Effects of grain growth on the interstellar polarization curve

TL;DR

This study models how grain growth processes in interstellar space influence the wavelength dependence of polarization, revealing that coagulation can significantly alter polarization curve parameters over tens of millions of years.

Contribution

It introduces a tuned model removing the silicate coagulation threshold, demonstrating how grain growth affects polarization curve parameters and explaining observed $K$ and $mbda_{ m max}$ trends.

Findings

Coagulation shifts $mbda_{ m max}$ to longer wavelengths.

Wider polarization curves develop over 30-50 Myr.

Model can reproduce narrow polarization curves with particle shape variations.

Abstract

We apply the time evolution of grain size distributions by accretion and coagulation found in our previous work to the modelling of the wavelength dependence of interstellar linear polarization. We especially focus on the parameters of the Serkowski curve $K$ and $\lambda_{\max}$ characterizing the width and the maximum wavelength of this curve, respectively. We use aligned silicate and non-aligned carbonaceous spheroidal particles with different aspect ratios $a/b$. The imperfect alignment of grains with sizes larger than a cut-off size $r_{V,\rm cut}$ is considered. We find that the evolutionary effects on the polarization curve are negligible in the original model with commonly used material parameters (hydrogen number density $n_\mathrm{H}=10^3$ cm$^{-3}$, gas temperature $T_\mathrm{gas}=10$~K, and the sticking probability for accretion $S_\mathrm{acc}=0.3$). Therefore, we apply the tuned model where the coagulation threshold of silicate is removed. In this model, $\lambda_{\max}$ displaces to the longer wavelengths and the polarization curve becomes wider ($K$ reduces) on time-scales $\sim (30 - 50) (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr. The tuned models at $T < 30 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1} $ Myr and different values of the parameters $r_{V,\rm cut}$ can also explain the observed trend between $K$ and $\lambda_{\max}$. It is significant that the evolutionary effect appears in the perpendicular direction to the effect of $r_{V,\rm cut}$ on the $K$ - $\lambda_{\max}$ diagram. Very narrow polarization curves can be reproduced if we change the type of particles (prolate/oblate) and/or vary $a/b$.