Open Access
Issue
A&A
Volume 686, June 2024
Article Number L15
Number of page(s) 11
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202450801
Published online 17 June 2024

© The Authors 2024

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.

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1. Introduction

Among the new species discovered in the last years with the QUIJOTE1 line survey of TMC-1 (Cernicharo et al. 2021a, 2023a,b, and references therein), it is worth noting that ten of them are the protonated form of abundant species. These are HC5NH+ (protonated HC5N, Marcelino et al. 2020), HC3O+ (protonated C3O, Cernicharo et al. 2020a), HCCNCH+ (protonated HCCNC, Agúndez et al. 2022), HCCS+ (protonated CCS, Cabezas et al. 2022a), HC3S+ (protonated CCCS, Cernicharo et al. 2021b), CH3CO+ (protonated H2CCO, Cernicharo et al. 2021c), HC7NH+ (protonated HC7N, Cabezas et al. 2022b), NC4NH+ (protonated form of the non-polar molecule NC4N, Agúndez et al. 2023a), C5H+ (protonated form of non-polar C5, Cernicharo et al. 2022a), and H2C3H+ (protonated l-H2C3, Silva et al. 2023). The observational and theoretical status of protonated molecules in cold dense clouds indicate that protonated-to-neutral abundance ratios MH+/M are in the range 10−3–10−1 for neutral molecules M with proton affinities above that of CO, and lower ratios for molecules M with lower proton affinities (Agúndez et al. 2022). It has also been found that chemical models tend to underestimate MH+/M ratios, which suggest the existence of alternative paths to direct protonation for the formation of these species MH+.

The abundant radicals CnH and CnN are species with a high electron affinity, leading to the formation – through electron attachment – of the anionic species C4H (Cernicharo et al. 2007), C6H (McCarthy et al. 2006), C8H (Brünken et al. 2007; Kawaguchi et al. 2007; Remijan et al. 2007), C10H (Remijan et al. 2023; Pardo et al. 2023), CN (Agúndez et al. 2010), C3N (Thaddeus et al. 2008), C5N (Cernicharo et al. 2008, 2020b), and C7N (Cernicharo et al. 2023a). The same radicals could also produce, via protonation, the HCnH+ and HCnN+ radical cations. These species have escaped detection due to the lack of a dipole moment for the HCnH+ symmetric species, and the lack of rotational spectroscopy for the HCnN+ ones. These species could play an important role in the chemistry of interstellar clouds and the detection of the polar HCnN+ species could be achieved with sensitive line surveys such as QUIJOTE.

In this Letter we present the discovery – for the first time in space – of the cationic radicals HC5N+ and HC7N+ through the observation of seven and eight of their rotational transitions, respectively. We discuss the possible formation and destruction paths of the cations and conclude that the adopted reaction rates of H2 with these cationic radicals limit their modelled abundances. Other possible routes for their formation, such as the reaction of CN with the cations C2nH2+, have also been explored.

2. Observations

The observational data used in this work are part of QUIJOTE (Cernicharo et al. 2021a), a spectral line survey of TMC-1 in the Q band carried out with the Yebes 40m telescope at the position αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″, corresponding to the cyanopolyyne peak (CP) in TMC-1. The receiver was built within the Nanocosmos project2 and consists of two cold high-electron mobility transistor amplifiers covering the 31.0–50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures vary between 16 K at 32 GHz and 30 K at 50 GHz. The back ends are 2 × 8 × 2.5 GHz fast Fourier transform spectrometers with a spectral resolution of 38 kHz, providing the whole coverage of the Q band in both polarizations. A detailed description of the system is given by Tercero et al. (2021), and details on the QUIJOTE line survey observations have been previously provided (Cernicharo et al. 2021a, 2023a,b, 2024a,b). The frequency switching method has been used for all observations. The data analysis procedure has been described by Cernicharo et al. (2022b). The total observing time on source is 1202 h and the measured sensitivity varies between 0.07 mK at 32 GHz and 0.2 mK at 49.5 GHz.

The main beam efficiency can be given across the Q band by ηeff = 0.797 exp[−(ν(GHz)/71.1)2]. The forward telescope efficiency is 0.95 and the beam size at half power intensity is 54.4″ and 36.4″ at 32.4 and 48.4 GHz, respectively.

The absolute intensity calibration uncertainty is 10%. However, the relative calibration between lines within the QUIJOTE survey is certainly better because all of them are observed simultaneously and have the same calibration uncertainties and systematic effects. The data were analysed with the GILDAS package3.

3. Results

Line identification in this work has been performed using the MADEX code (Cernicharo 2012), in addition to the CDMS and JPL catalogues (Müller et al. 2005; Pickett et al. 1998). The intensity scale utilized in this study is the antenna temperature (). Consequently, the telescope parameters and source properties have been used when modelling the emission of the different species to produce synthetic spectra in this temperature scale. In this work we assume a velocity for the source relative to the local standard of rest of 5.83 km s−1 (Cernicharo et al. 2020c). The source is assumed to be circular with a uniform brightness temperature and a radius of 40″ (Fossé et al. 2001). The procedure to derive line parameters is described in Appendix A. The observed line intensities have been modelled using a local thermodynamical equilibrium (LTE) hypothesis, or a large velocity gradient (LVG) approach. In the later case, MADEX uses the formalism described by Goldreich & Kwan (1974).

3.1. Discovery of HC5N+

Among the strongest unknown features in the QUIJOTE line survey, we have found a series of seven lines in harmonic relation with half-integer quantum numbers from Ju = 25/2 to 37/2 (see Fig. 1; Ju and Jl denote the upper and lower quantum numbers, respectively). The first two transitions (Ju = 25/2 and 27/2) of this molecule show four fine and/or hyperfine structure components, while the rest of the transitions show a remarkable line broadening. The frequency centroids of these lines have been derived and are given in Table A.1. Taking into account the considerable variation of line profiles among the transitions, we estimate an uncertainty of 40 kHz for these frequency centroids. They can be fitted with the standard formula ν = 2 BeffJ − 4 DeffJ3, with Beff = B0(1 ± B0/ASO) (Townes & Schawlow 1975). We derived Beff = 1336.662 ± 0.001 MHz and Deff = 27.4 ± 2.6 Hz. These parameters have to be considered as effective as they reproduce the centroid of the line, which can be affected by the fine and hyperfine structure of the observed transitions. The standard deviation of the fit is 26 kHz. The total integrated intensity, mean velocity, and equivalent line width of the observed lines are given in Table A.1. We have searched for other lines with a similar line profile over the entire QUIJOTE frequency coverage and none have been found. We have also checked that a molecule with B = Beff/2 and integer quantum numbers cannot be assigned to our data because all lines with J being even would be missing. Given the excellent fit obtained just with two parameters, we can conclude that the observed transitions are due to a linear open shell molecule with a 2Π electronic ground state and B ∼ 1336.7 MHz (hereafter, referred to as B1336) The lines are not detected towards the carbon-rich evolved star IRC+10216, a source rich in cyanopolyynes molecules (HC2n + 1N), CnH and CnN radicals, and their anions (see, e.g. Cernicharo et al. 2020b, 2023a; Remijan et al. 2023; Pardo et al. 2023, and references therein).

thumbnail Fig. 1.

Observed transitions of HC5N+ (left column) and HC7N+ (right column) in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milli Kelvin. Blanked channels correspond to negative features produced when folding the frequency-switched data. The red line shows the computed synthetic spectra for the lines of the two species (see Sects. 3.1 and 3.2). The physical parameters used for the models are given in Sect. 3.3. The centroids of the modelled lines of HC5N+ have been fixed to the observed ones (see Table A.1) and the simulated hyperfine components have been added to these centroids. For HC7N+ the line frequencies are given in Table A.2.

The derived rotational constant is 0.3% larger than that of HC5N, while the distortion constant is very close to that of this species (B = 1331.333 MHz, D = 30.1 Hz; Bizzocchi et al. 2004). The excellent agreement with the distortion constant of HC5N and that of other similar species (see Appendix B) gives strong support to a linear or quasi-linear open shell species. In Appendix B we discuss all possible candidates for the series of B1336 lines. The most promising candidate is the cationic radical HC5N+. Unfortunately, its rotational spectrum has not been observed in the laboratory. However, it has been observed in the optical through its electronic X2Π → A2Π transition by Sinclair et al. (1999a). Their spectral resolution allowed for the rotational structure of the electronic band providing a rotational constant B0 = 1337.5 ± 0.2 MHz to be resolved, in excellent agreement with that derived for B1336. They also derived the spin-orbit constant, ASO, to be −35.71 ± 0.41 cm−1 but no Λ doubling was observed. Hence, HC5N+ has an inverted 2Π electronic ground state, that is, the Ω = 3/2 ladder is the lowest in energy, with the Ω = 1/2 one more than 50 K above it. Hence, the lines from the 2Π1/2 ladder are extremely weak in TMC-1 due to the low kinetic temperature of this cloud (∼9 K, Agúndez et al. 2023b).

On this basis, we conclude that the lines observed in TMC-1 correspond to the rotational transitions of HC5N+ in its 2Π3/2 ladder. Taking into account the ASO constant derived by Sinclair et al. (1999a), the rotational constant B0 of HC5N+ in its ground vibrational state can be derived from the relation Beff = B0 (1 + B0/ASO) to be B0 = 1338.331 ± 0.001 MHz, which is in very good agreement with the value derived by these authors from less accurate optical observations. Quantum chemical calculations for HC5N+ indicate that the molecule has a large dipole moment between 6.5 and 6.9 D (see Appendix C, Zhang et al. 2012, and Gans et al. 2019).

The lack of nearby features similar to those observed indicates that the Λ-doubling, if any, is small (see Fig. 1). In fact, we can discard a significant fine structure as the first electronic 2Σ state has an energy above X2Π of ∼21 800 cm−1 (Gans et al. 2019). The Λ-doubling for a transition J+1→ J of a 2Π3/2 state produces a splitting between the fine structure components that can be approached as ±3qB/ASO(J+1/2)(J+3/2), with q = 2B2/Δ(2Σ−2Π), where Δ(2Σ−2Π) is the energy difference between the 2Σ and the 2Π electronic states (Mulliken & Christy 1931). Using the values given above, we can compute the expected splitting for the J = 37/2–35/2 transition to be ±8 kHz, that is, much smaller than our spectral resolution. Hence, the spectral pattern observed for the lowest frequency transitions of our series must correspond to the magnetic hyperfine effects due to the interaction between H and N nuclear spin with the electron orbital and electron spin angular momenta. The hyperfine splitting produces several lines, many of them mutually blended due to the high J values of the observed transitions.

Although the identification of HC5N+ appears robust, the assignment of the quantum numbers to the hyperfine lines observed in TMC-1 is rather difficult. Hence, we have simulated the expected spectrum with the SPFIT code (Pickett 1991) using the results of our quantum chemical calculations for the hyperfine constants as described in Appendix C. The rotational and distortion constants have been fixed to the values derived from the frequency centroids (see above). Figure C.1 shows the simulated spectra together with the observations for the first four transitions. Although the agreement between the synthetic spectrum and observations is not perfect, a similarity between the patterns can be observed. Slightly different values for H and N hyperfine parameters from those predicted by quantum chemical calculations reproduce the observed patterns and line shapes satisfactorily, as can be seen in Figs. 1 and C.1. The estimated hyperfine parameters are given in Table C.1. Although the agreement between the modelled and observed lines is good and fully supports the identification of B1336 with HC5N+, only laboratory observations of the low-J rotational transitions of this radical will permit accurate values of the hyperfine constants to be determined.

3.2. Discovery of HC7N+

Given the significant increase in the dipole moment of the HCnH+ series with the number of carbon atoms (Zhang et al. 2012), we could expect to detect HC7N+, the following member of the series with n being odd. This molecule has been observed in the optical with a high spectral resolution, allowing the rotational structure of its X2Π → A2Π transition to be resolved (Sinclair et al. 2000). These authors derived B0 = 568.6 ± 0.2 MHz and ASO = −36 cm−1, which provides an estimate for the effective rotational constant of the 2Π3/2 ladder of 568.3 MHz.

We have explored the QUIJOTE data around the expected position of the lines of HC7N+ and found eight lines in harmonic relation with half integer quantum numbers from Ju = 55/2 to 69/2. The lines are shown in the right column of Fig. 1 and their line parameters are given in Table A.2. The lines appear slightly broadened but without any obvious hyperfine splitting, as it could be expected for their high rotational quantum numbers. Moreover, for similarity with HC5N+, the expected Λ-doubling splitting should be very small. We have checked that a close shell molecule with B = Beff/2 and, hence, that integer quantum numbers cannot be assigned to the QUIJOTE data. If this were the case, all lines with J even would be missing in our data.

The eight observed lines can be fitted with Beff = 567.85036 ± 0.00037 MHz and Deff = 4.01 ± 0.19 Hz (hereafter, referred to as B568). The standard deviation of the fit is 9.3 kHz. These constants are very close to those of HC7N (B = 564.001 MHz and D = 4.04 Hz, Bizzocchi & Degli Esposti 2004) and those of C7N (B = 582.685 MHz and D = 4.0 Hz, Cernicharo et al. 2023a). The agreement with the optical observations is excellent. Consequently, we conclude that the series of lines (B568) pertain to the cation radical HC7N+. Other possible candidates, in particular the radical C7N, are discussed and discarded in Appendix D. We have performed quantum chemical calculations for HC7N+ and derived a dipole moment of 7.9 D (see Appendix C).

3.3. Column densities

Using the derived molecular constants, and for HC5N+ adopting the dipole moment of 6.5 D derived in this work (see Appendix C), we derived a rotational temperature, Trot, of 5.5 ± 0.5 K and a column density of (9.9 ± 1.0) × 1010 cm−2. In order to compare the column density of HC5N+ with those of C5N and C5N, we have analysed their lines in the QUIJOTE data. These two species were previously studied by Cernicharo et al. (2020b) with less sensitive data. The observed lines of C5N with the present version of QUIJOTE are shown in Fig. E.1 and those of C5N are shown in Fig. E.2 (see Appendix E). The line parameters for these two species are given in Table E.1. The best fit to the observed line intensities of C5N has been obtained for a rotational temperature of 8.6 ± 0.2 K and a column density of (4.8 ± 0.3) × 1011 cm−2. For C5N we derived Trot = 7.9 ± 0.4 K and N = (8.5 ± 0.5) × 1010 cm−2. The rotational temperatures for these two species have been obtained through a rotational diagram (see Appendix E). The value of the column density of C5N is a factor of two lower than that derived by Cernicharo et al. (2020b) due to the different rotational temperatures used for its determination, with the value previously reported being based on data with a significantly lower signal-to-noise ratio (S/N). The C5N/HC5N+ abundance ratio is 1.2 ± 0.2 and the C5N/HC5N+ one is 4.8 ± 0.8.

Finally, a molecule to consider in this analysis is HC5N. A detailed study using the weak hyperfine satellite lines of all transitions of cyanodiacetylene in the QUIJOTE line survey provides a rotational temperature of 7.5 ± 0.3 K and a column density of (6.6 ± 0.1) × 1013 cm−2 (Cernicharo et al., in prep.). Hence, the HC5N/HC5N+ abundance ratio is 670 ± 80.

For HC7N+ we have adopted a dipole moment of 7.9 D (see Appendix C) and derived Trot = 8.5 ± 0.5 K and N = (2.3 ± 0.2) × 1010 cm−2. The column density of HC7N can be derived from all lines of this species within the QUIJOTE data. We derived Trot = 7.8 ± 0.1 K and N = (2.3 ± 0.1) × 1013 cm−2 (Cernicharo et al. in prep.). Hence, the abundance ratio between HC7N and HC7N+ is 1000 ± 150.

The column density of the anion C7N is (5.0 ± 0.5) × 1010 cm−2 (Cernicharo et al. 2023a), hence the abundance ratio between C7N and HC7N+ is 2.2 ± 0.2, that is to say it is similar to that of C5N over HC5N+. Finally, the abundance ratio between HC5N+ and HC7N+ is 4.2 ± 0.5, which is of the order of the abundance ratio between the neutral species (2.9 ± 0.2). C7N has not been detected yet (see Appendix D), hence an abundance ratio between this neutral radical and its protonated species cannot be determined.

In addition to HC5N+ and HC7N+, other cyanopolyyne cations could be present in our data, in particular HC3N+. No high spectral resolution optical observations have been reported for this species in spite of its potential interest in the chemistry, not only of the interstellar medium (ISM), but also of Titan (Vuitton & Yelle 2006). The molecule will also have an inverted 2Π ground electronic state with ASO = −44 ± 2 cm−1 (Gans et al. 2016). Its rotational constant has been estimated to be between 4565 and 4594 MHz, depending of the level of theory used in the calculations (Zhang et al. 2012). The predicted dipole moment is ∼5 D. Only three transitions, J = 7/2–5/2, J = 9/2–7/2, and J = 11/2–9/2, will be within the frequency coverage of QUIJOTE. These low-J transitions will exhibit a significant hyperfine structure that, although helping in their identification, will dilute the corresponding line intensities. We could expect, similar to the case of HC5N+ and HC7N+, to have a column density ∼1/1000 that of the neutral HC3N. Hence, adopting the column density derived for HC3N of 1.9 × 1014 cm−2 (Tercero et al. 2024), the column density of the cation HC3N+ could be ∼1.9 × 1011 cm−2. Adopting the collisional rates of HC3N as a proxy for the unsplitted levels of HC3N+, then the expected intensities for the unsplitted transitions will be ∼40–60 mK (under a LVG calculation). Even when splitted, these lines will be prominent in our data and easily identified. We have analysed the QUIJOTE data for unidentified lines with frequencies in harmonic relation for a Beff between 4545 and 4615 MHz without success. Hence, we conclude that under the adopted assumptions, the abundance of HC3N+ has to be ≤1011 cm−2 if its effective rotational constant is within the explored range.

4. Discussion

The cations HC5N+ and HC7N+ can be seen as the protonated forms of C5N and C7N, respectively. These two radicals have a high proton affinity, 870 kJ mol−1 and 933.8 kJ mol−1, respectively, and thus reactions of proton transfer from abundant cations such as HCO+ are exothermic and can provide a formation pathway to HC5N+ and HC7N+ in TMC-1. Protonation of a neutral species M through efficient proton donors such as HCO+ results in abundance ratios MH+/M in the range 10−3-10−1 (Agúndez et al. 2022), while the abundance ratio HC5N+/C5N of ∼0.2 derived in TMC-1 is higher, which probably implies that proton transfer is not the main formation route to HC5N+. There is however one important aspect to consider that is related to the dipole moment of C5N. As discussed previously (Cernicharo et al. 2008, 2020b), the true dipole moment of C5N could be an average between that of its ground state 2Σ, which has been calculated to be 3.385 D (Botschwina 1996), and that of its first electronic excited state 2Π, which lies very low in energy (∼500 cm−1; Botschwina 1996) and has a dipole moment of ∼0.2–1 D (Pauzat et al. 1991; Cernicharo et al. 2008). Hence, C5N could have a dipole moment between these two values in the case of admixing between the 2Σ and the 2Π states. If so, the column density of C5N would increase by a factor of four and the HC5N+/C5N ratio would decrease to ∼0.05. In this case the C5N/C5N abundance ratio would also decrease by a factor of four, from 0.43 to 0.1, in better agreement with anion-to-neutral ratios of species of a similar size (Agúndez et al. 2023b). A similar effect has been analysed for C4H by Oyama et al. (2020), leading to a revised value of its dipole moment that removes previous inconsistencies in the column densities derived for this molecule in different astrophysical environments.

In order to shed additional light on the formation of HC5N+ and HC7N+, we have carried out chemical modelling calculations using typical parameters of cold dense clouds (Agúndez & Wakelam 2013) and the chemical network from the latest release of the UMIST database for astrochemistry (Millar et al. 2024). According to the chemical model, the most important reactions in the formation of cyanopolyynes cations HCnN+ (n = 1, 3, 5, 7, 9, ...) are

(1)

(2)

while the reactions of proton transfer from HCO+, H3O+, and to the radicals CnN contribute as well, but to a lower extent. The calculated abundances for the series of cations HCnN+ are shown as a function of time in Fig. 2, where we also plotted the abundances of HC5N+ and HC7N+ observed in TMC-1. It is seen that the cations HCnN+ appear early, after a few 103 yr, and last until approximately 105 yr. However, the peak abundances calculated for HC5N+ and HC7N+ lie about 20 times below the observed values.

thumbnail Fig. 2.

Calculated fractional abundances for the series of ionized cyanopolyynes HCN+, HC3N+, HC5N+, HC7N+, and HC9N+ as a function of time. Horizontal dotted lines correspond to the values observed in TMC-1 for HC5N+ and HC7N+, adopting a column density of H2 of 1022 cm−2 (Cernicharo & Guélin 1987).

We have explored the possibility that additional formation routes are missing in the UMIST network. For instance, the reactions of CN with the cations C4 and C6 could be a source of HC5N+ and HC7N+, respectively. These reactions are not included in the UMIST database and to our knowledge have not been studied. However, the analogous reaction CN + C2H2+ is included in the UMIST network with a high rate coefficient, estimated originally by Prasad & Huntress (1980), and theoretical calculations support that it is fast at low temperatures and yields HC3N+ as a main product (Redondo et al. 2000). If we include these reactions, they contribute somewhat to the formation of HC5N+ and HC7N+, although the calculated abundances of these cations do not experience any appreciable enhancement.

The main destruction path of the cations HCnN+ at times between 104 and 106 yr is the reaction with H2, and to a lower extent (10–50 times less) the dissociative recombination with electrons. There are only a few molecules detected in the ISM that react with H2. A good example is C3H+ (Pety et al. 2012; Cernicharo et al. 2022a), which is known to react with H2 with a moderate rate coefficient of 2.6 × 10−11 cm3 s−1 (McEwan et al. 1999). The larger analogous species C5H+ is assumed to react very slowly with H2. The cation HC3N+ is known to react with H2 with a relatively low rate coefficient of 7 × 10−12 cm3 s−1 (Knight et al. 1985), and longer analogous cations HCnN+ are assumed to react with H2 with a similar rate coefficient of 5 × 10−12 cm3 s−1 (the rate coefficient of the reaction HC5N+ + H2, taken as 10−9 cm3 s−1 in the UMIST network, was decreased to 5 × 10−12 cm3 s−1). It could happen that the underestimation of the abundances of HC5N+ and HC7N+ in the chemical model is not due to missing formation routes but to a too fast destruction through their reaction with H2. It would therefore be interesting to investigate the rate coefficient of the reactions of HC5N+ and HC7N+ (and also C5H+) with H2.


1

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

Acknowledgments

We thank Ministerio de Ciencia e Innovación of Spain (MICIU) for funding support through projects PID2019-106110GB-I00, and PID2019-106235GB-I00. We also thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. We thank the Consejo Superior de Investigaciones Científicas (CSIC; Spain) for funding through project PIE 202250I097. The present study was also supported by Ministry of Science and Technology of Taiwan and Consejo Superior de Investigaciones Científicas under the MoST-CSIC Mobility Action 2021 (Grants 11-2927-I-A49-502 and OSTW200006).

Note added to proofs. We have recently found a series of lines that we have been able to assign to HC3N+. The lines appear below the frequency ranges explored in this work. The result will be submitted soon (Cabezas et al. in prep.), and confirms the presence of cationic cyanopolyynes in TMC-1.

References

  1. Agúndez, M., & Wakelam, V. 2013, Chem. Rev., 113, 8710 [Google Scholar]
  2. Agúndez, M., Cernicharo, J., Guélin, M., et al. 2010, A&A, 517, L2 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  3. Agúndez, M., Cabezas, C., Marcelino, N., et al. 2022, A&A, 659, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  4. Agúndez, M., Cabezas, C., Marcelino, N., et al. 2023a, A&A, 669, L1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  5. Agúndez, M., Marcelino, N., Tercero, B., et al. 2023b, A&A, 677, A106 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  6. Aoki, K. 2014, Adv. Space Res., 54, 1651 [CrossRef] [Google Scholar]
  7. Bizzocchi, L., & Degli Esposti, C. 2004, ApJ, 614, 518 [NASA ADS] [CrossRef] [Google Scholar]
  8. Bizzocchi, L., Degli Esposti, C., & Botschwina, P. 2004, J. Mol. Spectrosc., 225, 145 [Google Scholar]
  9. Botschwina, P. 1996, Chem. Phys. Lett., 259, 627 [NASA ADS] [CrossRef] [Google Scholar]
  10. Botschwina, P., Horn, M., Markey, K., & Oswald, R. 1997, Mol. Phys., 92, 381 [NASA ADS] [CrossRef] [Google Scholar]
  11. Brünken, S., Gupta, H., Gottlieb, C. A., et al. 2007, ApJ, 664, L43 [CrossRef] [Google Scholar]
  12. Cabezas, C., Agúndez, M., Marcelino, N., et al. 2022a, A&A, 657, L4 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  13. Cabezas, C., Agúndez, M., Marcelino, N., et al. 2022b, A&A, 659, L8 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  14. 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]
  15. Cernicharo, J., & Guélin, M. 1987, A&A, 176, 299 [Google Scholar]
  16. Cernicharo, J., Guélin, M., Agúndez, M., et al. 2007, A&A, 467, L37 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  17. Cernicharo, J., Guélin, M., Agúndez, M., et al. 2008, ApJ, 688, L83 [NASA ADS] [CrossRef] [Google Scholar]
  18. Cernicharo, J., Marcelino, N., Agúndez, M., et al. 2020a, A&A, 642, L17 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  19. Cernicharo, J., Marcelino, N., Pardo, J. R., et al. 2020b, A&A, 641, L1 [Google Scholar]
  20. Cernicharo, J., Marcelino, N., Agúndez, M., et al. 2020c, A&A, 642, L8 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  21. Cernicharo, J., Agúndez, M., Kaiser, R. J., et al. 2021a, A&A, 652, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  22. Cernicharo, J., Cabezas, C., Endo, Y., et al. 2021b, A&A, 646, L3 [EDP Sciences] [Google Scholar]
  23. Cernicharo, J., Cabezas, C., Bailleux, S., et al. 2021c, A&A, 646, L7 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  24. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021d, A&A, 656, L21 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  25. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2022a, A&A, 657, L16 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  26. Cernicharo, J., Fuentetaja, R., Agúndez, M., et al. 2022b, A&A, 663, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  27. Cernicharo, J., Pardo, J. R., Cabezas, C., et al. 2023a, A&A, 670, L19 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  28. Cernicharo, J., Fuentetaja, R., Agúndez, M., et al. 2023b, A&A, 680, L4 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  29. Cernicharo, J., Tercero, B., Cabezas, C., et al. 2024a, A&A, 682, L13 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  30. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2024b, A&A, 682, L4 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  31. Cordiner, M. A., Charnley, S. B., Kisiel, Z., et al. 2017, ApJ, 850, A187 [NASA ADS] [CrossRef] [Google Scholar]
  32. Fossé, D., Cernicharo, J., Gerin, M., & Cox, P. 2001, ApJ, 552, 168 [Google Scholar]
  33. Frisch, M. J., Trucks, G. W., Schlegel, H. B., et al. 2016, Gaussian 16 Revision A.03 (Wallingford CT: Gaussian, Inc.) [Google Scholar]
  34. Frosch, R. A., & Foley, H. M. 1952, Phys. Rev., 88, 1337 [CrossRef] [Google Scholar]
  35. Fuentetaja, R., Agúndez, M., Cabezas, C., et al. 2023, A&A, 667, L4 [Google Scholar]
  36. Gans, B., Lamarre, N., Broquier, M., et al. 2016, J. Chem. Phys., 145, 234309 [NASA ADS] [CrossRef] [Google Scholar]
  37. Gans, B., Boyé-Péronne, S., & Liévin, J. 2019, J. Chem. Phys., 150, 244303 [NASA ADS] [CrossRef] [Google Scholar]
  38. Goldreich, P., & Kwan, J. 1974, ApJ, 189, 441 [CrossRef] [Google Scholar]
  39. Gordon, V. D., McCarthy, M. C., Apponi, A. J., & Thaddeus, P. 2002, ApJS, 138, 297 [CrossRef] [Google Scholar]
  40. Guélin, M., Neiningert, N., & Cernicharo, J. 1998, A&A, 335, L1 [Google Scholar]
  41. Hirahara, Y., Ohshima, Y., & Endo, Y. 1994, J. Chem. Phys., 101, 7342 [Google Scholar]
  42. Kasai, Y., Sumiyoshi, Y., Endo, Y., & Kawaguchi, K. 1997, ApJ, 477, L65 [NASA ADS] [CrossRef] [Google Scholar]
  43. Kawaguchi, K., Fujimori, R., & Aimi, S. 2007, PASJ, 59, L47 [NASA ADS] [CrossRef] [Google Scholar]
  44. Knight, J. S., Freeman, C. G., McEwan, M. J., et al. 1985, Int. J. Mass Spectrom. Ion Proc., 67, 317 [NASA ADS] [CrossRef] [Google Scholar]
  45. Kohguchi, H., Ohshima, Y., & Endo, Y. 1994, J. Chem. Phys., 101, 6463 [Google Scholar]
  46. Marcelino, N., Agúndez, M., Tercero, B., et al. 2020, A&A, 643, L6 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  47. McCarthy, M. C., Gottlieb, C. A., Gupta, H., et al. 2006, ApJ, 652, L141 [NASA ADS] [CrossRef] [Google Scholar]
  48. McEwan, M. J., Scott, G. B. I., Adams, N. G., et al. 1999, ApJ, 513, 287 [Google Scholar]
  49. McGuire, B. A., Burkhardt, A. M., Shingledecker, C. N., et al. 2017, ApJ, 843, L28 [NASA ADS] [CrossRef] [Google Scholar]
  50. Millar, T. J., Walsh, C., Van de Sande, M., & Markwick, A. J. 2024, A&A, 682, A109 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  51. Mohamed, S., McCarthy, M. C., Cooksy, A. L., et al. 2005, J. Chem. Phys., 123 [CrossRef] [Google Scholar]
  52. Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, J. Mol. Struct., 742, 215 [Google Scholar]
  53. Mulliken, R. S., & Christy, A. 1931, Phys. Rev., 38, 87 [Google Scholar]
  54. Oyama, T., Ozaki, H., Yoshihiro, S., et al. 2020, ApJ, 890, 39 [NASA ADS] [CrossRef] [Google Scholar]
  55. Pardo, J. R., Cabezas, C., Agúndez, M., et al. 2023, A&A, 677, A55 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  56. Pauzat, F., Ellinger, Y., & McLean, A. D. 1991, ApJ, 369, L13 [NASA ADS] [CrossRef] [Google Scholar]
  57. Pety, J., Gratier, P., Guzmán, V., et al. 2012, A&A, 548, A68 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  58. Pickett, H. M. 1991, J. Mol. Spectrosc., 148, 371 [Google Scholar]
  59. Pickett, H. M., Poynter, R. L., Cohen, E. A., et al. 1998, J. Quant. Spectrosc. Radiat. Transf., 60, 883 [Google Scholar]
  60. Prasad, S. S., & Huntress, W. T., Jr 1980, ApJS, 43, 1 [NASA ADS] [CrossRef] [Google Scholar]
  61. Redondo, P., Ruiz, J. M., Boronat, R., et al. 2000, Chem. Phys. Lett., Theor. Chem. Acc., 104, 199 [CrossRef] [Google Scholar]
  62. Remijan, A. J., Hollis, J. M., Lovas, F. J., et al. 2007, ApJ, 664, L47 [NASA ADS] [CrossRef] [Google Scholar]
  63. Remijan, A. J., Scolaty, H. N., Burkhardt, A. M., et al. 2023, ApJ, 944, L45 [NASA ADS] [CrossRef] [Google Scholar]
  64. Silva, W. G. D. P., Cernicharo, J., Schlemmer, S., et al. 2023, A&A, 676, L1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  65. Sinclair, W. E., Pfluger, D., & Maier, J. P. 1999a, J. Chem. Phys., 111, 9600 [NASA ADS] [CrossRef] [Google Scholar]
  66. Sinclair, W. E., Pfluger, D., Linnartz, H., & Maier, J. P. 1999b, J. Chem. Phys., 110, 296 [NASA ADS] [CrossRef] [Google Scholar]
  67. Sinclair, W. E., Pfluger, D., Verdes, D., & Maier, J. P. 2000, J. Chem. Phys., 112, 8899 [NASA ADS] [CrossRef] [Google Scholar]
  68. Tercero, F., López-Pérez, J. A., Gallego, J. D., et al. 2021, A&A, 645, A37 [EDP Sciences] [Google Scholar]
  69. Tercero, B., Marcelino, N., Roueff, E., et al. 2024, A&A, 682, L12 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  70. Thaddeus, P., Gottlieb, C. A., Gupta, H., et al. 2008, ApJ, 677, 1132 [NASA ADS] [CrossRef] [Google Scholar]
  71. Townes, C. H., & Schawlow, A. L. 1975, Microwave Spectroscopy (New York: Dover) [Google Scholar]
  72. Vuitton, V., & Yelle, R. V. 2006, ApJ, 647, L175 [NASA ADS] [CrossRef] [Google Scholar]
  73. Werner, H. J., Knowles, P. J., Knizia, G., et al. 2020, MOLPRO, version 2020.2 [Google Scholar]
  74. Zhang, Y., Guo, J., & Zhang, J. 2012, Int. J. Mass Spectom., 309, 56 [NASA ADS] [CrossRef] [Google Scholar]

Appendix A: Line parameters

Line parameters for all observed transitions with the Yebes 40m radio telescope have been derived by fitting a Gaussian line profile to them using the GILDAS package. A velocity range of ±20 km s−1 around each feature was considered for the fit after a polynomial baseline was removed. Negative features produced in the folding of the frequency switching data were blanked before baseline removal.

The derived frequency centroids of the lines of HC5N+ are given, together with their integrated intensities and equivalent line widths, in Table A.1. The derived line parameters for HC7N+ are given in Table A.2.

Table A.1.

Estimated frequency centroid of the observed lines of HC5N+.

Table A.2.

Observed line parameters of HC7N+.

Appendix B: Possible candidates for B1336

Although the agreement between our rotational constant for B1336 with that derived from the optical observations of Sinclair et al. (1999a) is excellent, it is important to discard other possible candidates that could fit the rotational constant and be abundant enough in TMC-1. The isotopologues of HC5N have rotational constants below that of the main isotopologue and have integer quantum numbers. Moreover, no vibrationally bending excited states of HC5N, which have rotational constants larger than that of the ground state, could be responsible for the observed lines as these states will also have integer quantum numbers. Another molecule with close rotational and distortion constants is the C5N anion (B = 1388.867 MHz, D = 35.7 Hz; Cernicharo et al. 2008, 2020b). However, this species is a closed shell molecule. An interesting species to consider is the radical C5N, which also has close rotational and distortion constants (B = 1403.08 MHz, D = 50±10 Hz; Kasai et al. 1997) and has been detected in TMC-1 (Guélin et al. 1998). Nevertheless, this radical has a 2Σ ground state with integer values for N. Some of the bending vibrationally excited states of C5N will have a 2Π vibronic character with half-integer quantum numbers. Such excited states have been found in TMC-1 for C6H in its low-energy ν11 bending mode (Cernicharo et al. 2023b). However, similarly to C6H, we expect that the vibrationally excited states of C5N will have rotational constants larger than that of the ground state. Moreover, C5N is less abundant than C6H. Hence, the possibility that our lines are produced by C5N in a bending state can be disregarded. Other possible candidates with a rotational constant close to our value are C6H+ and C5N+. However, their expected ground electronic state is a bent 3A″ with (B+C)/2∼1392.6 MHz and 1394.7 MHz, respectively (Aoki 2014). These two molecules will produce lines with integer quantum numbers and can be discarded.

Some species such as NC4N+ and HC6H+ have 2Π inverted ground electronic states, with B = 1339.7±0.4 MHz (Sinclair et al. 1999a) and B = 1336.9±0.1 MHz (Sinclair et al. 1999b), respectively. Unfortunately, these molecular species are symmetric and lack a permanent dipole moment. Hence, in spite of the good match in the rotational constant, they cannot be the carrier of our lines. HC6D+ will have a small dipole moment. However, its rotational constant will be too low and the required column density too large to explain our lines. Another species with similar rotational and distortion constants is NC4NH+ (B = 1293.9 MHz, D = 29.8 Hz; Agúndez et al. 2023a). Although this species is a close shell molecule, it gives, together with those discussed above, a good indication of the structure and composition of the carrier of our lines. Species containing sulphur such as HCCCCS, which has been found in TMC-1 (Fuentetaja et al. 2023), can be discarded as its rotational constant is too large, B = 1435.3 MHz (Hirahara et al. 1994). Its lines are too weak to permit the detection of its deuterated isotopologue which could have a rotational constant around our B. Carbon chains containing oxygen such as HC4O (B∼2245 MHz; Kohguchi et al. 1994) or HC5O (B∼1293.6 MHz, D = 36 Hz; Mohamed et al. 2005), can also be discarded. HC5O has been been detected in TMC-1 by McGuire et al. (2017).

Taking into account the observational facts and the arguments developed above, we conclude that the most likely carrier has to be very close in structure to HC5N (which has a very close rotational constant) and C5N. The most plausible candidate is the radical HC5N+, the protonated species of C5N.

Appendix C: Molecular parameters for HC5N+ and HC7N+ from quantum chemical calculations

Several theoretical calculations for HC5N+ and HC7N+ species have been previously reported. However, we carried out different quantum chemical calculations for HC5N+ and HC7N+ molecules in order to estimate some spectroscopic parameters not reported in the literature before. We optimized the geometry of the HC5N+ radical at the same level of theory employed by Gans et al. (2019), ic-MRCI+Q/AVTZ, using the MOLPRO 2020 ab initio program package (Werner et al. 2020). The values for the Be rotational constant and the electric dipole moment of HC5N+ using this level of theory are 1339.7 MHz and 6.5 D, respectively. These values slightly differ from those reported by Gans et al. (2019). At the optimized geometry, we calculated the values for the hyperfine coupling constants at the B3LYP/cc-pVTZ level of theory using the GAUSSIAN 16 package (Frisch et al. 2016). The Frosch and Foley (Frosch & Foley 1952) hyperfine constants were derived from the dipole-dipole interaction tensor T and the Fermi contact constant bF using the relations as

(C.1)

(C.2)

(C.3)

We utilized the approximate relation (Townes & Schawlow 1975)

(C.4)

between a, c, and d to estimate the a constant as

(C.5)

The main contribution to the hyperfine coupling in the 2Π3/2 state comes from the h1 constant, defined as

(C.6)

The hyperfine coupling constants obtained for HC5N+ are shown in Table C.1. They were employed to predict the expected hyperfine pattern of HC5N+, as shown in the left panels of Figure C.1. As it can be seen, the predicted hyperfine splittings are larger than those observed, which means that the h1 constant for H and N are larger than those predicted. To obtain a better reproduction of the observed splitting of HC5N+, we systematically decreased the h1 constants for H and N with the same factor. The best agreement between the simulation and the observations are obtained with h1 values 1.6 times smaller than those predicted by our calculations. The hyperfine coupling constants needed to reproduce the observations are shown in Table C.1. As it can be seen the bF, Taa and Tbb modified constants are not very different to those predicted by our calculations, which is additional proof supporting the identification of HC5N+ as the carrier of our observed lines.

thumbnail Fig. C.1.

Same as Fig. 1, but comparing observations with modelled spectra using the hyperfine constants of HC5N+ derived from our quantum chemical calculations (see App. C). The blue spectra in the left panels show the modelled lines. The panels of the right column show in red the final modelled spectra after fine-tuning the hyperfine constants (identical to those shown in Fig. 1). The centroids of the modelled lines of HC5N+ in both columns have been fixed to the observed ones (see Table A.1).

Table C.1.

Hyperfine coupling constants of HC5N+ (all in MHz).

In the case of HC7N+, we carried out molecular structure optimization calculations at the RCCSD(T)/cc-pVTZ level of theory using the MOLPRO 2020 ab initio program package (Werner et al. 2020). The calculated values for the rotational constant Be and the electric dipole moment are 561.8 MHz and 7.9 D, respectively. As in the case of HC5N+, we also estimated the values of the hyperfine coupling constants at the B3LYP/cc-pVTZ level of theory using the GAUSSIAN 16 package (Frisch et al. 2016). Our predictions using those constants show that the hyperfine structures are completely collapsed for all the expected lines of HC7N* in the Q band, in agreement with our observations.

Appendix D: Possible candidates for B568

Similar to HC5N+, several species with a 2Π electronic ground state could have rotational constants close to that of B568. As commented on in Sect. 3.2, the rotational constant of B568 is very close to that of the closed shell species HC7N and C7N, which suggests that B568 is structurally very similar to these species. Isotopologues or vibrationally excited states of these species have integer quantum numbers and can be discarded.

An important species that should be present, although no detected yet, is C7N. Its anionic form, C7N, has been detected in TMC-1 and IRC+10216 (Cernicharo et al. 2023a). Ab initio calculations by Botschwina et al. (1997) show a peculiar effect for this species, with a ground electronic state 2Π having a low dipole moment (∼1 D) and an electronic excited state 2Σ at ∼300 cm−1 above, and with a large dipole moment. The rotational constants for the 2Π and 2Σ states are ∼585 and ∼583.1 MHz, respectively. The estimated uncertainty on these constants is 2 MHz (Botschwina et al. 1997). It was not clear from these calculations what electronic ground state could have C7N. Even if C7N has a 2Π state, it does not fit B568 as the difference between the predicted and observed rotational constants is ∼20 MHz. Moreover, if the ground state of C7N is 2Π, then we could expect a significant Λ-doubling splitting in its 2Π1/2 and 2Π3/2 ladders due to the presence of the very close 2Σ state. Hence, C7N cannot be the carrier of B568.

We have explored our data searching for C7N but not obvious lines can be assigned at the present level of sensitivity. Due to the low dipole moment of the 2Π state, its lines will be well below our 3σ detection limit. We have also searched for a series of doublets with B = 580-590 MHz that could arise from the 2Σ state without success.

HC7O has a 2Π ground electronic state, but its rotational constant has been measured in the laboratory to be 549.2 MHz (Mohamed et al. 2005). This molecule has been detected in TMC-1 (Cordiner et al. 2017; Cernicharo et al. 2021d), but its isotopologues will have rotational constants that are too low. Finally, considering S-bearing species, the first molecule with a rotational constant close to that of B568 is the HC6S 2Π radical. It has been observed in the laboratory and its rotational constant is of 572.1 MHz (Gordon et al. 2002). The same authors also observed H2C6S which has (B+C)/2 = 559.8 MHz, but the molecule is a close shell asymmetric species. None of these two species are detected in our data. Hence, we can exclude their isotopologues or vibrationally excited states as possible carriers of B568. Hence, we have to conclude, as explained in Sect. 3.2, that the best candidate for the carrier of the lines of B568 is the cationic radical HC7N+.

Appendix E: C5N and C5N

In this work we have re-analysed the lines of C5N and C5N previously published by Cernicharo et al. (2020b). The lines of C5N are shown in Fig. E.1 and those of C5N in Fig. E.2. The derived line parameters are given in Table E.1. We have used a rotational diagram to derive the rotational temperature of these species and found Trot(C5N) = 8.6±0.2 K and Trot(C5N) = 7.9±0.4 K. The best fit to the column densities are N(C5N) = (4.8±0.3)×1011 and N(C5N) = (8.5±0.5)×1010 cm−2. The value of the column density of C5N is a factor two lower than that derived by Cernicharo et al. (2020b) due to the different rotational temperatures used for its determination. The value previously reported, Trot = 6.3±0.5 K, was derived from data with a significant lower sensitivity.

thumbnail Fig. E.1.

Observed transitions of C5N in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency adopting a velocity for the source of 5.83 km s−1 (Cernicharo et al. 2020c). The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milli Kelvin. Blank channels correspond to negative features produced when folding the frequency-switched data. The red line shows the modelled spectra for these lines (see App. E).

thumbnail Fig. E.2.

Observed transitions of C5N in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency adopting a velocity for the source of 5.83 km s−1 (Cernicharo et al. 2020c). The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. Blank channels correspond to negative features produced when folding the frequency-switched data. The red line shows the modelled spectra for these lines (see App. E).

Table E.1.

Observed line parameters for C5N and C5N.

The data used in this work for C7N are those of Cernicharo et al. (2023a). The present data do not provide a significant improvement to the S/N. Hence, the previously derived column density for this species does not require further revision.

All Tables

Table A.1.

Estimated frequency centroid of the observed lines of HC5N+.

Table A.2.

Observed line parameters of HC7N+.

Table C.1.

Hyperfine coupling constants of HC5N+ (all in MHz).

Table E.1.

Observed line parameters for C5N and C5N.

All Figures

thumbnail Fig. 1.

Observed transitions of HC5N+ (left column) and HC7N+ (right column) in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milli Kelvin. Blanked channels correspond to negative features produced when folding the frequency-switched data. The red line shows the computed synthetic spectra for the lines of the two species (see Sects. 3.1 and 3.2). The physical parameters used for the models are given in Sect. 3.3. The centroids of the modelled lines of HC5N+ have been fixed to the observed ones (see Table A.1) and the simulated hyperfine components have been added to these centroids. For HC7N+ the line frequencies are given in Table A.2.

In the text
thumbnail Fig. 2.

Calculated fractional abundances for the series of ionized cyanopolyynes HCN+, HC3N+, HC5N+, HC7N+, and HC9N+ as a function of time. Horizontal dotted lines correspond to the values observed in TMC-1 for HC5N+ and HC7N+, adopting a column density of H2 of 1022 cm−2 (Cernicharo & Guélin 1987).

In the text
thumbnail Fig. C.1.

Same as Fig. 1, but comparing observations with modelled spectra using the hyperfine constants of HC5N+ derived from our quantum chemical calculations (see App. C). The blue spectra in the left panels show the modelled lines. The panels of the right column show in red the final modelled spectra after fine-tuning the hyperfine constants (identical to those shown in Fig. 1). The centroids of the modelled lines of HC5N+ in both columns have been fixed to the observed ones (see Table A.1).

In the text
thumbnail Fig. E.1.

Observed transitions of C5N in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency adopting a velocity for the source of 5.83 km s−1 (Cernicharo et al. 2020c). The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milli Kelvin. Blank channels correspond to negative features produced when folding the frequency-switched data. The red line shows the modelled spectra for these lines (see App. E).

In the text
thumbnail Fig. E.2.

Observed transitions of C5N in TMC-1. Quantum numbers are indicated at the top right of each panel. The abscissa corresponds to the rest frequency adopting a velocity for the source of 5.83 km s−1 (Cernicharo et al. 2020c). The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. Blank channels correspond to negative features produced when folding the frequency-switched data. The red line shows the modelled spectra for these lines (see App. E).

In the text

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