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 (HCOOCH 3 ) 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 HCOOCH 3 -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 lowlying 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 (∼ 10 4 cm −3 ) and extended region surrounding Sgr B2(N). The derived column density of ∼ 4 × 10 14 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.Recently, rotational cross sections for methyl formate colliding with He atoms were computed (Faure et al. 2011). To date, these are the first available cross sections for an asymmetric top with eight atoms. In the present work, the corresponding rate coefficients are deduced in the temperature range 5-30 K. Encouraged by the availability of these rate coefficients, we have i) searched for all centimetre wave transitions of methyl formate toward the Sgr B2(N) region and ii) performed non-LTE radiative transfer calculations.All observational data are taken as part of the PRebiotic Interstellar MOlecular Survey (PRIMOS). The rate coefficient calculations are presented in Section 2. In Section 3, the PRIMOS observations are detailed. We compare non-LTE calculations with PRIMOS data in Section 4. Conclusions are drawn in Section 5. Rate coefficient calculationsQuantum scattering calculations for helium interacting with methyl formate were performed recently on the high-accuracy potential energy surface (PES) of Faure et al.(2011), where full details can be found. Helium was employed as a substitute for H 2 , which is five times more abundant than He. Calculations with He are, however, much less expensive and, in first approximation, cross sections with H 2 can be assumed to be equal to those for collisions with helium, as discussed below. The interaction energies were computed at the coupled cluster method with single, double, and noniterative triple excitations, CCSD(T).Symmetry-adapted perturbation theory based on the density-functional description of monomers, SAPT(DFT), was employed to compute the asymptotic, long-range part of the PES. Me...

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