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

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: 11 = 1, 15 = 1, 11 = 2, 10 = 1 ⇔ (11 = 1,15 = 1), 11 = 3/15 = 2/14 = 1, (11 = 1,10 = 1) ⇔ (11 = 2,15 = 1), 9 = 1, (11 = 1,15 = 2) ⇔ (10 = 1,15 = 1) ⇔ (11 = 1,14 = 1), and 11 = 4 are determined, as well as the detection of transitions in the 11 = 2a nd 11 = 3 states for the first time in Orion-KL and of those in the 10 = 1 ⇔ (11 = 1,15 = 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 11 = 1 (at 328.5 K), 15 = 1 (at 478.6 K), 11 = 2 (at 657.8 K), the 10 = 1 ⇔ (11 = 1,15 = 1) dyad (at 806.4/809.9 K), and 11 = 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 11 = 1, 11 = 2, and 15 = 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) × 10 15 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.