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A&A
Volume 667, November 2022
Article Number L4
Number of page(s) 7
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202245018
Published online 04 November 2022

© R. Fuentetaja et al. 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

The number of molecules discovered in the cold dark cloud TMC-1, both with the Yebes 40 m radio telescope through the QUIJOTE1 line survey (Cernicharo et al. 2021a) and the Green Bank 100 m radio telescope with the GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) survey (McGuire et al. 2018), demonstrates the great importance of this source for a complete understanding of the chemistry of the interstellar medium (ISM). Chemical models are not yet accurate enough to predict all the molecules that have been discovered, so we must continue to study the various chemical reactions in order to better constrain models of cold pre-stellar cores.

The TMC-1 cloud is remarkably rich in hydrocarbons, such as long carbon chains, propargyl (Agúndez et al. 2021), vinyl acetylene (Cernicharo et al. 2021b), and allenyl diacetylene (Fuentetaja et al. 2022). Furthermore, several cyclic molecules, such as indene, cyclopentadiene (Cernicharo et al. 2021c), ortho-benzyne (Cernicharo et al. 2021a), and fulvenallene (Cernicharo et al. 2022), have been discovered there.

The carbenes, a family of molecules discovered in the ISM, are characterized by having two non-bonded electrons located on a terminal carbon atom. The most relevant are the cumulene carbenes, whose formula is H2Cn. They have a chain with double bonds between their carbon atoms. The simplest and the first to be discovered was l-H2C3, in 1991 (Cernicharo et al. 1991a). That same year, l-H2C4 was also discovered in TMC-1 (Kawaguchi et al. 1991) and IRC+10216 (Cernicharo et al. 1991b) . The longest species detected so far is l-H2C6, which has been found in several sources, including TMC-1 (Langer et al. 1997) and IRC+10216 (Guélin et al. 2000). Finally, Cabezas et al. (2021) detected l-H2C5 in TMC-1. For all of these molecules, the most stable isomer corresponds to the non-polar species with the formula HCnH for an even n or a cyclic structure when n is odd. Several H2Cn isomers have been detected in the ISM, such as c-C3H2 (Thaddeus et al. 1987) and c-C3HCCH (Cernicharo et al. 2021c).

In the case of the C6H2 family, Sattelmeyer & Stanton (2000) performed the first theoretical study of the nine most stable isomers. They performed several types of ab initio calculations, the highest level being CCSD(T), using the cc-pVTZ basis set. The bent-chain isomer ethynylbutatrienyliden (HCCCHCCC) is 2.346 eV higher than the most stable isomer HC6H, but only 0.1648 eV above the l-H2C6 species detected by Langer et al. (1997). Moreover, it should be noted that there are two isomers whose energy is lower than those of these two species. The formula is c-C6H2 for both of them, with the hydrogens in the ortho and meta positions (1.7085 eV and 2.0078 eV above the more stable isomer, respectively), making them possible candidates for future discovery.

On the other hand, if we consider sulphur-bearing species, we see that the number of molecules detected in TMC-1 is small compared with C- and O-rich species. However, some of them, such as CCS or CCCS, present a high abundance (Saito et al. 1987; Yamamoto et al. 1987). Among the last sulphur-bearing molecules discovered in TMC-1 we have HC3S+ (Cernicharo et al. 2021d), H2CCS, H2CCCS, NCS, HCCS, C4S, and C5S (Cernicharo et al. 2021e), the cation radical HCCS+ (Cabezas et al. 2022), and the two derivatives of thioformaldehyde, HCSCN and HCSCCH (Cernicharo et al. 2021f).

In this Letter we report on the first identification of HCCCHCCC and HCCCCS towards TMC-1. We derive column densities and rotational temperatures for the two molecules and analyse them with our chemical models to explain which reactions can produce them.

2. Observations

The observational data used in this work are part of the ongoing QUIJOTE line survey, carried out with the Yebes 40 m telescope towards the cyanopolyyne peak of TMC-1 (αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″). We used a new receiver built within the Nanocosmos project2, which consists of two cold high electron mobility transistor amplifiers covering the 31.0–50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures achieved in the 2019 and 2020 runs vary from 22 K at 32 GHz to 42 K at 50 GHz. Some power adaptation in the down-conversion chains effectively reduced the receiver temperatures in 2021 to 16 K at 32 GHz and 30 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 full coverage of the Q band in both polarizations. A detailed description of the system is given by Tercero et al. (2021).

The data come from several observing runs carried out between December 2019 and May 2022, and correspond to 546 h of observing time on source, of which 293 and 253 h were acquired with a frequency-switching throw of 8 MHz and 10 MHz, respectively. A detailed description of the QUIJOTE line survey is provided in Cernicharo et al. (2021a). For each frequency throw, different local oscillator frequencies were used in order to remove possible side band effects in the down-conversion chain.

The telescope beam size is 56″ and 31″ at 31 and 50 GHz, respectively. 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). The adopted calibration uncertainties are at a level of 10%. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz (Tercero et al. 2021). All the data were analysed using the GILDAS package3.

3. Results

For line identification, we used various catalogues, including CDMS (Müller et al. 2005), JPL (Pickett et al. 1998), and MADEX (Cernicharo 2012), which, as of September 2022, contained a total of 6434 spectral entries corresponding to the isotopologues and the ground and excited states of 1734 molecules.

3.1. Discovery of HCCCHCCC

Ethynylbutatrienyliden is a planar molecule with a nearly prolate top symmetry and whose electronic ground state corresponds to a singlet 1A state (see Fig. 1). It was studied in the laboratory up to 40.3 GHz by McCarthy & Thaddeus (2002), who determined with high precision its rotational and centrifugal distortion constants (see Table B.1). The dipole moments, μa = 3.710D and μb = 1.320D, were obtained by Sattelmeyer & Stanton (2000). We fitted these lines and implemented the rotational constants and the dipole moment into MADEX (Cernicharo 2012). Twenty-nine lines of this species were found in our QUIJOTE data, with intensities between 0.5 and 1.2 mK. Derived line parameters are given in Table A.1 and the lines for Ka ≤ 1 are shown in Fig. 2, while those corresponding to Ka = 2 are shown in Fig. 3. Some lines are blended, but the large number of them allows for a solid detection. From the observed frequencies in TMC-1, together with those measured by McCarthy & Thaddeus (2002), we obtained a new set of rotational constants (see Appendix B and Table B.1) with the FITWAT code, which uses a Watson asymmetric rotor Hamiltonian in Ir representation (Cernicharo et al. 2018). The merged fit provides very good frequency predictions below 50 GHz and realistic ones in the 50–100 GHz frequency range.

thumbnail Fig. 1.

Structure of the two molecules detected in this work.

thumbnail Fig. 2.

Observed transitions Ka = 0 and 1 of HCCCHCCC in TMC-1. The abscissa corresponds to the rest frequency of the lines. 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 millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for Trot = 6±1 K and a column density of (1.3±0.2) × 1011 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 and FS8 correspond to a throw of 10 and 8 MHz, respectively).

thumbnail Fig. 3.

Selected Ka = 2 transitions of HCCCHCCC in TMC-1. The abscissa corresponds to the rest frequency of the lines. Frequencies and intensities for all observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for Trot = 6±1 K and a column density of (1.3±0.2) × 1011 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 and FS8 correspond to a throw of 10 and 8 MHz, respectively).

We used a model line fitting procedure (Cernicharo et al. 2018) to derive the column density and the rotational temperature assuming all rotational levels have the same Trot. We adopted a source of uniform brightness with a diameter of 80″ from the C6H maps presented in Fossé et al. (2001). We obtained Trot = 6±1 K and N(HCCCHCCC) = (1.3±0.2) × 1011 cm−2. To provide molecular abundances, we also assumed a H2 column density for TMC-1 of 1022 cm−2 (Cernicharo & Guélin 1987), so the abundance of HCCCHCCC is (1.3±0.2) × 10−11. Figures 2 and 3 show the synthesized spectrum obtained with these values. The column density for the ortho species l-H2C6 is 6 × 1010 cm−2, which, together with 2 × 1010 cm−2 for the para species, gives us a total column density N(l-H2C6) = 8 × 1010 cm−2 (Cabezas et al. 2021). Hence, the abundance ratio HCCCHCCC/l-H2C6 is 1.625. This species, with an angle between the carbon atoms of 123.63° (Sattelmeyer & Stanton 2000), could be a precursor of cyclic species.

3.2. Discovery of HCCCCS

Butadiynethionyl (HCCCCS) is a planar molecule with a 2Π ground state. Line frequencies and the value of the dipole moment used are from the CDMS catalogue. The laboratory study of HCCCCS was carried out by Hirahara et al. (1994), who determined the value of B = 1434.298 MHz. The value of the dipole moment is μ = 1.45D. We used these laboratory data to implement HCCCCS in the MADEX code and predict its rotational lines. We detected a total of six transitions, whose intensities ranged from 0.49 to 0.68 mK, as shown in Fig. 4. The derived line parameters are given in Appendix A. We obtained a rotational temperature of 10±1 K and a column density of N = (9.5±0.8) × 1010 cm−2. Smaller values of Trot underestimate the intensity of the transitions at the highest frequencies. These derived parameters were used to obtain the synthetic spectrum shown in Fig. 4. Within the family HCnS, the species HCS and HCCS have been detected with column densities of 5.5±0.5 × 1012 cm−2 and 6.8±0.6 × 1011 cm−2, respectively (Cernicharo et al. 2021e). Therefore, the abundance ratios HCS/HCCCCS and HCCS/HCCCCS are 58 and 7.2, respectively. Propadienthionyl, HC3S, has not been detected yet in the ISM. From the QUIJOTE line survey, we derived a 3σ upper limit to its column density of 5 × 1010 cm−2.

thumbnail Fig. 4.

Selected transitions of HCCCCS in TMC-1. The abscissa corresponds to the rest frequency of the lines. Frequencies and intensities for all observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectra for this species for Trot = 10 K and a column density of (9.5±0.8) × 1010 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 corresponds to a throw of 10 MHz).

4. Chemical model

We carried out chemical model calculations to shed light on the possible synthetic routes to HCCCHCCC and HC4S. The model parameters and chemical network used are similar to those employed by Cernicharo et al. (2021e) to model the chemistry of sulphur-bearing molecules. Modifications to the chemical network are described below.

We first focused on the hydrocarbon HCCCHCCC. This species is an isomer of the C6H2 family. The most stable isomer, HC6H, is probably the most abundant one, although it cannot be detected because it is non-polar, while the cumulene isomer H2C6 is detected in TMC-1 with an abundance somewhat lower than that of HCCCHCCC. Due to the lack of information on the chemical kinetics of the different C6H2 isomers, the chemical networks do not differentiate between them. In the chemical model, the most important reactions of C6H2 formation are: C6H− + H, the dissociative recombination of C6H, and the neutral-neutral reaction C4H + C2H2. The first and last reactions have been measured and determined to be rapid (Eichelberger et al. 2007; Berteloite et al. 2010), although information on the branching ratios of the different C6H2 isomers is not available. The dissociative reaction has not been measured, so the kinetic coefficient and the fragmentation channels used are an estimation determined by Herbst & Leung (1989). Given that the calculated abundance of all isomers of C6H2 is well above the sum of the observed abundances of H2C6 and HCCCHCCC (see Fig. 5) and that the undetected isomer HC6H is probably the most abundant one, it is likely that H2C6 and HCCCHCCC are formed by any of the aforementioned reactions, with branching ratios lower than the one that forms HC6H. Information on the product distribution for these reactions would allow light to be shed on the chemistry of C6H2 isomers.

thumbnail Fig. 5.

Calculated fractional abundances of C6H2 (including all possible isomers) and HC4S as a function of time. Horizontal dotted lines correspond to observed values in TMC-1; the dotted black lines account for the sum of the abundances of the two C6H2 isomers detected in TMC-1: H2C6 and HCCCHCCC. The most stable isomer, HC6H, is probably the most abundant one, although it cannot be probed since it is non-polar.

The sulphur-bearing species HC4S is not included in chemical networks (e.g., Vidal et al. 2017). We thus included it assuming standard destruction routes (through reactions with neutral atoms and cations) and formation through C + H2C3S (in analogy to the reaction C + H2CS, which is the main route to HCCS; Cernicharo et al. 2021e) and by the dissociative recombination of H2C4S+, where this cation is in turn assumed to form through the reactions S + C4H and S+ + C4H3. The calculated peak abundance of HC4S is in reasonable agreement with the value observed in TMC-1 (see Fig. 5), which supports the idea that this species is formed through neutral and ionic routes similar to those that form other S-bearing carbon chains (Vidal et al. 2017; Cernicharo et al. 2021e).

5. Conclusions

We report on the detection of HCCCHCCC and HCCCCS towards the cold dark cloud TMC-1. For the former, we derived a column density of (1.3±0.2) × 1011 cm−2 with Trot = 6±1 K. HCCCHCCC is thus more abundant than its cumulene isomer, which allows the chemistry of the C6H2 family to be constrained. For the sulphur-bearing molecule, we obtained a column density of (9.5±0.8)×1010 cm−2 with Trot = 10 K. According to our chemical model, this abundance can be correctly approximated with the standard destruction and formation routes.

1

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

Acknowledgments

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

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Appendix A: Observed line parameters

We obtained the observed line parameters for the two molecules detected in this work by fitting a Gaussian line profile to the observed data, using CLASS (GILDAS package). For this purpose, we considered a window of ±15 km s−1 around the VLSR (5.83 km s−1) of the source for each transition. The results are shown in Table A.1. Observed lines of HCCCHCCC are shown in Figs. 2 and 3, and lines of HCCCCS in Fig. 4. Some of these parameters correspond to lines with only frequency-switching data with a throw of 8 or 10 MHz. In other cases, the lines are partially blended, but we can derive good parameters. For lines that are marginally detected, an upper limit of 3σ is given (see Notes).

Table A.1.

Observed line parameters for the species studied in this work.

Appendix B: Improved rotational constants for HCCCHCCC

We merged the laboratory lines of HCCCHCCC obtained by McCarthy & Thaddeus (2002) with those observed in TMC-1. This improves the values of the rotational and distortion constants. The results are given in Table B.1. The values obtained from the merged fit are very similar to those obtained from the laboratory fit, with the greatest difference being less than 1 kHz.

Table B.1.

Improved rotational and distortion constants for HCCCHCCC.

All Tables

Table A.1.

Observed line parameters for the species studied in this work.

In the text
Table B.1.

Improved rotational and distortion constants for HCCCHCCC.

In the text

All Figures

thumbnail Fig. 1.

Structure of the two molecules detected in this work.
In the text
thumbnail Fig. 2.

Observed transitions Ka = 0 and 1 of HCCCHCCC in TMC-1. The abscissa corresponds to the rest frequency of the lines. 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 millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for Trot = 6±1 K and a column density of (1.3±0.2) × 1011 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 and FS8 correspond to a throw of 10 and 8 MHz, respectively).
In the text
thumbnail Fig. 3.

Selected Ka = 2 transitions of HCCCHCCC in TMC-1. The abscissa corresponds to the rest frequency of the lines. Frequencies and intensities for all observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for Trot = 6±1 K and a column density of (1.3±0.2) × 1011 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 and FS8 correspond to a throw of 10 and 8 MHz, respectively).
In the text
thumbnail Fig. 4.

Selected transitions of HCCCCS in TMC-1. The abscissa corresponds to the rest frequency of the lines. Frequencies and intensities for all observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectra for this species for Trot = 10 K and a column density of (9.5±0.8) × 1010 cm−2. Blanked channels correspond to negative features produced when folding the frequency-switched data. Green labels indicate the transitions for which only one of the frequency-switching data have been used (FS10 corresponds to a throw of 10 MHz).
In the text
thumbnail Fig. 5.

Calculated fractional abundances of C6H2 (including all possible isomers) and HC4S as a function of time. Horizontal dotted lines correspond to observed values in TMC-1; the dotted black lines account for the sum of the abundances of the two C6H2 isomers detected in TMC-1: H2C6 and HCCCHCCC. The most stable isomer, HC6H, is probably the most abundant one, although it cannot be probed since it is non-polar.
In the text

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