Modeling Rotational Disruption of Grains and Microwave Emission from Spinning Dust in AGB Envelopes

Le Ngoc Tram,Thiem Hoang,Archana Soam,William T Reach,Pierre Lesaffre

doi:10.3847/1538-4357/ab7b5e

Abstract

Abstract Radio observations of some asymptotic giant branch (AGB) star envelopes show excess emission at frequencies below 100 GHz that 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 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 the 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 how microwave emission from spinning dust in AGB envelopes could be observed with high-resolution upcoming radio telescopes such the Next Generation Very Large Array and Atacama Large Millimeter/submillimeter Array 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 via comparisons of C-rich and O-rich stars, because polycyclic aromatic hydrocarbons (PAHs) are formed in C-rich AGB stars, while silicates are formed in O-rich stars.

Highlights

Late in their evolution, low- and intermediate-mass stars (1–8 Me) reach the asymptotic giant branch (AGB) phase, before they become white dwarfs
Note that the overall purpose of this work is to demonstrate that spinning dust is one possible explanation for the anomalous microwave emission (AME) detections in AGB envelopes, which would allow us to use our model calculated for IRC +10216 and IK Tau to fit the observational data of two C-rich and O-rich AGB stars, respectively
We have studied rotational disruption of dust grains by radiative and mechanical torques in the AGB envelopes, performed detailed modeling of microwave emission from rapidly spinning nanoparticles, and applied the models to explain the observed excess microwave emission

Summary

Introduction

Low- and intermediate-mass stars (1–8 Me) reach the asymptotic giant branch (AGB) phase, before they become white dwarfs. CSEs of AGB stars can be considered the most significant chemical laboratories in the universe because their effective temperatures are usually low (Tå ; 2000 K–3500 K), and the mass-loss timescale is long, so molecules can form in the envelope through chemical and physical processes (e.g., Cernicharo et al 2000; Tenenbaum et al 2010)

Objectives

Discussion

Conclusion