# Modeling rotational disruption of grains and microwave emission from spinning dust in AGB envelopes

## TL;DR

This study models microwave emission from rapidly spinning nanoparticles in AGB star envelopes to explain observed anomalous microwave emission, highlighting the role of rotational disruption in nanoparticle size distribution and emission.

## Contribution

It introduces a detailed model of spinning dust emission considering rotational disruption effects specific to AGB envelopes, aligning with observed AME in six stars.

## Key findings

- Spinning dust emission dominates below 100 GHz in AGB envelopes.
- Model successfully reproduces observed AME in six AGB stars.
- Potential for future observations with ngVLA and ALMA to distinguish dust carriers.

## Abstract

Radio observations of some Asymptotic Giant Branch (AGB) star envelopes show the excess emission at frequencies below 100 GHz which cannot be explained by thermal dust emission (hereafter anomalous microwave emission-AME). Moreover, AGB envelopes are a common place where gas molecules condense to form nanoparticles (e.g., polycyclic aromatic hydrocarbons) and large grains. In this paper, we will study whether electric dipole emission from rapidly spinning nanoparticles can reproduce the AME observed toward AGB stars. To properly model the size distribution of nanoparticles in the AGB envelope, we take into account both the increase of nanoparticles due to rotational disruption of large grains spun-up by radiative torques and the decrease of smallest nanoparticles due to rotational disruption driven by stochastic gas collisions. We then perform detailed modeling of microwave emission from rapidly spinning nanoparticles from both C-rich and O-rich AGB envelopes using the grain size distribution constrained by rotational disruption. We find that spinning dust emission is dominant over thermal dust emission at frequencies below 100 GHz. We attempt to fit the observational data of AME using our spinning dust model and demonstrate that spinning dust can reproduce the observed AME in six AGB stars. Finally, we discuss that microwave emission from spinning dust in AGB envelopes could be observed with high-resolution upcoming radio telescopes such as ngVLA and ALMA Band 1. This would be a major leap for understanding AGB envelopes, formation, evolution, and internal structures of dust. Observations would help to distinguish the carrier of AME from comparing C-rich to O-rich stars, because PAHs are formed in C-rich AGB stars while silicates are formed in O-rich stars.