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A Study of the C3H2 Isomers and Isotopologues: first Interstellar Detection of HDCCC?

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A Study of the C3H2 Isomers and Isotopologues: first Interstellar Detection of HDCCC? A&A 586, A110 (2016) Astronomy DOI: 10.1051/0004-6361/201527460 & c ESO 2016 Astrophysics A study of the C3H2 isomers and isotopologues: first interstellar detection of HDCCC? S. Spezzano1;3, H. Gupta2;??, S. Brünken3, C. A. Gottlieb4, P. Caselli1, K. M. Menten5, H. S. P. Müller3, L. Bizzocchi1, P. Schilke3, M. C. McCarthy4, and S. Schlemmer3 1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany e-mail: [email protected] 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 Ka = 0 ladder and two new ones in the Ka = 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 −1 in the Ka ≤ 3 rotational ladders to be calculated to within 0.1 km s in radial velocity up to 400 GHz. Conclusive identification of the two astronomical lines of HDCCC was provided by the VLSR, 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. Key words. astrochemistry – line: identification – ISM: molecules – ISM: clouds 1. Introduction about 14 kcal mol−1 (7045 K) higher in energy than the more stable isomer c-C3H2 (Wu et al. 2010). The study of deuterated molecules in the radio band yields con- Propadienylidene (H2CCC) was detected in space by straints on the physical and chemical properties of the interstel- Cernicharo et al.(1991) and Kawaguchi et al.(1991) shortly lar gas in the early stages of low-mass star formation (Caselli after its millimetre-wave rotational spectrum was measured in & Ceccarelli 2012). Cyclopropenylidene (c-C3H2) is one of the the laboratory (Vrtilek et al. 1990). Initially, it was observed to- molecules frequently used in studies of deuterium fractionation wards the cold dark cloud TMC-1 in the centimetre and millime- in cold dark clouds, because lines of its rare isotopic species tre bands, and the carbon-rich asymptotic giant branch (AGB) are relatively intense, the degree of fractionation is high, and star IRC+10216 in the millimetre band. Here, we report the de- c-C3H2 is believed to form solely by gas phase reactions (Gerin tection of a line of HDCCC in the prestellar cores TMC-1C and et al. 1987; Bell et al. 1988; Talbi & Herbst 2001; Turner 2001; L1544 in the millimetre band at 96.9 GHz and a confirming line Liszt et al. 2012; Spezzano et al. 2013). During recent obser- in TMC-1 in the centimetre band at 19.38 GHz. vations of the doubly deuterated species c-C D towards the 3 2 Although H2CCC was first observed nearly 25 years ago in starless cores TMC-1C and L1544, Spezzano et al.(2013) ob- the laboratory and the interstellar gas, the deuterated species had served an unidentified line in the millimetre band that was ten- not been observed in any astronomical source and the rotational tatively assigned to the singly deuterated form of propadienyli- spectrum of the partially deuterated species HDCCC had not 1 dene (H2CCC) , a highly polar metastable carbene (µ = 4:17 D) been measured in the millimetre band prior to this work, but the two lowest transitions in the centimetre band had been measured ? Based on observations carried out with the IRAM 30 m Telescope. earlier (Kim & Yamamoto 2005). In support of the astronomi- IRAM is supported by INSU/CNRS (France), MPG (Germany) and cal identification of HDCCC and of future studies of the C3H2 IGN (Spain). isomeric system in the interstellar gas, the principal millimetre- ?? Current address: Division of Astronomical Sciences National wave rotational transitions of astronomical interest have now Science Foundation, 4201 Wilson Boulevard, Suite 1045, Arlington, been measured in high resolution laboratory spectra. VA 22230, USA. 1 Propadienylidene (Chemical Abstracts Services CAS #: 60731-10-4) and symmetry as the well known molecules formaldehyde (H2CO) is usually designated in the spectroscopic and astronomical literature as and ketene (H2CCO). Although propadienylidene is occasionally des- H2CCC (e.g., Vrtilek et al. 1990; Cernicharo et al. 1991) to distinguish ignated as l-C3H2, here we follow the example of spectroscopists and it from its cyclic isomer c-C3H2, and because it has a same structure astronomers and refer to propadienylidene as H2CCC. Article published by EDP Sciences A110, page 1 of9 A&A 586, A110 (2016) 2. Laboratory and astronomical observations Table 1. Laboratory frequencies of HDCCC. 2.1. Laboratory 0 00 a J 0 0 – J 00 00 Frequency Unc. O−C Ka;Kc Ka ;Kc Rotational lines of HDCCC were observed with the same 3 m (MHz) (kHz) (kHz) long free-space double-pass absorption spectrometer used to 10;1 – 00;0 19 384.5098 2 0:5 measure the millimetre-wave spectrum of c-C3D2 (Spezzano 21;2 – 11;1 38 283.5891 2 −0:6 et al. 2012). Guided by frequencies calculated with an effective 20;2 – 10;1 38 768.0019 2 1:0 rotational and centrifugal distortion constant derived from the 21;1 – 11;0 39 251.4274 2 −0:1 two lowest transitions measured at centimetre wavelengths in a 50;5 – 40;4 96 902.196 20 0:6 − supersonic molecular beam (Kim & Yamamoto 2005), lines of 60;6 – 50;5 116 271.438 20 6:5 70;7 – 60;6 135 634.586 20 −8:5 HDCCC were observed in the millimetre band. Following op- 8 – 7 154 990.661 20 26:6 timization of the normal isotopic species (H CCC), the most 0;8 0;7 2 90;9 – 80;8 174 338.544 20 −13:6 intense lines of HDCCC were observed in a low pressure 100;10 – 90;9 193 677.324 20 −40:1 (∼18 mTorr) DC discharge (140 mA) through a statistical mix- 101;9 – 91;8 196 204.056 24 −15:1 ture of deuterated acetylene (50% HCCD, 25% DCCD, and 25% 121;12 – 111;11 229 614.027 23 55:9 HCCH), carbon monoxide (CO), and argon (Ar) in a molar ratio 122;11 – 112;10 232 512.813 20 −6:9 of 10:5:1, with the walls of the discharge cell cooled to 150 K. 123;10 – 113;9 232 544.540 21 57:5 The acetylene sample was produced in real time by dropping 123;9 – 113;8 232 546.625 21 −34:7 122;10 – 112;9 232 775.855 20 −6:1 an equal mixture of normal (H2O) and heavy water (D2O) on 121;11 – 111;10 235 415.394 19 22:5 calcium carbide (CaC2). Under these conditions, the signal-to- − ≥ 131;13 – 121;12 248 731.752 19 24:1 noise ratio (S/N) of lines of HDCCC near 160 GHz was 10 in 13 – 12 251 629.269 20 32:3 15 min of integration. In addition, the two lowest K = 0 lines in 0;13 0;12 a 132;12 – 122;11 251 875.722 20 −15:2 the centimetre band at 19 and 38 GHz reported earlier by Kim 133;10 – 123;9 251 927.366 33 −42:7 & Yamamoto(2005) were remeasured with the FT microwave 131;12 – 121;11 255 015.024 20 −28:7 spectrometer described in Spezzano et al.(2012), and the two 141;14 – 131;13 267 845.608 37 −56:7 Ka = 1 transitions (21;2−11;1 and 21;1−11;0) were also measured. 142;13 – 132;12 271 235.509 12 8:1 In all, 27 rotational lines between 19 and 272 GHz with 143;12 – 133;11 271 303.986 41 41:2 − J ≤ 14 and K ≤ 3 (Table 1) are reproduced to an rms uncer- 143;11 – 133;10 271 308.664 28 29:1 a 14 – 13 271 653.245 20 13:6 tainty (28 kHz) that is comparable to the measurement uncer- 2;12 2;11 tainties with ten spectroscopic constants in Watson’s S reduced Notes. (a) Calculated with the spectroscopic constants in Table2. Hamiltonian: three rotational constants, five fourth-order distor- tion constants (DK was constrained to a theoretical value owing to the high correlation with A), and two sixth-order distortion Table 2. Spectroscopic constants of HDCCC (in MHz). constants (Table2). There is no evidence of deuterium hyperfine structure (hfs) Constanta This workb Expectedc when the fundamental transition of HDCCC is observed with high S/N at a resolution of 1 kHz in our molecular beam; there- A 199 370.(233) 199 755 fore, no such structure should be present when this transition is B 9 934.225 82(81) 9 936.8 observed in space.
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