INTERSTELLAR EXTINCTION AND POLARIZATION IN THE INFRARED

The wavelength dependences of interstellar continuum extinction and polarization in the range 0.35-5 μm are examined with attention to the extent to which data for diverse regions converge to a common or "universal" curve in the infrared, and to the appropriateness of a power-law representation of any common behavior. To within the limits of current data, the existence of a universal extinction curve extending from the near-infrared (I passband) to at least 5 μm appears to be established for both diffuse and dense cloud dust. A power law of index about 1.8 provides a good representation of this universal curve. Extinction curves normalized in the region of common behavior diverge at shorter wavelengths. The polarization yields intriguingly similar results, with evidence for some degree of universality in the 1.6-5 μm regime which may be represented by a power law with index in the range 1.5-2.0, encompassing that for extinction. The polarization data, like those for extinction, cover a wide range of environments from the low-density "diffuse" interstellar medium to denser dark clouds and molecular clouds in which the 3 μm ice band is seen. The form of the polarization curve in the infrared seems independent of the wavelength λmax at which the degree of polarization peaks in the optical, implying that variations in λmax, like the correlated variations in the ratio of total to selective extinction Rv, are caused by changes in the optical properties of the particles at blue-visible rather than infrared wavelengths. On the basis of these and other remarkable features (Cardelli, Clayton and Mathis), we argue that the more significant alterations of the grain size distribution from one environment to another occur for the smaller particles. This is not to say that the larger grains are completely unaffected, but we conclude that any change in the infrared opacity arises in such a manner that the shape of the extinction curve is preserved. Constraints on the details of accretion and coagulation theories are discussed. Among these, grain evolution by the larger particles sticking to one another seems ruled out. From the present data, one cannot conclude that the grains responsible for extinction and polarization are one and the same. However, it appears that the relevant grain populations respond similarly to changes in the environment.