BOK GLOBULES AND SMALL MOLECULAR CLOUDS - DEEP IRAS PHOTOMETRY AND (CO)-C-12 SPECTROSCOPY

The sample of 248 small molecular clouds (mostly Bok globules) cataloged by Clemens and Barvainis has been probed using deep, co-added IRAS image analysis and via millimeter-wavelength (CO)-C-12 spectroscopy. Co-adding of the IRAS data lowered the flux limits of all four IRAS bands to far below the detection thresholds for these clouds: virtually all of the clouds were detected in all four IRAS bands. The (CO)-C-12 spectroscopy revealed 244 (98%) detections, verifying the molecular nature of these clouds. In this paper the IRAS co-added fluxes and (CO)-C-12 J = 2 - 1 line parameters are presented. Our characterization of these Bok globules has revealed mean IRAS flux ratios (colors) of 0.70 for log (S100/S60), 0.68 for log (S60/S25), and 0.01 for log (S25/S12). These IRAS flux ratio distributions indicate average dust temperatures of 26 +/- 5 K for the 60 and 100-mu-m bands, 71 +/- 16 K for the 25 and 60-mu-m bands, and 254 +/- 72 K for the 12 and 25-mu-m bands. The average broad-band IRAS spectrum for this sample of clouds shows the 12-mu-m excess emission usually associated with small, warm, nonequilibrium grains in the cloud peripheries. The IRAS colors are virtually identical to those of other molecular clouds in our Galaxy. Hence, the IRAS data base cannot be used to select Bok globules based on their locations in color-color diagrams. For an assumed sample distance of 600 pc, the mean L(FIR) is about 6 L., which is close to the value expected of clouds externally heated by the interstellar radiation field. Similarly, using crude assumptions about the cloud structures, the CO observations were used to estimate mean cloud masses of about 11 M.. The mean L(FIR)/M(H-2) for this cloud sample is 0.5 L./M., much smaller than the mean of about 3 L./M. typical of the Galactic disk. We found only a mild correlation between integrated (CO)-C-12 emission and cloud-averaged dust optical depth for these clouds. Extrapolating our findings to the entire Galaxy, we estimate the total number of Bok globules to be around 3.2 X 10(5). The total mass of these clouds is roughly 3.5 X 10(6) M., representing approximately 0.1% of the Galaxy molecular mass. The total FIR luminosity of Bok globules in the Galaxy is about 2 X 10(6) L., roughly 0.014% of the total FIR luminosity of the Galaxy.