Interstellar polarization from 3 to 5 microns in reddened stars

Abstract

The study reports measurements of IR interstellar linear polarization in the spectral range 2-5 microns for 18 highly reddened stars. The observations covered lines of sight with wavelengths of peak polarization lambda(max) ranging from the blue-UV (0.35 micron) to the near-IR (0.78 micron). It is shown that excess polarization occurs at 3-5 microns relative to the standard Serkowski/Wilking empirical formula describing the wavelength dependence of polarization in the optical and near-IR out to 2.2 microns. The results of Nagata (1990) and Jones (1990) that significant excess exists at 3.6-3.8 microns are confirmed, and it is shown that it persists to 5 microns. It is found that neither the excess polarization relative to the Serkowski/Wilking formula nor the value of the power-law index is correlated to changes in lambda(max). Hence, the IR polarization does not respond to changes in the properties of the grains which give rise to dramatic variations in the wavelength dependence of polarization at shorter wavelengths, suggesting a degree of invariance in the form of the IR polarization law.