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Issue
A&A
Volume 669, January 2023
Article Number L1
Number of page(s) 6
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202245492
Published online 22 December 2022

© The Authors 2022

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

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

1. Introduction

Dicyanopolyynes are molecules with a linear unsaturated skeleton of carbon atoms that terminates at each edge by a cyano group, that is to say N≡C−(C≡C)n−C≡N. The simplest member of this family, cyanogen (NCCN), has been long known to be present in the atmosphere of Titan (Kunde et al. 1981; Coustenis et al. 1991; Sylvestre et al. 2020) and has been recently identified in the coma of comet 67P/Churyumov-Gerasimenko (Hänni et al. 2021). The second member of the series, dicyanoacetylene (NC4N), has been observed in solid state, although not in the gas phase, in the atmosphere of Titan (Samuelson et al. 1997; Jolly et al. 2015). It is also interesting to note that NCCP, a species chemically related to NCCN that results from the substitution of one N atom by one P atom, has been identified tentatively in the carbon star envelope IRC +10216 (Agúndez et al. 2014).

It has been hypothesized that dicyanopolyynes could be abundant in interstellar and circumstellar clouds (Kołos & Grabowski 2000; Petrie et al. 2003). Indeed, it is well known that monocyanopolyynes H−C≡C−(C≡C)n−C≡N, more commonly known as cyanopolyynes, are abundant in cold dense clouds and circumstellar envelopes around evolved stars, and thus it is conceivable that dicyanopolyynes are abundant as well. However, dicyanopolyynes are nonpolar and thus cannot be detected at radio wavelengths. In spite of that, in recent years we have learned that dicyanopolyynes are very likely abundant in interstellar clouds. The protonated form of cyanogen, NCCNH+, was detected in the cold dense clouds TMC-1 and L483 (Agúndez et al. 2015). Just as N2H+ is used to probe N2 (Linke et al. 1983), the detection of NCCNH+ was interpreted as indirect evidence of the presence of NCCN, for which a large abundance in the range (1–10) × 10−8 relative to H2 was inferred. A second piece of evidence in support of the presence of cyanogen in interstellar clouds was provided by the detection of isocyanogen (CNCN) in the cold dense clouds L483, TMC-1 (tentatively), and L1544 (Agúndez et al. 2018; Vastel et al. 2019). This species is a polar metastable isomer of NCCN, and thus it is very likely chemically connected to cyanogen. From the detection of CNCN, Agúndez et al. (2018) inferred an abundance of NCCN in the range (2.5–4.5) × 10−9 relative to H2.

Here we present additional evidence in support of the existence of abundant dicyanopolyynes in the interstellar medium. We identified a series of six harmonically related lines in the QUIJOTE (Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment) line survey of TMC-1 (Cernicharo et al. 2021c, 2022b), which are convincingly assigned to NC4NH+. This is the protonated form of dicyanoacetylene (NC4N), and thus indirectly probes the presence of the second member of the family of dicyanopolyynes. We discuss how abundant interstellar dicyanopolyynes are likely to be in comparison with their monocyano counterparts.

2. Observations

The observational data used here are based on QUIJOTE (Cernicharo et al. 2021c, 2022b), an ongoing line survey that is being carried out with the Yebes 40 m telescope at the position of the cyanopolyyne peak of TMC-1, αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″. QUIJOTE uses a 7 mm receiver covering the Q band (31.0–50.3 GHz) with horizontal and vertical polarizations. The back end is a fast Fourier transform spectrometer providing a spectral resolution of 38.15 kHz and a bandwidth of 8 × 2.5 GHz in each polarization, which allows for the whole Q band to be covered almost completely. The system is described by Tercero et al. (2021). The data presented here were acquired from November 2019 to May 2022 using the frequency-switching technique, and they comprise 546 h of on-source telescope time. The intensity scale used is antenna temperature, , which has an estimated uncertainty of 10% and can be converted to main beam brightness temperature, Tmb, by dividing by Beff/Feff, where Beff and Feff are the beam and forward efficiencies, respectively. For the Yebes 40 m telescope in the Q band1, Feff = 0.97 and Beff can be fitted as a function of frequency as Beff = 0.797 exp[−(ν(GHz)/71.1)2], where the expression corresponds to values measured during 2022 that represent a slight improvement over previous values due to a better alignment of mirrors in the receiver cabin. The half power beam width (HPBW) can also be fitted as a function of frequency as HPBW(″) = 1763/ν(GHz). Some spectra of TMC-1 presented in Sect. 4 were observed with the 30 m telescope of the Institut de RadioAstronomie Millimétrique (IRAM). These observations are described in Cabezas et al. (2022a). All data were analyzed using the GILDAS software2.

3. Results

3.1. Identification of NC4NH+

thumbnail Fig. 1.

Lines of NC4NH+ observed toward TMC-1. Blanked channels correspond to negative artifacts resulting from the frequency switching observing mode. Unidentified lines are labeled as U. The red lines correspond to the line profiles calculated under LTE adopting a column density of 1.2 × 1010 cm−2, a rotational temperature of 5.6 K, a line width of 0.60 km s−1, and an emission size of 40″ in radius (see text).

Table 1.

Observed line parameters of NC4NH+ in TMC-1.

We identified six lines in the QUIJOTE line survey whose frequencies show harmonic relations 13:14:15:16:17:18. Multiples of these relations are not possible because lines corresponding to their intermediate frequencies with similar intensities are missing. We could not assign these lines to any molecule with known rotational spectroscopy after inspection of the private catalog of J. Cernicharo MADEX3 (Cernicharo 2012), the Cologne Database for Molecular Spectroscopy (CDMS)4 (Müller et al. 2005), and the Jet Propulsion Laboratory (JPL) catalog5 (Pickett et al. 1998). The six lines are weak, with intensities around 1 mK in . However, given the noise level reached by the QUIJOTE data in the range 0.1–0.2 mK at frequencies below 45 GHz, most of the lines are detected with a high signal-to-noise ratio. The lines are shown in Fig. 1 and the frequencies measured, together with the rest of the line parameters derived from a Gaussian fit, are given in Table 1.

The line frequencies can be fitted as ν(J → J − 1) = 2BJ − 4DJ3, yielding a rotational constant of B = 1293.90840 ± 0.00060 MHz and a centrifugal distortion constant of D = 28.59 ± 1.21 Hz, with a rms of 13.5 kHz (see Table 2). The six lines show a nearly perfect harmonic relation and have similar intensities. Moreover, there is no missing line that should be detected and it is not detected. For example, the J = 12-11 line, predicted at 31 053.604 MHz, lies outside the frequency range covered and the J = 19-18 line, predicted at 49 167.735 MHz and expected with an intensity of mK, is not detected because the noise level in this frequency range is at the level of 0.3 mK. Given these facts, it is extremely unlikely that the six lines arise from different carriers.

Table 2.

Spectroscopic parameters of NC4NH+.

The rotational constant B derived here matches very well the theoretical rotational constant calculated for protonated dicyanoacetylene (NC4NH+), 1293.54 MHz (Marcelino et al. 2020). This value was calculated ab initio and scaled using the experimental-to-calculated ratio of B for NC4N, a method that has been used previously to detect molecules such as HC3O+ (Cernicharo et al. 2020b), HC3S+ (Cernicharo et al. 2021a), CH3CO+ (Cernicharo et al. 2021b), H2NC (Cabezas et al. 2021), HCCS+ (Cabezas et al. 2022a), C5H+ (Cernicharo et al. 2022a), HC7NH+ (Cabezas et al. 2022b), and HCCNCH+ (Agúndez et al. 2022). In all of these cases, the difference between the calculated and astronomical rotational constant was below 0.1%. Moreover, for HC3O+, HC3S+, and CH3CO+ the identification was definitively confirmed by laboratory spectra. Here, the difference between the calculated rotational constant of NC4NH+ and the astronomical value derived from the six lines observed in TMC-1 is 0.03%. The centrifugal distortion constant calculated for NC4NH+ by Marcelino et al. (2020), 27.8 Hz, is also very close to the value derived from the astronomical lines (see Table 2).

Other potential candidates, apart from NC4NH+, are HC5O+ and HC5NH+, as discussed by Marcelino et al. (2020). It is unlikely that HC5O+ is the carrier because its calculated rotational constant is significantly different, by 0.7%, from that derived here. The rotational constants calculated for NC4NH+ and HC5NH+ are close, although all levels of theory predict a rotational constant 2 MHz larger for HC5NH+ than for NC4NH+, which is consistent with the assignment of the series of lines observed here to NC4NH+ and that observed by Marcelino et al. (2020) to HC5NH+. In addition, if this is the case, the differences between the calculated and the astronomical rotational constant for NC4NH+ and HC5NH+ remain small, 0.03% and 0.02%, respectively, while if the assignment was the reverse, the differences would be significantly larger, 0.18% and 0.12%, respectively. That is, we conclude that the identification of HC5NH+ presented by Marcelino et al. (2020) is correct and that the series of harmonically related lines presented here correspond to NC4NH+. The excitation of the two molecules is in line with the assignment made (see Sect. 3.2), which in any case will require laboratory measurements for a definitive confirmation.

3.2. Abundance of NC4NH+

To evaluate how abundant NC4NH+ is in TMC-1, we adopted a dipole moment of 9.1 D, as calculated at the MP2/cc-pVTZ level by Marcelino et al. (2020). These authors calculated slightly higher values, in the range 9.5–9.9 D, using coupled cluster methods, although the MP2 level of theory probably yields a more accurate estimation of the dipole moment of NC4NH+, as suggested by the case of HC5N investigated in Marcelino et al. (2020).

thumbnail Fig. 2.

Excitation analysis of NC4NH+ in TMC-1. In the left panel we show the rotation diagram and in the right panel we show the χ2 resulting from a LTE calculation, where the contour corresponds to the 1σ level. The rotational temperature and column density derived through the two methods are very similar.

Using the velocity-integrated line intensities given in Table 1, we constructed a rotation diagram, which is shown in the left panel of Fig. 2. We assumed that the emission of NC4NH+ is distributed in the sky as a circle with a diameter of 80″, as observed for several hydrocarbons in TMC-1 (Fossé et al. 2001). We derived a rotational temperature of 6.1 ± 1.4 K, which is in the range of the rotational temperatures derived for other molecules observed in TMC-1, between 5 and 10 K. The column density derived from the rotation diagram is (1.1 ± 0.9) × 1010 cm−2. In order to get rid of some of the assumptions made by the rotation diagram method, such as the validity of the Rayleigh–Jeans limit, we carried out calculations assuming local thermodynamic equilibrium (LTE), with a single rotational temperature governing the excitation of all rotational levels, in which the rotational temperature and column density were varied. The best fit is then given by the minimum χ2, defined as χ2 = ∑[(Icalc − Iobs)/σ]2, where the sum extends over the six lines, Icalc and Iobs are the calculated and observed velocity-integrated intensities, and σ is the error in Iobs. In the right panel of Fig. 2, we plot χ2 as a function of the rotational temperature and the column density. The best-fit values of the rotational temperature and column density found this way are 5.6 ± 1.9 K and (1.1 ) × 1010 cm−2, which should be more precise than the values obtained through the rotation diagram method. In any case, the differences are small and only affect the rotational temperature since the column densities obtained through the two methods are identical. The calculated line profiles using these best-fit parameters are shown in Fig. 1.

By comparing the rotational temperature derived here for NC4NH+, 5.6 ± 1.9 K, with that derived for HC5NH+, 7.8 ± 0.7 K (Marcelino et al. 2020), we can find an additional argument in support of the assignment to NC4NH+ made here. That is, the fact that the rotational temperature of NC4NH+ is smaller than for HC5NH+ is consistent with the higher dipole moment of NC4NH+ compared to that of HC5NH+, 9.1 D vs. 3.6 D (Marcelino et al. 2020), because the larger the dipole moment, the higher the critical density for thermalization, and the lower the rotational temperature.

4. Discussion

thumbnail Fig. 3.

Lines of CNCN observed toward TMC-1. In red we show the Gaussian fits, which yield (in mK km s−1) of 2.27 ± 0.13, 4.47 ± 0.75, and 3.55 ± 0.33 for the J = 4-3, J = 8-7, and J = 9-8 lines, respectively.

The detection of NCCNH+ and CNCN (Agúndez et al. 2015, 2018) provided indirect evidence on the existence of the highly stable, but nonpolar, molecule NCCN in cold dense clouds. Similarly, the detection of NC4NH+ reported here provides indirect evidence on the presence of the nonpolar molecule NC4N.

In the case of cyanogen (NCCN), there are two indirect proxies: the protonated form NCCNH+ and the metastable isomer CNCN. The column density of NCCNH+ in TMC-1 is 8.6 × 1010 cm−2 (Agúndez et al. 2015), while for CNCN the detection in TMC-1 presented by Agúndez et al. (2018) was based on the line stacking of four lines in the λ 3 mm band and was therefore considered only as tentative. Here we present the confirmation of the detection of CNCN in TMC-1 thanks to the clear detection of three lines (see Fig. 3): the J = 4-3 line at 41 392.912 MHz in our QUIJOTE data and the J = 8-7 and J = 9-8 lines at 82 784.692 MHz and 93132.326 MHz, respectively, using IRAM 30 m data (observations are described in Cabezas et al. 2022a). We derived a rotational temperature of 10.6 ± 1.6 K and a column density of (8.0 ± 2.1) × 1011 cm−2, which is similar to the value of 9 × 1011 cm−2 derived by Agúndez et al. (2018). Therefore, the abundance ratio CNCN/NCCNH+ in TMC-1 is ∼9.

In the case of dicyanoacetylene (NC4N), the column density derived here for the protonated proxy is 1.1 × 1010 cm−2. In this case there is also a metastable isomer which can be used as a proxy, which is NC3NC. This molecule is polar, with a calculated dipole moment of 1.11 D, and its rotational spectrum has been measured in the laboratory (Huckauf et al. 1999). At the current level of sensitivity of the QUIJOTE data, we did not detect this species and we derived a 3σ upper limit to its column density of 7.3 × 1010 cm−2, assuming a rotational temperature of 7.5 K in the middle of the range 5–10 K typically found in TMC-1. Therefore, the abundance ratio NC3NC/NC4NH+ in TMC-1 is < 7, that is to say it is smaller than the ratio CNCN/NCCNH+, which is ∼9. If the abundance ratios between the protonated and metastable proxies are not that different for NCCN and NC4N, a deeper integration should lead to the detection of NC3NC in TMC-1.

thumbnail Fig. 4.

Column densities in TMC-1 for monocyanopolyynes (in green) and dicyanopolyynes (in blue), and their protonated forms. Observed column densities are taken from Agúndez et al. (2015), Cernicharo et al. (2020a), Marcelino et al. (2020), Cabezas et al. (2022b), and this work. The column densities of NCCN and NC4N were estimated from those of NCCNH+ and NC4NH+, respectively, assuming a protonated-to-neutral abundance ratio of 10−3 (see text).

The formation of NCCNH+ and NC4NH+ likely occurs by proton transfer to the neutral counterparts NCCN and NC4N, respectively. Indeed both NCCN and NC4N have high proton affinities, 674.7 kJ mol−1 (Hunter & Lias 1998) and 736 kJ mol−1 (Marcelino et al. 2020), respectively. The abundance ratio NCCNH+/NCCN was calculated to be ∼10−4 from a chemical model by Agúndez et al. (2015), although these authors noticed that the ratio may be closer to 10−3 because the chemical model underestimated – by a factor of ∼10 – the protonated-to-neutral abundance ratios of related cyanides, such as HCN/HNC and HC3N. From the detection of CNCN and arguments based on the expected similarity of the CNCN/NCCN and HCCNC/HCCCN ratios, Agúndez et al. (2018) estimated the abundance of NCCN to be around 4.5 × 10−9 relative to H2, which is consistent with a NCCNH+/NCCN ratio on the order of 10−3. The protonated-to-neutral abundance ratios derived from observations of cold dense clouds lie in the range 10−3–10−1 for neutral molecules with proton affinities larger than that of CO (Agúndez et al. 2022). For the monocyanopolyynes HC3N, HC5N, and HC7N, the protonated-to-neutral abundance ratios are a few 10−3. If we assume a protonated-to-neutral abundance ratio of 10−3 for NCCN and NC4N, the estimated column densities of these dicyanopolyynes are somewhat lower, by factors of 3 and 15, than those of the corresponding monocyanopolyynes HC3N and HC5N (see Fig. 4). For the next member of the series of dicyanopolyynes, NC6N, we can estimate its column density to be around ∼1012 cm−2, assuming that the decrease in the column density with size found for HC5N and HC7N also holds for NC4N and NC6N (see Fig. 4). Assuming a protonated-to-neutral ratio of 10−3, the column density of its protonated form, NC6NH+, would be ∼109 cm−2. According to the rotational constants and dipole moment calculated by Cabezas et al. (2022b), the most intense lines of NC6NH+ are expected to lie in the range 10–30 GHz, although the expected brightness temperatures are as low as 0.2 mK, which would require very sensitive observations.

Now the question is how are dicyanopolyynes formed in cold dense clouds. In the case of the simplest member of the series, NCCN, its formation is thought to occur by the reaction (Petrie et al. 2003)

(1)

Similarly, for NC4N one can consider the reaction

(2)

If this reaction is the main source of NC4N, we should expect the NC4N/NCCN ratio to reflect the C3N/CN ratio. In TMC-1 the C3N/CN ratio is 1.2 (Agúndez et al., in prep.; Pratap et al. 1997), while the NC4N/NCCN ratio can be assumed to a first approximation to be given by the NC4NH+/NCCNH+ ratio, which is 0.13. Given that C3N is as abundant as CN in TMC-1, one should expect NC4N to be as abundant as NCCN. From the observed abundances of NCCNH+ and NC4NH+, we infer that NC4N is somewhat less abundant than NCCN, which could happen if the rate coefficient of reaction (2) is lower than that of reaction (1) or if NC4N is destroyed more rapidly than NCCN. Other plausible reactions, such as

(3)

and

(4)

are unlikely to be behind the formation of NC4N in TMC-1 because the abundance ratios HNC3/HNC and HCCNC/HNC are 0.002 and 0.011, respectively (Pratap et al. 1997; Cernicharo et al. 2020a), which are much lower than the inferred NC4N/NCCN ratio of 0.13.

5. Conclusions

We observed six harmonically related lines in our QUIJOTE line survey of TMC-1, which we identify as due to protonated dicyanoacetylene (NC4NH+). The detection of this species, together with the previous detections of NCCNH+ and CNCN, support the hypothesis that dicyanopolyynes are abundant in cold dense clouds. We estimate that dicyanopolyynes are only a few times to one order of magnitude less abundant than their corresponding monocyanopolyynes.

Acknowledgments

We acknowledge funding support from Spanish Ministerio de Ciencia e Innovación through grants PID2019-106110GB-I00, PID2019-107115GB-C21, and PID2019-106235GB-I00 and from the European Research Council (ERC Grant 610256: NANOCOSMOS).

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All Tables

Table 1.

Observed line parameters of NC4NH+ in TMC-1.

In the text
Table 2.

Spectroscopic parameters of NC4NH+.

In the text

All Figures

thumbnail Fig. 1.

Lines of NC4NH+ observed toward TMC-1. Blanked channels correspond to negative artifacts resulting from the frequency switching observing mode. Unidentified lines are labeled as U. The red lines correspond to the line profiles calculated under LTE adopting a column density of 1.2 × 1010 cm−2, a rotational temperature of 5.6 K, a line width of 0.60 km s−1, and an emission size of 40″ in radius (see text).
In the text
thumbnail Fig. 2.

Excitation analysis of NC4NH+ in TMC-1. In the left panel we show the rotation diagram and in the right panel we show the χ2 resulting from a LTE calculation, where the contour corresponds to the 1σ level. The rotational temperature and column density derived through the two methods are very similar.
In the text
thumbnail Fig. 3.

Lines of CNCN observed toward TMC-1. In red we show the Gaussian fits, which yield (in mK km s−1) of 2.27 ± 0.13, 4.47 ± 0.75, and 3.55 ± 0.33 for the J = 4-3, J = 8-7, and J = 9-8 lines, respectively.
In the text
thumbnail Fig. 4.

Column densities in TMC-1 for monocyanopolyynes (in green) and dicyanopolyynes (in blue), and their protonated forms. Observed column densities are taken from Agúndez et al. (2015), Cernicharo et al. (2020a), Marcelino et al. (2020), Cabezas et al. (2022b), and this work. The column densities of NCCN and NC4N were estimated from those of NCCNH+ and NC4NH+, respectively, assuming a protonated-to-neutral abundance ratio of 10−3 (see text).
In the text

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