Interstellar extinction by spheroidal dust grains

¹ IUCAA, Post Bag 4, Ganeshkhind, Pune-411007, India e-mail: rag@iucaa.ernet.in
² The Graduate School of Science and Technology, Dept. of Earth & Planetary Sciences, Faculty of Science Kobe University, Nada, Kobe 657-8501, Japan
³ Gujarat College, Ahmedabad-380006, India
⁴ Department of Physics, Assam University Silchar-788011, India

Received: 17 January 2005
Accepted: 21 June 2005

Abstract

Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned non-spherical dust grains which show variation in the interstellar extinction curve for wavelengths ranging from NIR to UV. To model the extinction and polarization, one cannot use the conventional Mie theory which assumes the grains as solid spheres. We have used a T-matrix based method for computing the extinction efficiencies of spheroidal silicate and graphite grains of different shapes (axial ratios) and sizes and used these efficiencies to evaluate the interstellar extinction curve in the wavelength range m. A best fit linear combination of silicate and graphite grains of not very large axial ratio, fits the observed extinction curve reasonably well.
We calculate the volume extinction factor V_c, which is an important parameter from the point of view of the cosmic abundance, for the spheroidal grain models that reproduce the interstellar extinction curve. We find that the shape of the grains does not affect the volume extinction factor.
Finally we have also studied the extinction and linear polarization efficiencies for aligned spheroids. The results show that the shape of grains affects the linear polarization efficiencies considerably for various orientation angles of the spheroids.