Microwave and submillimeter spectroscopy and first ISM detection of ¹⁸O-methyl formate^⋆

¹ Centro de Astrobiología (CSIC-INTA). Laboratory of Molecular Astrophysics. Department of Astrophysics. Ctra de Ajalvir, Km 4, 28850 Torrejón de Ardoz, Madrid, Spain
² Laboratoire de Physique des Lasers, Atomes, et Molécules, UMR CNRS 8523, Université de Lille I, 59655 Villeneuve d’ Ascq Cedex, France
e-mail: laurent.margules@univ-lille1.fr
³ Departamento de Física Aplicada, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain
⁴ Institute of Radio Astronomy of NASU, Chervonopraporna Str., 4, 61002 Kharkov, Ukraine
⁵ 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
⁶ Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France
⁷ Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, PO Box 1033, Blindern, 0315 Oslo, Norway

Received: 13 April 2011
Accepted: 3 October 2011

Abstract

Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range 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 transition frequencies that were not measured in the laboratory very precisely.

Aims. We present an experimental study and a theoretical analysis of two ¹⁸O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL.

Methods. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH₃ internal rotation.

Results. We obtained spectroscopic constants of both ¹⁸O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL.