Near infrared polarimetry of cool stars

Broad band near infrared polarimetry of twenty-nine cool stars is presented. Measurements were made at the effective wavelengths I (0.911 μm), J (1.21 μm), H (1.65 μm) and K (2.18 μm). The program stars include M giants and supergiants, M and S type Mira variables and carbon stars. Theoretical calculations, interpreting the polarisation in terms of the scattering properties of circumstellar dust grains, are also presented. Correlations of the average degree of polarisation in each waveband with the colour indices (I-K) and (L-O) show that; in general, the polarisation (a) decreases between J and K (b) increases with decreasing photospheric temperature and (c) increases with the optical depth of the circumstellar dust. A provisional correlation between the polarisation and expansion velocity of the circumstellar material in the OH maser stars is also identified. This relation is consistent with the mechanism of radiation pressure being an important mass loss process. A number of the program stars show evidence of either silicate or graphite particles with sizes in the range 0.1 μm to 1.0 μm. These grains are significantly larger than those of the dominant component (mean size < 0.1 μm) responsible for the quasi-Rayleigh optical polarisation of red variable stars. Temporal variations in both the degree of polarisation and its position angle have been observed in most of the stars. A wide range of spectral behaviour was also found. (author)