Laboratory characterization and astrophysical detection of vibrationally excited states of vinyl cyanide in Orion-KL^⋆,⋆⋆

¹ Centro de Astrobiología (CSIC-INTA), Departamento de Astrofísica Molecular, Ctra. de Ajalvir Km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: lopezja@cab.inta-csic.es
² Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
⁴ Grupo de Espectroscopia Molecular (GEM), Unidad Asociada CSIC, Edificio Quifima, Laboratorios de Espectroscopia y Bioespectroscopia, Parque Cientfico UVa, Universidad de Valladolid, 47011, Valladolid, Spain
⁵ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6B, UK
⁶ NRAO, 520 Edgemont Road, Charlottesville, VA 22902, USA
⁷ Wright State University, 3640 Colonel Glenn Hwy, Dayton, OH 45435, USA
⁸ Ohio State University, 191 W. Wooddruff Ave., Columbus, OH 43210, USA

Received: 11 February 2014
Accepted: 12 July 2014

Abstract

Context. We perform a laboratory characterization in the 18–1893 GHz range and astronomical detection between 80–280 GHz in Orion-KL with IRAM-30 m of CH₂CHCN (vinyl cyanide) in its ground and vibrationally excited states.

Aims. Our aim is to improve the understanding of rotational spectra of vibrationally excited vinyl cyanide with new laboratory data and analysis. The laboratory results allow searching for these excited state transitions in the Orion-KL line survey. Furthermore, rotational lines of CH₂CHCN contribute to the understanding of the physical and chemical properties of the cloud.

Methods. Laboratory measurements of CH₂CHCN made on several different frequency-modulated spectrometers were combined into a single broadband 50–1900 GHz spectrum and its assignment was confirmed by Stark modulation spectra recorded in the 18–40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of vinyl cyanide detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions.

Results. Detailed characterization of laboratory spectra of CH₂CHCN in nine different excited vibrational states: ν₁₁ = 1, ν₁₅ = 1, ν₁₁ = 2, ν₁₀ = 1 ⇔ (ν₁₁ = 1,ν₁₅ = 1), ν₁₁ = 3/ν₁₅ = 2/ν₁₄ = 1, (ν₁₁ = 1,ν₁₀ = 1) ⇔ (ν₁₁ = 2,ν₁₅ = 1), ν₉ = 1, (ν₁₁ = 1,ν₁₅ = 2) ⇔ (ν₁₀ = 1,ν₁₅ = 1) ⇔ (ν₁₁ = 1,ν₁₄ = 1), and ν₁₁ = 4 are determined, as well as the detection of transitions in the ν₁₁ = 2 and ν₁₁ = 3 states for the first time in Orion-KL and of those in the ν₁₀ = 1 ⇔ (ν₁₁ = 1,ν₁₅ = 1) dyad of states for the first time in space. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature, which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The lowest energy vibrationally excited states of vinyl cyanide, such as ν₁₁ = 1 (at 328.5 K), ν₁₅ = 1 (at 478.6 K), ν₁₁ = 2 (at 657.8 K), the ν₁₀ = 1 ⇔ (ν₁₁ = 1,ν₁₅ = 1) dyad (at 806.4/809.9 K), and ν₁₁ = 3 (at 987.9 K), are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. The vibrational temperatures derived for the ν₁₁ = 1, ν₁₁ = 2, and ν₁₅ = 1 states are 252 ± 76 K, 242 ± 121 K, and 227 ± 68 K, respectively; all of them are close to the mean kinetic temperature of the hot core component (210 K). The total column density of CH₂CHCN in the ground state is (3.0 ± 0.9) × 10¹⁵ cm^-2. We report the detection of methyl isocyanide (CH₃NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH₂CHNC). We also give column density ratios between the cyanide and isocyanide isomers, obtaining a N(CH₃NC)/N(CH₃CN) ratio of 0.002.

Conclusions. Laboratory characterization of many previously unassigned vibrationally excited states of vinyl cyanide ranging from microwave to THz frequencies allowed us to detect these molecular species in Orion-KL. Column density, rotational and vibrational temperatures for CH₂CHCN in their ground and excited states, and the isotopologues have been constrained by means of a sample of more than 1000 lines in this survey.