On the dissipation of the rotation energy of dust grains in interstellar magnetic fields

TL;DR

This paper introduces a new mechanism for the dissipation of rotational energy in dust grains within magnetic fields, involving Lorentz force perturbations leading to vibrational IR radiation, with implications for molecular alignment.

Contribution

It presents a novel physical mechanism for rotational energy dissipation in interstellar molecules, supported by semi-empirical modeling and visualization of energy transfer processes.

Findings

Energy dissipation rate scales with field strength and grain size.

Vibrational IR radiation efficiently removes rotational energy.

Mechanism applies to both diamagnetic and paramagnetic molecules.

Abstract

A new mechanism is described, analyzed and visualized, for the dissipation of suprathermal rotation energy of molecules in magnetic fields, a necessary condition for their alignment. It relies upon the Lorentz force perturbing the motion of every atom of the structure, as each is known to carry its own net electric charge because of spatial fluctuations in electron density. If the molecule is large enough that the frequency of its lowest-frequency phonon lies near or below the rotation frequency, then the rotation couples with the molecular normal modes and energy flows from the former to the latter. The rate of this exchange is very fast, and the vibrational energy is radiated away in the IR at a still faster rate, which completes the removal of rotation energy. The energy decay rate scales like the field intensity, the initial angular velocity, the number of atoms in the grain and the inverse of the moment of inertia. It does not depend on the susceptibility. Here, the focus is on carbon-rich molecules which are diamagnetic. The same process must occur if the molecule is paramagnetic or bathes in an electric field instead. A semi-empirical method of chemical modeling was used extensively to illustrate and quantify these concepts as applied to a hydrocarbon molecule. The motion of a rotating molecule in a field was monitored in time so as to reveal the energy transfer and visualize the evolution of its orientation towards the stable configuration.