Constraining the Anomalous Microwave Emission Mechanism in the S140 Star
  Forming Region with Spectroscopic Observations Between 4 and 8 GHz at the
  Green Bank Telescope
  Anomalous microwave emission (AME) is a category of Galactic signals that
cannot be explained by synchrotron radiation, thermal dust emission, or
optically thin free-free radiation. Spinning dust is one variety of AME that
could be partially polarized and therefore relevant for ongoing and future
cosmic microwave background polarization studies. The Planck satellite mission
identified candidate AME regions in approximately $1^\circ$ patches that were
found to have spectra generally consistent with spinning dust grain models. The
spectra for one of these regions, G107.2+5.2, was also consistent with
optically thick free-free emission because of a lack of measurements between 2
and 20 GHz. Follow-up observations were needed. Therefore, we used the C-band
receiver (4 to 8 GHz) and the VEGAS spectrometer at the Green Bank Telescope to
constrain the AME mechanism. For the study described in this paper, we produced
three band averaged maps at 4.575, 5.625, and 6.125 GHz and used aperture
photometry to measure the spectral flux density in the region relative to the
background. We found if the spinning dust description is correct, then the
spinning dust signal peaks at $30.9 \pm 1.4$ GHz, and it explains the excess
emission. The morphology and spectrum together suggest the spinning dust grains
are concentrated near S140, which is a star forming region inside our chosen
photometry aperture. If the AME is sourced by optically thick free-free
radiation, then the region would have to contain HII with an emission measure
of $5.27^{+2.5}_{-1.5}\times 10^8$ $\rm{cm^{-6}\,pc}$ and a physical extent of
$1.01^{+0.21}_{-0.20} \times 10^{-2}\,\rm{pc}$. This result suggests the HII
would have to be ultra or hyper compact to remain an AME candidate.
