Abstract

Abstract In a previous work, Hoang and Tram discovered a new mechanism for destruction of nanoparticles due to suprathermal rotation of grains in stationary C-shocks, which is termed rotational disruption. In this paper, we extend our previous study for nonstationary shocks driven by outflows and young supernova remnants that have dynamical ages shorter than the time required to establish a stationary C-shock, which is composed of a C-shock and a J-shock tail (referred to as CJ-shock). For the C-shock component, we find that the smallest nanoparticles (size ≲1 nm) of weak materials (i.e., tensile strength S max ≲ 109 erg cm−3) can be rotationally disrupted owing to suprathermal rotation induced by supersonic neutral drift. For the J-shock component, although nanoparticles are rotating thermally, the smallest ones can still be disrupted because the gas is heated to higher temperatures by J-shocks. We then model microwave emission from rapidly spinning nanoparticles where the grain size distribution has the lower cutoff determined by rotational disruption for the different shock models. We also calculate the spectral flux of microwave emission from a shocked region at a distance of 100 pc from the observer for the different gas density, shock age, and shock velocities. We suggest that microwave emission from spinning dust can be used to trace nanoparticles and shock velocities in dense molecular outflows. Finally, we discuss a new way that can release molecules from the nanoparticle surface into the gas in the shocked regions, which we name rotational desorption.

Highlights

  • We extend our previous study for non-stationary shocks driven by outflows and young supernovae remnants that have dynamical ages shorter than the time required to establish a stationary C-shock, which is composed of a C-shock and a J-shock tail

  • For the C-shock component, we find that smallest nanoparticles of weak materials can be rotationally disrupted due to suprathermal rotation induced by supersonic neutral drift

  • In the case of shocks driven by outflows from (YSOs) and young supernova remnants (SNRs), shocks cannot reach the steady stage because the required timescale is longer than the dynamical age of outflows and SNRs

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Summary

INTRODUCTION

Very small dust grains, whose size is below 10 nm (hereafter referred to as nanoparticles), including Polycyclic Aromatic Hydrocarbons (PAHs), play an important role in the evolution of the interstellar medium (ISM). Weingartner & Draine (2001) and Akimkin et al (2013) demonstrated that nanoparticles can control the heating process of gas, while other studies (e.g., Mestel & Spitzer 1956, Zhao et al 2016) found that nanoparticles can influence the dynamics of molecular clouds and star formation due to their dominant charge carrier in dense and low ionization molecular clouds. Most of observations show the lack of strong PAH emission features in supernova remnants (Smith et al 2009) and outflows of massive young stellar objects (YSOs) (Smith et al 2006) where shocks are present It suggests that PAHs/nanoparticles are perhaps efficiently destroyed in the shocked regions. Smallest nanoparticles (a 1 nm) can be disrupted into tiny fragments when the centrifugal stress induced by grain rotation exceeds the maximum tensile strength of the grain material This mechanism is found to be the most efficient in destroying nanoparticles in C-shocks compared to previously known mechanisms such as thermal sputtering and grain shattering (see Hoang & Tram 2019 for details).

STRUCTURES OF NON-STATIONARY SHOCKS
CJ-Shock structures and physical parameters
Drifting velocities in C-shock component
Rotational temperature and rate
Dynamic and disruption timescales
Grain disruption size
SPINNING DUST EMISSION FROM NANOPARTICLES IN NON-STATIONARY
Emission spectrum
Emission spectral flux
DISCUSSION
Implications for mid-IR emission from shock regions
Constraining the shock velocity in dense regions with spinning dust
Tracing nanoparticles in shocks with spinning dust
Implication for grain chemistry: rotational desorption
Effects of charge fluctuations on grain rotation in CJ-shocks
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