Free Access
Issue
A&A
Volume 643, November 2020
Article Number L6
Number of page(s) 9
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202039251
Published online 27 October 2020

© ESO 2020

1. Introduction

Although gas phase chemistry in cold interstellar clouds is dominated by ion-neutral reactions, only around 15% of the detected species are cations (see the Cologne Database for Molecular Spectroscopy; Müller et al. 2005). Observed polyatomic cations are usually protonated forms of stable molecules. The rather reduced number of them detected in space is due to the fact that a dissociative recombination with electrons is rapid and depletes cations, producing most of the neutrals observed in these objects. In addition to the widespread ions HCO+ and N2H+, other interesting protonated species are HCS+ (Thaddeus et al. 1981), HCNH+ (Schilke et al. 1991), HC3NH+ (Kawaguchi et al. 1994), HCO (Turner et al. 1999; Sakai et al. 2008), NH3D+ (Cernicharo et al. 2013), NCCNH+ (Agúndez et al. 2015), H2COH+ (Bacmann et al. 2016), and H2NCO+ (Marcelino et al. 2018).

The abundance ratio between a protonated molecule and its neutral counterpart, [MH+]/[M], is sensitive to the degree of ionisation and to the proton affinity of the neutral. The higher the density is, the lower the ionisation fraction is and thus the lower the importance of protonated molecules. It is interesting to note that both the chemical models and the observations suggest a trend in which the abundance ratio [MH+]/[M] increases with an increasing proton affinity of M (Agúndez et al. 2015).

Protonated nitriles and dinitriles are observed in cold dense clouds because their neutral counterparts are abundant and have high proton affinities. The nitriles HCN and HC3N have proton affinities of 712.9 kJ mol−1 and 751.2 kJ mol−1, respectively (Hunter & Lias 1998), and abundances in excess of 10−8 relative to H2 (Agúndez & Wakelam 2013). Whereas the dinitrile NCCN has a proton affinity of 674.7 kJ mol−1 (Hunter & Lias 1998) and an inferred abundance as large as that of HCN and HC3N (Agúndez et al. 2015, 2018). The next larger members in the series of cyanopolyynes and dicyanopolyynes are also good candidates to be detected in their protonated form. Cyanodiacetylene (HC5N) has a high proton affinity (770 ± 20 kJ mol−1; Edwards et al. 2009) and it is just a few times less abundant than HC3N in cold dark cores (Agúndez & Wakelam 2013). Dicyanoacetylene (NC4N) also has a high proton affinity (736 kJ mol−1; this work) and it is likely to be present with a high abundance based on the large inferred abundance of NCCN. Hence, we could expect the protonated forms of HC5N and NC4N to be present in cold dense clouds with moderately high abundances.

In this Letter, we report the detection of a new series of harmonically related lines belonging to a molecule with a 1Σ ground electronic state towards the cold dark core TMC-1. Two of the molecular species discussed above, HC5NH+ and NC4NH+, could be the carriers. From ab initio calculations and the expected intensities of the lines for each of these species, we conclude that we have discovered the cation HC5NH+. No laboratory data are available for it, hence, this is the first time this species has been observed. Abundance ratios between HC3N and HC5N and their protonated forms were derived and compared with predictions from chemical models. We searched for lines that could be attributed to NC4NH+ without success. A search for NC3NC in our data also provides upper limits to the abundance of this species.

2. Observations

New receivers, which were built within the Nanocosmos project1 and installed at the Yebes 40 m radio telescope, were used for the observations of TMC-1. The Q-band receiver consists of two HEMT cold amplifiers covering the 31.0–50.3 GHz band with horizontal and vertical polarisations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers are 2 × 8 × 2.5 GHz FFTs with a spectral resolution of 38.1 kHz, providing the whole coverage of the Q-band in both polarisations. The main beam efficiency and the half power beam width (HPBW) of the Yebes 40m telescope range from 0.6 and 55″ (at 32 GHz) to 0.43 and 37″ (at 49 GHz), respectively.

The observations leading to the line survey in the Q-band towards TMC-1 (αJ2000 = 4h41m41.9s, δJ2000 = +25° 41′27.0″) were performed during several sessions between November 2019 and February 2020. The observing procedure used was frequency switching with a frequency throw of 10 MHz. The nominal spectral resolution of 38.1 kHz was used for the final spectra. A study of the velocity structure of the source (see, e.g. Lique et al. 2006; Xue et al. 2020) could require a higher spectral resolution. However, the determination of the total column density in the line of sight for a given molecule is not affected by our spectral resolution of 38.1 kHz. The sensitivity varies along the Q-band between 1 and 3 mK, which considerably improves previous line surveys in the 31–50 GHz frequency range (Kaifu et al. 2004).

The intensity scale, antenna temperature () was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM, Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties have been adopted to be 10%. All data have been analysed using the GILDAS package2.

3. Results

One of the most surprising results from the line survey in the Q-band in TMC-1 is the presence of a forest of weak lines. Most of them can be assigned to known species and their isotopologues, and only a few remain unidentified (Marcelino et al., in prep.). As previously mentioned, the level of sensitivity has been increased by a factor of 5–10 with respect to previous line surveys performed with other telescopes at these frequencies (Kaifu et al. 2004). Frequencies for the unknown lines have been derived by assuming a local standard of rest velocity of 5.83 km s−1, which is a value that was derived from the observed transitions of HC5N and its isotopologues in our line survey.

3.1. Harmonically related lines

Among the unidentified features in our survey, we have found a series of seven harmonically related lines to a precision better than 2 × 10−7. This strongly suggests that the carrier is a linear molecule with a 1Σ ground electronic state. Fig. 1 shows these lines and the quantum numbers that were obtained. The derived line parameters are given in Table 1. Using the MADEX code (Cernicharo et al. 2012), we have verified that none of these features can be assigned to lines from other species. For a linear molecule, the frequencies of its rotational transitions follow the standard expression ν(J → J − 1) = 2B0J − 4D0J3. By fitting the frequencies of the lines given in Table 1, we derive

thumbnail Fig. 1.

Observed lines of the new molecule found in the 31–50 GHz domain towards TMC-1. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J = 19 − 18 line is only detected at a 3.5σ level. Spectral resolution is 38.1 kHz.

Open with DEXTER

Table 1.

Observed line parameters for the new molecule in TMC-1.

where values between parentheses represent the 1σ uncertainty on the parameters in units of the last digit. The standard deviation of the fit is 5.6 kHz.

The rotation constant is ∼36 MHz lower than that of HC5N (1331.332 MHz; Bizzocchi et al. 2004). It is also lower than that of dicyanoacetylene, NCCCCN (1336.7 MHz; Winther et al. 1992). The rotational constants of the 13C and 15N isotopologues of HC5N are well known from laboratory measurements (Bizzocchi et al. 2004; Giesen et al. 2020). The isotopologues HCN and H13CCCCCN have rotational constants of 1298.640 MHz and 1296.676 MHz, respectively, that is to say they are very close to that of the new species, suggesting that a slightly heavier species could be the carrier. The isomer HC4NC has a larger rotational constant of 1401.182 MHz (Botschwina et al. 1998) and it can be excluded as a carrier. Moreover, this species has been detected in our line survey (Cernicharo et al. 2020) and also by Xue et al. (2020). Although the isomer HNC5 has not been observed in the laboratory, ab initio calculations indicate that the molecule is bent with a (B + C)/2 value of ∼1363 MHz (Gronowski & Kolos 2006; Cernicharo et al. 2020), which is ∼70 MHz above the observed rotational constant.

In TMC-1, only polyatomic molecules containing H, C, N, O, and S have been found so far. We could consider a linear chain containing sulphur as a possible carrier. However, the best candidate in this case is HC4S, which is linear but has a 2Πi ground electronic state and a rotational constant of 1435.326 MHz that is too high (Hirahara et al. 1994). Another potential species is NCCCS, which is linear, but also with a 2Πi ground electronic state and a rotational constant of 1439.186 MHz (McCarthy et al. 2003). The neutral species HC5O was observed in the laboratory by Mohamed et al. (2005). It has a rotational constant of 1293.6 MHz and has been observed in TMC-1 (McGuire et al. 2017). Hence, good candidates for the carrier of the observed lines are molecular species having a structure and mass close to that of HC5N, HC5O, or NC4N.

The cation HC3NH+ was detected towards TMC-1 by Kawaguchi et al. (1994); additionally, protonated cyanogen, HNCCN+, was also detected in this source by Agúndez et al. (2015). Hence, good candidates for the carrier of the series of harmonically related lines are HC5NH+ and NC4NH+. An additional candidate is HC5O+, which could be the product of protonation of C5O. However, C5O has not been detected so far in space and ab initio calculations (see below) indicate a rotational constant several MHz above the observed one. For protonated cyanodiacetylene, HC5NH+, ab initio calculations by Botschwina et al. (1997) indicate an equilibrium rotational constant very close to ours of ∼1294.1 MHz with a dipole moment of 3.811 D. It is a very good candidate indeed as the difference between the predicted and the observed rotational constant is less than 0.1%. Nevertheless, since we have a good second candidate, NC4NH+, and also a third (less clear) possibility, HC5O+, additional calculations are needed to help with the assignment of the lines.

3.2. Quantum chemical calculations

As mentioned before, HC5NH+, NC4NH+, and HC5O+ are the three candidates that have a rotational constant compatible with that derived from the lines observed in TMC-1. In order to obtain precise geometries and spectroscopic molecular parameters that help with the assignment of the observed lines, we carried out high-level ab initio calculations for these three species. Other structural isomers of HC5NH+ and NC4NH+ species were discarded as candidates because of their rotational constant values and their energetics; all metastable isomers lie at least at 18 kcal mol−1 over the HC5NH+ and NC4NH+ species, see Table B.3. Therefore, the calculations presented below are restricted to HC5NH+, NC4NH+, and HC5O+ molecules. Details regarding the calculations can be found in Appendix B.

The experimental rotational parameters for HC5N, NC4N, and C5O have been determined before (Bizzocchi et al. 2004; Winther et al. 1992; Ogata et al. 1995) and, therefore, they can be used to calibrate the computational results of their corresponding protonated forms. The Be rotational constant for HC5N, NC4N, and C5O, which were calculated at the CCSD(T)-F12/cc-pCVTZ-F12 level of theory, are 1329.57, 1334.69, and 1363.74 MHz, respectively. The zero-point vibrational contributions to the rotational constant at the MP2/cc-pVTZ level of theory were calculated to be 0.62, 0.72, and 1.82 MHz, respectively. Adding this contribution to the above Be, B0 thus takes values of 1330.19, 1335.42, and 1365.16 MHz, respectively, which are very close to the experimental values of 1331.332687(20), 1336.68433(30), and 1366.84709(6) MHz. The calculated constants show deviations from experimental values by 0.1% for all the species, which gives confidence as to the accuracy of our calculations.

The B0 rotational constant calculated for HC5NH+, NC4NH+, and HC5O+ were each scaled using the ratio Bexp/Bcal for HC5N, NC4N, and C5O, respectively. These values are shown in Table 2 together with the estimated values for the centrifugal distortion constant (D), which were derived from the frequency calculations at CCSD/cc-pVTZ level of theory and scaled in the same manner as the rotational constants. The results of the same calculations at different levels of theory are given in Tables B.1 and B.2. Independent of the level of theory employed, our calculations provide a rotational constant value for HC5NH+ around 1295.7 MHz, while that for NC4NH+ is about 2.0 MHz lower, around 1293.6 MHz. In the case of HC5O+, the value is larger, around 1303.0 MHz. No large differences were found for the predicted values of the centrifugal distortion constants.

Table 2.

Rotational constants and electric dipole moments calculated for HC5NH+ and NC4NH+.

4. Discussion

4.1. Assignment to HC5NH+ and derived column density

The calculated rotational constants of HC5NH+ and NC4NH+ are very close to the value of B0 observed. A definitive assignment requires observations in the laboratory. Nevertheless, we could give some support to the assignment to HC5NH+. First, the ab initio calculations at all levels of theory predict a rotational constant for HC5NH+ around 1295 MHz, while that of NC4NH+ is systematically 2 MHz below. Second, the rotational constant corrected for the ratio theory-to-observation of the reference species, HC5N and NC4N, provide an excellent match with the observed B0 for protonated cyanodiacetylene within 0.05%, while the difference for protonated dicyanoacetylene reaches 0.2%.

Additional support for the assignment to HC5NH+ comes from the comparison of the proton affinities of HC5N and NC4N. As previously indicated, the abundance of a protonated species depends on the abundance and proton affinity of the neutral counterpart. A large proton affinity permits the transfer of H+ to the neutral species, M, through the reactions M + XH+ → MH+ + X, where XH+ is an abundant proton donor, such as HCO+ or H. For cyanodiacetylene, HC5N, the proton affinity was measured to be 770 ± 20 kJ mol−1 (Edwards et al. 2009). The abundance of this species is large in cold dark cores, hence, we could expect a moderately high abundance for its protonated form. Due to the lack of a permanent dipole, species such as NCCN and NC4N have not been observed in dark clouds so far. Nevertheless, the detection of NCCNH+ in these objects by Agúndez et al. (2015) suggests that NCCN has an abundance as large as (1 − 10) × 10−8 relative to H2, that is to say it is similar to that of HC3N. Assuming an abundance for NC4N that is similar to that of HC5N, then the relative abundances of HC5NH+ and NC4NH+ depend on the proton affinities of the neutrals and on their electronic dissociative recombination rates, which in principle could be assumed to be similar. For NC4N, there is not an experimental value in the literature, so we calculated it at CCSD/cc-pVTZ level of theory. We used the energy balance between NC4N + H+ and NC4NH+, considering NC4N and H+ are independent species. We found a proton affinity value for NC4N of 736 kJ mol−1. Using the same scheme, we obtained a proton affinity value for HC5N of 783 kJ mol−1, which is very close to the experimental one (see above). Hence, the proton affinity of NC4N is lower than that of HC5N, which favours the protonated form of HC5N as a carrier of our lines.

Another argument supporting this assignment concerns the very different dipole moment of the two species. While, HC5NH+ has a predicted dipole moment of ∼3.3 D, the corresponding value for NC4NH+ is ∼9.5 D (see Table 2). Hence, the abundance resulting from the observed line intensities would be significantly different if the carrier were to be one versus the other species. In Fig. 2 we show the rotational diagram obtained from the observed intensities (see Table 1) assuming that the carrier is HC5NH+. We adopted a source radius of 40″ (Fossé et al. 2001). The parameter that is really interesting from this plot is the derived rotational temperature as the column density depends on the assumed dipole moment. The observed lines can be reproduced with a Trot of 7.8 ± 0.7 K. This is a typical value of the rotational temperature for most molecules detected so far in TMC-1, but that could require a very large H2 volume density if the dipole moment were 9.5 D, that is, if the carrier were NC4NH+. The derived column density is (7.5 ± 2.2) × 1011 if the carrier is HC5NH+, and ∼9 × 1010 cm−2 if the carrier is NC4NH+.

thumbnail Fig. 2.

Rotational diagram of the observed lines in TMC-1 assuming a dipole moment of 3.3 D, i.e. assuming the carrier of the lines is HC5NH+.

Open with DEXTER

Hence, we consider that we have arguments to support the assignment of the lines to protonated cyanodiacetylene. Cernicharo et al. (2020) performed a rotational analysis to all the transitions of HC5N observed in our line survey. They derived Trot = 8.6 ± 0.2 K and N(HC5N) = (1.8 ± 0.2) × 1014 cm−2. This column density was derived from the observed weak hyperfine components in the HC5N transitions from J = 12–11 up to J = 16–15 in order to take line opacity effects into account. Hence, we derived an abundance ratio of ∼240 for HC5N/HC5NH+.

Another indirect indication supporting our identification could arise from the comparison of the abundances of other protonated species, in particular HC3NH+ and NCCNH+. Both species are present in TMC-1 (Kawaguchi et al. 1994; Agúndez et al. 2015) and two rotational lines of each one are within our line survey. Figure 3 shows the observed lines of both species. The corresponding line parameters are given in Table 3. From the observed intensities, we derived N(HC3NH+) = 1.0 × 1012 cm−2 and N(HNCCN+) = 9.0 × 1010 cm−2, that is, N(HC3NH+)/N(HNCCN+) ∼ 14. Cernicharo et al. (2020) also derived a column density for HC3N of (2.3 ± 0.2) × 1014 cm−2, which was corrected for line opacity effects. Therefore, the derived abundance ratio for HC3N/HC3NH+ is ≃230, which is a value that is in good agreement with the one found by Kawaguchi et al. (1994) ∼160 and it is very similar to what was found above for HC5N/HC5NH+.

thumbnail Fig. 3.

Observed lines of HC3NH+ (a) and of HNCCN+ (b). The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 3. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J = 4 − 3 line of HNCCN+ shows two components due to the hyperfine structure introduced by the external nitrogen atom. Spectral resolution is 38.1 kHz.

Open with DEXTER

Table 3.

Observed line parameters for HC3NH+ and HNCCN+.

Between our unidentified features, we searched for lines that could be assigned to NC4NH+ using our ab initio calculations without success. A search for lines of NCCCNC also provides an upper limit to its column density of ≤1012 cm−2.

The case for HC5NH+ is similar to that of the discovery of C3H+ by Pety et al. (2012), which was done from the reasonable agreement between the observed rotational constant and the best ab initio calculations available at that moment for C3H+.

4.2. Chemistry of protonated species

The chemistry of protonated molecules in cold dense clouds has been discussed by Agúndez et al. (2015). Here, we revisit the pseudo time-dependent gas-phase chemical model of a cold dark cloud carried out by these authors. The chemical network adopted is largely based on the UMIST RATE12 reaction network (McElroy et al. 2013). For more details on the chemical model, we refer the reader to Agúndez et al. (2015). In cold dense clouds, protonated molecules MH+ form mainly by the proton transfer from a proton donor XH+ to M:

(1)

while they are destroyed through the dissociative recombination with electrons

(2)

Therefore, at the steady state, the neutral-to-protonated abundance ratio is

(3)

where kDR and kPT are the rate constants of the reactions of the dissociative recombination and proton transfer, respectively. The chemical model indicates that this scheme holds for HC3NH+ and HC5NH+, where the main proton donor XH+ is HCO+. Calculated neutral-to-protonated abundance ratios are about ten times higher than observed for HC3NH+ and HC5NH+ (see Fig. 4). That is to say, the chemical model underestimates the abundance of the protonated form with respect to its neutral counterpart. The rate constants of the dissociative recombination and proton transfer from HCO+ used in the chemical model are just estimates for HC5NH+; although, in the case of HC3NH+, they are well known from experiments (Anicich 2003; Geppert et al. 2004). We note that the chemical model also underestimates the abundance of HCNH+ (see Fig. 4) and other protonated species, such as HCS+ and NH3, regardless of the cosmic-ray ionisation adopted (Agúndez et al. 2015). We suspect that the most likely reason for the underestimation of protonated molecules in the chemical model is the existence of additional formation routes that are not considered in the model.

thumbnail Fig. 4.

Abundance ratios between neutral and protonated species for the series of cyanopolyynes. Calculated values correspond to a chemical model of a cold dark cloud at the steady state and observed ones to TMC-1.

Open with DEXTER


Acknowledgments

The Spanish authors thank Ministerio de Ciencia e Innovación for funding support through project AYA2016-75066-C2-1-P. We also thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. M.A. thanks Ministerio de Ciencia e Innovación for Ramón y Cajal Grant RyC-2014-16277.

References

  1. Adler, T. B., Knizia, G., & Werner, H.-J. 2007, J. Chem. Phys., 127, 221106 [NASA ADS] [CrossRef] [Google Scholar]
  2. Agúndez, M., & Wakelam, V. 2013, Chem. Rev., 113, 8710 [Google Scholar]
  3. Agúndez, M., Cernicharo, J., de Vicente, P., et al. 2015, A&A, 579, L10 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  4. Agúndez, M., Marcelino, N., & Cernicharo, J. 2018, ApJ, 861, L22 [NASA ADS] [CrossRef] [Google Scholar]
  5. Alexander, A. J., Kroto, H. W., & Walton, D. R. M. 1976, J. Mol. Spectr., 62, 175 [NASA ADS] [CrossRef] [Google Scholar]
  6. Anicich, V. G. 2003, An index of the literature for bimolecular gas phase cation-molecule reaction kinetics (Pasadena, CA, USA: JPL Publication) [Google Scholar]
  7. Bacmann, A., García-García, E., & Faure, A. 2016, A&A, 588, L8 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  8. Bizzocchi, L., Degli Esposti, C., & Botschwina, P. 2004, J. Mol. Spectr., 225, 145 [NASA ADS] [CrossRef] [Google Scholar]
  9. Botschwina, P., Heyl, Ä., Oswald, R., & Hirano, T. 1997, Spectrochim. Acta A: Molecular Biomolecular Spectr., 53, 1079 [NASA ADS] [CrossRef] [Google Scholar]
  10. Botschwina, P., Heyl, A., Chen, W., et al. 1998, J. Chem. Phys., 109, 3108 [CrossRef] [Google Scholar]
  11. Cernicharo, J. 1985, Internal IRAM report (Granada: IRAM) [Google Scholar]
  12. Cernicharo, J. 2012, in ECLA 2011: Proc. of the European Conference on Laboratory Astrophysics, eds. C. Stehl, C. Joblin, & L. d’Hendecourt (Cambridge: Cambridge Univ. Press), EAS Pub. Ser., 2012, 251, https://nanocosmos.iff.csic.es/?page_id=1619 [Google Scholar]
  13. Cernicharo, J., Tercero, B., Fuente, A., et al. 2013, ApJ, 771, L10 [NASA ADS] [CrossRef] [Google Scholar]
  14. Cernicharo, J., Marcelino, N., Agúndez, M., et al. 2020, A&A, 642, L8 [CrossRef] [EDP Sciences] [Google Scholar]
  15. Cížek, J. 1969, in Advances in Chemical Physics, ed. P. C. Hariharan (New York: Wiley Interscience), 14, 35 [Google Scholar]
  16. Edwards, S. J., Freeman, C. G., & McEwan, J. 2009, Int. J. Mass Spectrom., 279, 82 [CrossRef] [Google Scholar]
  17. Fossé, D., Cernicharo, J., Gerin, M., & Cox, P. 2001, ApJ, 552, 168 [NASA ADS] [CrossRef] [Google Scholar]
  18. Frisch, M. J., Trucks, G. W., Schlegel, H. B., et al. 2013, Gaussian 09, revision D.01 [Google Scholar]
  19. Geppert, W. D., Ehlerding, A., Hellberg, F., et al. 2004, ApJ, 613, 1302 [NASA ADS] [CrossRef] [Google Scholar]
  20. Giesen, T. F., Harding, M. E., Gauss, J., et al. 2020, J. Mol. Spectr., 371, 111303 [CrossRef] [Google Scholar]
  21. Gordy, W., & Cook, R. L. 1984, Microwave Molecular Spectra, Chapter V (New York: Wiley) [Google Scholar]
  22. Gronowski, M., & Kolos, R. 2006, J. Mol. Struct., 834, 102 [Google Scholar]
  23. Hirahara, Y., Ohshima, Y., & Endo, Y. 1994, J. Chem. Phys., 101, 7342 [CrossRef] [Google Scholar]
  24. Hill, J. G., Mazumder, S., & Peterson, K. A. 2010a, J. Chem. Phys., 132, 054108 [NASA ADS] [CrossRef] [Google Scholar]
  25. Hill, J. G., & Peterson, K. A. 2010b, Phys. Chem. Chem. Phys., 12, 10460 [CrossRef] [Google Scholar]
  26. Hunter, E. P., & Lias, S. G. 1998, J. Phys. Chem. Ref. Data, 27, 413 [NASA ADS] [CrossRef] [Google Scholar]
  27. Kawaguchi, K., Kasai, Y., Ishikawa, S., et al. 1994, ApJ, 420, L95 [NASA ADS] [CrossRef] [Google Scholar]
  28. Kaifu, N., Ohishi, M., Kawaguchi, K., et al. 2004, PASJ, 56, 69 [NASA ADS] [CrossRef] [Google Scholar]
  29. Knizia, G., Adler, T. B., & Werner, H.-J. 2009, J. Chem. Phys., 130, 054104 [NASA ADS] [CrossRef] [Google Scholar]
  30. Lique, F., Cernicharo, J., & Cox, P. 2006, ApJ, 653, 1342 [NASA ADS] [CrossRef] [Google Scholar]
  31. Marcelino, N., Agúndez, M., Cernicharo, J., et al. 2018, A&A, 612, L10 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  32. McCarthy, M. C., Cooksy, A. L., Mohamed, S., et al. 2003, ApJS, 194, 287 [CrossRef] [Google Scholar]
  33. McElroy, D., Walsh, C., Markwick, A. J., et al. 2013, A&A, 550, A36 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  34. McGuire, B. A., Burkhardt, A. M., & Shingledecker, C. N. 2017, ApJ, 843, L28 [CrossRef] [Google Scholar]
  35. Mohamed, S., McCarthy, M. C., Cooksky, A. L., et al. 2005, J. Chem. Phys., 123, 234301 [CrossRef] [Google Scholar]
  36. Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, J. Mol. Struct., 742, 215 [NASA ADS] [CrossRef] [Google Scholar]
  37. Ogata, T., Ohshima, Y., & Endo, Y. 1995, J. Am. Chem. Soc., 117, 3593 [CrossRef] [Google Scholar]
  38. Pardo, J. R., Cernicharo, J., & Serabyn, E. 2001, IEEE Trans. Antennas Propag., 49, 12 [NASA ADS] [CrossRef] [MathSciNet] [Google Scholar]
  39. Pety, J., Gúzman, V., Roueff, E., et al. 2012, A&A, 548, A68 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  40. Pritchard, B. P., Altarawy, D., Didier, B., et al. 2019, J. Chem. Inf. Model., 59, 4814 [CrossRef] [Google Scholar]
  41. Raghavachari, K., Trucks, G. W., Pople, J. A., & Head-Gordon, M. 1989, Chem. Phys. Lett., 157, 479 [Google Scholar]
  42. Sakai, N., Sakai, T., Aikawa, Y., & Yamamoto, S. 2008, ApJ, 675, L89 [NASA ADS] [CrossRef] [Google Scholar]
  43. Schilke, P., Walmsley, C. M., Millar, T. J., & Henkel, C. 1991, A&A, 247, 487 [Google Scholar]
  44. Thaddeus, P., Guélin, M., & Linke, R. A. 1981, ApJ, 246, L41 [NASA ADS] [CrossRef] [Google Scholar]
  45. Turner, B. E., Terzieva, R., & Herbst, E. 1999, ApJ, 518, 699 [NASA ADS] [CrossRef] [Google Scholar]
  46. Werner, H. J., Knowles, P. J., Knizia, G., et al. 2018, MOLPRO, version 2018.1 [Google Scholar]
  47. Winther, F., Schönhoff, M., LePrince, R., et al. 1992, J. Mol. Spectr., 152, 205 [CrossRef] [Google Scholar]
  48. Xue, C., Willis, E. R., Loomis, R. A., et al. 2020, ApJ, 900, L9 [CrossRef] [Google Scholar]

Appendix A: Frequencies, intensities, and energies of HC5NH+ transitions

In order to facilitate the search of HC5NH+ towards other sources, we provide the frequencies, uncertainties, line strenghts, Einstein coefficients, and upper energy levels for all transitions of this species below 100 GHz in Table A.1. The denegeracies of the energy levels are 2J + 1. The rotational partition function for HC5NH+ can be derived for a given temperature T from the standard expresion Qrot = kT/hB (Gordy & Cook 1984), to be Qrot = 16.087 × T.

Table A.1.

Frequencies, upper energy levels, Einstein coefficients, and line strengths of HC5NH+ transitions.

Appendix B: Additional quantum chemical calculation data

All structure optimisation calculations reported in this work were performed using the closed-shell coupled cluster with singles and doubles (CCSD; Cížek et al. 1969) and perturbative triple excitations (CCSD(T), Raghavachari et al. 1989) with and without an explicitly correlated (F12) approximation (Adler et al. 2007; Knizia et al. 2009). For CCSD and CCSD(T) calculations, we used Dunning’s correlation consistent polarised valence (and valence-core) triple-ζ basis sets cc-pVTZ (cc-pCVTZ), which were also augmented with diffuse functions (aug-cc-pVTZ, Pritchard et al. 2019). On the other hand, with the calculations at the CCSD(T)-F12 level, the Dunning’s correlation consistent basis sets with polarised core-valence correlation triple-ζ for explicitly correlated calculations (cc-pCVTZ-F12; Hill et al. 2010a; Hill & Peterson 2010b) was used. In this latter case, all electrons (valence and core) are correlated. In order to achieve an estimate of the B0 rotational constant, vibration-rotation interaction constants were calculated using second-order perturbation theory at the MP2/cc-pVTZ level. All the calculations were carried out using the Molpro 2018.1 (Werner et al. 2018) and Gaussian 09 (Frisch et al. 2013) programme packages.

The molecular structure for all the linear isomers of HC5NH+ and NC4NH+ species were calculated using CCSD/cc-pVTZ level of theory. We did not considered asymmetric structures because the carrier of the observed lines is a linear molecule. The relative energies of all plausible isomers, as well as their rotational constants and dipole moments obtained at the CCSD/cc-pVTZ level, are summarised in Table B.3. In both cases (C5H2N+ and C4HN), the considered species HC5NH+ and NC4NH+ are the lowest energy isomers, respectively.

Vibrational calculations were also conducted in order to estimate the energies, the IR intensities, and the first order vibration-rotation coupling constant (Table B.4). From these values, we derived the rotational constants Bν for each vibrational state following the expression (Gordy & Cook 1984):

(B.1)

where Be is the rotational constant at the equilibrium position and αi represents the first order vibration-rotation coupling constants for each i vibrational mode. Keeping in mind that the rotational constant of the vibrational ground state (B0) is obtained when the v for every vibrational mode is equal to zero (), Eq. (B.1) can be also expressed as Bν = B0 − ∑iαivi.

The values for the rotational constants of the fundamental vibrations, Bν, of HC5NH+ and HNC4N+ were corrected by the scale factor for HC5N and NC4N, respectively, which were obtained as follows: B0, the experimental value (Bizzocchi et al. 2004; Winther et al. 1992, respectively), was divided by the theoretical values (MP2/cc-pVTZ level of theory for Bν estimations). In the case of the B0 values reported in Table B.2, we initially calculated the theoretical ground state rotational constant B0 (), using the Be value of the corresponding level of theory and the vibration-rotation coupling constants (α) from the MP2/cc-pVTZ anharmonic calculations, and, finally, we corrected the B0 values using a scale factor of HC5N and NC4N at the corresponding ab initio level of calculation.

From the energies of the vibrational modes, which were obtained theoretically, we could also estimate the vibrational partition function for the HC5NH+ and HNC4N+ species. They were calculated using the expression (Gordy & Cook 1984):

(B.2)

where ωi and di represent the energy and the degeneracy of each i vibrational mode. These energies for the vibrational modes are taken from the results of the ab initio calculations evaluated at the MP2/cc-pVTZ level of theory, under the anharmonic correction.

Table B.1.

Theoretical values for spectroscopic parameters of HC5NH+, NC4NH+, and HC5O+ at different levels of theory.

Table B.2.

Comparison of the experimental and theoretical values for spectroscopic parameters of HC5N, NC4N, and C5O at different levels of theory.

Table B.3.

Relative energies, rotational constants, and electric dipole moments for HC5NH+ and NC4NH+ and all their isomers calculated at the CCSD/cc-pVTZ level of theory.

Table B.4.

Rotational constants, energies, and IR Intensities for the vibrational modes of HC5NH+ and HNC4N+.

Table B.5.

Vibrational partition function for HC5NH+ and HNC4N+ species.

All Tables

Table 1.

Observed line parameters for the new molecule in TMC-1.

Table 2.

Rotational constants and electric dipole moments calculated for HC5NH+ and NC4NH+.

Table 3.

Observed line parameters for HC3NH+ and HNCCN+.

Table A.1.

Frequencies, upper energy levels, Einstein coefficients, and line strengths of HC5NH+ transitions.

Table B.1.

Theoretical values for spectroscopic parameters of HC5NH+, NC4NH+, and HC5O+ at different levels of theory.

Table B.2.

Comparison of the experimental and theoretical values for spectroscopic parameters of HC5N, NC4N, and C5O at different levels of theory.

Table B.3.

Relative energies, rotational constants, and electric dipole moments for HC5NH+ and NC4NH+ and all their isomers calculated at the CCSD/cc-pVTZ level of theory.

Table B.4.

Rotational constants, energies, and IR Intensities for the vibrational modes of HC5NH+ and HNC4N+.

Table B.5.

Vibrational partition function for HC5NH+ and HNC4N+ species.

All Figures

thumbnail Fig. 1.

Observed lines of the new molecule found in the 31–50 GHz domain towards TMC-1. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J = 19 − 18 line is only detected at a 3.5σ level. Spectral resolution is 38.1 kHz.

Open with DEXTER
In the text
thumbnail Fig. 2.

Rotational diagram of the observed lines in TMC-1 assuming a dipole moment of 3.3 D, i.e. assuming the carrier of the lines is HC5NH+.

Open with DEXTER
In the text
thumbnail Fig. 3.

Observed lines of HC3NH+ (a) and of HNCCN+ (b). The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 3. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J = 4 − 3 line of HNCCN+ shows two components due to the hyperfine structure introduced by the external nitrogen atom. Spectral resolution is 38.1 kHz.

Open with DEXTER
In the text
thumbnail Fig. 4.

Abundance ratios between neutral and protonated species for the series of cyanopolyynes. Calculated values correspond to a chemical model of a cold dark cloud at the steady state and observed ones to TMC-1.

Open with DEXTER
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.