The G11.11-0.12 infrared-dark cloud: Anomalous dust and a nonmagnetic isothermal model

The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in absorption against the diffuse 8 mumm Galactic background and in emission from cold dust at 850 mum. A 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) less than or equal to 1010, which is well below that expected for the diffuse interstellar medium where kappa(8)/kappa(850). 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 850 mum 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(-alpha), where alpha greater than or equal to 3, indicating that G11.11-0.12 is the first observed filament with a profile similar to that of a nonmagnetic isothermal cylinder.