Collisional Pumping of OH Masers near Supernova Remnants

TL;DR

This paper investigates the conditions under which OH masers are collisionally pumped near supernova remnants, focusing on shock parameters that influence maser emission in specific OH lines.

Contribution

It provides a detailed analysis of how shock speed, gas density, ionization rate, and magnetic field affect OH maser emission, highlighting the conditions for maximum optical depth.

Findings

Maximum optical depth in 1720 MHz line occurs at high ionization rates and specific densities.

Level population inversion is predicted for 6049 and 4765 MHz transitions.

No maser emission detected in these excited lines due to small optical depths.

Abstract

The collisional pumping of OH masers in non-dissociative C-type shocks near supernova remnants is considered. The emergence of maser emission in OH lines is investigated for various shock parameters - the shock speed, the preshock gas density, the cosmic-ray ionization rate, and the magnetic field strength. The largest optical depth in the 1720 MHz line is reached at high gas ionization rates $\zeta \geq 10^{-15}$ s$^{-1}$, an initial density $n_{\rm H,0} \leq 2 \times 10^4$ cm$^{-3}$, and a shock speed $u_{\rm s} \geq 20$ km s$^{-1}$. According to our calculations, there is also a level population inversion for the 6049 and 4765 MHz transitions of excited OH rotational states. However, the optical depth in these lines is small for all of the investigated shock parameters, which explains the non-detection of maser emission in these lines in supernova remnants.