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A&A
Volume 663, July 2022
Article Number L2
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202244063
Published online 01 July 2022

© C. Cabezas et al. 2022

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe-to-Open model. Subscribe to A&A to support open access publication.

1. Introduction

Carbon can be considered the most versatile element for building molecules in the interstellar medium (ISM). In fact, about 80% of the nearly 260 molecules detected to date in space (CDMS1, Müller et al. 2005) contain at least one carbon atom, and one-fourth are hydrocarbons. The chemistry of hydrocarbons is dominated to a large extent by highly unsaturated (low H/C ratios) carbon chain molecules such as the CnH family. The hydrocarbon radicals CnH detected in space range from the methylidene radical, CH (Dunham 1937), to C8H (Cernicharo & Guélin 1996; Bell et al. 1999). The smaller ones, such as C2H, C3H, and C4H, are observed in star-forming regions (Tucker et al. 1974), photon-dominated regions (Teyssier et al. 2004), cold dark clouds (Wootten et al. 1980; Irvine et al. 1981; Thaddeus et al. 1985), translucent molecular clouds (Turner et al. 2000), circumstellar envelopes (Guélin et al. 1987; Pardo & Cernicharo 2007), and the diffuse medium (Bell et al. 1983; Nyman 1984). The larger ones, C5H, C6H, C7H, and C8H, are mainly observed in cold dense molecular clouds (Cernicharo et al. 1987; Saito et al. 1987; Bell et al. 1999; Araki et al. 2017) and in the expanding envelope of the carbon-rich star IRC+10216 (Cernicharo et al. 1986a, 1986b; Suzuki et al. 1986; Cernicharo & Guélin 1996; Guélin et al. 1996).

The C3H radical shows a particular behaviour because it exists in two isomeric forms, a cyclic one (c-C3H) and a linear one (l-C3H), with c-C3H lower in energy. Both have been widely observed in the ISM, and in general the c-C3H isomer (Yamamoto et al. 1987; Mangum & Wootten 1990; Turner et al. 2000; Cernicharo et al. 2000; Zhang et al. 2009; Liszt et al. 2014) is found to have a larger abundance than l-C3H (Thaddeus et al. 1985; Turner et al. 2000; Pardo & Cernicharo 2007). The C5H radical is predicted to adopt up to seven different structures, including linear, cyclic, and bent ones (Crawford et al. 1999). Conversely to C3H, the linear structure, l-C5H, is the most stable one. It has been detected in the ISM by Cernicharo et al. (1986a,b, 1987). The cyclic isomer, c-C5H, which is analogous to the c-C3H isomer but with the H atom replaced by a ethynyl (-CCH) group, is the second most stable isomer (6.1 kcal mol−1; Crawford et al. 1999). As l-C5H (McCarthy et al. 1999), c-C5H was characterized in the laboratory (Apponi et al. 2001), but it had not yet been detected in the ISM.

In this Letter we report the detection of the c-C5H radical towards the cold dark cloud TMC-1. The derived abundance is compared with that of the linear isomer l-C5H, and the plausible reactions that could lead to the formation of this species are discussed with the aid of a chemical model.

2. Observations

The observational data used in this article consist of spectra of TMC-1 taken with the Yebes 40m telescope towards the cyanopolyyne peak of TMC-1, αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″. The observations are part of the ongoing QUIJOTE2 line survey (Cernicharo et al. 2021a) carried out during different observing runs between November 2019 and January 2022. The observations were performed using the frequency-switching mode with a frequency throw of 10 MHz in the very first observing runs, in November 2019 and February 2020, 8 MHz in the observations of January–November 2021, and 10 MHz again in the last observing run that took place between October 2021 and January 2022. The total on-source telescope time is 430 h in each polarization (twice this value after averaging the two polarizations), which can be split into 238 and 192 h for the 8 MHz and 10 MHz frequency throws. The QUIJOTE line survey uses a 7 mm receiver covering the Q band (31.0–50.3 GHz) with horizontal and vertical polarizations. Receiver temperatures in 2019 and 2020 varied from 22 K at 32 GHz to 42 K at 50 GHz. In 2021, some power adaptation carried out in the down-conversion chains changed the receiver temperatures to 16 K at 32 GHz and 25 K at 50 GHz. The backends are 16 Fourier transform spectrometers, which provide a bandwidth of 8 × 2.5 GHz in each polarization, thus covering practically the whole Q band, with a spectral resolution of 38.15 kHz. The system is described in detail by Tercero et al. (2021).

The intensity scale used is antenna temperature, , which is calibrated using two absorbers at different temperatures and the ATM package (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties were assumed to be 10% based on the observed repeatability of the line intensities between different observing runs. All data were analysed using the software GILDAS3.

3. Results

3.1. New laboratory data for c-C5H

Apponi et al. (2001) observed the rotational spectrum of the c-C5H radical in the laboratory. It has a 2B2 electronic ground state with C2v symmetry. Owing to the Bose statistics of the two equivalent off-axis carbon nuclei, only rotational levels with odd Ka occur, and rotational transitions with Ka = 0 are thus forbidden. Apponi et al. (2001) found two series of lines, with Ka = 0 and Ka = 1 values, which were fitted separately. The Ka = 1 series was ascribed to the ground state of c-C5H, while the origin of the Ka = 0 lines was not totally clear. Our hypothesis is that the Ka = 0 lines arise from a low frequency vibrationally excited state with different vibrational symmetry (either B1 or B2) from that for the ground state (A1). To gain some insight into this, we carried out Fourier transform microwave (FTMW) spectroscopy experiments using as carrier gases either neon or argon.

The rotational spectrum of the c-C5H radical was observed using a Balle–Flygare narrow-band-type FTMW spectrometer operating in the frequency region 4–40 GHz (Endo et al. 1994; Cabezas et al. 2016). We observed some of the rotational transitions reported before by Apponi et al. (2001). The short-lived species c-C5H was produced in a supersonic expansion by a pulsed electric discharge of a gas mixture of C2H2 (0.3%) diluted in neon or argon. The gas mixture was flowed through a pulsed-solenoid valve that is accommodated in the backside of one of the cavity mirrors and aligned parallel to the optical axis of the resonator. A pulse voltage of 1000 V with a duration of 450 μs was applied between stainless-steel electrodes attached to the exit of the pulsed discharge nozzle, resulting in an electric discharge synchronized with the gas expansion. The resulting products generated in the discharge were then probed by FTMW spectroscopy, which allowed small hyperfine splittings to be resolved.

Previous studies have shown that heavier inert gases have an enhanced cooling efficiency in supersonic expansions due to the larger collision energies they provide (Cabezas et al. 2018). Consequently, an argon-seeded expansion is expected to favour the population of the lower frequency vibrational states, while higher vibrational states will be populated in a neon-seeded expansion. Figure 1 shows the spectra recorded using argon and neon gases. When argon is used, only the Ka = 1 lines are observed, but in the neon experiments both Ka = 0 and Ka = 1 are observed. We note that the intensity of the Ka = 1 lines decreases when neon is used due to a fraction of its population being distributed between higher vibrationally excited states. In light of our experimental observations, we infer that the Ka = 0 series of lines come from a low frequency vibrationally excited state, probably an in-plane or out-of-plane bending mode (Crawford et al. 1999).

thumbnail Fig. 1.

FTMW spectra for c-C5H. Coloured lines show spectra using argon (magenta) and neon (black) as the carrier gas in the supersonic expansion. Each line is split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry–Pérot cavity of the spectrometer. The difference in the magnitude of Doppler splitting when different carrier gases – neon and argon – are used is related to the velocity of the molecule in the jet; it is reciprocal to the square root of the atomic mass of the carrier gas.

3.2. Identification of c-C5H in TMC-1

Line identification in this work was done using the catalogues MADEX (Cernicharo 2012), CDMS (Müller et al. 2005), and JPL (Pickett et al. 1998). As of May 2022, the MADEX code contained 6434 spectral entries corresponding to the ground and vibrationally excited states, together with the corresponding isotopologues, of 1734 molecules. Once the assignment of all known molecules and their isotopologues is done, QUIJOTE will permit us to search for molecules for which frequencies are known. Moreover, QUIJOTE also allows us to perform rotational spectroscopy in space of new species for which no previous rotational spectroscopic laboratory information is available, such as HC5NH+ (Marcelino et al. 2020), HC3O+(Cernicharo et al. 2020a), HC3S+ (Cernicharo et al. 2021b), CH3CO+ (Cernicharo et al. 2021c), HCCS+ (Cabezas et al. 2022a), C5H+ (Cernicharo et al. 2022), HC7NH+ (Cabezas et al. 2022b), and HCCNCH+ (Agúndez et al. 2022).

Only the Ka = 1 series of lines of c-C5H are expected to be observed in TMC-1. We used the rotational parameters reported by Apponi et al. (2001) to predict, using the SPFIT program (Pickett 1991), the frequency transition lines in the Q band. We used a dipole moment of 3.39D, following Crawford et al. (1999). The molecule was implemented in the MADEX code (Cernicharo 2012), which was used to search for the Ka = 1 lines. A total of 17 lines of c-C5H were detected in TMC-1 above the 3σ level; they are shown in Fig. 2. These 17 lines correspond to 20 hyperfine components, three of which are not resolved, from five Ka = 1 rotational transitions with N = 5, 6, and 7. The derived line parameters are given in Table 1. A fit to the observed line profiles assuming a source diameter of 40″ (Fossé et al. 2001) provides a rotational temperature of 6.0 ± 0.5 K and a column density of N(c-C5H) = (9.0 ± 0.9) × 1010 cm−2. The synthetic spectra are compared with observations in Fig. 2 (red line). No lines with Ka = 0 were detected, which is in line with our conclusion that those lines belong to a vibrationally excited state that would be under-populated in TMC-1 due to the low kinetic temperature of the cloud.

thumbnail Fig. 2.

Observed lines of c-C5H in TMC-1 in the 31.0–50.3 GHz range. Frequencies and line parameters are given in Table 1. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 6 K and a column density of 9.0 × 1010 cm−2 (see text). Blanked channels correspond to negative features created in the folding of the frequency-switching data. The label U corresponds to unidentified features above 4σ.

Table 1.

Observed line parameters for c-C5H in TMC-1.

Using the new observed frequencies in TMC-1 and those measured in the laboratory (Apponi et al. 2001), we have derived, using the SPFIT program (Pickett 1991), a new set of molecular constants for c-C5H, which are shown in Table 2. The new molecular constants are practically identical to those provided by Apponi et al. (2001), with the exception of the dipole–dipole constants, and . We believe that the definition of these parameters in the fitting code used by Apponi et al. (2001) may be different to that employed by the Pickett program, used in this work, which results in the values being different.

Table 2.

Spectroscopic parameters of c-C5H (all in MHz).

We also observed several lines for the l-C5H isomer. A total of 12 hyperfine components for the J = 15/2 − 13/2, 17/2–15/2, and 19/2–17/2 transitions, which belong to the 2Π1/2 spin sub-level, were detected (see Fig. 3 and Table 3) with antenna temperatures around 30 mK. An analysis of the line intensities through a line model fitting procedure (Cernicharo et al. 2021d) provides a rotational temperature of 6.0 ± 0.5 K and a column density of N(l-C5H) = (1.3 ± 0.3) × 1012 cm−2. We assumed a dipole moment of 4.88D for l-C5H (Woon 1995). Therefore, the abundance ratio c-C5H/l-C5H is 0.069 in TMC-1. This value is far from that found in TMC-1 for the analogue system c-C3H/l-C3H, whose ratio is 5.5 (Loison et al. 2017).

thumbnail Fig. 3.

Observed lines of C5H in TMC-1 in the 31.0–50.3 GHz range. Frequencies and line parameters are given in Table 3. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 6 K and a column density of 1.3 × 1012 cm−2 (see text). Blanked channels correspond to negative features created in the folding of the frequency-switching data.

Table 3.

Observed line parameters for l-C5H in TMC-1.

4. Chemistry

In order to shed some light on the formation of c-C5H in TMC-1, we carried out gas-phase chemical modelling calculations similar to those presented in Cabezas et al. (2021). Briefly, we adopted physical conditions typical of cold dark clouds: a volume density of H nuclei of 2 × 104 cm−3, a gas kinetic temperature of 10 K, a visual extinction of 30 mag, a cosmic-ray ionization rate of H2 of 1.3 × 10−17 s−1, and the so-called set of low-metal elemental abundances (e.g., Agúndez & Wakelam 2013). We employed the chemical network RATE12 from the UMIST database (McElroy et al. 2013), which has been updated with results from Lin et al. (2013) and expanded with the subset of gas-phase chemical reactions involving C3H and C3H2 isomers revised by Loison et al. (2017).

The cyclic isomer of C5H is not included in either the UMIST (McElroy et al. 2013) or KIDA (Wakelam et al. 2015) databases, and information on reactions involving it are lacking in the literature. Our main aim here is thus to explore whether plausible reactions of formation of c-C5H can account for the abundance observed in TMC-1. Since c-C5H can be viewed as the result of substituting an H atom in c-C3H with a C2H group, an obvious reaction of formation of c-C5H is

(1)

We thus included this reaction with a rate coefficient of 3 × 10−10 cm3 s−1, which is similar to rate coefficients measured at low temperature for other reactions of C2H with closed-shell unsaturated hydrocarbons (Chastaing et al. 1998). Another interesting potential source of c-C5H is the reaction between atomic carbon and diacetylene,

(2a)

(2b)

where both the linear and cyclic isomers of C5H are in principle possible, although chemical networks only consider the linear isomer as product (Smith et al. 2004; Loison et al. 2014). Reaction (2) is thought to occur rapidly at low temperatures based on experimental studies of reactions of C with closed-shell unsaturated hydrocarbons such as C2H2 (Chastaing et al. 2001). Takahashi (2000) studied this reaction theoretically and concluded that both the linear and cyclic isomers of C5H can be formed without an entrance barrier. On the other hand, calculations by Sun et al. (2008) show that only the linear isomer of C5H should be formed. The different conclusions are likely due to the fact that Reaction (2b) was found to be exothermic by 1 kcal mol−1 by Takahashi (2000) but endothermic by 0.7 kcal mol−1 by Sun et al. (2008). That is, formation of c-C5H is nearly thermo-neutral. Here we assume that Reaction (2b) occurs with a branching ratio of just 10%. Finally, we assumed that c-C5H reacts fast with neutral atoms (H, C, N, and O) and cations that are known or expected to be abundant in TMC-1, such as C+ and HCO+.

In Fig. 4 we show the calculated abundances of the cyclic and linear isomers of C3H and C5H. It is seen that the peak calculated abundances, reached in the 105–106 yr time range, agree well with the observed values. If we focus on cyclic C5H, according to the chemical model the main reaction of formation is C + C4H2. That is, if this reaction produces cyclic C5H with only a branching ratio of 10%, then it can by itself explain the abundance of c-C5H observed in TMC-1. Further studies to evaluate the branching ratios of the reaction C + C4H2 would allow the chemistry of the linear and cyclic isomers of C5H in cold dark clouds to be better constrained.

thumbnail Fig. 4.

Calculated fractional abundances of the cyclic and linear isomers of C3H and C5H as a function of time. The abundances observed in TMC-1 for the isomers of C3H (Loison et al. 2017) and C5H (this study) are indicated by dotted horizontal lines.

5. Conclusions

We have reported the first identification of the c-C5H radical towards TMC-1. We observed 17 rotational transitions within the 31.0–50.3 GHz range using the Yebes 40m radio telescope. The derived c-C5H/l-C5H abundance ratio of 0.069 is very different from that found for c-C3H/l-C3H, whose ratio is 5.5 in TMC-1. A state-of-the-art chemical model reproduces the observed abundance of c-C5H and indicates that this radical can probably be formed in the reaction of atomic carbon with diacetylene.

2

Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment.

Acknowledgments

We acknowledge funding support from Spanish Ministerio de Ciencia e Innovación through grants PID2019-106110GB-I00, PID2019-107115GB-C21, and PID2019-106235GB-I00 and from the European Research Council (ERC Grant 610256: NANOCOSMOS). We would like to thank María Eugenia Sanz for her useful comments on the laboratory experiments of c-C5H radical.

References

  1. Agúndez, M., & Wakelam, V. 2013, Chem. Rev., 113, 8710 [Google Scholar]
  2. Agúndez, M., Cabezas, C., Marcelino, N., et al. 2022, A&A, 659, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  3. Apponi, A. J., Sanz, M. E., Gottlieb, C. A., et al. 2001, ApJ, 547, L65 [NASA ADS] [CrossRef] [Google Scholar]
  4. Araki, M., Takano, S., Sakai, N., et al. 2017, ApJ, 847, 51 [Google Scholar]
  5. Bell, M. B., Feldman, P. A., & Matthews, H. E. 1983, ApJ, 273, L35 [NASA ADS] [CrossRef] [Google Scholar]
  6. Bell, M. B., Feldman, P. A., Watson, J. K. G., et al. 1999, ApJ, 518, 740 [NASA ADS] [CrossRef] [Google Scholar]
  7. Cabezas, C., Guillemin, J.-C., & Endo, Y. 2016, J. Chem. Phys., 145, 184304 [Google Scholar]
  8. Cabezas, C., Guillemin, J.-C., & Endo, Y. 2018, J. Chem. Phys., 149, 084309 [NASA ADS] [CrossRef] [Google Scholar]
  9. Cabezas, C., Tercero, B., Agúndez, M., et al. 2021, A&A, 650, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  10. Cabezas, C., Agúndez, M., Marcelino, N., et al. 2022a, A&A, 657, L4 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  11. Cabezas, C., Agúndez, M., Marcelino, N., et al. 2022b, A&A, 659, L8 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  12. Cernicharo, J. 1985, Internal IRAM report (Granada: IRAM) [Google Scholar]
  13. Cernicharo, J. 2012, in European Conference on Laboratory Astrophysics, eds. C. Stehlé, C. Joblin, & L. d’Hendecourt, EAS Pub. Ser., 58, 251 [Google Scholar]
  14. Cernicharo, J., & Guélin, M. 1996, A&A, 309, L27 [NASA ADS] [Google Scholar]
  15. Cernicharo, J., Kahane, K., Gómez-Gónzalez, J., & Guélin, M. 1986a, A&A, 164, L1 [NASA ADS] [Google Scholar]
  16. Cernicharo, J., Kahane, K., Gómez-Gónzalez, J., & Guélin, M. 1986b, A&A, 167, L5 [NASA ADS] [Google Scholar]
  17. Cernicharo, J., Guélin, M., & Walmsley, C. M. 1987, A&A, 172, L5 [NASA ADS] [Google Scholar]
  18. Cernicharo, J., Guélin, M., & Kahane, C. 2000, A&AS, 142, 181 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  19. Cernicharo, J., Marcelino, N., Agúndez, M., et al. 2020a, A&A, 642, L17 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  20. Cernicharo, J., Agúndez, M., Kaiser, R., et al. 2021a, A&A, 652, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  21. Cernicharo, J., Cabezas, C., Endo, Y., et al. 2021b, A&A, 646, L3 [EDP Sciences] [Google Scholar]
  22. Cernicharo, J., Cabezas, C., Bailleux, S., et al. 2021c, A&A, 646, L7 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  23. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021d, A&A, 649, L15 [EDP Sciences] [Google Scholar]
  24. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2022, A&A, 657, L16 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  25. Chastaing, D., James, P. L., Sims, I. R., & Smith, I. W. M. 1998, Faraday Discuss., 109, 165 [NASA ADS] [CrossRef] [Google Scholar]
  26. Chastaing, D., Le Picard, S., Sims, I. R., & Smith, I. W. M. 2001, A&A, 365, 241 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  27. Crawford, T. D., Stanton, J. F., Saeh, J. C., & Schaefer, H. F. 1999, JACS, 121, 1902 [CrossRef] [Google Scholar]
  28. Dunham, T., Jr 1937, PASP, 49, 26 [CrossRef] [Google Scholar]
  29. Endo, Y., Kohguchi, H., & Ohshima, Y. 1994, Faraday Discuss., 97, 341 [Google Scholar]
  30. Fossé, D., Cernicharo, J., Gerin, M., & Cox, P. 2001, ApJ, 552, 168 [Google Scholar]
  31. Guélin, M., Green, S., & Thaddeus, P. 1987, ApJ, 224, L27 [Google Scholar]
  32. Guélin, M., Cernicharo, J., Travers, M. J., et al. 1996, A&A, 317, L1 [Google Scholar]
  33. Irvine, W. M., Hoglund, B., Friberg, P., et al. 1981, ApJ, 248, L113 [NASA ADS] [CrossRef] [Google Scholar]
  34. Lin, Z., Talbi, D., Roueff, E., et al. 2013, ApJ, 765, 80 [NASA ADS] [CrossRef] [Google Scholar]
  35. Liszt, H. S., Pety, J., Gerin, M., & Lucas, R. 2014, A&A, 564, A64 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  36. Loison, J.-C., Wakelam, V., Hickson, K. M., et al. 2014, MNRAS, 437, 930 [Google Scholar]
  37. Loison, J.-C., Agúndez, M., Wakelam, V., et al. 2017, MNRAS, 470, 4075 [Google Scholar]
  38. Mangum, J. G., & Wootten, A. 1990, A&A, 239, 319 [NASA ADS] [Google Scholar]
  39. Marcelino, N., Agúndez, M., Tercero, B., et al. 2020, A&A, 643, L6 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  40. McCarthy, M. C., Chen, W., Apponi, A. J., et al. 1999, ApJ, 520, 158 [NASA ADS] [CrossRef] [Google Scholar]
  41. McElroy, D., Walsh, C., Markwick, A. J., et al. 2013, A&A, 550, A36 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  42. Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, J. Mol. Struct., 742, 215 [Google Scholar]
  43. Nyman, L.-Å. 1984, ApJ, 141, 323 [NASA ADS] [Google Scholar]
  44. Pardo, J. R., & Cernicharo, J. 2007, ApJ, 654, 978 [NASA ADS] [CrossRef] [Google Scholar]
  45. Pardo, J. R., Cernicharo, J., & Serabyn, E. 2001, IEEE Trans. Antennas and Propagation, 49, 12 [Google Scholar]
  46. Pickett, H. M. 1991, J. Mol. Spectr., 148, 371 [Google Scholar]
  47. Pickett, H. M., Poynter, R. L., Cohen, E. A., et al. 1998, J. Quant. Spectrosc. Radiat. Transfer, 60, 883 [Google Scholar]
  48. Saito, S., Kawaguchi, K., Suzuki, H., et al. 1987, PASJ, 39, 193 [NASA ADS] [Google Scholar]
  49. Smith, I. W. M., Herbst, E., & Chang, Q. 2004, MNRAS, 350, 323 [Google Scholar]
  50. Sun, B. J., Huang, C. Y., Kuo, H. H., et al. 2008, J. Chem. Phys., 128 [Google Scholar]
  51. Suzuki, H., Ohishi, M., Kaifu, N., et al. 1986, PASJ, 38, 911 [NASA ADS] [Google Scholar]
  52. Takahashi, J. 2000, PASJ, 52, 401 [NASA ADS] [CrossRef] [Google Scholar]
  53. Tercero, F., López-Pérez, J. A., Gallego, J. D., et al. 2021, A&A, 645, A37 [EDP Sciences] [Google Scholar]
  54. Teyssier, D., Fossé, D., Gerin, M., et al. 2004, A&A, 417, 135 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  55. Thaddeus, P., Gottlieb, C. A., Hjalmarson, Å., et al. 1985, ApJ, 294, L49 [NASA ADS] [CrossRef] [Google Scholar]
  56. Tucker, K. D., Kutner, M. L., & Thaddeus, P. 1974, ApJ, 193, L115 [NASA ADS] [CrossRef] [Google Scholar]
  57. Turner, B. E., Herbst, E., & Terzieva, R. 2000, ApJS, 126, 427 [Google Scholar]
  58. Wakelam, V., Loison, J.-C., Herbst, E., et al. 2015, ApJS, 217, 20 [Google Scholar]
  59. Woon, D. E. 1995, Chem. Phys. Lett., 244, 45 [NASA ADS] [CrossRef] [Google Scholar]
  60. Wootten, A., Bozyan, E. P., Garrett, D. B., et al. 1980, ApJ, 239, 944 [NASA ADS] [Google Scholar]
  61. Yamamoto, S., Saito, S., Ohishi, M., et al. 1987, ApJ, 322, L55 [Google Scholar]
  62. Zhang, Y., Kwok, S., & Nakashima, J.-I. 2009, ApJ, 700, 1262 [NASA ADS] [CrossRef] [Google Scholar]

All Tables

Table 1.

Observed line parameters for c-C5H in TMC-1.

In the text
Table 2.

Spectroscopic parameters of c-C5H (all in MHz).

In the text
Table 3.

Observed line parameters for l-C5H in TMC-1.

In the text

All Figures

thumbnail Fig. 1.

FTMW spectra for c-C5H. Coloured lines show spectra using argon (magenta) and neon (black) as the carrier gas in the supersonic expansion. Each line is split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry–Pérot cavity of the spectrometer. The difference in the magnitude of Doppler splitting when different carrier gases – neon and argon – are used is related to the velocity of the molecule in the jet; it is reciprocal to the square root of the atomic mass of the carrier gas.
In the text
thumbnail Fig. 2.

Observed lines of c-C5H in TMC-1 in the 31.0–50.3 GHz range. Frequencies and line parameters are given in Table 1. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 6 K and a column density of 9.0 × 1010 cm−2 (see text). Blanked channels correspond to negative features created in the folding of the frequency-switching data. The label U corresponds to unidentified features above 4σ.
In the text
thumbnail Fig. 3.

Observed lines of C5H in TMC-1 in the 31.0–50.3 GHz range. Frequencies and line parameters are given in Table 3. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 6 K and a column density of 1.3 × 1012 cm−2 (see text). Blanked channels correspond to negative features created in the folding of the frequency-switching data.
In the text
thumbnail Fig. 4.

Calculated fractional abundances of the cyclic and linear isomers of C3H and C5H as a function of time. The abundances observed in TMC-1 for the isomers of C3H (Loison et al. 2017) and C5H (this study) are indicated by dotted horizontal lines.
In the text

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