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