Flares and Proper Motions of Ground-State OH Masers in W75N

TL;DR

This study investigates the variability, proper motions, and magnetic fields of ground-state OH masers in W75N, revealing flaring activity, associations with outflows and dense regions, and consistency with theoretical models over a 7-year period.

Contribution

It provides new high-resolution observations of OH masers in W75N, demonstrating their flaring behavior, proper motions, and magnetic field variations, and compares these with theoretical predictions.

Findings

1720-MHz masers are associated with dense outflows near VLA 1.

Flaring masers exceed 400 Jy, indicating intense activity.

Maser proper motions are median 3.5 km/s, consistent with turbulence.

Abstract

The star-forming region W75N hosts bright OH masers that are observed to be variable. We present observations taken in 2008 of the ground-state OH maser transitions with the Very Long Baseline Array (VLBA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN) and with the Nancay Radio Telescope in 2011. Several of the masers in W75N were observed to be flaring, with the brightest 1720-MHz maser in excess of 400 Jy. The 1720-MHz masers appear to be associated with the continuum source VLA 1, unlike the bright flaring 1665- and 1667-MHz masers, which are associated with VLA 2. The 1720-MHz masers are located in an outflow traced by water masers and are indicative of very dense molecular material near the H II region. The magnetic field strengths are larger in the 1720-MHz maser region than in most regions hosting only main-line OH masers. The density falls off along the outflow, and the order of appearance of different transitions of OH masers is consistent with theoretical models. The 1665- and 1667-MHz VLBA data are compared against previous epochs over a time baseline of over 7 years. The median maser motion is 3.5 km/s, with a scatter that is comparable to thermal turbulence. The general pattern of maser proper motions observed in the 1665- and 1667-MHz transitions is consistent with previous observations.