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Description
OH masers trace diverse physical processes, from the expanding envelopes around evolved stars to star-forming regions or supernovae remnants. Providing a survey of the ground-state OH maser transitions in the northern hemisphere inner Milky Way facilitates the study of a broad range of scientific topics. We want to identify the ground-state OH masers at 18 cm wavelength in the area covered by "The HI/OH/Recombination line survey of the Milky Way (THOR)". We will present a catalogue of all OH maser features and their possible associated environments. The THOR survey covers longitude and latitude ranges of 14.3{deg}<l<66.8{deg} and b<+/-1.25{deg}. All OH ground state lines ^2^{PI}_3/2_(J=3/2) at 1612 (F=1-2), 1665 (F=1-1), 1667 (F=2-2) and 1720MHz (F=2-1) have been observed, employing the Very Large Array (VLA) in its C configuration. 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. We identify 1585 individual maser spots (corresponding to single spectral features) distributed over 807 maser sites (regions of size ~10^3^-10^4^AU). 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 ^13^CO(1-0) emission as well as to other surveys of class II CH_3_OH and H_2_O maser emission. The catalogue attempts to present associations to astrophysical sources where available, and the full catalogue is available in electronic form. This OH maser catalogue presents a unique resource of stellar and interstellar masers in the northern hemisphere. It provides the basis for a diverse range of follow-up studies from envelopes around evolved stars to star-forming regions and Supernova remnants.
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