Spinning dust emission from circumstellar disks and its role in excess microwave emission

TL;DR

This study models microwave emission from spinning PAHs and nanosilicates in protoplanetary disks, showing that such spinning dust can explain observed microwave excesses and offering a new method to probe nanoparticles in circumstellar environments.

Contribution

It provides the first detailed physical modeling of microwave emission from spinning PAHs and nanosilicates in protoplanetary disks, supporting the spinning dust hypothesis for excess microwave emission.

Findings

Spinning dust dominates over thermal emission below 60 GHz.

Spinning dust models successfully fit observed microwave excesses in certain disks.

Spinning nanoparticles can be probed via future radio observations.

Abstract

Electric dipole emission from rapidly spinning polycyclic aromatic hydrocarbons (PAHs) is widely believed as an origin of anomalous microwave emission (AME), but recently it encounters a setback due to the non-correlation of AME with PAH abundance seen in a full-sky analysis. Microwave observations for specific regions with well-constrained PAH features would be crucial to test the spinning dust hypothesis. In this paper, we present physical modeling of microwave emission from spinning PAHs from protoplanetary disks (PPDs) around Herbig Ae/Be stars and T-Tauri stars where PAH features are well observed. Guided by the presence of 10 $\mu$m silicate features in some PPDs, we also model microwave emission from spinning nanosilicates. Thermal emission from big dust grains is computed using the Monte Carlo radiative transfer code. Our numerical results demonstrate that microwave emission from either spinning PAHs or spinning nanosilicates dominates over thermal dust at frequencies $\nu< 60$ GHz, even in the presence of significant grain growth. Finally, we attempt to fit mm-cm observational data with both thermal dust and spinning dust for several disks around Herbig Ae/Be stars that exhibit PAH features and find that spinning dust can successfully reproduce the observed excess microwave emission (EME). Future radio observations with ngVLA, SKA and ALMA Band 1 would be valuable for elucidating the origin of EME and potentially open a new window for probing nanoparticles in circumstellar disks.