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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 (~104 cm-3), and extended region surrounding Sgr B2(N). The derived column density of ~4 × 1014 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 centimeter wavelengths.
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https://hal-insu.archives-ouvertes.fr/insu-03618596
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Submitted on : Thursday, March 24, 2022 - 2:11:41 PM
Last modification on : Friday, March 25, 2022 - 4:02:56 AM

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A. Faure, A. J. Remijan, K. Szalewicz, L. Wiesenfeld. Weak Maser Emission of Methyl Formate toward Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey. The Astrophysical Journal, 2014, 783, ⟨10.1088/0004-637X/783/2/72⟩. ⟨insu-03618596⟩

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