2013ApJS..207...12R

We report the results of a survey in the CH₃ CN J = 12 ⟶ 11 transition toward a sample of massive proto-stellar candidates. The observations were carried out with the 10 m Submillimeter Telescope on Mount Graham, AZ. We detected this molecular line in 9 out of 21 observed sources. In six cases this is the first detection of this transition. We also obtained full beam sampled cross-scans for five sources which show that the lower K-components can be extended on the arcminute angular scale. The higher K-components, however, are always found to be compact with respect to our 36'' beam. A Boltzmann population diagram analysis of the central spectra indicates CH₃CN column densities of about 10¹⁴/cm2, and rotational temperatures above 50 K, which confirms these sources as hot molecular cores. Independent fits to line velocity and width for the individual K-components resulted in the detection of an increasing blueshift with increasing line excitation for four sources. Comparison with mid-infrared (mid-IR) images from the SPITZER GLIMPSE/IRAC archive for six sources show that the CH₃ CN emission is generally coincident with a bright mid-IR source. Our data clearly show that the CH₃CN J = 12 ⟶ 11 transition is a good probe of the hot molecular gas near massive protostars, and provide the basis for future interferometric studies.