The MUSTANG Galactic Plane Survey (MGPS90) Pilot

We report the results of a pilot program for a Green Bank Telescope MUSTANG-2 Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a typical 1σ depth of 1-2 mJy beam^-1 with a 9″ beam. We describe the survey parameters, quality assessment process, cataloging, and comparison with other data sets. We have identified 709 sources over seven observed fields selecting some of the most prominent millimeter-bright regions between 0 deg < ℓ < 50 deg (total area ≈7.5 deg ²). The majority of these sources have counterparts at other wavelengths. By applying flux selection criteria to these sources, we successfully recovered several known hypercompact H II (HCH II) regions but did not confirm any new ones. We identify 126 sources that have mm-wavelength counterparts but do not have cm-wavelength counterparts and are therefore candidate HCH II regions; of these, 10 are morphologically compact and are strong candidates for new HCH II regions. Given the limited number of candidates in the extended area in this survey compared to the relatively large numbers seen in protoclusters W51 and W49, it appears that most HCH II regions exist within dense protoclusters. Comparing the counts of HCH II to ultracompact H II (UCH II) regions, we infer the HCH II region lifetime is 16%-46% that of the UCH II region lifetime. We additionally separated the 3 mm emission into dust and free-free emission by comparing with archival 870 $\mu {\rm{m}}$ and 20 cm data. In the selected pilot fields, most (≳80%) of the 3 mm emission comes from plasma, either through free-free or synchrotron emission.