Observations of MSX Infrared-Dark Clouds

Redman, Feldman (NRC Canada), Carey (Boston College) and Egan (AFRL) observed dust and gas in a selection of MSX infrared-dark clouds. These clouds are identified by their substantial mid-infrared (8-25 um) extinctions in MSX Galactic Plane survey images (Egan et al., ApJL, 494, L199, 1998). From their high mid-infrared opacities and lack of emission between 8 and 100 um, Egan et al. concluded that the IRDCs exhibit hundreds of magnitudes of visual extinction, contain large column densities of cold dust, and are dense molecular cores.

Subsequent observations (Carey et al., ApJ, 508, 721, 1998) of millimeter transitions of H₂CO toward ten IRDCs confirmed that these objects contain dense molecular gas. Large Velocity Gradient (LVG) modeling of several transitions of H₂CO indicate that IRDCs have kinetic temperatures of 10-20 K, H₂ number densities ~10⁶ cm^-3, and H₂ column densities ranging up to 10²³ cm^-2. The available data strongly suggest that the gas and dust are in thermal equilibrium.

SCUBA was used to map a number of IRDCs and revealed filamentary/flocculent clouds in emission at 850 and 450 um, with bright, compact sources along the filaments. Many of these compact sources are prolate, extending along the direction of the filament (see Figure 1). These cores appear to be in a variety of early stages of star formation. Some of them contain bright infrared sources, while others are completely dark in the MSX 8 um images, suggesting that they are Class 0 or earlier. In addition, molecular line data taken with receivers A3 and B3 show evidence for outflow from all of the cores observed, and for infall in several cases.

The SCUBA polarimeter was also used to observe several bright filamentary regions; all showed high degrees of linear polarization. The magnetic fields derived from the polarization vectors tend to run along the filaments except near the brightest compact sources where the fields often switch direction to be perpendicular to the filaments. The observed high-percentage polarization and large-scale organization of the linear polarization imply that magnetic fields in the filaments are well organized on the 14" scale of the SCUBA beamsize at 850 um.

From the molecular line and SCUBA observations, the estimated masses of the IRDCs are comparable to those of the largest star-forming cores. The individual compact sources have masses that range from tens to hundreds of solar masses. It is interesting to note that the bright IRAS source 18402-0403, immediately adjacent to the bright SCUBA source P2 in G28.34+0.06, has a bolometric luminosity of about 10³ solar luminosities. If this luminosity is due to accretion onto a protostar, the central mass would be that of a typical OB star. It is plausible that massive IRDCs may be sites where high-mass star formation is occurring.

These results were presented at the 33^rd ESLAB Symposium, "Star Formation from the Small to the Large Scale" and are in press in the Proceedings (eds. F. Favata, A. A. Kaas, and A. Wilson).