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

Abstract

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.

33 pages, 9 figures, accepted in The Astrophysical Journal (january 4 2014)

References in corpus (5)

• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)
• Detection of E-cyanomethanimine towards Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey
• Rotational Excitation of HC_3N by H_2 and He at low temperatures
• Non-Thermal Continuum toward SGRB2(N-LMH)

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