Discovery of HC₄NC in TMC-1: A study of the isomers of HC₃N, HC₅N, and HC₇N

Abstract

We present a study of the isocyano isomers of the cyanopolyynes HC₃N, HC₅N, and HC₇N in TMC-1 and IRC+10216 carried out with the Yebes 40m radio telescope. This study has enabled us to report the detection, for the first time in space, of HCCCCNC in TMC-1 and to give upper limits for HC₆NC in the same source. In addition, the deuterated isotopologues of HCCNC and HNCCC were detected, along with all ¹³C substitutions of HCCNC, also for the first time in space. The abundance ratios of HC₃N and HC₅N, with their isomers, are very different in TMC-1 and IRC+10216, namely, N(HC₅N)/N(HC₄NC) is ~300 and ≥2100, respectively. We discuss the chemistry of the metastable isomers of cyanopolyynes in terms of the most likely formation pathways and by comparing observational abundance ratios between different sources.

1. Introduction

Millimeter-wave observations of lines from isomers of abundant molecular species provide key information for studying the chemical paths leading to their formation in interstellar and circumstellar clouds, an important piece of the astrochemical puzzle. The best known example is that of HCN and HNC, which are formed in interstellar clouds by the dissociative recombination of a common species, the HCNH⁺ cation, in a branching ratio of 1:1. Once these species are formed, they undergo various reactions that can lead to changes in the initial abundances, in particular, reactions of HNC with atoms and radicals (see, e.g. Hacar et al. 2020 and references therein). In circumstellar clouds, HCN and HNC are produced very close to the photosphere of the central star under thermodynamical chemical equilibrium with a branching ratio of ~ 1000:1 (Cernicharo et al. 2013). While HCN maintains its abundance across the envelope, HNC disappears very quickly when moving away from the star. It reappears again in the zone where galactic UV photons are able to penetrate the envelope, initiating a very rich photochemistry (Cernicharo et al. 2013).

Some isomers may not necessarily be formed from a common precursor but as an effect of radiation as it is thought to occur for the cis conformer of HCOOH in the Orion Bar photodissociation region (Cuadrado et al. 2016). This is an interesting case, in which the absorption of a UV photon by the trans conformer leads, through radiative decay, to the cis conformer. This photochemical switch can only work in regions with enough UV illumination. In fact, cis HCOOH has been detected in the cold dark clouds B5 and L483 (Taquet et al. 2017; Agúndez et al. 2019). The lack of UV photons in these environments together with their derived lower cis-to-trans abundance ratios, as compared to the Orion Bar, indicate that a different mechanism must operate in these cold clouds.

However, most of the isomers known in space involve radical CN. In addition to HCN and HNC, the isocyanide isomers of CH₃CN (Cernicharo et al. 1988), HC₃N (Kawaguchi et al. 1992a,b), and NCCN (Agúndez et al. 2018) have been detected in space. HCCNC and HNCCC, the two isomers of HC₃N, have also been detected towards the circumstellar envelope IRC+10216 (Gensheimer 1997a,a). A comparative study of the abundances of isomers in interstellar and circumstellar clouds provide an opportunity to understand the chemical processes leading to their formation, as the chemistry prevailing in cold interstellar clouds (ion-neutral reactions) and in external layers of circumstellar envelopes (radical-neutral and photochemical reactions) are very different. In this letter, we present a systematic study of the HC₃N, HC₅N, and HC₇N isomers, along with their isotopologues, towards TMC-1 and IRC+10216 making use of the observations described in Section 2. In Section 3 we report on the first detection in space of HC₄NC¹, Isocyanodiacetylene, which is the most stable isomer after HC₅N. This new molecular species has been detected only towards TMC-1, whereas a very low upper limit has been obtained towards IRC+10216, which is a rather interesting finding and which we discuss in Section 4.

2. Observations

New receivers, built within the Nanocosmos project² and installed at the Yebes 40m radio telescope, (hereafter, Yebes 40m) were used for the observations of TMC-1. The Q-band receiver consists of two HEMT cold amplifiers covering the 31.0-50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers are 2 × 8 × 2.5 GHz FFTs with a spectral resolution of 38.1 kHz providing the whole coverage of the Q-band in both polarizations. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz. The beam size of the telescope is 56″ and 36″ at 31 and 50 GHz, respectively. Pointing errors are always within 2-3″.

The observations leading to the Q-band line survey towards TMC-1 were performed in several sessions, between November 2019 and February 2020. The observing procedure was frequency switching with a frequency throw of 10 MHz. The nominal spectral resolution of 38.1 kHz was left unchanged for the final spectra. The sensitivity, σ, along the Q-band varies between ~0.6 (31 GHz), ~1.0 (43 GHz), and ~2.5 mK (50 GHz). It was derived by removing a polynomial baseline in velocity windows of -6.2 to 17.8 km s^–1 centered on each observed line.

The intensity scale, antenna temperature $\left({\text{T}}_A^{\ast }\right)$, was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM, Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties are estimated to be within 10 %. All data were analyzed using the GILDAS package³. The observations in the Q-band of IRC+10216 were previously described by Cernicharo et al. (2019) and Pardo et al. (2020).

3. Results

The sensitivity of our observations towards TMC-1 (see section 2) is a factor of 5-10 better than previously published line surveys of this source at the same frequencies (Kaifu et al. 2004). This has allowed us to detect a forest of weak lines, most of them arising from the isotopologues of abundant species such as HC₃N and HC₅N. For the observed line parameters of HC₃N and its isotopologues, see Appendix B. With regard to HC₅N, in addition to its deuterated species, it has five ¹³C and one ¹⁵N isotopologues and is responsible for 50 features detected within our survey with a signal to noise ratio (S/N) between 10 for ${\text{HC}}_5^{15}\text{N}$, 20-40 for the ¹³C isotopologues, and >1000 for the main isotopo- logue (see also Appendix B for their observed line parameters). From the seven rotational transitions of HC₅N observed within the Q-band (J _up=12 to J _up=18), we obtained a local standard of rest velocity of the source, namely, v_LSR, of 5.83±0.01 km s^–1. From the ¹³C and ¹⁵N isotopologes of HC₅N, which provide 43 different transitions to estimate this same velocity, we get v_LSR = 5.84±0.01 km s^–1. Hence, we adopt a value v_LSR 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 observational uncertainties.

3.1. The isomers of HC₃N

The most stable isomers of HC₃N are HCCNC and HNCCC. They have been extensively observed in the laboratory. Rotational transitions of HCCNC have been measured up to J _up=33 at a frequency of 327.8 GHz (Guarnieri et al. 1992). HNCCC is a quasi-linear species, with the hydrogen atom slightly bent with respect to the NCCC axis. Its rotational transitions have been measured in the microwave domain (Hirahara et al. 1993) and at millimeter wavelengths (Vastel et al. 2018). Hence, accurate frequencies are available for these species. The J=1-0 line of nine HCCNC isotopologue (D, ¹³C and ¹⁵N) was measured in the laboratory by Krüger et al. (1992). The millimeterwave spectrum of DCCNC was measured by Huckauf et al. (1998) up to J _up=51. The microwave spectrum of DNCCC was measured by Hirahara et al. (1993) up to J _up=2, and ν _max=19 GHz. The derived rotational constants for all these species have been implemented in the MADEX code (Cernicharo 2012) in order to work for their detection and analysis in interstellar and circumstellar sources.

The HCCNC and HNCCC isomers were previously detected towards TMC-1 by Kawaguchi et al. (1992a,b) and in IRC +10216 by Gensheimer (1997a,b). A detailed study of these two species in L1544, including their relative abundances, formation paths, and comparison with the abundances observed in TMC-1 has been conducted by Vastel et al. (2018) based on observations in the λ 3 mm domain.

The derived line parameters for the HCCNC and HNCCC transitions observed in TMC-1 are given in Table A.1. Selected lines are shown in Fig. 1. The deuterated counterparts DNCCC and DCCNC, together with the three ¹³C substitutions of HCCNC, have also been detected for the first time in space. Its derived line parameters are given in Table A.1 (see also Fig. 1). Improved rotational constants for DNCCC, H¹³CCNC, HC¹³CNC, and HCCN¹³C were obtained from the observed frequencies. They are given in Appendix A. The isotopic abundance ratios of the HC₃N isomers are discussed in Appendix B.

Fig. 1.

J=4-3 transition of HNCCC, HCCNC, and some of their isotopologues, observed towards TMC-1. The abscissa corresponds to the local standard of rest velocity in kms^–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. Cyan stars indicate the position of ghost negative features in the spectra produced by the frequency switching observation mode. Other spectral features arising from other molecular species, or otherwise unidentified, are labeled in the different panels.

Assuming a TMC-1 source size of 40″ and that the rotational temperature of the observed transitions is identical to the kinetic temperature (10 K), we derive N(HNCCC)=(5.2±0.3)×10¹¹ cm^–2, and N(HCCNC)=(3.0±0.3)×10¹² cm^–2. We can check the validity of the assumed rotational temperature by adopting for the isomers the same collisional rates of HC₃N (Wernli et al. 2007), and a value of (4-10)×10⁴ cm^–3 for the H₂ volume density, in the large velocity gradient (LVG) module integrated in mAdEX (Cernicharo 2012). For the lowest value of the H₂ density, the derived excitation temperatures for the J=4-3 and 5-4 transitions are 9.5 and 8.2 K for HCCNC, whereas they are 10 and 7 K for HNCCC. For n(H₂)=10⁵ cm^–3, these rotational transitions will have excitation temperatures very close to 10 K. For HC₃N, we also performed LVG calculations leading to derived excitation temperatures very close to 10 K for the J=4-3 and 5-4 transitions. However, these HC₃N transitions exhibit line opacity problems due to the high abundance of this molecule in TMC-1. The very weak hyperfine components F=4-4 and F=3-3 of the J=4-3 transition, and the F=5-5 and F=4-4 of the J=5-4 transition, were detected. We used those components to derive N(HC₃N)=(2.3±0.2)×10¹⁴ cm^–2, which is a value 1.5 times larger than the one obtained using only the strongest hyperfine components of the observed rotational transitions. We can also estimate the column density of cyanoacetylene by using its ¹³C isotopologues (see Table B.2) and a ¹²C/¹³C abundance ratio of 93±10 (derived from all isotopologues of HC₅N, see Appendix B). In this case, the result is N(HC₃N)=(1.9±0.2)×10¹⁴ cm^–2. Our value for the isotopic abundance ratio agrees rather well with the one derived by Takano et al. (1998) in the same source. The derived column densities for HNCCC, HCCNC, and HC₃N in TMC-1 (see Table 1) are in reasonable agreement with those obtained previously (Kawaguchi et al. 1992a,b; Takano et al. 1998). We note that our assumed rotational temperatures are higher than those reported in the literature (5-7 K), but the differences in the estimated column densities for optically thin lines involving energy levels between 5-8 K is is rather small. For example, for T_rot=5 K and N(HNCCC)=10¹¹ cm^–2, the expected brightness intensity for its J=4-3 transition is ~43.5 mK, while for T_rot=10 K, it is 50.4 mK.

Table 1. Column densities and abundance ratios for the isomers of HC₃N and HC₅N towards TMC-1 and IRC+10216.

Molecule	TMC-1	IRC+10216
N(HC3N)	(2.3±0.2)×1014	(4.5±0.2)×1014
N(HCCNC)	(3.0±0.3)×1012	(1.1±0.1)×1012
N(HNCCC)	(5.2±0.3)×1011	(3.4±0.2)×1011
N(HC5N)	(1.8±0.2)×1014	(4.2±0.4)×1014 b
N(HC4NC)	(3.0±0.7)×1011	≤2.1×1011
N(HNC5)	≤1.5×1011	≤1.1×1011
N(HC7N)	(6.4±0.2)×1013	(1.9±0.2)×1014 c
N(HC6NC)	≤9.0×1010	≤2.0×1011
N(HC3N)/N(HCCNC)	77.0±8.0	392±22
N(HC3N)/N(HNCCC)	442.0±70.0	1305±45
N(HCCNC)/N(HNCCC)	5.8±1.0	4.0±0.9
N(HC5N)/N(HC4NC)	600±70.0	≥2000
N(HC5N)/N(HNC5)	≥1200	≥3800
N(HC4NC)/N(HNC5)	≥2	≥1.9
N(HC7N)/N(HC6NC)	≥710.0	≥950
N(HC3N)/N(HC5N)	1.3±0.2	1.1±0.2
N(HC5N)/N(HC7N)	2.8±0.3	2.2±0.5

Notes.

^(a)All column densities are given in units of cm^–2.

^(b)For IRC+10216, we adopted the value derived by Pardo et al. (2020), which corresponds to the gas component with T _rot=10.1 K. A second component, with similar column density, was found for high-J levels with a T _rot=24.5 K.

^(c)For IRC+10216, we adopted the value derived by Pardo etal. (2020), which corresponds to the gas component with T _rot=16.8 K. A second component, with a column density 2.5 lower, was found for high-J levels with a T _rot=31.6 K.

The Q-band survey of IRC+10216 carried out with the Yebes 40m also reached an unprecedented sensitivity, which has al-lowed the detection of new molecular species such as MgC₃N and MgC₄H (Cernicharo et al. 2019), and of previously undetected vibrational excited states of abundant species such as HC₇N and HC₉N (Pardo et al. 2020). In these data, we find two transitions of HNCCC and HCCNC, with their observed line parameters, which are given together with those of HC₃N and their isotopologues in Table A.2. The isotopologues of the isomers are, however, below the sensitivity limit of the data. We performed an LVG model to derive the column densities of the three HC₃N isomers from the observations. The results are given in Table 1. To avoid line opacity effects, the column density of HC₃N has been derived from the observed intensities of its three ¹³C isotopologues using an isotopic abundance ratio of 45 (Cernicharo et al. 2000).

3.2. The isomers of HC₅N. Detection of HC₄NC

HC₅N has different isomers shown in Figure C. The most stable one, after HC₅N, is HC₄NC (see Appendix C). Both of them could be formed in TMC-1 through the dissociative recombination of the cation HC₅NH⁺, recently detected in TMC-1 by Marcelino et al. (2020). Through its similarity to HC₃N and HNCCC, the isomer HNC₅ could also be formed in the dissociative recombination of HC₅NH⁺. While HC₄NC has been observed in the laboratory (Botschwina et al. 1998) and precise rotational frequencies are available from the Cologne Database for Molecular Spectroscopy (CDMS) catalogue (Müller et al. 2005) or from the MADEX code (Cernicharo 2012), no laboratory data are available for HNC₅. Nevertheless, we estimated, using ab initio calculations, its rotational constants and searched for this species in TMC1 and IRC+10216.

Five lines of HC₄NC were detected towards TMC-1 (see Fig 2). This is the first time that this species has been detected in space. From the rotational diagram obtained with its observed line intensities, we derived T _rot=9.8±0.9 K, and N(HC₄NC)=(3.0±0.7)×10¹¹ cm^–2. In order to compare the derived column density with that of the most stable isomer, HC₅N, we used the observed intensities of this last species (see Table B.3) to build another rotational diagram. We obtained T _rot=8.6±0.2 K, and N(HC₅N)=(1.8±0.2)×10¹⁴ cm^–2 (corrected for line opacity effects, see Appendix B). Hence, the HC₅N/HC₄NC abundance ratio in TMC-1 is ~600. Using the observed frequencies of HC₄NC in TMC-1 and those obtained in the laboratory (Botschwina et al. 1998), we improved the rotational constants of this species (see Appendix A).

Fig. 2.

Observed lines of HC₄NC found in the 31-50 GHz frequency range towards TMC-1. 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 2. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J=17-16 line is not detected within a 3σ level of 5.7 mK. Spectral resolution is 38.1 kHz.

We searched for the lines of HC₄NC towards IRC+10216 without success. By averaging all the data from the expected line positions within the Q-band, we derived a column density upper limit of 2×10¹¹ cm^–2. Using the same dataset, the column density of HC₅N was recently derived as (4.2±0.4)×10¹⁴ cm^–2 (Pardo et al. 2020). Hence, in this carbon-rich circumstellar envelope, the HC₅N/HC₄NC abundance ratio is ≥2000.

Precise rotational constants were estimated through our ab initio calculations for HNC₅ (scaled values B₀ = 1358.8±1.0 MHz, D ₀ ~32 Hz; see Appendix C). Although slightly asymmetrical, it is a quasi-linear species (see Appendix C) such that we could expect the J _0J series of lines to be in harmonic relation. The K =1 lines could be at an energy too high to be detected in TMC1. We searched for such an harmonic pattern around ±100 MHz of the predicted line frequencies without success. Using the dipole moment we derived from our calculations, we get N(HNC₅)≤1.5×10¹¹ cm^–2 and ≤1.1×10¹¹ cm^–2 towards TMC-1 and IRC+10216, respectively.

3.3. The isomers of HC₇N

HC₇N has 17 rotational transitions within the frequency range of our Q-band data. All of them have been detected in TMC1 and in IRC+10216. For the latter source, the data were presented and analyzed by Pardo et al. (2020); the derived HC₇N column density is given in Table 1. From the TMC1 data (see Table B.3), we built a rotational diagram that gives the following results: T _rot=7.6±0.1 K and N(HC₇N)=(6.4±0.4)×10¹³ cm^–2.

The most stable isomer of HC₇N is HC₆NC, which was previously observed in the laboratory by Botschwina et al. (1998). We searched for its lines towards both sources without success. The derived upper limits, which are based on a stacking of the J=28-27 up to J=31-30 transitions, are given in Table 1.

4. Discussion

The chemistry of HC₃N isomers in cold dense clouds has been discussed by Osamura et al. (1999) and Vastel et al. (2018). In the scenario depicted by these authors, the main formation route to the HC₃N isomers relies on the dissociative recombination of HC₃NH⁺. The rate constant of this reaction has been measured, using the deuterated variant of the ion, by Geppert et al. (2004). Calculations by Osamura et al. (1999) indicate that rearrangement is possible so that the various linear or nearly-linear isomers of HC₃N, that is, HNC₃, HCCNC, and HCNCC, can be formed. There is some experimental information on the different fragments that are formed (Geppert et al. 2004), but precise branching ratios for the different exit channels are not known and introduce important uncertainties when modeling the chemistry of HC₃N isomers. The chemical models constructed for TMC-1 (Osamura et al. 1999) and for L1544 (Vastel et al. 2018) reproduce quite well the observed abundances of HC₃N isomers, although they tend to overestimate the abundances of HNC₃ and HCCNC with respect to HC₃N. In these models, the most stable isomer, HC₃N, is also formed through neutral-neutral reactions, while the different metastable isomers are mostly formed through dissociative recombination of HC₃NH⁺ and, to a lesser extent, of its less stable isomer HCCNCH⁺. The higher abundance observed for the isomer HCCNC with respect to HNC₃ is explained in terms of a higher reactivity of the latter isomer with neutral H and C atoms (Osamura et al. 1999). By way of an analogy with HC₃N, the main source of HC₅N and its isomers is most likely the dissociative recombination of HC₅NH⁺, recently detected in TMC-1 (Marcelino et al. 2020). However, branching ratios for the formation of the different isomers HC₅N, HC₄NC, and HNC₅ are unknown.

Looking at how the relative abundances of HC₃N and HC₅N isomers behave in different sources can provide clues on the underlying chemical processes. In Fig. 3, we compare the abundance ratios between HC₃N isomers derived here for TMC-1 and IRC + 10216 with those obtained in the molecular clouds L1544 and L483. It can be seen that the HC₃N/HCCNC and HC₃N/HNC₃ ratios observed in TMC-1 are similar to those seen in the Class 0 molecular cloud L483, although they are higher than those derived in the starless core L1544 and lower than the values in the C-star envelope IRC +10216. The change in these abundance ratios may be related to the temperature of the source. The low temperature of the starless core L1544 seems to favor the formation of metastable isomers while these are less efficiently formed in the warmer ejecta of IRC +10216. Indeed, as temperature increases, the presence of high-energy isomers is less favorable because chemical reactions, including isomerization, are expected to favor the most stable isomer. The same is expected to be true of isotopic fractionation. For example, in TMC-1, the isotopologue HCC¹³CN is twice as abundant as H¹³CCCN and HC¹³CCN (see also Takano et al. 1998), whereas, in the case of HCCNC, its HC¹³CNC isotopologue is 1.8 times more abundant than H¹³CCNC and HCCN¹³C (see Table B.1). A slight overabundance of HCCCC¹³CN with respect to the other four ¹³C isotopologues of HC₅N has also been observed (see Appendix B). These kinds of isotopic anomalies have been also seen for HC₃N in other cold dense clouds (Araki et al. 2016; Taniguchi et al. 2017; Agúndez et al. 2019), but they are not seen in IRC +10216.

Fig. 3.

Observed abundance ratios between isomers of HC₃N and HC₅N in different molecular sources. Values for TMC-1 and IRC +10216 are from this study, for L1544 from Vastel et al. (2018), and for L483 from AgUndez et al. (2019).

The column densities and abundance ratios derived for the isomers of HC₃N, HC₅N, and HC₇N in TMC-1 and IRC +10216 are summarized in Table 1. The abundance ratios given in Table 1 (see also Fig. 3), suggest a trend in which the relative abundance of metastable isomers decreases as we move to larger molecular sizes. In TMC-1, we have N(HCN)/N(HNC)≃1, N(HC₃N)/N(HCCNC)≃77, N(HC₃N)/N(HNC₃)≃442, N(HC₅N)/N(HC₄NC)~600, and N(HC₇N)/N(HC₆NC)≥710. These ratios are increased by a factor of ~4-5 in IRC + 10216. This suggests that metastable isomers of longer carbon chains will be hardly detectable in these sources.

Table 2. Observed line parameters of HCCCCNC in TMC-1.

J u	$v_{obs}^a$ (MHz)	${\text{T}}_A^{*}$ b (mK)	Δvc(kms–1)	${\displaystyle \int {\text{T}}_A^{*}\text{dv}}$ d (mK kms–1)
12	33628.151±0.010	3.6±0.5	0.71±0.12	2.68±0.3
13	36430.429±0.010	3.8±0.5	0.66±0.09	2.68±0.3
14	39232.730±0.010	3.1±0.6	0.85±0.12	2.76±0.3
15	42035.040±0.010	3.7±1.0	0.47±0.14	2.08±0.4
16	44837.295±0.010	4.9±1.2	0.42±0.19	2.24±0.5

Notes.

^(a)Observed frequencies for a v_LSR of 5.83 km s^–1. The J= 17-16 line has not been detected within a 3σ level of 5.7 mK.

^(b)Antenna temperature in mK.

^(c)Linewidth at half intensity derived by fitting a Gaussian line profile to the observed transitions (in kms^–1).

^(d)Integrated line intensity in mK kms^–1.