2003/12/11 by William D. Watson, W. D. Watson, D. S. Wiebe +3 · 1 citation
Physics and Astronomy · #Astrophysics and Star Formation Studies #Solar and Space Plasma Dynamics #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/382022
published as Astrophys.J. 604 (2004) 707-716 · accepted for Astrophysical Journal
arxiv created 2003/12/11 · openalex publication_date 2004/03/26 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We consider astrophysical maser radiation that is created in the presence of mildly supersonic, magnetohydrodynamic (MHD) turbulence. The focus is on the OH masers for which the magnetic field is strong enough that the separations of the Zeeman components are greater than the spectral line breadths. A long-standing puzzle has been the absence of the Zeeman π-components and the high circular polarization in the observed spectra of these masers. We first argue that the elongation of eddies along the field that has recently been recognized in MHD turbulence will enhance the optical depth parallel to the magnetic field in comparison with that perpendicular to the magnetic field. We then simulate maser emission with a numerical model of MHD turbulence to demonstrate quantitatively how the intensities of the linearly polarized π-components are suppressed and the intensities of the nearly circularly polarized σ-components are enhanced. This effect is also generic in the sense that most spectral lines in MHD turbulence with Mach number M ~ 1 should have larger optical depth parallel to the magnetic field than perpendicular. The effect is reduced considerably when M < 1. The simulations also demonstrate that the velocity and magnetic field variations due to the turbulence can (but do not necessarily) cause one of the σ-components to be much more intense than the other, as is often observed for mainline OH masers.