Terry Jay Jones

Question of the Month...

Alas, it does indeed appear that the dust grains in cold dark clouds do not polarize light in transmission. The evidence is strong that there is not just a reduction in the polarization efficiency within cold dark clouds, but that there is no contribution at all! Not only is there no increase in polarization accross the face of cold dark clouds, but there is no measureable change in the position angle distribution (Goodman et al., ApJ 448, 748). Yet, at the same time, many lines of sight into molecular clouds with a luminous imbedded source show very high polarization. This interesting observation is illustrated below in a P vs. Extinction diagram at K (2.2 microns) adapted from Jones, Klebe & Dickey, ApJ, 389, 602.

The observed trend in the filimentary cloud L1755, shown as a dashed line, is clearly at odds with the general trend seen in the more diffuse ISM. However, BN is very highly polarized for its extinction and follows the trend. Both lines of sight pass through molecular gas, so what is the difference between these two lines of sight that causes grains to polarize in one case, but not in the other? The gas and dust along the line of sight to BN are somewhat warmer than in L1755, but otherwise the conditions are probably very similar. A scematic representation of the situation is shown below. DISM refers to the diffuse ISM outside the molecular cloud.

In the top panel, the current observational results for cold dark clouds with a background source is illustrated. In the second panel, a luminous imbedded source such as BN or W33 is shown within a much deeper molecular cloud. Far infrared observations of polarized emission show a clear trend of reduced polarization with increasing optical depth at 100 microns. This could be explained by having the Good Grains relatively local to the imbedded source, with the rest of the path contributing emission, but not polarization. At near IR wavelengths, only Good Grains are between the source and the observer. If the cloud is viewed from the other direction, the source would be too extinguished to be measured in polarization in the near IR. The bottom panel illustrates the case that has yet to be observed, a luminous background star behind a very thick, but quiescent molecular cloud.

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