On the alignment of diamagnetic molecules in interstellar magnetic fields

• Published in: Monthly notices of the Royal Astronomical Society Vol. 467; no. 1; pp. 684 - 693
• Main Author: Papoular, R.
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.05.2017
• Subjects: dust, extinction; ISM: lines and bands; ISM: molecules; astrochemistry; ISM: magnetic fields
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx100

Abstract This paper reports the results of new chemical modelling measurements of the Faraday rotation braking mechanism operating on a diamagnetic molecule in a magnetic field. The time length of the experiment is extended, more relevant variables are measured (rotation, vibration, drift energies; molecule orientation), and more accurately, as a function of time. The polarization of light by the moving molecule is computed. The observed behaviour of the molecule may be understood, and the rotation damping time more accurately deduced by fitting a mathematical model built upon the classical equations of motion in a field. This model, meant to include the essential physics involved in the experiment, with the minimum number of parameters, also allows the chemical modelling experimental results to be extrapolated to other molecular structures, shapes and sizes, and other magnetic fields. For a given particle, the rotation damping time scales like 1/H and is independent of the rotation frequency. As an example, we follow the motion of a rod of homogeneous material, 10−5 cm in length, moving in a field 5 × 10−6 G in intensity. Its angular rotation is found to decrease to 0, while its axis settles perpendicularly to the field within a few years. Molecular vibrations appear as an illustration of the fluctuation–dissipation theorem: they absorb friction heat and, at the same time, are the very cause of this friction.