Other
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...431..783k&link_type=abstract
The Astrophysical Journal, vol. 431, no. 2, pt. 1, p. 783-796
Other
44
Cosmic Dust, Infrared Radiation, Interstellar Extinction, Linear Polarization, Particle Size Distribution, Polarized Radiation, Cosmology, Maximum Entropy Method, Refractivity, Silicates
Scientific paper
To extract the size distribution of polarizing dust grains from the wavelength dependence of interstellar linear polarization as objectively as possible, we have used the maximum-entropy method (MEM), as in an earlier study of size distributions based on extinction (Kim, Martin, & Hendry). There are additional complications using polarization data since polarization depends on shape and alignment. In this first investigation, we adopted infinite cylinders with perfect spinning alignment. To constrain a wide range of sizes it is necessary to use data from the infrared to the far-ultraviolet. Much of our analysis is based on bare silicate grains. The modified Serkowski law represents interstellar polarization quite well for the wavelength range 0.3 to 2 micrometers using one parameter, lambdamax, the wavelength at which the polarization is maximum. Recent ultraviolet polarimetric observations of eight stars of differing lambdamax indicate that extrapolation of the modified Serkowski curve into the ultraviolet produces a reasonable approximation for larger lambdamax (greater than or approximately 0.55 micrometer), but for smaller lambdamax there is an excess of polarization observed. Therefore, we have investigated how the size distribution of the polarizing grains changes with lambdamax simply by fitting the modified Serkowski curve evaluated for lambdamax = 0.55, 0.615, and 0.68 micrometers. But for HD 25443 (lambdamax = 0.49 micrometer) which shows super-Serkowski behavi or, and for HD 197770 (lambdamax = 0.51 micrometers) which might exhibit a 2175 A polarization bump, we combined the modified Serkowski curve in the infrared and optical with the actual far-ultraviolet data. The size distributions found for silicates bear little resemblance to a power law. Instead, when expressed as contributions to the total mass, they peak roughly at 0.14 micrometer and are skewed, with the relative rate of decrease to larger and smaller sizes depending on lambdamax. For the particles larger than 0.1 micrometer, the size distribution does not change much with lambdamax because the infrared polarization is fairly invariant; on the other hand there is a remarkable change for the smaller particles--the drop-off gets faster as lambdamax increases, explaining the decreasing ultraviolet polarization. In terms of the evolution of the size distribution, there is nothing to signal the transition from Serkowski to super-Serkowski behavior; this distinction is probably artificial. We have explored size distributions and polarization fitting using homogeneous grains of organic refractory material and other more highly absorbing materials like iron, magnetite, and an artificial one introduced by Chlewicki & Greenberg. Among three organic refractories studied, two give a fairly good fit to the average polarization curve; but in detail, one of these gives excess polarization in the infrared and the other predicts unobserved spectral structure in the ultraviolet. The third more processed organic refractory, thought to be most characteristic of interstellar grains, does not have features in the ultraviolet; however it produces a very flat polarization curve in the far ultraviolet, unlike that observed. On the whole, these organic refractories appear less satisfactory than silicates. Iron and magnetite have features in the visible and near-infrared which do not agree with the polarization data.
Kim Sang-Hee
Martin Peter G.
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