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 inabsorption against the diffuse 8micron Galactic background, and in emissionfrom cold dust at 850micron. Detailed comparison of the dust properties atthese two wavelengths reveals that standard models for the diffuse interstellardust in the Galaxy are not consistent with the observations. The ratio ofabsorption coefficients within the cloud is kappa_8/kappa_850 <= 1010, which iswell 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 graincoagulation, both of which are expected within dense regions of molecularclouds. The 850micron emission probes the underlying radial structure of thefilament. The profile is well represented by a marginally resolved centralregion 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 aprofile similar to that of a non-magnetic isothermal cylinder.Comment: 13 pages, 4 figures, accepted for publication by ApJ Letter