The G11.11-0.12 Infrared-Dark Cloud: Anomalous Dust and a Non-Magnetic Isothermal Model

The G11.11-0.12 Infrared-Dark Cloud has a filamentary appearance, both in absorption against the diffuse 8micron Galactic background, and in emission from cold dust at 850micron. Detailed comparison of the dust properties at these two wavelengths reveals that standard models for the diffuse interstellar dust in the Galaxy are not consistent with the observations. The ratio of absorption coefficients within the cloud is kappa_8/kappa_850 <= 1010, which is well below that expected for the diffuse ISM where kappa_8/kappa_850 ~ 1700. This may be due to the formation of ice mantles on the dust and grain coagulation, both of which are expected within dense regions of molecular clouds. The 850micron emission probes the underlying radial structure of the filament. The profile is well represented by a marginally resolved central region and a steeply falling envelope, with Sigma(r) proportional to r^(-a), where a <= 3, indicating that G11.11-0.12 is the first observed filament with a profile similar to that of a non-magnetic isothermal cylinder.