Implications of Grain Size Distribution and Composition for the Correlation Between Dust Extinction and Emissivity

TL;DR

This study investigates how dust grain size distribution and composition influence the correlation between optical extinction properties and infrared dust emission, revealing that composition and grain size affect observed correlations but do not fully explain them.

Contribution

The paper introduces a detailed modeling approach combining dust size distribution and composition to analyze their effects on extinction-emissivity correlations, highlighting the importance of priors in these relationships.

Findings

Larger grains correlate with higher R_V values.

Composition with more carbonaceous grains leads to higher R_V and lower beta.

Explicit priors are necessary to reproduce observed correlations.

Abstract

We study the effect of variations in dust size distribution and composition on the correlation between the spectral shape of extinction (parameterized by $R_{\textrm{V}}$) and far-infrared dust emissivity (parameterized by the power-law index $\beta$). Starting from the size distribution models proposed by Weingartner & Draine (2001), using the dust absorption and emission properties derived by Laor & Draine (1993) for carbonaceous and silicate grains, and by Li & Draine (2001) for PAH grains, we calculate the extinction and compare it with the reddening vector derived by Schlafly et al. (2016). An optimizer and an MCMC are used to explore the space of available parameters for the size distributions. We find that larger grains are correlated with high $R_{\textrm{V}}$. However, this trend is not enough to explain the emission-extinction correlation observed by Schlafly et al. (2016). For the $R_{\textrm{V}}-\beta$ correlation to arise, we need to impose explicit priors for the carbonaceous and silicate volume priors as functions of $R_{\textrm{V}}$. The results show that a composition with higher ratio of carbonaceous to silicate grains leads to higher $R_{\textrm{V}}$ and lower $\beta$. A relation between $E(\textrm{B}-\textrm{V})/\tau_{353}$ and $R_{\textrm{V}}$ is apparent, with possible consequences for the recalibration of emission-based dust maps as a function of $R_{\textrm{V}}$.