Dust Rotational Dynamics in C-shocks: Rotational Disruption of Nanoparticles by Stochastic Mechanical Torques and Spinning Dust Emission

TL;DR

This paper introduces a new rotational disruption mechanism for nanoparticles in C-shocks caused by stochastic torques, affecting their survival, size distribution, and microwave emission, with implications for astrophysical observations.

Contribution

It proposes a novel rotational disruption process for nanoparticles in C-shocks and models its impact on dust emission and size distribution.

Findings

Nanoparticles can be spun-up to destructive speeds by supersonic neutral drift.

Rotational disruption surpasses thermal sputtering in controlling nanoparticle sizes.

Microwave emission from spinning nanoparticles increases with shock velocity.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and nanoparticles are expected to play an important role in many astrophysical processes due to its dominant surface area, including gas heating, chemistry, star formation , and anomalous microwave emission. In dense magnetized molecular clouds where C-shocks are present, PAHs and nanoparticles are widely believed to originate from grain shattering due to grain-grain collisions. The remaining question is whether these nanoparticles can survive in the dense and hot shocked regions, and how to constrain their size and abundance with observations. In this paper, we present a new mechanism to destroy nanoparticles in C-shocks based on centrifugal stress within rapidly spinning nanoparticles spun-up by stochastic atomic bombardment, which is termed rotational disruption. We find that, due to supersonic neutral gas-charged grain drift in C-shocks, nanoparticles can be spun-up to suprathermal rotation by stochastic torques exerted by supersonic neutral flow. The resulting centrifugal stress within suprathermally rotating nanoparticles can exceed the maximum tensile strength of grain material ($S_{\max}$), resulting in rapid disruption of nanoparticles smaller than $a\sim 1$ nm for $S_{\max}\sim 10^{9}\erg\cm^{-3}$. The proposed disruption mechanism is shown to be more efficient than thermal sputtering in controlling the lower cutoff of grain size distribution in C-shocks. We model microwave emission from spinning nanoparticles in C-shocks subject to supersonic neutral drift and rotational disruption. We find that suprathermally rotating nanoparticles can emit strong microwave radiation, and both peak flux and peak frequency increase with increasing the shock velocity. We suggest spinning dust as a new method to constrain nanoparticles and trace shock velocities in dense, shocked regions.