Composite Interstellar Grains

TL;DR

This study models composite interstellar grains with silicate and graphite components, calculating their extinction and polarization properties to better match observed interstellar extinction curves and estimate cosmic abundances.

Contribution

It introduces a detailed composite grain model using DDA to compute extinction and polarization, providing improved estimates of cosmic element abundances.

Findings

Composite grains reproduce observed extinction curves.

Estimated abundances are lower than bare grain models.

Polarization peaks at specific wavelengths.

Abstract

The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA). The interstellar extinction curve is evaluated in the spectral region 3.40--0.10$\mu m$ using the extinction efficiencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve. We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and find the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than that are predicted by the bare silicate/graphite grain models but these values are still higher than that are implied from the recent ISM values.