Open Access
Issue
A&A
Volume 672, April 2023
Article Number L12
Number of page(s) 6
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202346462
Published online 21 April 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

Metal-containing molecules are almost exclusively found in space towards circumstellar envelopes around evolved stars, except for a few recent detections towards protostars (Ginsburg et al. 2019; Tachibana et al. 2019). Most of these molecules have been discovered towards IRC +10216, the carbon-rich envelope of the star CW Leo. The first metal-bearing molecules detected in IRC +10216 were diatomic halides such as NaCl, KCl, AlCl, and AlF (Cernicharo & Guélin 1987). These species are formed in the hot inner parts of the envelope, close to the asymptotic giant branch star. The discovery and mapping of MgNC (Kawaguchi et al. 1993; Guélin et al. 1993) revealed that metal-bearing molecules are also formed in the cool outer regions of the envelope. Other metal cyanides and isocyanides, such as NaCN (Turner et al. 1994), MgCN (Ziurys et al. 1995), AlNC (Ziurys et al. 2002), KCN (Pulliam et al. 2010), FeCN (Zack et al. 2011), HMgNC (Cabezas et al. 2013), and CaNC (Cernicharo et al. 2019a), were detected later on in the same source and are probably formed in the cool outer regions of the envelope. Indeed, metal atoms have been observed to survive in the gas phase in these regions (Mauron & Huggins 2010), allowing for the formation of these metal-bearing molecules, specially those containing magnesium. Indeed, a variety of Mg-containing carbon chains of increasing length, such as MgC2, MgCCH, MgC4H, MgC6H, MgC3N, MgC5N, MgC4H+, MgC6H+, MgC3N+ and MgC5N+ have been recently detected (Agúndez et al. 2014; Cernicharo et al. 2019b; Pardo et al. 2021; Changala et al. 2022; Cernicharo et al. 2023a).

In this Letter we present the discovery in space of two new metal-bearing carbon chains towards IRC +10216. These molecules are HMgCCCN, a further member of the family of Mg-bearing molecules, and NaCCCN, the first long carbon chain of the family of Na-bearing molecules. These two molecules were detected thanks to a deep integration in the Q band (31.0–50.3 GHz) with the Yebes 40 m telescope. The identification of HMgCCCN is based on high-level quantum chemical calculations while that of NaCCCN relies on previous laboratory data (Cabezas et al. 2019). In addition, we provide an upper limit to the abundance of the aluminium cyanoacetylide, AlCCCN, in IRC +10216.

2. Observations

New receivers, built within the Nanocosmos1 project and installed at the Yebes 40 m radiotelescope, were used for the observations of IRC +10216 (αJ2000 = 9h47m57.36s and δJ2000 = +13 ° 16′44.4″) as part of the Nanocosmos survey of evolved stars (Pardo et al. 2022). Data from the QUIJOTE2 line survey (Cernicharo et al. 2021a, 2022, 2023b) towards TMC-1 were also used to discriminate between possible carriers of the lines found in IRC +10216.

A detailed description of the telescope, receivers, and backends can be found in Tercero & López-Pérez (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 polarizations. Receiver temperatures range from 16 K at 31 GHz to 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 the whole coverage of the Q band in both linear polarizations. The data were smoothed to 228.9 kHz (six channels), which corresponds to a velocity resolution of 1.7 km s−1 at 40 GHz. This spectral resolution is good enough to resolve the broad U-shaped lines of IRC +10216 which exhibit a full velocity width of 29 km s−1 (Cernicharo et al. 2000). The intensity scale used in this work, antenna temperature (), was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM; Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties were adopted to be 10%. Additional uncertainties could arise from the line intensity fluctuation with time induced by the variation of the stellar infrared flux (Cernicharo et al. 2014; Pardo et al. 2018). The beam efficiency of the Yebes 40 m telescope in the Q band is given as a function of frequency by Beff = 0.797 exp[−(ν(GHz)/71.1)2] and the forward telescope efficiency is 0.97. The emission size is assumed to have a radius of 15 arcsec, as observed for MgNC IRC +10216 (Guélin et al. 1993). Pointing corrections were obtained by observing the SiO masers of R Leo. Pointing errors were always within 2–3″. All data were analysed using the GILDAS package3.

3. Results

Our current data of IRC +10216 have never before reached a sensitivity (σ) as low as 0.15 mK at some frequencies in the scale for a spectral resolution of 229 kHz. This exceptional data quality has already been shown in recent publications presenting the detection of MgC5N and MgC6H (Pardo et al. 2021), and of C7N (Cernicharo et al. 2023b). The data show a large number of lines coming from isotopologues and vibrationally excited states of abundant species that are identified using the catalogues MADEX (Cernicharo et al. 2012), CDMS (Müller et al. 2005), and JPL (Pickett et al. 1998).

In order to derive line parameters, we used the SHELL method of the GILDAS package, which is well adapted for the line profiles observed in circumstellar envelopes. A variable window of 50–100 MHz around the line centre was used in this process. In all cases we fixed the terminal expansion velocity of the envelope to 14.5 km s−1 (Cernicharo et al. 2000).

3.1. Identification of HMgCCCN

Among the unidentified lines that arise with a high signal-to-noise ratio in the current Q-band spectrum of IRC +10216, six of them are in a harmonic relation from Ju = 12 to Ju = 18. They have been reported as unidentified features in the previous, much noisier, published version of the survey (Pardo et al. 2022). The lines are shown in Fig. 1 and the line parameters are provided in Table 1. Most of the observed lines of this series are free of blending, except for J = 15-14 which is fully blended with C3N. Two of them (J = 13-12 and J = 16-15) have a marginal overlap with another feature but line fit is still possible. The lines do not show any evidence for spectroscopic broadening due to a fine and/or hyperfine structure. They can be fitted to a standard line profile for an expanding envelope with a terminal expansion velocity of 14.5 km s−1 (Cernicharo et al. 2000). Hence, the carrier of the lines has a 1Σ ground electronic state. The lines can be fitted to the standard Hamiltonian of a linear rotor providing the spectroscopic constants shown in Table 2. We name this carrier B1320. We checked that values of B divided by integers ranging from two to six do not match the observations as many lines would be missing.

thumbnail Fig. 1.

Lines of HMgCCCN observed with the Yebes 40 m telescope towards IRC +10216. Line parameters are given in Table 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 mK. The red lines show the fitted line profiles adopting an expanding terminal velocity of 14.5 km s−1 (Cernicharo et al. 2000).

Table 1.

Observed line parameters of HMgCCCN and NaCCCN towards IRC +10216.

Table 2.

Spectroscopic parameters of HMgCCCN.

The derived rotational constant is close to that of HC5N (1331.333 MHz). All the isotopologues of this species can be discarded since they have already been reported in this source (Pardo et al. 2022). In addition, all possible isomers of HC5N have rotational constants larger than that of B1320 (Cernicharo et al. 2020a). Other potential species could be the isotopologues of the anions C6H and C5N, which have rotational constants B of 1376.8630 MHz and 1388.8673 MHz, respectively. However, the intensities of C6H in our survey are 10–15 mK, and those of C5N are 4–6 mK. Hence, all the isotopologues of these species will have intensities well below the mK level, thus being undetectable at the current level of sensitivity. Moreover, the distortion constants for all these possible candidates are around 30 Hz which is a factor of three lower than that of B1320. Distortion constants as large as the observed one are only provided by molecules containing a metal. Some species containing Si could have rotational constants around the observed value. For example SiC4 has a rotational constant of 1510 MHz and exhibits lines of 10–15 mK in the Q-band. All its istopologues have been observed in the laboratory and none match the rotational constant of B1320. Species such as SiCCCN (Umeki et al. 2014) or CCCCS (Hirahara et al. 1993) are open-shell species and cannot be considered as potential carriers.

We note that AlCCCN and NaCCCN have been observed in the laboratory and have rotational and distortion constants similar to those found for B1320. Also, NaCCCN has been detected in this work (see Sect. 3.2), and all its possible isomers have rotational constants larger than that of B1320 (Cabezas et al. 2019). The intensities of the NaCCCN lines are two to three times lower than those of B1320. Although the rotational constants of the NaCCCN isotopologues could be close to that of B1320, the expected intensities are too low to match those observed for B1320. Furthermore, AlCCCN is not detected at the present level of sensitivity and its isomers, similar to NaCCCN, have rotational constants that are too large. Hence, we have to consider another metal such as Mg for which several species containing it have been detected in IRC +10216. Ab initio calculations by Cabezas et al. (2014, 2019) also exclude other possibilities such as molecules containing Ca.

Taking into account that MgCCCN and MgCCCCH are both detected in IRC +10216 with column densities of ∼1013 cm−2 (Cernicharo et al. 2019b; Pardo et al. 2021), we proceeded in our search with high-level ab initio calculations for their hydrogenated counterparts HMgCCCN and HMgCCCCH using the same level of calculation as in previous studies (Cernicharo et al. 2019b): CCSD(T)-F12 (Knizia et al. 2009) with all electrons (valence and core) correlated and the Dunning’s correlation consistent basis sets with polarized core-valence correlation triple ζ for explicitly correlated calculations (cc-pCVTZ; Hill et al. 2010). All the calculations were carried out using the Molpro 2020.2 program (Werner et al. 2020). The calculated rotational constants for the two species are very similar: 1319.1 MHz and 1319.3 MHz for HMgCCCN and HMgCCCCH, respectively. However, the low dipole moment of 0.3 D calculated for HMgCCCCH excludes it as a possible carrier because a column density as high as ∼8 × 1014 cm−2 would be needed to reproduce the observed intensities. In contrast, the dipole moment calculated for HMgCCCN is 4.5 D, which makes it the best candidate for B1320. The centrifugal distortion constant D calculated at the same level of theory for HMgCCCN is 62.1 Hz, which is a bit different from the one derived in our fit. However, the hydridomagnesium derivatives are more floppy molecules, due to the HMgC bending mode, than the non-hydrogenated analogues and the prediction of this parameter is less reliable (Cernicharo et al. 2019b; Pardo et al. 2021). This is also seen for HMgNC, whose D value is 2.94 kHz, while the one calculated using the mentioned level of calculation is 2.3 kHz. We used this experimental/theoretical ratio to scale the theoretical value of D for HMgCCCN with the result of 79.0 Hz, which is closer to the one derived from our astronomical data considering 3σ uncertainty of the experimental value. It must be said that we obtained a reliable fit also keeping D constant fixed to 79.0 Hz. From the observed line intensities of HMgCCCN, we derived a rotational temperature of 17.1 ± 2.8 K and a column density of (3.0 ± 0.6) × 1012 cm−2. The derived rotational temperature agrees well with the value derived for the cold MgC3N component, Trot = 15 ± 2 K (Cernicharo et al. 2019b). However, the lines of MgC3N in the 3 mm domain, which involve high-energy rotational levels, require an additional gas component with a rotational temperature of 34±6 K.

3.2. Detection of NaCCCN and non detection of AlCCCN

The rotational constants of AlCCCN and NaCCCN are available from laboratory experiments (Cabezas et al. 2014, 2019). For NaCCCN seven lines were found in our IRC +10216 Q-band data with frequencies in very good agreement with those predicted from the laboratory measurements. They are shown in Fig. 2 and their line parameters are given in Table 1. Using the IRC +10216 frequencies and those measured in the laboratory (Cabezas et al. 2019), we derived a new set of molecular constants for NaCCCN, using SPFIT (Pickett 1991), which are given in Table 3.

thumbnail Fig. 2.

Same as Fig. 1, but for NaCCCN.

Table 3.

Spectroscopic parameters of NaCCCN.

The calculated dipole moment of NaCCCN is 12.9 D, using the same level of theory as for HMgCCCN. The rotational temperature and column density derived for NaCCCN are 13.5 ± 1.7 K and (1.2 ± 0.2) × 1011 cm−2, respectively.

In spite of the high accuracy of the frequency predictions for AlCCCN in the Q band (better than 25 kHz), we failed to detect any of its lines at the predicted frequencies. Assuming an expanding velocity of 14.5 km s−1 (Cernicharo et al. 2000), we derived a 3σ upper limit to its column density of 8 × 1011 cm−2.

4. Discussion

It is interesting to compare the abundance derived for HMgCCCN with those derived for other Mg-bearing cyanides detected in IRC +10216. The column densities reported in IRC +10216 for these species are N(MgNC) = 1.3 × 1013 cm−2 (Kawaguchi et al. 1993; Cabezas et al. 2013), N(HMgNC) = 6 × 1011 cm−2 (Cabezas et al. 2013), N(MgCN) = 7.4 × 1011 cm−2 (Ziurys et al. 1995; Cabezas et al. 2013), N(MgCCCN) = 9.3 × 1012 cm−2 (Cernicharo et al. 2019b), and N(MgC5N) = 4.7 × 1012 (Pardo et al. 2021). Therefore, HMgCCCN is just three times less abundant than MgCCCN, in contrast with the smaller hydridomagnesium analogue HMgNC, which is 20 times less abundant than MgNC.

In the case of sodium, the number of Na-bearing cyanides detected in IRC +10216 is much smaller than for magnesium. The only such species detected in IRC +10216, apart from NaCCCN, is NaCN, which was first detected in IRC +10216 by Turner et al. (1994) and later on mapped by Guélin et al. (1997). Unlike most metal-bearing cyanides detected in IRC +10216, which are most likely formed in the outer layers, NaCN is formed in the inner regions. The studies by Agúndez et al. (2012) and Quintana-Lacaci et al. (2017) constrained the abundance of NaCN to (3–9) × 10−9 relative to H2, which implies that it is significantly more abundant than NaCCCN, as discussed below.

To shed light on the formation mechanism of HMgCCCN and NaCCCN in IRC +10216, we expanded the chemical model presented in Pardo et al. (2021). The chemical scheme, based on the work of Petrie (1996) and Dunbar & Petrie (2002), starts with the injection of neutral metal atoms into the expanding wind. These atoms are then ionized by the interstellar radiation field and the resulting ionized metal atoms associate radiatively with long neutral carbon chains to form cationic complexes, which then recombine dissociatively with electrons to yield neutral fragments as products that are detected in IRC +10216. The formation scheme of HMgCCCN is the following:

(1)

(2)

where n = 2, 3, 4 and the second reaction also yields other Mg-bearing neutral fragments in addition to HMgCCCN. For Mg we adopted an initial abundance of 3 × 10−6 relative to H, which allowed us to reproduce the observed column densities of Mg-bearing molecules, while for Na we adopted an abundance of 4.2 × 10−7 relative to H, as measured in the outer layers of IRC +10216 by Mauron & Huggins (2010). The radiative association rate coefficients for ionized metal atoms and carbon chains are taken from the calculations of Dunbar & Petrie (2002). The other critical input data in the chemical model are the branching ratios yielding the different fragments upon dissociative recombination of the cationic complexes. These ratios are not known and are difficult to predict. We therefore tuned them to reproduce the relative abundances of metal-bearing molecules observed in IRC +10216.

The results from the chemical model are shown in Fig. 3. The calculated abundances of Mg-bearing cyanides are relatively high, in the range 10−9 − 10−8 relative to H2. In the case of sodium, we assumed that the main channel in the dissociative recombination of NaHC2n + 1N+ yields NaCN, while NaCCCN is formed with a branching ratio of ∼1%. Under this assumption, the column density of NaCCCN agrees with the observed one and NaCN is predicted to have an abundance of ∼10−8 relative to H2, within the range of values derived by Agúndez et al. (2012) and Quintana-Lacaci et al. (2017). Therefore, the observed abundance of NaCCCN is consistent with NaCN being around 100 times more abundant than it and with Na being injected into the expanding envelope with the abundance constrained by the observations of Mauron & Huggins (2010).

thumbnail Fig. 3.

Abundances calculated with the chemical model for Mg- and Na-bearing cyanides in IRC +10216. The initial abundance of Mg and the branching ratios of the dissociative recombination of metal-bearing cation complexes were chosen to reproduce the observed values. For example, the observed column density of MgNC is 1.3 × 1013 cm−2 (Cabezas et al. 2013) while the calculated value, evaluated as twice the radial column density, is 1.1 × 1013 cm−2. Closed shell species, such as HMgNC, HMgC3N, NaCN, and NaC3N, extend further than open shell ones because we assume that they do not react with H atoms and electrons.

5. Conclusions

We have reported the first identification in space of two new metal-bearing carbon chains, HMgCCCN and NaCCCN, towards IRC +10216, the carbon-rich circumstellar envelope of CW Leo. The detection of HMgCCCN adds to the long list of Mg-bearing molecules already known to be present in IRC +10216. On the other hand, the detection of NaCCCN implies that long carbon chains containing metals other than magnesium are also formed in IRC +10216, which opens the door for future detections of carbon chains containing metals such as Na, Al, K, Fe, or Ca.


1

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

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

2

Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment.

Acknowledgments

We thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS, and 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. We also thank Ministry of Science and Technology of Taiwan and Consejo Superior de Investigaciones Científicas for funding support under the MOST-CSIC Mobility Action 2021 (Grant 11-2927-I-A49-502 and OSTW200006).

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All Tables

Table 1.

Observed line parameters of HMgCCCN and NaCCCN towards IRC +10216.

Table 2.

Spectroscopic parameters of HMgCCCN.

Table 3.

Spectroscopic parameters of NaCCCN.

All Figures

thumbnail Fig. 1.

Lines of HMgCCCN observed with the Yebes 40 m telescope towards IRC +10216. Line parameters are given in Table 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 mK. The red lines show the fitted line profiles adopting an expanding terminal velocity of 14.5 km s−1 (Cernicharo et al. 2000).

In the text
thumbnail Fig. 2.

Same as Fig. 1, but for NaCCCN.

In the text
thumbnail Fig. 3.

Abundances calculated with the chemical model for Mg- and Na-bearing cyanides in IRC +10216. The initial abundance of Mg and the branching ratios of the dissociative recombination of metal-bearing cation complexes were chosen to reproduce the observed values. For example, the observed column density of MgNC is 1.3 × 1013 cm−2 (Cabezas et al. 2013) while the calculated value, evaluated as twice the radial column density, is 1.1 × 1013 cm−2. Closed shell species, such as HMgNC, HMgC3N, NaCN, and NaC3N, extend further than open shell ones because we assume that they do not react with H atoms and electrons.

In the text

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