THz spectroscopy and first ISM detection of excited torsional states of ¹³C-methyl formate^⋆

¹ Laboratoire de Physique des Lasers, Atomes, et Molécules, UMR CNRS 8523, Université Lille I, 59655 Villeneuve d’Ascq Cedex France
e-mail: laurent.margules@univ-lille1.fr
² Departamento de Física Aplicada Unidad Asociada CSIC, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain
³ Centro de Astrobiología (CSIC-INTA), Department of Astrophysics. Ctra de Ajalvir, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS/IPSL 7583, Université Paris 7 et Université Paris Est, 61 Av. Charles de Gaulle, 94010 Créteil Cedex, France
⁵ Institut des Sciences Chimique de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France

Received: 29 October 2013
Accepted: 24 April 2014

Abstract

Context. An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory.

Aims. We present the experimental study and its theoretical analysis for two ¹³C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130 cm^-1 (200 K) in Orion-KL.

Methods. New spectra of HCOO¹³CH₃ (¹³C₂) methyl formate were recorded with the mm- and submm-wave spectrometer in Lille from 50 to 940 GHz. A global fit for v_t = 0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy.

Results. We analysed 5728 and 2881 new lines for v_t = 0 and 1 for HCOO¹³CH₃. These new lines were globally fitted with 846 previously published lines for v_t = 0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V₃ = 369.93168(395) cm^-1) was improved. We report the detection of the first excited torsional states (v_t = 1) in Orion-KL for the ¹³C₂ and ¹³C₁ methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species.

Conclusions. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of ¹³C-methyl formate isotopologues in Orion-KL in the 80−280 GHz frequency range, without missing lines.