Weak maser emission of methyl formate toward Sagittarius B2(N) in the
  Green Bank Telescope PRIMOS Survey
  A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3)
rotational lines is presented for the first time using a set of theoretical
collisional rate coefficients. These coefficients have been computed in the
temperature range 5-30 K by combining coupled-channel scattering calculations
with a high accuracy potential energy surface for HCOOCH3-He. The results are
compared to observations toward the Sagittarius B2(N) molecular cloud using the
publicly available PRIMOS survey from the Green Bank Telescope. A total of 49
low-lying transitions of methyl formate, with upper levels below 25 K, are
identified. These lines are found to probe a presumably cold (~30 K),
moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The
derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the
column density of the trans conformer in the same source. Provided that the two
conformers have the same spatial distribution, this result suggests that
strongly non-equilibrium processes must be involved in their synthesis.
Finally, our calculations show that all detected emission lines with a
frequency below 30 GHz are (collisionally pumped) weak masers amplifying the
continuum of Sgr B2(N). This result demonstrates the importance and generality
of non-LTE effects in the rotational spectra of complex organic molecules at
centimetre wavelengths.
