L}$, we have determined the spectral type of a zero-age main sequence star capable of producing such a flux of ionizing photons. In addition, we have compared the far-infrared and 6 cm luminosities of the sources to try to determine the high-mass stellar content of the clouds.

From the BIMA array CO maps, we have determined the clump mass spectrum for two of the three clouds and find that the slope of the spectrum is flatter (-1.0) than that found locally (-1.5). This result is somewhat surprising, since Garmany et al. (1982, ApJ, 263, 777) found that the slope of the massive star IMF is steeper in the outer Galaxy (-2.1) than in the inner Galaxy (-1.3). Thus, our results challange the na\"\i ve assumption that the mass of a star is directly proportional to the mass of the clump from which it formed, and suggest that fragmentation of clumps plays an important role in the star formation process.

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