MOLECULAR CLOUDS IN THE OUTER GALAXY .4. STUDIES OF STAR FORMATION

We report the results of continuum observations of outer Galaxy molecular clouds for the purpose of determining their high-mass star content. Radio observations, at 6 and 20 cm, were carried out using the VLA. Far-infrared flux densities were determined from IRAS data. In a study of 17 molecular clouds, 14 thermal radio sources were found, with 6 cm flux densities ranging from 0.9 to 3700 mJy. The corresponding range in rates of ionizing photons is 6 × 1045 s-1 (B1) to 5.5 × 1049 s-1 (O4). Far-infrared luminosites range from 500 to 1.0 × 106 L⊙. The required rate of ionizing photons correlates with far-infrared luminosity, and many sources have infrared and radio luminosities that are consistent with ionization by single main-sequence stars. In some cases there is an infrared excess, suggesting the presence of dust in the H II regions or the formation of clusters of stars. This pattern is similar to that seen in inner Galaxy molecular clouds. There is a very large range (at least two orders of magnitude) in the ratio of LFIR/LCO, also similar to what is seen in the inner Galaxy. This large range suggests that the formation of massive stars does not proceed at a constant rate per unit cloud mass. Even after accounting for a slightly lower LCO/M ratio in the outer Galaxy, the average star formation rate per molecular cloud mass in the outer Galaxy is only slightly lower than in the inner Galaxy, indicating that similar mechanisms are operative throughout the Galaxy.