A study of the C₃H₂ isomers and isotopologues: first interstellar detection of HDCCC^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
e-mail: spezzano@mpe.mpg.de
² California Institute of Technology, 770 S. Wilson Ave., M/C 100-22, Pasadena, CA 91125, USA
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, and School of Engineering & Applied Sciences, Harvard University, Cambridge, MA 02138, USA
⁵ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 28 September 2015
Accepted: 12 November 2015

Abstract

The partially deuterated linear isomer HDCCC of the ubiquitous cyclic carbene (c-C₃H₂) was observed in the starless cores TMC-1C and L1544 at 96.9 GHz, and a confirming line was observed in TMC-1 at 19.38 GHz. To aid the identification in these narrow line sources, four centimetre-wave rotational transitions (two in the previously reported K_a = 0 ladder and two new ones in the K_a = 1 ladder) and 23 transitions in the millimetre band between 96 and 272 GHz were measured in high-resolution laboratory spectra. Ten spectroscopic constants in a standard asymmetric top Hamiltonian allow the main transitions of astronomical interest in the K_a ≤ 3 rotational ladders to be calculated to within 0.1 km s^-1 in radial velocity up to 400 GHz. Conclusive identification of the two astronomical lines of HDCCC was provided by the V_LSR, which is the same as for the normal isotopic species (H₂CCC) in the three narrow line sources. In these sources, deuterium fractionation in singly substituted H₂CCC (HDCCC/H₂CCC ~4−19%) is comparable to that in c-C₃H₂ (c-C₃H₂/c-C₃HD ~5−17%) and similarly in doubly deuterated c-C₃H₂ (c-C₃D₂/c-C₃HD ~3−17%), implying that the efficiency of the deuteration processes in the H₂CCC and c-C₃H₂ isomers are comparable in dark clouds.