Mathematics – Probability
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...183.1415k&link_type=abstract
American Astronomical Society, 183rd AAS Meeting, #14.15; Bulletin of the American Astronomical Society, Vol. 25, p.1312
Mathematics
Probability
Scientific paper
To extract the sd of polarizing dust grains we have used the Maximum Entropy Method (MEM). In this first investigation we adopted infinite cylinders with perfect spinning alignment. Only bare silicate particles were considered. The modified Serkowski law represents interstellar polarization quite well for the wavelength range 0.3 si 2 micron using one parameter, lmax, the wavelength at which the polarization is maximum. For large lmax it extrapolates reasonably into the ultraviolet and so we have investigated how the sd changes with lmax by fitting the mser curve evaluated for lmax = 0.55, 0.60, and 0.65 micron. For HD 25443 which shows sser behavior and for HD 197770 which might exhibit a 2200 Angstroms polarization bump, we combined the mser curve in the optical with the actual far-ultraviolet data. The sd s found bear little resemblance to a power law. Instead they peak at si 0.1 micron and are skewed, with the relative rate of decrease to larger and smaller sizes depending on lmax. For the particles larger than 0.1 micron, the sd does not change much with lmax; on the other hand there is a remarkable change for the smaller ones -- the drop-off gets faster as lmax increases. Compared to the sd based on extinction, there is a similarity for large particles, but not for small ones: to fit the polarization curve it is not necessary to have as many small particles. This means that the (excess) smaller particles which contribute to the extinction are either not as well aligned or rounder. Mathis (1986) developed an alignment theory based on superparamagnetic inclusions in coagulated grains; the probability of a grain having such an inclusion (and therefore being aligned) decreases for smaller grains. While this is qualitatively correct, our MEM results indicate even fewer small particles than predicted. It might be that smaller particles tend to be rounder, becoming more aspherical as they grow (by coagulation).
Kim Sang-Hee
Martin Peter G.
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