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A&A
Volume 641, September 2020
Article Number L9
Number of page(s) 9
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202039231
Published online 23 September 2020

© ESO 2020

1. Introduction

The importance of anions in the chemistry of interstellar clouds was analyzed in the early years of astrochemistry by Dalgarno & McCray (1973). The presence of carbon chain negative ions in space was predicted on the ground that electron radiative attachment is efficient for molecules with large electron affinities and dense vibrational spectra (Sarre 1980; Herbst 1981).

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 45 m telescope by Kawaguchi et al. (1995). Their assignation to C6H− was not possible until the laboratory observations of McCarthy et al. (2006) became available. Nevertheless, Aoki (2000) suggested that the carrier of these lines was C6H− from ab initio calculations. The laboratory and space detection of this species prompted attention to the abundance of 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 and TMC-1 (Sakai et al. 2008; Agúndez et al. 2008a; Cordiner et al. 2013). C6H− was also detected towards other interstellar sources (Sakai et al. 2007; Gupta et al. 2009; Cordiner et al. 2011, 2013). Following the observation of C8H− in the laboratory (Gupta et al. 2007) this anion was found in TMC-1 (Brünken et al. 2007) and IRC +10216 (Kawaguchi et al. 2007; Remijan et al. 2007).

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). While accurate laboratory frequencies were available for CN− and C3N− (Gottlieb et al. 2007; Thaddeus et al. 2008; Amano 2008), the assignment of C5N− was based on the coincidence of the observed rotational constants with ab initio calculations by Botschwina & Oswald (2008) and Aoki (2000). Although this species is the best candidate for this identification, the lack of precise laboratory frequencies prevents us from ruling out other species involving metals, which are present in IRC +10216. For example, MgC3N and MgC4H have been recently detected in IRC +10216 based on ab initio calculations (Cernicharo et al. 2019) and they have rotational constants B just a few megahertz below that of C5N−. Moreover, there is controversy about the formation of CnN− anions through radiative electron attachment to CnN radicals, for which calculated rate constants differ by orders of magnitude (Walsh et al. 2009; Khamesian et al. 2016; Millar et al. 2017). Hence, the detection of these anions in cold dark clouds and the determination of their abundances are an important step forward in understanding their chemistry in different astronomical environments.

In this Letter we present the detection of C3N− and C5N− in the cold dark core TMC-1. This is the first time nitrile anions are observed in the interstellar medium. The C3N/C3N− and C5N/C5N− abundance ratios derived are discussed within the frame of a chemical model of a cold dense cloud.

2. Observations

The Q-band (31.0−50.3 GHz) observations of TMC-1 (α2000 = 4h41m42.0s, δ2000 = 25°41′27.6″) were carried out during the winter 2019/2020 with the 40 m radio telescope of the Yebes observatory (IGN, Spain), hereafter Yebes 40 m. Observations of IRC +10216 in the same frequency band were performed in spring 2019 and have been previously described by Cernicharo et al. (2019) and Pardo et al. (2020). New receivers were built within the Nanocosmos project1 and installed at the telescope (Tercero 2020). They were used for the observations presented in this work. The Q-band receiver consists of two HEMT cold amplifiers covering the 31.0−50.3 GHz band with horizontal and vertical polarisations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The backends are 16 × 2.5 GHz fast Fourier transform spectrometers (FFTS) with a spectral resolution of 38.1 kHz providing the whole coverage of the Q band in both polarisations. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz. Pointing corrections obtained by observing strong nearby quasars and were always within 2−3″.

For IRC +10216 the observing mode was position switching with an off position at 300″ in azimuth. The final spectra were smoothed to a resolution of 0.15 MHz, that is a velocity resolution of ≈1.5 and 0.9 km s−1 at 31 and 50 GHz, respectively. The sensitivity of the final spectra varies between 0.4 mK and 1 mK per 0.15 MHz channel across the Q band, which is a factor of ≈10 better than previous observations in the same frequency range with the Nobeyama 45 m telescope taken with a spectral resolution of 0.5−0.625 MHz (Kawaguchi et al. 1995).

The TMC-1 Q-band observations were performed using the frequency switching technique with a frequency throw of 10 MHz. The nominal spectral resolution of 38.1 kHz was left unchanged for the final spectra because of the low temperature of this source and, therefore, narrowness of its lines. The average noise at this spectral resolution in the Q band ranges from ∼0.7 mK at 31 GHz to ∼2 mK at 49.5 GHz, which considerably improves previous line surveys in this frequency range for this source (Kaifu et al. 2004).

In order to improve the rotational constants of C5N− we used all lines of this species observed with the IRAM 30 m telescope since its detection (Cernicharo et al. 2008). These observations in the λ3 mm band have been described in detail by Cernicharo et al. (2019). They correspond to observations acquired during the last 35 years covering the 70−116 GHz range with very high sensitivity (1−3 mK) over 1 MHz wide channels. Examples of these data can be found in Cernicharo et al. (2004, 2007, 2008, 2019) and Agúndez et al. (2008b, 2014).

The beam size of the Yebes 40 m in the Q band is in the range 36−56″, while that of the IRAM 30 m telescope in the 3 mm domain is 21−30″. Pointing corrections were obtained by observing strong nearby quasars or SiO masers for both sources. Pointing errors were always within 2−3″. The intensity scale for the observations with both telescopes, antenna temperature ( T A * $ T_{\rm A}^* $), was obtained after a calibration procedure that uses two absorbers at different temperatures and the atmospheric transmission model (ATM; Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties are ∼10%. Additional uncertainties could arise, in the case of IRC +10216, from the line intensity fluctuation induced by the time variation of the stellar infrared flux (Cernicharo et al. 2014; Pardo et al. 2018). All data were analyzed via the GILDAS package2.

3. Results

One of the most remarkable results from the Q-band observations in TMC-1 and IRC +10216 is the presence of a forest of weak lines. Most of these can be assigned to known species and only a few remain unidentified in IRC +10216 (Cernicharo et al. 2019; Pardo et al. 2020) and TMC-1 (Marcelino et al., in prep.). For both sources the level of sensitivity in this work has been increased, as commented previously, by a factor 5−10 with respect to previous works with other telescopes at the same frequencies (Kawaguchi et al. 1995; Kaifu et al. 2004). This high sensitivity has also allowed us to detect all known C2nH and C2n + 1N anions in TMC-1. In this Letter we focus on C3N− and C5N− lines in TMC-1. In addition, we use the observed frequencies of C5N− transitions towards IRC +10216 for an improved determination of the rotational and distortion constants of this anion.

3.1. C3N−

Laboratory work exists for this anion and frequencies are well determined with accuracies of ≃2 kHz (Thaddeus et al. 2008). This species has two rotational transitions in the 31.0−50.3 GHz frequency range. Figure 1 shows both of these transitions towards TMC-1, while the derived line parameters are given in Table 1. This species was searched towards TMC-1 by Thaddeus et al. (2008) without success, and to the best of our knowledge, this is the first time this anion has been detected in an interstellar cloud.

thumbnail Fig. 1.

Lines of C3N− observed towards TMC-1 in the 31.0−50.3 GHz frequency range. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 1. The ordinate axis represents the antenna temperature in mK corrected for atmospheric and telescope losses.

Table 1.

Observed line parameters for C3N− in TMC-1.

3.2. C5N−

The detected lines of C5N− towards TMC-1 are shown in Fig. 2. Only the J = 18−17 transition at 49998.4 MHz has not been detected. This is, however, justified since the sensitivity of the data at 50 GHz is 3 mK, whereas the expected J = 18−17 line intensity is below 5 mK (see also Fig. 2). No C5N− lines were detected with the Nobeyama 45 m telescope by Kaifu et al. (2004).

thumbnail Fig. 2.

Same as Fig. 1 but for C5N−. Observed frequencies and intensities are given in Table 2.

In order to derive precise frequencies for C5N− in TMC-1 we fitted, in our Yebes 40 m data, the central vLSR of the line emission from well-known species for which accurate laboratory rotational frequencies exist. From the seven HC5N rotational transitions in the Q band (Jup = 12 to Jup = 18), Cernicharo et al. (in prep.) derive a vLSR of 5.83 ± 0.01 km s−1. From the 13C and 15N isotopologues of HC5N, they obtain vLSR = 5.84 ± 0.01 km s−1. Hence, we adopt a vLSR of 5.83 km s−1 for further frequency determinations in TMC-1. The value given by Kaifu et al. (2004) is 5.85 km s−1, which is practically identical to our result within the uncertainties. Derived line parameters for C5N− in TMC-1 are given in Table 2.

Table 2.

Observed line parameters for C5N− in TMC-1.

The characteristic U-shaped line profiles exhibited by molecular lines in IRC +10216 allow an accurate central frequency determination, within ≃50 kHz (Cernicharo et al. 2018), in spite of the broad emission that covers 29 km s−1 (Cernicharo et al. 2000). The vLSR of the source, −26.5 km s−1, has been well determined from the observation of hundreds of lines (Cernicharo et al. 2000). The observed C5N− lines in IRC +10216 at λ ∼ 7 mm (Q band) are shown in Fig. A.1. Frequencies and other line parameters are given in Table A.1. This table also includes lines observed in the 3 mm data obtained after the detection of this species in 2008. All lines at 3 mm re-observed after Cernicharo et al. (2008) have a spectral resolution of 0.198 MHz; these lines are shown in Fig. A.2.

3.3. New rotational constants for C5N−

The frequencies of a linear molecular species can be fitted to this standard expression involving the rotational quantum number J, rotational constant B0, and the distortion constant D0:

ν ( J → J − 1 ) = 2 B 0 J − 4 D 0 J 3 . $$ \begin{aligned} \nu (J\rightarrow J{-}1)=2B_0J - 4D_0J^3. \end{aligned} $$

Using all observed C5N− lines in both sources, the fit provides the following results:

B 0 = 1388.86681 ( 19 ) MHz D 0 = 34.44 ( 13 ) × 10 − 6 MHz , $$ \begin{aligned}&B_0=1388.86681(19)\,\mathrm{MHz}\\&D_0=34.44(13)\times 10^{-6}\,\mathrm{MHz}, \end{aligned} $$

where values between parentheses represent the 1σ uncertainty for the fitted parameters. The data were weighted in the fit according 1/δv2, where δv is the estimated uncertainty on the observed frequencies. The correlation coefficient between B0 and D0 is 0.738 and the standard deviation between the predicted and observed frequencies is 66 kHz. These values significantly improve those derived from previous observations by Cernicharo et al. (2008), B0 = 1388.860(2) and D0 = 33(1) × 10−6 MHz. We tried to fit the distortion constant of order six (H0), but the derived value is only marginally significant. Its inclusion reduces the weighted deviation from 1.03 to 0.89 and the standard deviation from 66 to 53.7 kHz. However, it increases the correlation between the fitted parameters. The derived values in this case, are written as

B 0 = 1388.86734 ( 25 ) MHz D 0 = 35.68 ( 44 ) × 10 − 6 MHz H 0 = 5.0 ( 1.7 ) × 10 − 10 MHz . $$ \begin{aligned}&B_0=1388.86734(25)\,\mathrm{MHz}\\&D_0=35.68(44)\times 10^{-6}\,\mathrm{MHz}\\&H_0=5.0(1.7)\times 10^{-10}\,\mathrm{MHz}. \end{aligned} $$

The differences between observed and calculated frequencies are given in Tables 2 and A.1 for TMC-1 and IRC+10216, respectively. New rotational parameters were also derived for C5N (see Appendix A.1).

4. Discussion

Only two rotational C3N− lines have been detected in TMC-1. We assumed a volume density of H2 of 4 × 104 cm−3 (see e.g. Fossé et al. 2001), a kinetic temperature of 10 K, a dipole moment for the molecule of 2.27 D (Pascoli & Lavendy 1999), and the collisional rates of C3N−/H2 from Lara-Moreno et al. (2019). For simulating the source we assumed a circular uniform brightness distribution with a radius of 40″ (see e.g. the intensity maps of different carbon chains presented by Fossé et al. 2001). With this assumed size the source completely covers the main beam of the Yebes 40 m at 50 GHz.

Using the large velocity gradient approximation (LVG) implemented in MADEX (Cernicharo 2012), and correcting the intensities of all observed transitions for the beam efficiency and the source beam dilution, we derive a column density of N(C3N−) = 1.3 × 1011 cm−2.

For C5N− we have enough observed lines to build the rotational diagram shown in Fig. 3. A dipole moment of 5.2 D was adopted for this species (Botschwina & Oswald 2008). A source size identical to that of C3N− was adopted. We derive a rotational temperature of 6.4 ± 0.6 K and a column density of (2.6 ± 0.9) × 1011 cm−2. We also performed an LVG calculation with MADEX adopting for C5N− the collisional rates of C6H−/p-H2 (Walker et al. 2017). For a kinetic temperature of 10 K, the observed intensities corrected for beam efficiency and source beam dilution can be reproduced with a column density of 9 × 1011 cm−2. If the collisional rates of HC5N/p-H2 are adopted (F. Lique, priv. comm.), then the column density is 1.5 × 1011 cm−2. Hence, the difference by a factor two of the latter value with respect to that from the rotational diagram is due to the uncertainties on both the collisional rates and the assumed H2 volume density. We adopt for C5N− the column density derived from the rotational diagram of Fig. 3.

thumbnail Fig. 3.

Rotational diagram for the observed lines of C5N− in TMC-1.

The derived line parameters for C3N and C5N are given in Tables A.2 and A.3. For C3N collisional rates are not available and we assumed a rotational temperature of 7 K (that is similar to that of C5N−) to compute line intensities assuming local thermodynamical equilibrium conditions (LTE). Assuming the same source size as for the anions, we derive N(C3N) = 1.8 × 1013 cm−2. For C5N we can build a rotational diagram based on the data provided in Table A.3, which yields Trot = 10.1 ± 1.90 K and N(C5N) = (6.0 ± 2.5) × 1011 cm−2. The adopted dipole moment for this species is 3.385 D (Botschwina 1996). Hence, the N(C3N)/N(C3N−) and N(C5N)/N(C5N−) abundance ratios in TMC-1 are ∼140, and ∼2.3, respectively.

In IRC +10216 Thaddeus et al. (2008) derived a N(C3N)/N(C3N−) abundance ratio of ≃194, and Cernicharo et al. (2008) obtained a N(C5N)/N(C5N−) abundance ratio around 2. Hence, the observed abundance ratios between neutral radicals CnN and their anions are very similar in interstellar and circumstellar clouds.

One of the problems in obtaining the CnN/CnN− abundance ratio is the assumed permanent dipole moment for the neutrals. In the case of C5N, the dipole moment for the ground 2Σ electronic state was calculated by Botschwina (1996). However, as discussed by Cernicharo et al. (2008), the C5N radical has a low lying 2Π electronic state with a dipole moment of ∼1 D (Pauzat et al. 1991). Detailed calculations by Botschwina (1996) indicate that it lies 500 cm−1 above the 2Σ ground state. Hence, C5N could have a dipole moment between these two values in the case of admixing between the 2Σ and the 2Π states. A dipole moment averaged over both electronic states (i.e., twice as small as that calculated for the unperturbed 2Σ state) would raise the C5N/C5N− abundance ratio to ∼8 in TMC-1 and IRC +10216, which is very similar to the C6H/C6H− ratio in IRC +10216 (Cernicharo et al. 2007).

The same situation applies to C4H and C4H−. The neutral radical also has a 2Σ+ ground electronic state with a first excited 2Π electronic state very close in energy. The case has been recently discussed by Oyama et al. (2020) who computed a dipole moment for C4H 2.4 times larger than the value of the 2Σ+ state alone. The effect is a decrease by a factor ∼6 on the derived C4H column densities and an increase of the C4H/C4H− abundance ratio by the same factor.

The chemistry of negative ions in cold interstellar clouds has been discussed by Walsh et al. (2009) and Millar et al. (2017). In the light of the discovery of interstellar C3N− and C5N−, we revisit the chemistry of these species in this work. We performed a chemical model of a cold dark cloud with typical physical parameters (Tk = 10 K, nH = 2 × 104 cm−3, ζ = 1.3 × 10−17 s−1, AV = 30 mag) and “low metal” elemental abundances (see Agúndez & Wakelam 2013). We adopted the University of Manchester Institute of Science and Technology RATE12 reaction network (McElroy et al. 2013), with a subset of reactions involving HCCN from Loison et al. (2015). According to the model, formation of C3N− does not occur via radiative electron attachment to C3N, which is slow (Petrie & Herbst 1997), but through reactions between N atoms and bare carbon-chain anions C n − $ \rm{C}_n^- $ (with n ≥ 6), which are rapid and produce several nitrile radical and anions (Eichelberger et al. 2007). In the case of C5N−, the reaction of radiative electron attachment to C5N is rapid (Walsh et al. 2009) and dominates the synthesis of this anion. Formation of C3N− through dissociative electron attachment to metastable isomers such as HNC3 (Harada & Herbst 2008) is included and occurs to some extent, although most of C3N− is formed by N + C n − $ \rm{C}_n^- $ reactions. Reactions of H− with HC3N and HC5N could be a source of C3N− and C5N−, although they are likely to be too slow at 10 K based on calculations for similar reactions of H− with C2H2 and C4H2 (Gianturco et al. 2016). Destruction of C3N− and C5N− is dominated by reactions with neutral atoms H, O, and C in cold dark clouds. Nitrile anions have been shown to react rapidly with polar molecules at low temperatures (Joalland et al. 2016). However, it is unlikely that this is a major loss channel for anions in cold dense clouds because, apart from CO, molecules have much lower abundances than neutral atoms.

The much larger rate constant of radiative electron attachment to C5N compared to C3N is the main reason why C5N− is calculated to be much more abundant with respect to the neutral than C3N− (see Fig. 4). The results of our model are similar to the theoretical predictions of Walsh et al. (2009). Calculated neutral-to-anion abundance ratios during the so-called early time (105–106 yr), at which calculated abundances agree better with observations (see e.g. Agúndez & Wakelam 2013), agree well with the values retrieved from observations for C3N−, although they are somewhat higher for C5N− (see Fig. 4). We note that, as discussed above, the observed C5N/C5N− ratio could be as high as ∼8, in which case model and observations would agree much better. Finally, we point out that the chemical model predicts an abundance of ∼3 × 10−11 relative to H2 for CN−. Adopting a column density of 1022 cm−2 for H2 (Cernicharo & Guélin 1987), the predicted column density of CN− would be 3 × 1011 cm−2, which is below the 3σ upper limit of 1.4 × 1012 cm−2 derived by Agúndez et al. (2008a).

thumbnail Fig. 4.

Neutral-to-anion abundance ratios calculated with the chemical model are shown as a function of time and compared with observed values. The early time (105–106 yr) at which calculated abundances agree better with observations (see e.g. Agúndez & Wakelam 2013) is indicated. A conservative uncertainty of a factor of 2 is adopted for the observed ratios.

This work definitively excludes metal-bearing species, or vibrationally excited states of other known species, as carriers for the series of lines assigned to C5N− by Cernicharo et al. (2008), and gives strong support to this identification.

Acknowledgments

We thank Spanish Ministerio de Ciencia e Innovación for funding support through project AYA2016-75066-C2-1-P. We also thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. MA and CB thank Ministerio de Ciencia e Innovación for Ramón y Cajal grant RyC-2014-16277 and Juan de la Cierva grant FJCI-2016-27983. We thank support from French National Research Agency through project Anion Cos Chem (ANR-14-CE33-0013). We would like to thank our referee, Prof. S. Yamamoto, for useful comments and suggestions.

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Appendix A: Additional tables and figures

Line parameters for C5N− towards IRC +10216 are given in Table A.1. The observed lines are shown in Figs. A.1 and A.2. The data in the 3 mm domain (see Fig. A.2) considerably improve the signal to noise ratio of the lines reported by Cernicharo et al. (2008). The new rotational constants derived from these frequencies and those of TMC-1 are discussed in Sect. 3.3.

thumbnail Fig. A.1.

Rotational transitions of C5N− observed towards IRC +10216 in the 31−50 GHz range. Derived frequencies and intensities are given in Table A.1. Figure A.2 shows the lines of the same species observed with the IRAM 30 m telescope at λ = 3 mm.

thumbnail Fig. A.2.

Observed lines of C5N− towards IRC +10216 in the 3 mm domain with the IRAM 30 m telescope. Spectral resolution is 0.19 MHz for all lines. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table A.1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK.

The observed line parameters for C3N and C5N in TMC-1, which are needed to compute the column density of these species and to compare with that of their anions, are given in Tables A.2 and A.3. For C3N, accurate frequency predictions can be found in the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al. 2005). However, we discuss below the case of C5N, as significant deviations between predicted and measured frequencies in space are observed.

Table A.1.

Observed line parameters for C5N− in IRC +10216.

Table A.2.

Observed line parameters for C3N in TMC-1.

Table A.3.

Observed line parameters for C5N in TMC-1.

A.1. New rotational parameters for C5N

For C5N the frequencies of rotational lines up to J = 6 (νmax = 16.842 GHz) were measured in the laboratory by Kasai et al. (1997). These authors provide frequency predictions up to 98.2 GHz that are systematically below the observed frequencies of this species towards TMC-1 (data provided in Table A.3) and IRC +10216 (data provided in Table A.4). In order to derive more accurate predictions we fitted the observed astronomical frequencies to the same Hamiltonian than Kasai et al. (1997). The fits only consider frequencies measured in astronomical (space) sources that have an unresolved hyperfine structure and a merged set of space and laboratory frequencies. The results are provided in Table A.5. The poorly determined distortion constant from the laboratory data alone is responsible for the observed discrepancies. The new D value derived from space data alone is 44.2 ± 0.2 Hz, compared with the laboratory value of 50 ± 10 Hz. In IRC +10216 several additional doublets of C5N are detected between Jup = 40 to Jup = 48. However, their line intensity is weak and the derived frequencies have uncertainties of ∼1 MHz. These have not been included in the fit.

Table A.4.

Observed line parameters for C5N in IRC +10216.

In addition we combined in one fit all the rotational transitions measured so far for C5N, including those from laboratory measurements and those observed in TMC-1 and IRC +10216. We used the SPFIT program (Pickett 1991). The results are shown in Table A.5. In all these fits the data were weighted according 1/σ2, where σ is the estimated uncertainty on the measured frequencies. We obtained new values for the rotational and centrifugal distortion constants for the three spin-rotation constants and also for the nuclear quadruple coupling constant. In total we analyzed 69 rotational transitions. We tried to determine additional parameters such as the distortion constant H or the spin-rotation constant γD, but the attempts resulted to be unfruitful. The uncertainty in the rotational constant, B, improves from 0.54 kHz at Kasai et al. (1997) to 0.15 kHz. The most significant change corresponds to the distortion constant, which has an uncertainty of 10 Hz from the laboratory data alone, and 0.3 Hz when the lines from TMC-1 and IRC +10216 are included.

Table A.5.

New derived rotational parameters for C5N from TMC-1 & IRC +10216.

The fit to the TMC-1 and IRC +10216 alone allows us to predict frequencies without hyperfine splitting with an accuracy better than 0.1 MHz up to Nup = 55 (ν ∼ 154.3 GHz). Observable hyperfine splitting in TMC-1 could occur for transitions with N ≤ 8 (ν ∼ 22.4 GHz). In this case, the merged fit to the laboratory and space data is recommended to compute the expected frequencies.

All Tables

Table 1.

Observed line parameters for C3N− in TMC-1.

In the text
Table 2.

Observed line parameters for C5N− in TMC-1.

In the text
Table A.1.

Observed line parameters for C5N− in IRC +10216.

In the text
Table A.2.

Observed line parameters for C3N in TMC-1.

In the text
Table A.3.

Observed line parameters for C5N in TMC-1.

In the text
Table A.4.

Observed line parameters for C5N in IRC +10216.

In the text
Table A.5.

New derived rotational parameters for C5N from TMC-1 & IRC +10216.

In the text

All Figures

thumbnail Fig. 1.

Lines of C3N− observed towards TMC-1 in the 31.0−50.3 GHz frequency range. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table 1. The ordinate axis represents the antenna temperature in mK corrected for atmospheric and telescope losses.
In the text
thumbnail Fig. 2.

Same as Fig. 1 but for C5N−. Observed frequencies and intensities are given in Table 2.
In the text
thumbnail Fig. 3.

Rotational diagram for the observed lines of C5N− in TMC-1.
In the text
thumbnail Fig. 4.

Neutral-to-anion abundance ratios calculated with the chemical model are shown as a function of time and compared with observed values. The early time (105–106 yr) at which calculated abundances agree better with observations (see e.g. Agúndez & Wakelam 2013) is indicated. A conservative uncertainty of a factor of 2 is adopted for the observed ratios.
In the text
thumbnail Fig. A.1.

Rotational transitions of C5N− observed towards IRC +10216 in the 31−50 GHz range. Derived frequencies and intensities are given in Table A.1. Figure A.2 shows the lines of the same species observed with the IRAM 30 m telescope at λ = 3 mm.
In the text
thumbnail Fig. A.2.

Observed lines of C5N− towards IRC +10216 in the 3 mm domain with the IRAM 30 m telescope. Spectral resolution is 0.19 MHz for all lines. The abscissa corresponds to the local standard of rest velocity in km s−1. Frequencies and intensities for the observed lines are given in Table A.1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK.
In the text

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