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

Weak maser emission of methyl formate toward Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey