The bolocam galactic plane survey. III. Characterizing physical properties of massive star-forming regions in the Gemini OB1 molecular cloud

We present the 1.1 mm Bolocam Galactic Plane Survey (BGPS) observations of the Gemini OB1 molecular cloud complex, and targeted NH₃ observations of the BGPS sources. When paired with molecular spectroscopy of a dense gas tracer, millimeter observations yield physical properties such as masses, radii, mean densities, kinetic temperatures, and line widths. We detect 34 distinct BGPS sources above 5σ = 0.37 Jy beam^-1 with corresponding 5σ detections in the NH₃(1,1) transition. Eight of the objects show water maser emission (20%). We find a mean millimeter source FWHM of 1.12 pc and a mean gas kinetic temperature of 20K for the sample of 34 BGPS sources with detections in the NH₃(1,1) line. The observed NH₃ line widths are dominated by non-thermal motions, typically found to be a few times the thermal sound speed expected for the derived kinetic temperature. We calculate the mass for each source from the millimeter flux assuming the sources are isothermal and find a mean isothermal mass within a 120″ aperture of 230±180 M _·. We find a total mass of 8400 M _· for all BGPS sources in the Gemini OB1 molecular cloud, representing 6.5% of the cloud mass. By comparing the millimeter isothermal mass to the virial mass calculated from the NH₃ line widths within a radius equal to the millimeter source size, we find a mean virial parameter (M _vir/M _iso) of 1.0±0.9 for the sample. We find mean values for the distributions of column densities of 1.0 × 10²² cm^-2 for H₂, and 3.0 × 10¹⁴ cm ^-2 for NH₃, giving a mean NH₃ abundance of 3.0 × 10^-8 relative to H₂. We find volume-averaged densities on the order of 10³-10⁴cm^-3. The sizes and densities suggest that in the Gem OB1 region the BGPS is detecting the clumps from which stellar clusters form, rather than smaller, higher density cores where single stars or small multiple systems form.