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Open Access
Issue
A&A
Volume 670, February 2023
Article Number L19
Number of page(s) 6
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202245816
Published online 15 February 2023

The Authors 2023

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

The presence of carbon-chain negative ions in space was predicted on the grounds that electron radiative attachment is efficient for open-shell molecules with large electron affinities (Dalgarno & McCray 1973; Sarre 1980; Herbst 1981). The predicted abundance ratio between an anion and its corresponding neutral species increases with the size of the molecule, which is a consequence of the increasing density of vibrational states as the number of atoms in the molecule increases.

The first anion detected in space, C6H−, was observed towards TMC-1 (McCarthy et al. 2006). Lines from this species were already reported as unidentified features in the line survey of IRC +10216 performed with the Nobeyama 45m telescope by Kawaguchi et al. (1995). Aoki (2000) suggested that the carrier of these lines was C6H− from ab initio calculations, which was finally confirmed by the laboratory observations of McCarthy et al. (2006). The presence of this anion in space attracted significant attention and motivated searches for other hydrocarbon anions in interstellar and circumstellar clouds. C4H− was first discovered in the circumstellar cloud IRC +10216 by Cernicharo et al. (2007) and then in the interstellar clouds L1527, Lupus-1A, and TMC-1 (Agúndez et al. 2008; Sakai et al. 2008, 2010; Cordiner et al. 2013). Following the observation of C8H− in the laboratory (Gupta et al. 2007), this anion was found in TMC-1 (Brünken et al. 2007), IRC +10216 (Kawaguchi et al. 2007; Remijan et al. 2007), and Lupus-1A (Sakai et al. 2010). Remijan et al. (2023) recently announced the detection of C10H− in TMC-1. Although we searched for it and other anions, we cannot confirm any line of this species in the QUIJOTE1 line survey, probably due to the high-J levels involved in the rotational transitions of C10H− in our data (Ju ≥ 52 and Eu > 40 K).

The nitrile anions CN−, C3N−, and C5N− were first detected in the circumstellar envelope of the carbon-rich star IRC +10216 (Agúndez et al. 2010; Thaddeus et al. 2008; Cernicharo et al. 2008, 2020). CN− and C3N− were detected thanks to accurate laboratory frequencies (Gottlieb et al. 2007; Thaddeus et al. 2008; Amano 2008). However, the assignment of C5N− was based on ab initio calculations by Botschwina & Oswald (2008) and Aoki (2000). Although C5N− was the best candidate, the lack of precise laboratory frequencies prevented metal-bearing molecules being ruled out, such as MgC3N and MgC4H. Later on, these two molecules were identified in IRC +10216 based on ab initio calculations (Cernicharo et al. 2019). Moreover, a series of lines with the same rotational constant as that assigned to C5N− in IRC +10216 were recently observed in TMC-1 (Cernicharo et al. 2020), which ruled out metal-bearing molecules and provided strong support in favour of C5N−.

There is controversy over the formation of CnN− anions through radiative electron attachment to CnN radicals because calculated rate coefficients can differ by orders of magnitude depending on the study (Petrie & Herbst 1997; Walsh et al. 2009; Khamesian et al. 2016; Millar et al. 2017). Hence, the detection of nitrile anions and the determination of their abundances in different astronomical environments are an important step forward in understanding the chemistry of anions in space.

In this Letter, we present the discovery of C7N− in the cold dark core TMC-1 and in the external layers of the circumstellar envelope of the carbon-rich star IRC +10216. The assignment is based on the excellent agreement between the derived rotational constant and that from high-level ab initio calculations performed in this work (see also Botschwina & Oswald 2008). The assignment to C7N− is strengthened by the lack of other plausible candidates common to both sources.

2. Observations

New receivers built within the Nanocosmos2 project and installed at the Yebes 40m radiotelescope were used for observations of TMC-1 (αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″) and IRC +10216 (αJ2000 = 9h47m57.36s and δJ2000 = +13° 16′44.4″). The observations of TMC-1 belong to the QUIJOTE line survey (Cernicharo et al. 2021) and those of IRC +10216 to the Nanocosmos2 survey of evolved stars (Pardo et al. 2022). A detailed description of the telescope, receivers, and backends is given by Tercero et al. (2021). Briefly, the receiver consists of two cold, high-electron-mobility transistor amplifiers covering the 31.0–50.3 GHz band with horizontal and vertical polarisations. Receiver temperatures in the runs achieved during 2020 vary from 22 K at 32 GHz to 42 K at 50 GHz. Some power adaptation in the down-conversion chains reduced the receiver temperatures during 2021 to 16 K at 32 GHz and 25 K at 50 GHz. The backends are 2 × 8 × 2.5 GHz fast Fourier transform spectrometers with a spectral resolution of 38.15 kHz, providing coverage of the whole Q-band in both polarisations. For the observations of TMC-1, the original spectral resolution was used, while for IRC +10216, the data have been smoothed to 220 kHz, which corresponds to a velocity resolution of 1.7 km s−1 at 40 GHz. This resolution is large enough to resolve the U-shaped lines of IRC +10216, which exhibit a full velocity width of 29 km s−1 (Cernicharo et al. 2000).

All observations of TMC-1 were performed in the frequency switching mode with frequency throws of 8 and 10 MHz. Details of the survey and the data-analysis procedure are given in Cernicharo et al. (2021, 2022). The observations of IRC +10216 were performed in position switching and are described by Pardo et al. (2022). The main beam efficiency of the telescope varies from 0.6 at 32 GHz to 0.43 at 50 GHz. The intensity scale used in this work, namely antenna temperature (), was calibrated using two absorbers at different temperatures and the atmospheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties of 10% are adopted. However, some systematic effects were observed between the data acquired during 2022 and 2021. The intensity of the lines in the earliest observations of QUIJOTE are typically 10%–15% weaker than those of the most recent observations. No frequency effects are observed in the derived calibration factors. This systematic change is probably related to the surface adjustement after holography measurements at the end of 2021. The whole set of data has been recalibrated and co-added to produce the QUIJOTE line survey used for this work. The new line intensities, and hence the column densities, are ∼15% larger than those previously reported. The data for TMC-1 and IRC +10216 presented here correspond to 758 h and 696 h of on-source telescope time, respectively. All data were analysed using the GILDAS package3.

3. Detection of C7N−

Line identification in this work was done using the catalogues MADEX (Cernicharo 2012), CDMS (Müller et al. 2005), and JPL (Pickett et al. 1998). By November 2022, the MADEX code contained 6446 spectral entries corresponding to the ground and vibrationally excited states –together with the corresponding isotopologues– of 1746 molecules.

The new data of IRC +10216 have an unprecedented sensitivity of 0.1 mK in the scale and for a spectral resolution of 220 kHz, and the survey shows a forest of unidentified lines. A series of doublets and triplets are found in very good harmonic relation at slightly higher frequencies than the rotational lines of HC9N; their rotational constants are around 291 MHz. These results will be reported in a forthcoming paper (Pardo et al., in prep.). These lines correspond to the ν19 = 2, 3 vibrational levels of HC9N. Their quantum numbers go from Ju = 54 up to Ju = 80. Some of them are shown in Fig. 1. In addition, we find that an extra line was detected every two transitions of the HC9N ν19 = 2 transitions following a perfect harmonic relation from J = 27 − 26 up to J = 42 − 43 with a rotational constant of B = 582.6 MHz. The possibility that these lines also belong to a vibrational excited state of HC9N is immediately ruled out because if B were 291.3, then all lines with odd Ju would be missing. The lines are shown in Fig. 1 and appear as single features; the carrier is therefore a 1Σ molecule, or a 2Σ molecule with a very small value of γ. A fit to the observed lines provides the rotational and distortion constants given in Table 1. Taking into account the observed line profile of the unblended lines, the carrier has to be produced in the external layers of the circumstellar envelope of IRC +10216, that is, in the region where chemistry is controlled by the Galactic UV photons and where radicals, cyanopolyynes, carbon chains, metal-bearing species, and anions are detected (see e.g. Cernicharo et al. 2000).

thumbnail Fig. 1.

Observed lines of C7N− towards IRC +10216. Line parameters are given in Table A.1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of −26.5 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in milliKelvin. The red lines show the fitted line profiles.

Table 1.

Rotational and distortion constants for C7N−.

In order to obtain additional insight into the carrier of the lines we checked if they are present in the QUIJOTE line survey of TMC-1. Six of them are clearly detected, corresponding to the transitions J = 27 − 26 up to J = 32 − 31. These are shown in Fig. 2. None of them show signs of fine or hyperfine structure and they therefore definitively belong to a 1Σ molecule. The rotational and distortion constant are given in Table 1 and agree very well with those derived from the 17 lines of IRC +10216. Moreover, the common measured frequencies in both sources are identical within the measured uncertainties for the IRC +10216 lines. The carrier is therefore common to both sources and consequently we can exclude a metal-bearing species or a vibrationally excited state as these are not expected in TMC-1. As we find no evidence of fine structure, we can also discard a radical as the carrier. Moreover, we can also discard an isotopologue of an abundant species such as HC7N for which B = 564 MHz, and C8H for which B is 587.264 MHz (McCarthy et al. 1999). An isotopologue of C8H can be discarded due to the fact that the carrier has to be a singlet state. The rotational constants of the isotopologues of HC7N have been measured in the laboratory (McCarthy et al. 2000) and all them have rotational constants below that of the carrier. Hence, we have to conclude that we are dealing with a new unknown molecular species. Nevertheless, its structure has to be similar to that of HC7N and C8H. An isomer of HC7N can be excluded as HC6NC, the most likely candidate, has been observed in the laboratory to have a rotational constant of 582.5203 MHz (Botschwina et al. 1998) and is not detected in either TMC-1 or IRC+10216. The other isomers, HCxNC7 − x, are expected to have larger rotational constants, except HNC7 for which the expected rotational constant is around 567 MHz (da Silva & Neto 1997; Lee et al. 2000). Therefore, the most plausible candidates are C7N and C7N−. The former can be discarded as its ground electronic state is predicted to be 2Π with a 2Σ state close in energy (Botschwina et al. 1999). This leaves the anion C7N− as the most likely carrier of the lines in TMC-1 and IRC +10216. Lines of the previous member of the series, C5N−, have been found in both sources (Cernicharo et al. 2008, 2020). Ab initio calculations by Botschwina & Oswald (2008) indicate a rotational constant for this anion of 582 MHz, very close indeed to the observed value, and a dipole moment of 7.545 D.

thumbnail Fig. 2.

Observed lines of C7N− towards TMC-1. Line parameters are given in Table A.2. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of +5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in milliKelvin. The red line shows the synthetic spectrum derived for Trot = 6 K and N(C7N−) = 5 × 1010 cm−2. Blank channels correspond to negative features produced in the folding of the frequency-switching data.

We performed geometry ab initio calculations (Werner et al. 2020) using the coupled cluster method with single, double, and perturbative triple excitations with an explicitly correlated approximation (CCSD(T)-F12; Knizia et al. 2009) and all electrons (valence and core) correlated together with the Dunning’s correlation consistent basis sets with polarized core-valence correlation triple-ζ for explicitly correlated calculations (cc-pCVTZ; Hill et al. 2010). The values for the centrifugal distortion constant were calculated (Frisch et al. 2016) using the MP2 perturbation theory method (Møller & Plesset 1934) and the correlation is consistent with polarized valence triple-ζ basis set (cc-pVTZ; Woon & Dunning 1993). The values of B and D calculated for C7N− were scaled by the corresponding experimental and calculated ratios obtained for HC7N using the same level of theory. For HC7N, we obtained B = 562.9 MHz and D = 3.50 Hz, while for C7N− the uncorrected values are B = 581.6 MHz and D = 3.70 Hz. The scaled values for C7N− are presented in Table 1 and perfectly match those measured for the lines in TMC-1 and IRC +10216.

We therefore conclude that we have discovered the C7N− anion in the interstellar and circumstellar media. A fit to the lines observed in both sources provides B = 582.68495 ± 0.0024 MHz and D = 4.02 ± 0.13 Hz (see Table 1), which are the recommended rotational parameters with which to predict the spectrum of C7N−.

4. Discussion

Assuming a source emission diameter of 80″ for C7N− in TMC-1 (Fossé et al. 2001), a fit to the observed line profiles results in a rotational temperature of 6.0 ± 0.5 K and a column density of (5.0 ± 0.5) × 1010 cm−2. Adopting an averaged column density of H2 over the source of 1022 cm−2 (Cernicharo et al. 1987), the abundance of C7N− is 5×10−12. The column density of C5N− in TMC-1 is (2.6 ± 0.9) × 1011 cm−2 (Cernicharo et al. 2020), and therefore the abundance ratio C5N−/C7N− is ∼5. However, we note that due to the high energy of the rotational levels associated to the observed transitions, small variations of the rotational temperature can be compensated by a significant variation of the column density with a negligible variation of χ2. This means that, in light of the lack of information regarding the intensity of low-J transitions, Trot and N are strongly correlated.

For IRC +10216, we assume that the emitting region has a radius of 15″, which corresponds to the observed spatial distribution of cyanopolyynes and radicals with ALMA (Agúndez et al. 2017). A rotation diagram of the observed intensities towards IRC +10216 provides a rotational temperature of 26.6 ± 1.8 K and a column density of (2.4 ± 0.2)×1012 cm−2 for C7N−. The column density of C5N− was previously derived in this source from lines observed at 3 mm to be 3.4 × 1012 cm−2 (Cernicharo et al. 2008). However, several lines of C5N− have been observed in the Q-band in our survey with the Yebes 40m telescope and those at 3 mm observed with the IRAM 30m telescope have been significantly improved (Cernicharo et al. 2020). From all these data, we derived a column density of C5N− in IRC +10216 from a rotation diagram analysis, which provides N(C5N−) = (5.7 ± 0.3) × 1012 cm−2 and Trot = 29.9 ± 0.6 K. The C5N−/C7N− abundance ratio is therefore ∼2.4, which is similar to that found in TMC-1.

The detection of C7N− in interstellar and circumstellar environments implies that C7N must also be present in both media if the anion is formed via electron attachment to the radical. For the neutral species C7N, ab initio calculations by Botschwina et al. (1999) predict an electronic ground state 2Π with a moderate dipole moment of ∼1. However, due to the possible admixing of the ground state with a low-lying 2Σ state, which has a dipole moment of ∼3.6 (Botschwina et al. 1999), the real value of the dipole moment could be between those of both states. This mixing of two low-lying electronic states was previously discussed for C5N (Cernicharo et al. 2008) and C4H (Oyama et al. 2020). Taking into account the observed intensities of C7N− (μ = 7.545 D) in TMC-1 and IRC +10216, those of C7N could be much lower due to the lower dipole moment of the neutral and to the larger partition function of the radical caused by the presence of two ladders, 2Π1/2 and 2Π3/2, each one exhibiting Λ-doubling and hyperfine structure. Adopting an averaged dipole for C7N of between those of the 2Π and 2Σ states of 2.3 D (Botschwina et al. 1999) and a rotational partition function four times larger than for C7N−, the lines of the radical could be ∼40 × N(C7N−)/N(C7N) times weaker than those of C7N−. Adopting a N(C7N−)/N(C7N) similar to the N(C5N−)/N(C5N), which is found to be ∼0.5 in IRC +10216 and TMC-1 (Cernicharo et al. 2008, 2020), the expected intensities will be approximately 20 times weaker for C7N than for C7N−, which is well below the present sensitivity of our data towards the two sources. We can also make a prediction of the expected intensities of C7N based on those of the analogue radical C8H, which has a similar partition function to C7N (excluding the hyperfine structure), although a larger dipole moment of 6.5 D. The lines of C7N will have an intensity of N(C7N)/N(C8H) × [(μ(C7N)/μ(C8H))]2 times those of C8H; that is, ∼N(C7N)/N(C8H)/8. The lines of the 2Π3/2 ladder of C8H have an intensity of ∼5 mK in TMC-1 and of 4 mK in IRC +10216 at 31 GHz. Therefore, detecting C7N in our data will require an abundance for this species of similar to or larger than that of C8H, which is very unlikely in view of the observed abundance ratios of other members of these two families of radicals. For example, the abundance ratios C3N/C4H and C5N/C6H are on the order of 0.1 in both IRC +10216 and TMC-1 (Cernicharo et al. 2000; Agúndez & Wakelam 2013).

The detection of C7N− is in line with previous observational constraints of the series of hydrocarbon and nitrile anions CnH− and CnN− in which long carbon chains are favoured with respect to the short ones. Although we do not have access to the C7N−/C7N abundance ratio, we expect this ratio to be in the range of 0.1–0.5, as found for other large anions, such as C8H− and C5N− (Cernicharo et al. 2008, 2020; Brünken et al. 2007; Remijan et al. 2007). This scenario is in agreement with the proposed formation mechanism of negative ions from electron attachment to the corresponding radical (Herbst 1981), which becomes increasingly efficient as the size of the chain increases, as calculated for the series of hydrocarbon radicals CnH (Herbst & Osamura 2008). It would be interesting to compute the rate coefficient of electron attachment for the series of nitrile radicals CnN. Calculated values for C3N (Petrie & Herbst 1997; Harada & Herbst 2008) and C5N (Walsh et al. 2009) indicate that, similarly to the case of hydrocarbons, the rate coefficient increases considerably as the size of the chain increases, although Khamesian et al. (2016) find much lower rate coefficients for C3N and C5N.

It is interesting to note that, although the anion-to-neutral ratio increases considerably with increasing chain length, the column densities of negative ions are on the same order along the series of hydrocarbons CnH− and nitriles CnN− (see Fig. 3). This is interesting because if this behaviour is maintained for larger sizes, we could expect the anions C10H− and C9N− to be present in significant amounts, allowing their detection in TMC-1 and IRC +10216 at centimetre wavelengths. Moreover, looking at anions would allow us to probe the extent to which carbon chains can grow, and this would more straightforward than looking at neutrals. This is due to the fact that, for large carbon chains, the detection of the anion is favoured over the radical because of the collapse of the fine and hyperfine structure and eventually also because of the more favourable dipole moment, as perfectly illustrated in the case of C7N− and C7N.

thumbnail Fig. 3.

Column densities of negative ions derived through observations toward TMC-1 and IRC +10216. Values are taken from this work, from Agúndez et al., (in prep.), and from the literature (Cernicharo et al. 2007, 2020; Remijan et al. 2007; Thaddeus et al. 2008; Agúndez et al. 2008, 2010).

5. Conclusions

We report in this work the detection of the anion C7N− in TMC-1 and IRC +10216. The lack of detection of the neutral C7N is due to its much lower dipole moment and larger partition function.

1

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

2

ERC grant ERC-2013-Syg-610256-NANOCOSMOS.

https://nanocosmos.iff.csic.es/

Acknowledgments

We thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. M.A. thanks MICIU for grant RyC-2014-16277. We also thank Ministerio de Ciencia e Innovación of Spain (MICIU) for funding support through projects PID2019-106110GB-I00, PID2019-107115GB-C21/AEI/10.13039/501100011033, and PID2019-106235GB-I00.

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Appendix A: Line parameters

Line parameters for TMC-1 were obtained by fitting a Gaussian line profile to the observed data. A window of ± 15 km s−1 around the vLSR of the source was considered for each transition. The derived line parameters are given in Table A.2. For IRC +10216, we used the SHELL method of the GILDAS3 package which is well adapted to reproducing the observed line profiles in circumstellar envelopes. A variable window of 50-100 MHz was used to derive the line parameters in this source; these are given in Table A.1.

Table A.1.

Observed lines of C7N− in IRC +10216.

Table A.2.

Observed lines of C7N− in TMC-1.

All Tables

Table 1.

Rotational and distortion constants for C7N−.

In the text
Table A.1.

Observed lines of C7N− in IRC +10216.

In the text
Table A.2.

Observed lines of C7N− in TMC-1.

In the text

All Figures

thumbnail Fig. 1.

Observed lines of C7N− towards IRC +10216. Line parameters are given in Table A.1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of −26.5 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in milliKelvin. The red lines show the fitted line profiles.
In the text
thumbnail Fig. 2.

Observed lines of C7N− towards TMC-1. Line parameters are given in Table A.2. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of +5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in milliKelvin. The red line shows the synthetic spectrum derived for Trot = 6 K and N(C7N−) = 5 × 1010 cm−2. Blank channels correspond to negative features produced in the folding of the frequency-switching data.
In the text
thumbnail Fig. 3.

Column densities of negative ions derived through observations toward TMC-1 and IRC +10216. Values are taken from this work, from Agúndez et al., (in prep.), and from the literature (Cernicharo et al. 2007, 2020; Remijan et al. 2007; Thaddeus et al. 2008; Agúndez et al. 2008, 2010).
In the text

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