OH maser emission in the THOR survey of the northern Milky Way
  Context: OH masers trace diverse physical processes, from the expanding
envelopes around evolved stars to star-forming regions or supernovae remnants.
Aims: We identify the ground-state OH masers at 18cm wavelength in the area
covered by ``The HI/OH/Recombination line survey of the Milky Way (THOR)''. We
present a catalogue of all OH maser features and their possible associated
environments. Methods: The THOR survey covers longitude and latitude ranges of
14.3<l<66.8 and b<1.25 deg. All OH ground state lines at 1612, 1665, 1667 and
1720MHz have been observed, employing the Very Large Array. The spatial
resolution of the data varies between 12.5'' and 19'', the spectral resolution
is 1.5km/s, and the rms sensitivity of the data is ~10mJy/beam per channel.
Results: We identify 1585 individual maser spots distributed over 807 maser
sites. Based on different criteria from spectral profiles to literature
comparison, we try to associate the maser sites with astrophysical source
types. Approximately 51\% of the sites exhibit the double-horned 1612MHz
spectra typically emitted from the expanding shells of evolved stars. The
separations of the two main velocity features of the expanding shells typically
vary between 22 and 38km/s. In addition to this, at least 20% of the maser
sites are associated with star-forming regions. While the largest fraction of
1720MHz maser spots (21 out of 53) is associated with supernova remnants, a
significant fraction of the 1720MHz maser spots (17) are also associated with
star-forming regions. We present comparisons to the thermal 13CO(1-0) emission
as well as to other surveys of class II CH3OH and H2O maser emission. The
catalogue attempts to present associations to astrophysical sources where
available, and the full catalogue is available in electronic form.
