Free Access
Issue
A&A
Volume 646, February 2021
Article Number L7
Number of page(s) 12
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202040076
Published online 05 February 2021

© ESO 2021

1. Introduction

The cold dark core TMC-1 presents an interesting chemistry. It produces a significant number of the molecules detected in space, in particular long neutral carbon-chain radicals and their anions (see e.g. Cernicharo et al. 2020a; Marcelino et al. 2020, and references therein) as well as cyanopolyynes (see Cernicharo et al. 2020b and Xue et al. 2020, and references therein). The presence in this object of O-bearing carbon chains, such as C2O (Ohishi et al. 1991), C3O (Matthews et al. 1984), HC5O (McGuire et al. 2017), HC7O (Cordiner et al. 2017), HCCO, and HC3O+ (Cernicharo et al. 2020c), is a surprising result that has not yet been fully accounted for by chemical models.

The abundance of polyatomic cations in cold interstellar clouds is relatively low because they react fast with electrons. Interestingly, all polyatomic cations detected in cold clouds are protonated forms of stable and abundant molecules. Chemical models and observations suggest a trend in which the protonated-to-neutral abundance ratio [MH+]/[M] increases with the proton affinity of M (Agúndez et al. 2015; Cernicharo et al. 2020c, 2021; Marcelino et al. 2020).

It has been suggested that some O-bearing cations are sufficiently long-lived to be abundant (Petrie et al. 1993). We have recently reported the discovery of the cation HC3O+ in TMC-1 (Cernicharo et al. 2020c). In this Letter, we report the detection of two series of lines that are harmonically related towards the cold dark core TMC-1. These lines can be fitted as the K = 0 and K = 1 lines of a symmetric rotor. From the astronomical data and the derived rotational constants, together with high-level ab initio calculations, we suggest CH3CO+ as the best possible carrier. We have performed microwave laboratory experiments that fully support this hypothesis: we detected 79 rotational transitions near the predicted frequencies from the astronomical constants. Hence, we report the discovery in space and in the laboratory of CH3CO+ (acetyl cation), which is the most stable isomer resulting from the protonation of ketene (H2CCO). The presence of CH3CO+ can be expected on the basis of the high abundance of H2CCO in TMC-1 and its large proton affinity (825.3 kJ mol−1; Traeger et al. 1982). An anomalous abundance ratio of 2.2 is found between the A and E symmetry species of CH3CO+. We discuss these results in the context of state-of-the-art chemical models and in terms of the interconversion of E-CH3CO+ into A-CH3CO+ through the formation process of the molecule or by collisions with H and/or H2.

2. Observations

New receivers, built as part of the Nanocosmos project1 and installed at the Yebes 40 m radio telescope, were used for the observations of TMC-1. The Q-band receiver consists of two high electron mobility transistor (HEMT) cold amplifiers that cover 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 fast Fourier transform spectrometers (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 (Tercero et al. 2020).

The observations that led to the line survey in the Q-band towards TMC-1 ( and ) 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 used for the final spectra. In these runs, two different frequency coverages were observed, 31.08–49.52 GHz and 31.98–50.42 GHz. This permits the user to check that no spurious ghosts are produced in the down-conversion chain, in which the signal coming from the receiver is down-converted to 1–19.5 GHz and then split into eight bands with a coverage of 2.5 GHz, each of which are analysed by the FFTs. Additional data were taken in October 2020 to improve the line survey at some frequencies and to further check the consistency of all observed spectral features. These observations were also performed in frequency switching but with a throw of 8 MHz. The sensitivity varies along the Q-band between 0.5 and 2.5 mK, which is a considerable improvement compared to previous line surveys in the 31–50 GHz frequency range (Kaifu et al. 2004).

The IRAM 30 m data come from a line survey performed towards TMC-1 and B1, and the observations have been described by Marcelino et al. (2007) and Cernicharo et al. (2012). The observations of L1527 and L1544 were obtained as part of the IRAM 30 m Large Program ASAI and were described by Lefloch et al. (2018). The intensity scale and antenna temperature () for the two telescopes used in this work were calibrated using two absorbers at different temperatures as well as the atmospheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties were adopted to be 10%. All data were analysed using the GILDAS package2.

3. Results and discussion

The assignment of the observed features in our line surveys was done using the CDMS and JPL catalogues (Müller et al. 2005; Pickett et al. 1998) and the MADEX code (Cernicharo 2012). Most of the weak lines found in our survey of TMC-1 can be assigned to known species and their isotopologues. Nevertheless, many features remain unidentified. Frequencies for the unknown lines were derived by assuming a local standard of rest velocity of 5.83 km s−1, a value that was derived from the observed transitions of HC5N and its isotopologues in our line survey (Cernicharo et al. 2020a,b). Our new data towards TMC-1 allowed us to detect C3N and C5N (Cernicharo et al. 2020a), as well as new species such as the isocyano isomer of HC5N, HC4NC (Cernicharo et al. 2020b), the cation HC3O+ (Cernicharo et al. 2020c), the cation HC3S+ (Cernicharo et al. 2021), and the cation HC5NH+ (Marcelino et al. 2020), in addition to several tens of already known molecules and their isotopologues.

Within the unidentified features in our surveys in the 3 mm band and the Q-band, we found two series of four lines with a harmonic relation of 2:4:5:6 (see Fig. 1). Taking into account the line density in TMC-1, the possibility that the observed pattern is fortuitous is very small. The observed lines are shown in Fig. 1, and the derived line parameters are given in Table 1. In fact, the J = 5−4 line at 91 342 MHz has intrigued us since 2017 when we detected it in TMC-1, L483, L1527, and L1544. We interpreted the K = 0, 1 lines and the U line at ∼91 344 (see Fig. A.1) as the hyperfine structure of a J = 1−0 or J = 2−1 transition of a molecule containing a nucleus with a spin of 1. Using the old receivers of the Yebes 40 m telescope, and assuming that the three lines around 91 342 MHz could correspond to J = 2−1, we searched for lines at 45 671 MHz without success. Only when the new receivers covering the whole Q-band were available at the telescope, and we detected the doublet at 36 537 MHz (see Fig. 1), did we realized that two of the lines around 91 342 MHz correspond to a J = 5−4 transition in harmonic relation 2:5 with the 36 537 MHz doublet. Moreover, the U line at 91 344 MHz is produced by another carrier as it is detected in B1, while the other lines are not. Once we relaxed the initial idea that these features were the hyperfine structure of a low-J transition, other features were found in the 3 mm domain (J = 4−3 and J = 6−5, as well as J = 7−6 in L1527).

thumbnail Fig. 1.

Observed lines of CH3CO+ towards TMC-1. The abscissa corresponds to rest frequencies (in MHz) assuming a local standard of rest velocity of 5.83 km s−1 (Cernicharo et al. 2020a,b). Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature (in mK). Spectral resolution is 38.1 kHz below 50 GHz and 48.8 kHz above. The blue labels correspond to the series of lines we assign to the A species of CH3CO+, while the red ones correspond to those of the E species.

Table 1.

Observed line parameters for CH3CO+ in TMC-1.

The two series of lines can be fitted to two linear rotors with rotational constants B = 9134.4738 ± 0.0006 MHz and B = 9134.2860 ± 0.0020 MHz. The distortion constant is exactly the same for both series, D = 4.00 ± 0.02 kHz. The observed spectra is reminiscent of the K = 0 and K = 1 components of the rotational transitions of a symmetric rotor. In fact, the eight observed lines in TMC-1 can be fitted with a single rotational constant and two distortion constants if we assume that the carrier is the same for both series and that it has a C3v symmetry (i.e. that it is a symmetric rotor). Using the standard Hamiltonian for this kind of molecular rotor (Gordy & Cook 1984), we derived the rotational and distortion constants provided in Table 2.

Table 2.

Derived spectroscopic parameters (in MHz) for CH3CO+.

From the derived rotational constant, 9134 MHz, the molecule should contain at least three atoms between C, N, and O. We analysed the possible candidates that could have a rotational constant similar to the observed one. Detailed ab initio calculations for the possible linear and asymmetric carriers are given in Appendix B. Concerning symmetric rotors, it is amazing to realize that all CH3X, with X = CN, NC, and CCH, have rotational constants close to our rotational and distortion constants. For example, CH3CN has a rotational constant of 9198.9 MHz (Müller et al. 2009), which is really very close to our result. The other possible candidates, CH3CNH+ (B = 8590.5 MHz; Amano et al. 2006) and CH3NCH+ (see, Table B.1), are too heavy. Hence, the best symmetric rotor candidate seems to be a species similar to CH3CN. The acetyl radical, CH3CO, has been observed in the laboratory by Hirota et al. (2006), but it is asymmetric and its lines show a very complex hyperfine structure. However, CH3CO+ is a symmetric rotor (Mosley et al. 2014) and the lowest energy isomer of H3C2O+. Its possible precursor, if formed through protonation, is ketene, which is one of the most abundant O-bearing species in TMC-1 (see Cernicharo et al. 2020c).

3.1. Quantum chemical calculations and assignment to CH3CO+

Precise geometries and spectroscopic molecular parameters for the species mentioned above were computed using high-level ab initio calculations. The first screening for all plausible candidates (see Appendix B) was done at the CCSD/cc-pVTZ level of theory (Cížek et al. 1969; Dunning 1989). These results are shown in Table B.1. In a second stage, the most promising candidates, namely CH3CO+, CH2COH+, and CH3NCH+, were calculated at the CCSD(T)-F12b/aug-cc-pVQZ levels of theory (Raghavachari et al. 1989; Adler et al. 2007; Knizia et al. 2009). To obtain more precise values for the rotational parameters of these three species, we calibrated our calculations using experimental to theoretical scaling ratios for analogue molecular species. This method has been proved to be suitable to accurately reproduce the molecular geometry of other identified molecules (Cernicharo et al. 2019, 2020c; Marcelino et al. 2020). In our present case, we used CH3CN, CH2CNH, and CH3NC, which are isoelectronic species of CH3CO+, CH2COH+, and CH3NCH+, respectively, for this purpose. Table B.2 shows the results of these calculations, which are summarized in Table 3. As can be seen, the employed level of theory reproduces the rotational parameters for CH3CN, CH2CNH, and CH3NC very well, with relative discrepancies around 0.08% and 0.04% for B in the cases of CH3CN and CH3NC, respectively. After correcting the calculated parameters for CH3CO+, CH2COH+, and CH3NCH+ using the derived scaling ratios for CH3CN, CH2CNH, and CH3NC, respectively, we obtained a B constant of 9129.62 MHz for CH3CO+, which shows the best agreement with that derived from the TMC-1 lines. The centrifugal distortion values, obtained in the same manner but at the MP2/aug-cc-pVQZ level of theory for CH3CO+ and CH3NCH+, are both compatible with those obtained from the fit of the lines. The agreement between the experimental constants and those calculated for CH2COH+ is substantially worse. The calculated dipole moments for CH3CO+ and CH3NCH+ are 3.5 D and 2.0 D, respectively, while the μa and μb values for CH2COH+ are 0.8 and 1.7 D, respectively.

Table 3.

Scaled theoretical values for the spectroscopic parameters of CH3CO+, CH2COH+, and CH3NCH+ together with the experimental values obtained in this work (all in MHz).

In addition to the geometry optimizations, we calculated the energy associated with the plausible formation of CH3CO+, starting from ketene and three proton donors; , H3O+, and HCO+. All these calculations were carried out at the CCSD/cc-pVTZ level of theory. We found a total energy change in the protonation of ketene to form CH3CO+ of −421.8, −130.9, and −244.8 kJ mol−1 when ketene reacts with , H3O+, and HCO+, respectively. More details can be found in Appendix C.

3.2. Laboratory detection of CH3CO+

We conducted an experiment to detect the CH3CO+ cation in the laboratory using rotational spectroscopy below 500 GHz. The experimental setup was similar to the one used to detect NS+ (Cernicharo et al. 2018). The cation was produced in a liquid-nitrogen-cooled Pyrex absorption cell by glow-discharging a mixture of CH4, CO (1:1), and Ar. A solenoid coil wound on the cell can generate an axial magnetic field (up to 300 G) to magnetically extend the negative glow, the region known to produce the highest concentrations of cations (compared to the positive column discharge; De Lucia et al. 1983). We also tried acetone and acetaldehyde as precursors (Mosley et al. 2014), but without success.

To optimize the experimental setup, we first observed the J = 2 ← 1 transition of HCO+ at 178 375.056 MHz, which was produced in the same gas mixture. We then searched for the J = 10 ← 9, K = 0−2 series of lines of CH3CO+ between 182.658 and 182.675 GHz based on the rotational constants derived from the lines observed in TMC-1. Weak spectra were observed within 500 kHz. The best experimental conditions were found to be P(CH4) = P(CO) = 1.5 mTorr, P(Ar) = 5.5 mTorr (gas mixture cooled using liquid nitrogen but pressures measured at room temperature), an electric discharge of 3.5 kV/10 mA, and an axial magnetic field of 200 G. These lines disappeared when one of the precursors was suppressed, or when the axial magnetic field was cut off. The latter phenomenon confirmed almost unambiguously that they were due to a cation. Subsequent measurements of higher-J transitions fully support the astrophysical assignment of the observed lines to CH3CO+.

In total, 79 lines were observed in the laboratory with quantum numbers in the ranges J = 10−27 and K ≤ 6 (see Table E.1). Transitions occurring below 330 GHz were measured by standard frequency modulation absorption spectroscopy, resulting in second-derivative lineshapes. These lines (K ≤ 3) were found unshifted from the first prediction. Those from 400 to 500 GHz were measured by emission spectroscopy (Zou & Motiyenko 2020), giving Voigt-profile lineshapes. Compared to the prediction, some deviations were observed up to 1 MHz for K = 6; these measurements led us to determine the HJK and HKJ centrifugal distortion terms. For maximum sensitivity, these lines were measured using the single frequency excitation method with 5–20 million acquisitions (which took 1 to 5 min.). Additionally, a 120 MHz wide chirped excitation spectrum, measured with 67 million acquisitions, is given in Fig. 2 for illustration and comparison purposes. The uncertainty of the laboratory frequency measurements are estimated to be 50 kHz. Given the mass of the cation, and that the negative glow is a nearly electric field-free region, the reported laboratory frequencies are expected to be unshifted by the Doppler effect. The separate and merged least-squares analysis of all (astronomical and laboratory) measured transitions are provided in Table 2. The measured frequencies and the observed minus calculated values are given in Table E.1. Frequency predictions are given in Table E.2.

thumbnail Fig. 2.

J = 22 → 21, K = 0−3 transitions obtained by chirped-pulsed excitation. The record corresponds to the average of 67 million spectra acquired in ∼20 min.

3.3. Chemistry of CH3CO+

From the observed line intensities of CH3CO+, we derived a rotational temperature of ∼5 K and a total column density of (3.2 ± 0.3) × 1011 cm−2 (see Appendix D). The column densities for the A and E species are (2.2 ± 0.2) × 1011 cm−2 and (9.7 ± 0.9) × 1010 cm−2, respectively. Adopting the column density for ketene derived by Cernicharo et al. (2020c), we obtained a H2CCO/CH3CO+ ratio of 44. Assuming the H2 column density derived by Cernicharo & Guélin (1987), the abundance of CH3CO+ is 3.2 × 10−11.

The chemistry of protonated molecules in cold dense clouds has been discussed by Agúndez et al. (2015). Chemical model calculations similar to those that they presented predict that the abundance of protonated ketene is controlled by the typical routes operating for protonated molecules. That is, CH3CO+ is mostly formed by proton transfer to H2CCO from HCO+, , and H3O+, while it is destroyed through dissociative recombination with electrons. The radiative association between and CO is also an important route to CH3CO+. The abundance ratio H2CCO/CH3CO+ predicted by the model is in the range 250–450 and depends on whether the UMIST RATE12 (McElroy et al. 2013) or KIDA kida.uva.2014 (Wakelam et al. 2015) chemical networks are used. As occurs for most protonated molecules observed in cold dense clouds, the abundance of the protonated form with respect to the neutral is underestimated by the chemical model. In this case, there is a factor of 5–10 difference between the model and observations. Incorrect estimates for the rate constants of the dominant reactions of the formation and destruction of CH3CO+ may be behind this disagreement. Alternatively, the chemical network may miss some important formation route to CH3CO+, although it is difficult to identify reactions producing this ion from abundant reagents. For example, plausible reactions of ions with CO, H2CO, or CH3OH tend to form products other than CH3CO+ (Adams & Smith 1978). In this context, it is worth noting that not all species resulting from the protonation of abundant molecules in TMC-1 are detected. For example, CH3CNH+ is not detected in TMC-1 despite the CH3CN proton affinity of 787.4 ± 5.9 kJ mol−1 (Williams et al. 2001). The 3σ upper limit to the column density of CH3CNH+ is 2.5 × 1011 cm−2. The column density of CH3CN is (3.2 ± 0.2) × 1012 cm−2 (see Appendix A); hence, the abundance ratio between the neutral and its protonated form is ≥13. The low dipole moment of CH3CNH+ compared to that of CH3CN (1.01 D versus 3.93 D) limits the chances of detecting this species.

3.4. A-CH3CO+/E-CH3CO+ abundance ratio

The column densities derived for the A and E species of CH3CO+ are not identical, as would be expected for a symmetric top. The A/E abundance ratio for this molecule is 2.27. However, all symmetric molecules of CH3X detected in TMC-1 have an abundance ratio between their A and E species that is close to unity (see Appendix D and Fig. D.2). In a symmetric top, the two symmetry states A and E are not connected radiatively nor through inelastic collisions with H2. Unlike the rest of the CH3X molecules detected in TMC-1, CH3CO+ is a cation, and its reactive collisions with H2 or H could produce a proton interchange if there is no barrier to the reaction. The lowest energy level of the E symmetry state is the J = 1, K = 1, which is 7.8 K above the ground J = 0, K = 0 level of the A state. Hence, the reaction of interchange of a proton

E–CH3CO+ + H2/H → A-CH3CO+ + H2/H + 7.8 K

is exothermic, although it is unknown if there is a barrier; this is something that has to be established via detailed theoretical calculations. At thermal equilibrium, and for a kinetic temperature of 10 K, the A/E abundance ratio could be e0.78 = 2.18, which is very close to the observed value of 2.27. For neutral molecules with two or more symmetric hydrogens, the proton interchange could be mainly produced through collisions with H+, , HCO+, and H3O+, which are much less abundant than H2 and H. In Appendix D, we discuss the A/E abundance ratio of all neutral symmetric rotors that have been detected so far in TMC-1, including CH3NC, which has previously only been observed in two cold dense clouds: L1544 (Jiménez-Serra et al. 2016) and L483 (Agúndez et al. 2019). For all these species, the A/E abundance ratio is close to unity.

Alternatively, we could also consider the possibility that the collisional rates of the acetyl cation with H2 or He are higher for the A species than for the E species. As the acetyl cation is isoelectronic to CH3CN, we could use the collisional rates of the latter species (Khalifa et al. 2020) to estimate possible differences in the excitation temperature of the K = 0 and K = 1 lines. We explored a density range of (4−10) × 104 cm−3 and a kinetic temperature range of 5–10 K. No significant differences were found in the predicted brightness temperature between these lines. Of course, if the effect is due to inelastic collisions, then methyl cyanide (CH3CN) would also show a similar behaviour. Nevertheless, although both species are isoelectronic, the fact that CH3CO+ is positively charged could result in very different collisional rates with H2 compared to CH3CN.

We could also consider that the A/E abundance ratio is affected by the formation process of the molecule. As shown in Sect. 3.1, the reaction of ketene with is the most favourable for protonation from the thermodynamical point of view. Both species, ketene and , could also have their ortho/para ratio affected by the low temperature of dense dark clouds, which will introduce a non-trivial spin statistic into the formation process of CH3CO+. Additional calculations are needed to evaluate the role of collisional excitation and of spin interchange in order to understand the anomalous behaviour exhibited by the A and E symmetry species of CH3CO+.


3

Access to the entire PRIMOS data set, specifics on the observing strategy, and overall frequency coverage information is available at http://www.cv.nrao.edu/~aremijan/PRIMOS/.

Acknowledgments

The Spanish authors thank Ministerio de Ciencia e Innovación for funding support through project AYA2016-75066-C2-1-P, PID2019-106235GB-I00 and PID2019-107115GB-C21/AEI/10.13039/501100011033. We also thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. MA and CB thanks Ministerio de Ciencia e Innovación for grants RyC-2014-16277 and FJCI-2016-27983, respectively. Y. Endo thanks Ministry of Science and Technology of Taiwan through grant MOST108-2113-M-009-25. We would like to thank Evelyne Roueff and Octavio Roncero for useful comments and suggestions.

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Appendix A: CH3CO+ in other sources

The acetyl cation, CH3CO+, has also been detected towards L483, L1544, and L1527 (see Fig. A.1). However, it is not observed towards Sgr B2 (PRIMOS3 line survey; Neill et al. 2012), towards Orion-KL (Tercero et al. 2010, 2011), or in our line survey of B1 (Marcelino et al. 2007, 2009, 2010; Cernicharo 2012). In the PRIMOS data on SgrB2, a very tentative detection of the J = 1−0 K = 0 line could be claimed at a velocity of 80 km s−1. However, only an upper limit can be obtained for the J = 2−1 transition as this line is heavily blended with a strong line of acetone. It seems, hence, that CH3CO+ is typical of cold interstellar clouds.

thumbnail Fig. A.1.

Observations of the J = 5−4 transition of CH3CO+ towards L483, L1544, L1527, and B1 (top panels); bottom panel: J = 7−6 line towards L1527. The abscissa corresponds to the rest frequency (in MHz) and the ordinate is the antenna temperature (in mK). The spectral resolution is 48.8 kHz for all sources except L1527, for which it is 198 kHz. The vertical dashed blue lines indicate the position of the K = 0 and K = 1 lines (detected in all sources except B1), and the cyan line corresponds to the U feature at 91 344 MHz (detected in all sources except L1527). The rest velocities of L1527 and L1544 were taken as vLSR = 5.9 km s−1 and 7.2 km s−1, respectively, based on Sakai et al. (2009) and Vastel et al. (2015), respectively.

Appendix B: Potential carriers of the series of lines

All known diatomic species in cold dark clouds have overly large rotational constants compared with those derived from the lines in TMC-1. For example, a molecule containing one S (CS, NS, SO) will have a rotational constant that is too high by more than 12 GHz. Adding one or two H atoms to these combinations of sulphur produces radicals (with overly high rotational constants, such as HCS) or asymmetric rotors such as H2CS, which is too heavy (Müller et al. 2019). A molecule with two S atoms is, of course, too heavy (for example, B(S2) = 8831 MHz; Pickett & Boyd 1979), and we do not expect to have Si- or P-bearing polyatomic species in this cloud. The first step in finding candidates is to exclude the possibility of having a slightly asymmetric species. In that case, we could expect to have lines corresponding to K= ± 1 at roughly ±(BC) from the K = 0 lines. We searched in the Q-band survey (J = 2−1) for such a pattern. No lines up to one-fourth of the intensity of the K = 0 line are observed. Moreover, taking into account that there is no evidence for a radical as a possible carrier, the species resulting from the addition of one hydrogen to the closed-shell asymmetric species HNCO, HCOOH, and H2CCC have to be excluded. However, their protonated species are also, at least in principle, closed-shell species. Hence, possible candidates are H2CCCH+, , HCNOH+, HNCOH+, , and CH3OO+, all of which, with the exception of the last one, are protonated forms of known neutral and abundant species in TMC-1. Nevertheless, the resulting molecular structures will be highly asymmetric for most of them, or they are too light or too heavy, as are the cases for H2CCCH+ and , respectively. HNCOH+ is a linear species characterized in the laboratory with a rotational constant of 9955 MHz (Latanzi et al. 2012) and is not detected in our data. Other exotic species, such as NH2CHOH+, , and CH3NOH+, which could result from the protonation of interesting molecules (NH2CHO for example), are discarded for their molecular asymmetry and because the neutral species have not been observed in TMC-1. Ab initio calculations have been performed for the most promising candidates (see Table B.1), and their isomers and the results are discussed in Sect. 3.1.

Table B.1.

Rotational constants and electric dipole moments of potential candidates.

Table B.2.

Scaled theoretical values for the spectroscopic parameters of CH3CO+, CH2COH+, and CH3NCH+ (all in MHz).

Appendix C: Additional quantum chemical calculations for CH3CO+

The potential energy surface (PES) for the protonation of ketene has been explored at the CCSD/cc-pVTZ level of theory. In the calculations, we considered three possible proton donors: , H3O+, and HCO+, as well as the formation of the two isomers of protonated ketene, CH3CO+ and CH2COH+. Figure C.1 depicts the PES along the reaction coordinate for the protonation of ketene and the relative energies for all the stationary points when ketene reacts with , H3O+, or HCO+. For each reaction, the two reactants, ketene and the proton donor, that separated from each other were assumed to be the energy zero. The protonation of ketene in the CH2 in the three cases is exothermic, and it proceeds without any transition state (TS) to form CH3CO+. This formation is more favourable in the case of . On the other hand, the formation of CH2COH+ is exothermic in the cases of and HCO+, but endothermic in the case of H3O+. The less stable isomer, CH2COH+, can interconvert through a hydrogen migration to CH3CO+, which has a TS barrier height of 210.1 kJ mol−1. As shown in Fig. C.1, the TS for this interconversion lies over the energy of the reactants in the protonation of ketene with H3O+ and HCO+. In contrast, this TS lies below the energy of the reactants when ketene reacts with .

thumbnail Fig. C.1.

Energy diagram for the protonation of ketene. Total energies relative to those of the separated reactants, ketene and the proton donor X, are given in the enclosed table in kJ mol−1. C* is the TS energy for the interconversion between CH2COH+ and CH3CO+ isomers. Y** is the energy difference between the reactants and the interconversion TS; a negative value indicates that the TS is submerged below the reactant energy, and a positive value implies that the TS lies above the reactant energy.

Appendix D: CH3X species in TMC-1

As noted above, the intensity of the J = 2−1 K = 1 line in TMC-1 is well below the expected value if the rotational temperature for the A and E species is the same. In order to check this point, we show in Fig. D.1 the rotational diagrams for the A and E species of CH3CO+ using the observed line parameters given in Table 1. The observed intensities have been corrected for beam dilution and the beam efficiencies of the Yebes 40 m and IRAM 30 m telescopes. We assumed a uniform source of radius 40″ (Fossé et al. 2001). The derived rotational temperatures, Trot(A) = 4.4 ± 0.4 K and Trot(E) = 5.0 ± 0.5 K, are consistent with a common excitation through collisions with H2. The derived column densities are N(A-CH3CO+) = (2.2 ± 0.2) × 1011 cm−2 and N(E-CH3CO+) = (9.7 ± 0.9) × 1010 cm−2. Hence, as discussed in Sect. 3.4, the A/E abundance ratio for CH3CO+ has been modified through collisions with H+, , HCO+, and H3O+. This is a similar effect to that found in cold molecular clouds for molecules having ortho and para symmetry species.

thumbnail Fig. D.1.

Rotational diagrams for the A (black line) and E (red line) symmetry species of CH3CO+ in TMC-1.

In order to check this peculiar result, we analysed all symmetric rotors having transitions within our line survey: CH3CCH, CH3C4H, CH3CN, and CH3NC. The cation CH3CNH+ has not been detected in TMC-1 (see Sect. 3.3). The symmetric top CH3C3N is discussed in Marcelino et al. (2021). Figure D.2 shows the J = 2−1 transition for CH3CN, CH3CCH, CH3NC, and CH3CO+ (for this species, see also Fig. 1). The K = 0 and K = 1 lines of CH3CN exhibit the typical hyperfine structure introduced by the quadruple moment of the N nucleus. For all these additional molecules, we assumed a rotational temperature of 10 K and a source radius of 40″ (Fossé et al. 2001), and we produced a synthetic spectrum that is compared to the observations. We found that the A/E abundance ratio is ≃1 for all species but CH3CO+. Adopting a lower rotational temperature has little effect on the derived A/E abundance ratio for these symmetric rotors.

thumbnail Fig. D.2.

Observed lines in the transition J = 2−1 K = 0,1 of different symmetric rotors in TMC-1. The colour lines represent the expected line profiles for the A species (red) and E species (blue). The abundance ratio between them in the model is indicated in each panel.

Appendix E: Observed and calculated frequencies of CH3CO+

The frequencies observed in space and in the laboratory were merged to obtain the recommended rotational and distortion constants. A total of 89 rotational transitions, ten in space (see Table 1) and 79 in the laboratory (see Table E.1), were fitted to the standard Hamiltonian of a symmetric rotor (Gordy & Cook 1984). For the lines observed in TMC-1 and other dark clouds, only B, DJ, and DJK can be obtained as only rotational transitions with K = 0 and 1 have been observed. The results are given in Table 2. For the 79 lines observed in the laboratory, the constants HJK and HKJ were included in the fit, and the results are given in Table 2. Finally, the merged fit to the astronomical and laboratory lines produces the recommended set of rotational constants given in the last column of Table 2. The observed and calculated frequencies, together with the observed minus calculated values for the merged fit, are given in Table E.1.

Table E.1.

Observed and calculated frequencies (in MHz) for CH3CO+.

We used the rotational and distortion constants that resulted from the merged fit to the astronomical and laboratory lines (see Table 2) to produce frequency predictions, frequency uncertainties, line strengths, upper energy levels, and Einstein coefficients for all transitions involving levels with energies below 2000 K. They are given in Table E.2. The whole table is electronically available at the CDS. It should be noted that this table contains the transitions for the A and E species, and that the E lowest energy level, JK = 11, is 7.8 K above the 00 level of the A species.

Table E.2.

Frequency predictions for CH3CO+.

All Tables

Table 1.

Observed line parameters for CH3CO+ in TMC-1.

Table 2.

Derived spectroscopic parameters (in MHz) for CH3CO+.

Table 3.

Scaled theoretical values for the spectroscopic parameters of CH3CO+, CH2COH+, and CH3NCH+ together with the experimental values obtained in this work (all in MHz).

Table B.1.

Rotational constants and electric dipole moments of potential candidates.

Table B.2.

Scaled theoretical values for the spectroscopic parameters of CH3CO+, CH2COH+, and CH3NCH+ (all in MHz).

Table E.1.

Observed and calculated frequencies (in MHz) for CH3CO+.

Table E.2.

Frequency predictions for CH3CO+.

All Figures

thumbnail Fig. 1.

Observed lines of CH3CO+ towards TMC-1. The abscissa corresponds to rest frequencies (in MHz) assuming a local standard of rest velocity of 5.83 km s−1 (Cernicharo et al. 2020a,b). Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature (in mK). Spectral resolution is 38.1 kHz below 50 GHz and 48.8 kHz above. The blue labels correspond to the series of lines we assign to the A species of CH3CO+, while the red ones correspond to those of the E species.

In the text
thumbnail Fig. 2.

J = 22 → 21, K = 0−3 transitions obtained by chirped-pulsed excitation. The record corresponds to the average of 67 million spectra acquired in ∼20 min.

In the text
thumbnail Fig. A.1.

Observations of the J = 5−4 transition of CH3CO+ towards L483, L1544, L1527, and B1 (top panels); bottom panel: J = 7−6 line towards L1527. The abscissa corresponds to the rest frequency (in MHz) and the ordinate is the antenna temperature (in mK). The spectral resolution is 48.8 kHz for all sources except L1527, for which it is 198 kHz. The vertical dashed blue lines indicate the position of the K = 0 and K = 1 lines (detected in all sources except B1), and the cyan line corresponds to the U feature at 91 344 MHz (detected in all sources except L1527). The rest velocities of L1527 and L1544 were taken as vLSR = 5.9 km s−1 and 7.2 km s−1, respectively, based on Sakai et al. (2009) and Vastel et al. (2015), respectively.

In the text
thumbnail Fig. C.1.

Energy diagram for the protonation of ketene. Total energies relative to those of the separated reactants, ketene and the proton donor X, are given in the enclosed table in kJ mol−1. C* is the TS energy for the interconversion between CH2COH+ and CH3CO+ isomers. Y** is the energy difference between the reactants and the interconversion TS; a negative value indicates that the TS is submerged below the reactant energy, and a positive value implies that the TS lies above the reactant energy.

In the text
thumbnail Fig. D.1.

Rotational diagrams for the A (black line) and E (red line) symmetry species of CH3CO+ in TMC-1.

In the text
thumbnail Fig. D.2.

Observed lines in the transition J = 2−1 K = 0,1 of different symmetric rotors in TMC-1. The colour lines represent the expected line profiles for the A species (red) and E species (blue). The abundance ratio between them in the model is indicated in each panel.

In the text

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