Thermal Formaldehyde Emission in NGC7538 IRS1
  Spectral lines from formaldehyde (H2CO) molecules at cm wavelengths are
typically detected in absorption and trace a broad range of environments, from
diffuse gas to giant molecular clouds. In contrast, thermal emission of
formaldehyde lines at cm wavelengths is rare. In previous observations with the
100m Robert C. Byrd Green Bank Telescope (GBT), we detected 2 cm formaldehyde
emission toward NGC7538 IRS1 - a high-mass protostellar object in a prominent
star-forming region of our Galaxy. We present further GBT observations of the 2
cm and 1 cm H2CO lines to investigate the nature of the 2 cm H2CO emission. We
conducted observations to constrain the angular size of the 2 cm emission
region based on a East-West and North-South cross-scan map. Gaussian fits of
the spatial distribution in the East-West direction show a deconvolved size (at
half maximum) of the 2 cm emission of 50" +/- 8". The 1 cm H2CO observations
revealed emission superimposed on a weak absorption feature. A non-LTE
radiative transfer analysis shows that the H2CO emission is consistent with
quasi-thermal radiation from dense gas (~10^5 to 10^6 cm^-3). We also report
detection of 4 transitions of CH3OH (12.2, 26.8, 28.3, 28.9 GHz), the (8,8)
transition of NH3 (26.5 GHz), and a cross-scan map of the 13 GHz SO line that
shows extended emission (> 50").
