# Discovery of CH3CHCO in TMC-1 with the QUIJOTE line survey

R. Fuentetaja, C Bermúdez, C. Cabezas, M. Agúndez, B. Tercero, N.

Marcelino, J. R Pardo, Laurent Margules, R.A. Motiyenko, J. C. Guillemin, et al.

► To cite this version:

R. Fuentetaja, C Bermúdez, C. Cabezas, M. Agúndez, B. Tercero, et al.. Discovery of CH3CHCO in TMC-1 with the QUIJOTE line survey. Astronomy & Astrophysics - A&A, 2023, 671, pp.L6. 10.1051/0004-6361/202245732 . hal-04057653

HAL Id: hal-04057653

https://hal.science/hal-04057653v1

Submitted on 16 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
LETTER TO THE EDITOR

# Discovery of CH<sub>3</sub>CHCO in TMC-1 with the QUIJOTE line survey*

R. Fuentetaja<fnref n="1" />, C. Bermúdez<fnref n="2" />, C. Cabezas<fnref n="1" />, M. Agúndez<fnref n="1" />, B. Tercero<fnref n="3" />,<fnref n="4" />, N. Marcelino<fnref n="3" />,<fnref n="4" />, J. R. Pardo<fnref n="1" />, L. Margulès<fnref n="5" />, R. A. Motiyenko<fnref n="5" />, J.-C. Guillemin<fnref n="6" />, P. de Vicente<fnref n="3" />, and J. Cernicharo<fnref n="1" />

Received 20 December 2022 / Accepted 2 February 2023

## ABSTRACT

We report the detection of methyl ketene towards TMC-1 with the QUIJOTE line survey. Nineteen rotational transitions with rotational quantum numbers ranging from $J=3$ up to $J=5$ and $K_a \le 2$ were identified in the frequency range 32.0–50.4 GHz, 11 of which arise above the $3\sigma$ level. We derived a column density for CH<sub>3</sub>CHCO of $N = 1.5 \times 10^{11} \text{ cm}^{-2}$ and a rotational temperature of 9 K. Hence, the abundance ratio between ketene and methyl ketene, CH<sub>2</sub>CO/CH<sub>3</sub>CHCO, is 93. This species is the second C<sub>3</sub>H<sub>4</sub>O isomer detected. The other, trans-propenal (CH<sub>2</sub>CHCHO), corresponds to the most stable isomer and has a column density of $N = (2.2 \pm 0.3) \times 10^{11} \text{ cm}^{-2}$, which results in an abundance ratio CH<sub>2</sub>CHCHO/CH<sub>3</sub>CHCO of 1.5. The next non-detected isomer with the lowest energy is cis-propenal, which is therefore a good candidate for future discovery. We have carried out an in-depth study of the possible gas-phase chemical reactions involving methyl ketene to explain the abundance detected, achieving good agreement between chemical models and observations.

**Key words.** molecular data – line: identification – ISM: molecules – ISM: individual objects: TMC-1 – astrochemistry

## 1. Introduction

The QUIJOTE<fnref n="1" /> line survey (Cernicharo et al. 2021a) toward TMC-1 performed in recent years with the Yebes 40 m radio telescope has allowed us to detect more than 40 new molecules in space. This underlines the importance of this source for a deep understanding of the different chemical processes in cold dense cores.

Among the latest discoveries, there are several cycles such as cyclopentadiene, indene, ortho-benzyne, or fulvenal-lene (Cernicharo et al. 2021a,b, 2022). Cyano and ethynyl derivatives of cyclopentadiene (McCarthy et al. 2021; Lee et al. 2021; Cernicharo et al. 2021c) and cyano derivatives of benzene, naphthalene, and indene (McGuire et al. 2018, 2021; Sita et al. 2022) have also been detected. We have also detected propargyl (Agúndez et al. 2021a), one of the most abundant radicals found. Furthermore, long carbon chains such as vinyl acetylene (Cernicharo et al. 2021d), allenyl acetylene (Cernicharo et al. 2021e), butadiynyllallene (Fuentetaja et al.

2022a), and ethynylbutatrienylidene (Fuentetaja et al. 2022b) have also been discovered toward TMC-1. Many of these species were not expected because they did not show a high abundance in chemical models. This highlights the importance of further study of the dark cloud TMC-1 in order to understand the chemical processes at work in this kind of environment.

Oxygen-bearing complex organic molecules (COMs) are also an important molecular family present in diverse interstellar environments. The star-forming regions, such as Sgr B2 and Orion KL, are the sources with the highest abundance of COMs. On the contrary, dark clouds like TMC-1 are characterized by carbon-rich chemistry, resulting in long carbon chains with low oxygen content. Agúndez et al. (2021b) reported the detection of O-bearing species, such as CH<sub>2</sub>CHCHO, CH<sub>2</sub>CHOH, HCOOCH<sub>3</sub>, and CH<sub>3</sub>OCH<sub>3</sub> in TMC-1. Long carbon chain O-bearing molecules with formulae HC<sub>n</sub>O and C<sub>n</sub>O (e.g., HC<sub>3</sub>O, HC<sub>7</sub>O, HC<sub>5</sub>O, and C<sub>5</sub>O) have also been detected (McGuire et al. 2017; Cordiner et al. 2017; Cernicharo et al. 2021f). O-bearing cations, such as HC<sub>3</sub>O<sup>+</sup> and CH<sub>3</sub>CO<sup>+</sup>, have also been recently detected in this source (Cernicharo et al. 2020, 2021g).

In the family of C<sub>3</sub>H<sub>4</sub>O isomers, the most stable is trans-propenal (trans-CH<sub>2</sub>CHCHO), whose detection was reported by Agúndez et al. (2021b). To date, this is the only isomer in this family detected towards TMC-1. Close in energy, with a difference of 2.8 kJ mol<sup>-1</sup> compared to trans-propenal, is methyl ketene (CH<sub>3</sub>CHCO). Bermúdez et al. (2018) studied this

<footnote n="1">¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: r.fuentetaja@csic.es; jose.cernicharo@csic.es</footnote>

<footnote n="2">² Dept. de Química Física y Química Inorgánica, Facultad de Ciencias – I.U. CINQUIMA, Universidad de Valladolid, Paseo de Belén 7, 47011 Valladolid, Spain</footnote>

<footnote n="3">³ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain</footnote>

<footnote n="4">⁴ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain</footnote>

<footnote n="5">⁵ Université de Lille, Faculté des Sciences et Technologies, Département Physique, Laboratoire de Physique des Lasers, Atomes et Molécules, UMR CNRS 8523, 59655 Villeneuve d'Ascq Cedex, France</footnote>

<footnote n="6">⁶ Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, 35000 Rennes, France</footnote>

<footnote>* Based on observations carried out with the Yebes 40m telescope (projects 19A003, 20A014, 20D023, 21A011, and 21D005). The 40m radiotelescope at Yebes Observatory is operated by the Spanish Geographic Institute (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana).</footnote>

<footnote n="1">¹ Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment.</footnote>
species in different environments and made a theoretical study of the stability of all C₃H₄O isomers using the coupled cluster (CCSD(T)) ab initio method and the aug-cc-pVTZ basis set. The next member in the series is cis-propenal. It is also close in energy to CH₃CHCO, with a difference of 5.8 kJ·mol⁻¹, and therefore it is a candidate for future detection in TMC-1. The last isomer we refer to is cyclopropanone (c-H₄C₃O). Its energy is considerably higher than that of the previously named molecules, specifically 78.1 kJ·mol⁻¹ with respect to cis-propenal (1 kJ·mol⁻¹ = 120.3 K), but it is a candidate for detection because the similar and smaller species cyclopropenone (c-H₂C₃O) has been reported by Loison et al. (2016).

In this Letter we report the first clear detection of CH₃CHCO (methyl ketene) in TMC-1 using the line survey QUIJOTE (Cernicharo et al. 2021a) performed with the Yebes 40 m telescope. The formation of this species is investigated in detail using state-of-the-art gas-phase chemical models.

## 2. Observations

The observational data used in this work are part of QUIJOTE, a spectral line survey of TMC-1 in the *Q* band carried out with the Yebes 40 m telescope at the position $\alpha_{J2000} = 4^{\mathrm{h}}41^{\mathrm{m}}41.9^{\mathrm{s}}$ and $\delta_{J2000} = +25^{\circ}41'27.0''$. The receiver was built within the Nanocosmos project<fnref n="2" /> and 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 have reduced the receiver temperatures during 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 the whole coverage of the *Q* band in both polarizations. A more detailed description of the system is given by Tercero et al. (2021).

The QUIJOTE line survey was carried out in several observing runs between December 2019 and May 2022. All observations are performed using frequency-switching observing mode with a frequency throw of 8 and 10 MHz. The total observing time on the source for data taken with frequency throws of 8 MHz and 10 MHz is 293 and 253 h, respectively. Hence, the total observing time on source by May 2022 is 546 h. The measured sensitivity varies between 0.12 mK at 32 GHz and 0.25 mK at 49.5 GHz. The sensitivity of QUIJOTE is around 50 times better than that of previous line surveys in the *Q* band of TMC-1 (Kaifu et al. 2004). For each frequency throw, different local oscillator frequencies were used in order to remove possible side band effects in the down conversion chain. A detailed description of the QUIJOTE line survey is provided in Cernicharo et al. (2021a).

The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz (Tercero et al. 2021). The telescope beam size is 56" and 31" at 31 and 50 GHz, respectively. The intensity scale used in this work, antenna temperature ($T_A^*$), was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM; Cernicharo 1985; Pardo et al. 2001). The calibration uncertainties adopted were 10%. All data were analysed using the GILDAS package<fnref n="3" />.

## 3. Results

Methyl ketene is a nearly prolate molecule having a planar molecular skeleton ($C_s$ frame) with only the two hydrogen atoms of the methyl group out of the plane ($C_{3v}$ top), and thus, due to the internal rotation of the methyl top, it belongs to the $G_6$ symmetry group. As was shown in the previous rotational analysis of methyl ketene (Bermúdez et al. 2018), its $\rho$ parameter, which accounts for the coupling of the methyl torsion with the overall rotation of the molecule, is relatively high ($\rho$ = 0.196). For this reason, it was necessary to employ a Hamiltonian aligned to the $\rho$ vector ($\rho$-axis-method, RAM) that accounts for the coupling of the two movements. This employed method is incorporated in the RAM36 software (Rho-axis method for 3- and 6-fold barriers; Ilyushin et al. 2010). The previous model for CH₃CHCO reported by Bermúdez et al. (2018) was slightly adapted to account for the laboratory observed transitions from Bak et al. (1966) and Bermúdez et al. (2018). The model presented in this work also contains the observed transitions in TMC-1 between 32.0 and 50.4 GHz since no lines at those frequencies were accounted for in the previous model. A comparison of the parameters obtained in the previous model (Bermúdez et al. 2018) with the current parameters is presented in Table 1. The parameters of the model have barely changed, showing that the transitions were perfectly incorporated in the fit. Furthermore, for all the observed lines in TMC-1 with $K_a$ = 0 or 1, the predictions lie within the experimental error. Some transitions of higher $K_a$ show slightly higher uncertainty than expected; however, this can be explained by the difficulties of the model to account for the lower energy transitions, already observed in the previous work. These issues are related to the strong coupling of the internal motion and the overall rotation of the molecule, and hence to the high complexity of the model. The dipole moments used in this work, $\mu_a$ = 1.65 D and $\mu_b$ = 0.33 D, were reported by Bermúdez et al. (2018).

The line identification was achieved using the MADEX catalogue (Cernicharo 2012). We detected a total of 11 lines (divided into *A* and *E* components, due to an internal rotation of the methyl group) within the *Q* band, together with eight lines having an intensity lower than $3\sigma$. The intensities range from 0.26 to 0.9 mK. The quantum numbers involved range from *J* = 3 to *J* = 5 and $K_a \le 2$. The derived line parameters are given in Table A.1. To obtain the column density, we used a model line fitting procedure, with the LTE approach for the thin optical lines (see e.g., Cernicharo et al. 2021d). We obtained $N(\mathrm{CH}_3\mathrm{CHCO}) = 1.5 \times 10^{11} \mathrm{cm}^{-2}$ with a rotational temperature of 9 K. The models predict the line intensities in antenna temperature taking into account the assumed size of the source to correct for beam dilution, and the beam efficiency of the telescope at the different frequencies of the observations. We assume a source of uniform brightness with a diameter of 80" (Fossé et al. 2001). The H₂ column density for TMC-1 is $10^{22} \mathrm{cm}^{-2}$ (Cernicharo & Guélin 1987), so the abundance of CH₃CHCO is $1.5 \times 10^{-11}$. The predicted synthetic lines for these data are shown in Fig. 1 for $K_a$ = 0, 1 and in Fig. 2 for $K_a$ = 2.

There are several molecules related to CH₃CHCO, so it is interesting to compare their abundances. The most obvious is trans-propenal, which is its more stable isomer. The abundance ratio CH₂CHCHO/CH₃CHCO is 1.5. This means that the abundance ratio between the two most stable isomers of the C₃H₄O family is similar to that of the two most stable isomers of the C₂H₄O family, in which case C₂H₃OH/CH₃CHO ~1 (Agúndez et al. 2021b). We can also compare CH₃CHCO with ketene, one of the most abundant O-bearing molecules in

<footnote n="2">² https://nanocosmos.iff.csic.es/</footnote>

<footnote n="3">³ http://www.iram.fr/IRAMFR/GILDAS</footnote>
<table><caption>**Table 1.** Ground torsional state molecular parameters of methyl ketene obtained from the fit using the RAM36 program (A-reduction, I'-representation).</caption><thead><tr><th>Parameter <sup>(a)</sup></th><th>Operator <sup>(b)</sup></th><th>n<sub>op</sub>t<sub>op</sub>r<sub>op</sub> <sup>(c)</sup></th><th>Previous work (MHz) <sup>(d)(j)</sup></th><th>This work (MHz) <sup>(d)(k)</sup></th></tr></thead><tbody><tr><td>A<sub>RAM</sub></td><td>J<sup>2</sup><sub>a</sub></td><td>2<sub>0,2</sub></td><td>38 587.00(11)</td><td>38 586.91(11)</td></tr><tr><td>B<sub>RAM</sub></td><td>J<sup>2</sup><sub>b</sub></td><td>2<sub>0,2</sub></td><td>4773.813(29)</td><td>4773.833(27)</td></tr><tr><td>C<sub>RAM</sub></td><td>J<sup>2</sup><sub>c</sub></td><td>2<sub>0,2</sub></td><td>4139.199(46)</td><td>4139.249(39)</td></tr><tr><td>D<sub>ab</sub></td><td>{J<sub>a</sub>, J<sub>b</sub>}</td><td>2<sub>0,2</sub></td><td>-3015.26(39)</td><td>-3015.62(35)</td></tr><tr><td>ρ <sup>(e)</sup></td><td>p<sub>α</sub>J<sub>a</sub></td><td>2<sub>1,1</sub></td><td>0.194354(13)<sup>e</sup></td><td>0.194364(12)<sup>e</sup></td></tr><tr><td>F <sup>(f)</sup></td><td>p<sup>2</sup><sub>α</sub></td><td>2<sub>2,0</sub></td><td>6.4215(13)<sup>f</sup></td><td>6.419877(84)<sup>f</sup></td></tr><tr><td>V<sub>3</sub> <sup>(f)</sup></td><td><sup>1</sup>&frasl;<sub>2</sub>(1 &minus; cos 3α)</td><td>2<sub>2,0</sub></td><td>429.38(11)<sup>f</sup></td><td>429.2326(32)<sup>f</sup></td></tr><tr><td>Δ<sub>J</sub></td><td>&minus;J<sup>4</sup></td><td>4<sub>0,4</sub></td><td>0.46682(81) &times; 10<sup>&minus;2</sup></td><td>0.46730(77) &times; 10<sup>&minus;2</sup></td></tr><tr><td>Δ<sub>JK</sub></td><td>&minus;J<sup>2</sup>J<sup>2</sup><sub>a</sub></td><td>4<sub>0,4</sub></td><td>&minus;0.17168(62) &times; 10<sup>0</sup></td><td>&minus;0.17161(59) &times; 10<sup>0</sup></td></tr><tr><td>Δ<sub>K</sub></td><td>&minus;J<sup>4</sup><sub>a</sub></td><td>4<sub>0,4</sub></td><td>0.47291(60) &times; 10<sup>1</sup></td><td>0.47296(59) &times; 10<sup>1</sup></td></tr><tr><td>δ<sub>J</sub></td><td>&minus;2J<sup>2</sup>(J<sup>2</sup><sub>b</sub> &minus; J<sup>2</sup><sub>c</sub>)</td><td>4<sub>0,4</sub></td><td>0.15904(40) &times; 10<sup>&minus;2</sup></td><td>0.15927(38) &times; 10<sup>&minus;2</sup></td></tr><tr><td>δ<sub>K</sub></td><td>&minus;2{J<sup>2</sup><sub>a</sub>, (J<sup>2</sup><sub>b</sub> &minus; J<sup>2</sup><sub>c</sub>)}</td><td>4<sub>0,4</sub></td><td>0.2120(19) &times; 10<sup>&minus;1</sup></td><td>0.2120(19) &times; 10<sup>&minus;1</sup></td></tr><tr><td>D<sub>abJ</sub></td><td>{J<sub>a</sub>, J<sub>b</sub>}J<sup>2</sup></td><td>4<sub>0,4</sub></td><td>0.3983(19) &times; 10<sup>&minus;1</sup></td><td>0.3993(18) &times; 10<sup>&minus;1</sup></td></tr><tr><td>D<sub>abK</sub></td><td>{J<sub>a</sub>, J<sub>b</sub>}J<sup>2</sup><sub>a</sub></td><td>4<sub>0,4</sub></td><td>0.5329(38) &times; 10<sup>0</sup></td><td>0.5320(36) &times; 10<sup>0</sup></td></tr><tr><td>ρ<sub>J</sub></td><td>p<sub>α</sub>J<sub>a</sub>J<sup>2</sup></td><td>4<sub>1,3</sub></td><td>&minus;0.4253(14) &times; 10<sup>0</sup></td><td>&minus;0.4254(13) &times; 10<sup>0</sup></td></tr><tr><td>ρ<sub>K</sub></td><td>p<sub>α</sub>J<sup>3</sup><sub>a</sub></td><td>4<sub>1,3</sub></td><td>0.21648(59) &times; 10<sup>2</sup></td><td>0.21687(53) &times; 10<sup>2</sup></td></tr><tr><td>F<sub>J</sub></td><td>p<sup>2</sup><sub>α</sub>J<sup>2</sup></td><td>4<sub>2,2</sub></td><td>0.15159(31) &times; 10<sup>0</sup></td><td>0.15136(26) &times; 10<sup>0</sup></td></tr><tr><td>F<sub>K</sub></td><td>p<sup>2</sup><sub>α</sub>J<sup>2</sup><sub>a</sub></td><td>4<sub>2,2</sub></td><td>&minus;0.4535(31) &times; 10<sup>2</sup></td><td>&minus;0.4580(14) &times; 10<sup>2</sup></td></tr><tr><td>F<sub>ab</sub></td><td><sup>1</sup>&frasl;<sub>2</sub>{J<sub>a</sub>, J<sub>b</sub>}p<sup>2</sup><sub>α</sub></td><td>4<sub>2,2</sub></td><td>0.902(35) &times; 10<sup>0</sup></td><td>0.898(30) &times; 10<sup>0</sup></td></tr><tr><td>V<sub>3J</sub></td><td>J<sup>2</sup>(1 &minus; cos 3α)</td><td>4<sub>2,2</sub></td><td>&minus;0.13759(47) &times; 10<sup>2</sup></td><td>&minus;0.13807(40) &times; 10<sup>2</sup></td></tr><tr><td>V<sub>3K</sub></td><td>J<sup>2</sup><sub>a</sub>(1 &minus; cos 3α)</td><td>4<sub>2,2</sub></td><td>&minus;0.1229(28) &times; 10<sup>3</sup></td><td>&minus;0.1246(27) &times; 10<sup>3</sup></td></tr><tr><td>V<sub>3ab</sub></td><td><sup>1</sup>&frasl;<sub>2</sub>({J<sub>a</sub>, J<sub>b</sub>})(1 &minus; cos 3α)</td><td>4<sub>2,2</sub></td><td>&minus;0.647(15) &times; 10<sup>2</sup></td><td>&minus;0.647(14) &times; 10<sup>2</sup></td></tr><tr><td>V<sub>3bc</sub></td><td>(J<sup>2</sup><sub>b</sub> &minus; J<sup>2</sup><sub>c</sub>)(1 &minus; cos 3α)</td><td>4<sub>2,2</sub></td><td>0.2134(47) &times; 10<sup>1</sup></td><td>0.2184(39) &times; 10<sup>1</sup></td></tr><tr><td>D<sub>3ac</sub></td><td><sup>1</sup>&frasl;<sub>2</sub>{J<sub>a</sub>, J<sub>c</sub>} sin 3α</td><td>4<sub>2,2</sub></td><td>0.4186(50) &times; 10<sup>3</sup></td><td>0.4193(49) &times; 10<sup>3</sup></td></tr><tr><td>ρ<sub>m</sub></td><td>p<sup>3</sup><sub>α</sub>J<sub>a</sub></td><td>4<sub>1,3</sub></td><td>0.514(11) &times; 10<sup>2</sup></td><td>0.5324(15) &times; 10<sup>2</sup></td></tr><tr><td>F<sub>m</sub></td><td>p<sup>4</sup><sub>α</sub></td><td>4<sub>4,0</sub></td><td>&minus;0.443(15) &times; 10<sup>2</sup></td><td>&minus;0.46728(72) &times; 10<sup>2</sup></td></tr><tr><td>V<sub>6</sub></td><td><sup>1</sup>&frasl;<sub>2</sub>(1 &minus; cos 6α)</td><td>4<sub>4,0</sub></td><td>&minus;0.3088(44) &times; 10<sup>6</sup></td><td>&minus;0.30159(12) &times; 10<sup>6</sup></td></tr><tr><td>Φ<sub>J</sub></td><td>J<sup>6</sup></td><td>6<sub>0,6</sub></td><td>0.21428(94) &times; 10<sup>&minus;7</sup></td><td>0.21471(90) &times; 10<sup>&minus;7</sup></td></tr><tr><td>Φ<sub>JKK</sub></td><td>J<sup>2</sup>J<sup>4</sup><sub>a</sub></td><td>6<sub>0,6</sub></td><td>&minus;0.7678(62) &times; 10<sup>&minus;4</sup></td><td>&minus;0.7686(60) &times; 10<sup>&minus;4</sup></td></tr><tr><td>Φ<sub>K</sub></td><td>J<sup>6</sup><sub>a</sub></td><td>6<sub>0,6</sub></td><td>0.8161(75) &times; 10<sup>&minus;3</sup></td><td>0.8154(73) &times; 10<sup>&minus;3</sup></td></tr><tr><td>φ<sub>J</sub></td><td>2J<sup>4</sup>(J<sup>2</sup><sub>b</sub> &minus; J<sup>2</sup><sub>c</sub>)</td><td>6<sub>0,6</sub></td><td>0.9235(85) &times; 10<sup>&minus;8</sup></td><td>0.9245(80) &times; 10<sup>&minus;8</sup></td></tr><tr><td>ρ<sub>JK</sub></td><td>p<sub>α</sub>J<sup>3</sup>J<sup>2</sup><sub>a</sub></td><td>6<sub>1,5</sub></td><td>0.5936(53) &times; 10<sup>&minus;3</sup></td><td>0.5941(51) &times; 10<sup>&minus;3</sup></td></tr><tr><td>ρ<sub>KK</sub></td><td>p<sub>α</sub>J<sup>5</sup><sub>a</sub></td><td>6<sub>1,5</sub></td><td>&minus;0.6917(54) &times; 10<sup>&minus;2</sup></td><td>&minus;0.6895(52) &times; 10<sup>&minus;2</sup></td></tr><tr><td>F<sub>JK</sub></td><td>p<sup>2</sup><sub>α</sub>J<sup>2</sup>J<sup>2</sup><sub>a</sub></td><td>6<sub>2,4</sub></td><td>&minus;0.1522(14) &times; 10<sup>&minus;2</sup></td><td>&minus;0.1524(13) &times; 10<sup>&minus;2</sup></td></tr><tr><td>F<sub>KK</sub></td><td>p<sup>2</sup><sub>α</sub>J<sup>4</sup><sub>a</sub></td><td>6<sub>2,4</sub></td><td>0.1585(15) &times; 10<sup>&minus;1</sup></td><td>0.1578(15) &times; 10<sup>&minus;1</sup></td></tr><tr><td>V<sub>3JK</sub></td><td>J<sup>2</sup>J<sup>2</sup><sub>a</sub>(1 &minus; cos 3α)</td><td>6<sub>2,4</sub></td><td>0.1922(51) &times; 10<sup>&minus;1</sup></td><td>0.1954(46) &times; 10<sup>&minus;1</sup></td></tr><tr><td>ρ<sub>mbc</sub></td><td>p<sup>3</sup><sub>α</sub>J<sub>a</sub>(J<sup>2</sup><sub>b</sub> &minus; J<sup>2</sup><sub>c</sub>)</td><td>6<sub>3,3</sub></td><td>0.677(15) &times; 10<sup>&minus;3</sup></td><td>0.671(14) &times; 10<sup>&minus;3</sup></td></tr><tr><td>V<sub>6ab</sub></td><td>{J<sub>a</sub>, J<sub>b</sub>}(1 &minus; cos 6α)</td><td>6<sub>4,2</sub></td><td>0.425(10) &times; 10<sup>2</sup></td><td>0.4207(97) &times; 10<sup>2</sup></td></tr><tr><td>σ<sub>rms/wrms</sub> <sup>(g)</sup></td><td></td><td></td><td>4.42 &times; 10<sup>&minus;2</sup>/0.74</td><td>4.18 &times; 10<sup>&minus;2</sup>/0.73</td></tr><tr><td>J<sub>max</sub>/K<sub>d</sub>max <sup>(h)</sup></td><td></td><td></td><td>38/18</td><td>38/18</td></tr><tr><td>N<sub>lines</sub> <sup>(i)</sup></td><td></td><td></td><td>3040</td><td>3059</td></tr></tbody></table>

Notes. <sup>(a)</sup>Parameter nomenclature. <sup>(b)</sup>Torsional-rotation operators employed in the model in the I' representation. <sup>(c)</sup>n<sub>op</sub> is the total order operator, which is n<sub>op</sub> = t<sub>op</sub> + r<sub>op</sub>, with t<sub>op</sub> the order of the torsional part and r<sub>op</sub> that of the rotational part. <sup>(d)</sup>Unless indicated, all constants are expressed in MHz. <sup>(e)</sup>ρ value is unitless. <sup>(f)</sup>F and V<sub>3</sub> values are expressed in cm<sup>-1</sup>. <sup>(g)</sup>Fit root mean square error in MHz / weighted root mean square. <sup>(h)</sup>Maximum value of quantum number J and K<sub>d</sub> included in the fit. <sup>(i)</sup>Number of transitions included in the fit. <sup>(j)</sup>Ground torsional state molecular parameters from Bermúdez et al. (2018). <sup>(k)</sup>Ground torsional state molecular parameters derived from this work.

TMC-1, with an abundance of 1.4×10<sup>-9</sup> relative to H<sub>2</sub>, reported by Cernicharo et al. (2020). This gives an abundance ratio of CH<sub>2</sub>CO/CH<sub>3</sub>CHCO ~ 93, which means that the methylated form of ketene is about two orders of magnitude less abundant than ketene itself. Finally, we compared the abundance of methyl ketene with acetaldehyde (CH<sub>3</sub>CHO), which has an abundance of 3.5×10<sup>-10</sup> reported by Cernicharo et al. (2020). This gives an abundance ratio of CH<sub>3</sub>CHO/CH<sub>3</sub>CHCO ~ 23.

## 4. Chemical model

To investigate the formation of methyl ketene in TMC-1 we carried out gas-phase chemical modelling calculations. The model parameters and chemical network are the same used in Cernicharo et al. (2021g) to model the chemistry of O-bearing molecules following the discovery of HC<sub>3</sub>O and C<sub>5</sub>O. We added CH<sub>3</sub>CHCO as a new species, with a simple chemical scheme of formation and destruction. Although some
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Observed transitions with $K_a = 0,1$ of CH$_3$CHCO 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 $T_{\text{rot}} = 9$ K and a column density of $1.5 \times 10^{11} \text{ cm}^{-2}$. Blanked channels correspond to negative features produced in the folding of the frequency-switching data. Green labels indicate the transitions for which only one of the frequency-switching data points has been used (FS10 and FS8 correspond to a throw of 10 and 8 MHz, respectively). The dotted line in each panel indicates the $3\sigma$ value.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Observed transitions with $K_a = 2$ of CH$_3$CHCO 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 milli Kelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for $T_{\text{rot}} = 9$ K and a column density of $1.5 \times 10^{11} \text{ cm}^{-2}$. Blanked channels correspond to negative features produced in the folding of the frequency-switching data. The dotted line in each panel indicates the $3\sigma$ value.</figcaption></figure>

chemical models (e.g., Garrod et al. 2022) indicate that grain surface processes can help to explain the presence of some complex organic molecules (e.g., methyl formate and dimethyl ether; Agúndez et al. 2021b) in cold sources such as TMC-1, here we aim to evaluate whether purely gas phase processes can account for the formation of CH$_3$CHCO in TMC-1.

Methyl ketene is not included in the UMIST<fnref n="4" /> (McElroy et al. 2013) or KIDA<fnref n="5" /> (Wakelam et al. 2015) databases, although

some information is available in the NIST Chemical Kinetics database<fnref n="6" />. There are several plausible reactions of formation. The reaction between OH and CH$_3$CCH is a potential source of methyl ketene because it is relatively rapid at low temperatures, with a measured rate coefficient of $5.08 \times 10^{-12} \text{ cm}^3 \text{ s}^{-1}$ at 69 K (Taylor et al. 2008). However, no information is available on the product distribution, and thus here we assume that CH$_3$CHCO and H are the main products. The reaction between OH and the non-polar isomer allene (CH$_2$CCH$_2$) has also been found to

<footnote n="4">⁴ http://udfa.ajmarkwick.net/</footnote>

<footnote n="5">⁵ https://kida.astrochem-tools.org/</footnote>

<footnote n="6">⁶ https://kinetics.nist.gov/</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Calculated fractional abundance of CH<sub>3</sub>CHCO and its relevant precursors CH<sub>3</sub>CCH and CH<sub>3</sub>CHO as a function of time. The horizontal dotted line corresponds to the observed abundance in TMC-1.</figcaption></figure>

be rapid at low temperatures, although the main products seem to be H<sub>2</sub>CCO and CH<sub>3</sub> (Daranlot et al. 2012), and we thus do not include it as a source of methyl ketene. Another reaction that can provide an efficient formation route to methyl ketene is CH + CH<sub>3</sub>CHO. This reaction was studied by Goulay et al. (2012), who found that CH<sub>3</sub>CHCO is formed with a branching ratio of 0.39. However, the rate coefficient has not been measured, although Wang et al. (2017) studied the reaction theoretically and found that the formation of CH<sub>3</sub>CHCO is barrierless. We thus adopted a rate coefficient of 2.41 × 10<sup>-10</sup> cm<sup>3</sup> s<sup>-1</sup>, as measured for CH and H<sub>2</sub>CO (Hancock & Heal 1992), and the branching ratio measured by Goulay et al. (2012).

There are other reactions that could potentially form CH<sub>3</sub>CHCO, although they are unlikely to be efficient in TMC-1. For example, the reaction between HCO and C<sub>2</sub>H<sub>4</sub> has a barrier (Lesclaux et al. 1986; Xie et al. 2005a), and the same happens for the reaction between C<sub>2</sub>H<sub>3</sub> and H<sub>2</sub>CO (Xie et al. 2005b). The reaction between CH<sub>3</sub> and H<sub>2</sub>CCO could provide a simple pathway to CH<sub>3</sub>CHCO by simply substituting one H atom of ketene by a methyl group, although ketene does not show a high reactivity with radicals. Semenikhin et al. (2018) studied theoretically the reaction between CH<sub>3</sub> and H<sub>2</sub>CCO; although they did not consider the formation of CH<sub>3</sub>CHCO and H, they found that all the explored channels have barriers. Therefore, we did not include this reaction. Finally, we considered that in TMC-1 methyl ketene is mostly destroyed through reactions with the most abundant cations, such as C<sup>+</sup>, HCO<sup>+</sup>, and H<sub>3</sub>O<sup>+</sup>.

The fractional abundance calculated for CH<sub>3</sub>CHCO is shown in Fig. 3 as a function of time. It is seen that the peak abundance, reached at a time of some 10<sup>5</sup> yr, agrees very well with the abundance derived from the observations. The two formation reactions considered here (i.e., OH + CH<sub>3</sub>CCH and CH + CH<sub>3</sub>CHO) contribute to the formation of methyl ketene. The abundances calculated for CH<sub>3</sub>CCH and CH<sub>3</sub>CHO are in good agreement with those obtained in Cabezas et al. (2021) and Cernicharo et al. (2020). Further research on the low temperature kinetics and the product distribution of these two reactions will be of great interest to shed light on the origin of methyl ketene in TMC-1.

## 5. Conclusions

We have presented the first detection of CH<sub>3</sub>CHCO towards TMC-1. We used the QUIJOTE line survey taken with the Yebes 40 m radiotelescope, with which we observed a total of 11 lines with an intensity higher than 3σ and another 8 lines with an

intensity lower than 3σ, involving *J* = 3 to *J* = 5 and *K*<sub>a</sub> ≤ 2. The rotational temperature is 9 K and the derived column density *N*(CH<sub>3</sub>CHCO) = 1.5×10<sup>11</sup> cm<sup>-2</sup>. These results imply that methyl ketene is 1.46 less abundant than its most stable isomer (trans-CH<sub>2</sub>CHCHO), a value quite similar to the abundance ratio of the isomers vinyl alcohol to acetaldehyde of 1. The observed abundance of methyl ketene is well explained using our gas-phase chemical model, considering the formation reactions from propyne and acetaldehyde, and the destruction reactions with the most abundant cations of TMC-1.

**Acknowledgements.** 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. C.B. thanks Ministerio de Universidades for her "Maria Zambrano" grant at UVa (CONVREC-2021-317).

## References

Agúndez, M., Cabezas, C., Tercero, B., et al. 2021a, A&A, 647, L10

Agúndez, M., Marcelino, N., Tercero, B., et al. 2021b, A&A, 649, L4

Bak, B., Christiansen, J., Kunstmann, K., et al. 1966, ApJ, 45, 883

Bermúdez, C., Tercero, B., Motiyenko, R. A., et al. 2018, A&A, 619, A92

Cabezas, C., Endo, Y., Roueff, E., et al. 2021, A&A, 646, L1

Cernicharo, J. 1985, Internal IRAM report (Granada: IRAM)

Cernicharo, J. 2012, in ECLA 2011: Proc. of the European Conference on Laboratory Astrophysics, eds. C. Stehl, C. Joblin, & L. d'Hendecourt (Cambridge: Cambridge Univ. Press), EAS Publications Series, 251

Cernicharo, J., & Guélín, M. 1987, A&A, 176, 299

Cernicharo, J., Marcelino, N., Agúndez, M., et al. 2020, A&A, 642, L17

Cernicharo, J., Agúndez, M., Kaiser, R., et al. 2021a, A&A, 652, L9

Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021b, A&A, 649, L15

Cernicharo, J., Agúndez, M., Kaiser, R. I., et al. 2021c, A&A, 655, L1

Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021d, A&A, 647, L2

Cernicharo, J., Cabezas, C., Agúndez, M., et al. 2021e, A&A, 647, L3

Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021f, A&A, 656, L21

Cernicharo, J., Cabezas, C., Bailleux, S., et al. 2021g, A&A, 646, L7

Cernicharo, J., Fuentetaja, R., Agúndez, M., et al. 2022, A&A, 663, L9

Cordiner, M. A., Charnley, S. B., Kisiel, Z., et al. 2017, ApJ, 850, 187

Daranlot, J., Hickson, K. M., Loison, J.-C., et al. 2012, J. Phys. Chem. A, 116, 10871

Fossé, D., Cernicharo, J., Gerin, M., & Cox, P. 2001, ApJ, 552, 168

Fuentetaja, R., Cabezas, C., Agúndez, M., et al. 2022a, A&A, 663, L3

Fuentetaja, R., Agúndez, M., Cabezas, C., et al. 2022b, A&A, 667, L4

Garrod, R. T., Jin, M., Matis, K. A., et al. 2022, ApJS, 259, 1

Goulay, F., Trevitt, A. J., Savee, J. D., et al. 2012, J. Phys. Chem. A, 116, 6091

Hancock, G., & Heal, M. R. 1992, J. Chem. Soc. Faraday Trans., 88, 2121

Ilyushin, V. V., Kisiel, Z., Pszczókowski, L., et al. 2010, J. Mol. Spectrosc., 259, 26

Kaifu, N., Ohishi, M., Kawaguchi, K., et al. 2004, PASJ, 56, 69

Lee, K. L. K., Changala, P. B., Loomis, R. A., et al. 2021, ApJ, 910, L2

Lesclaux, R., Roussel, P., Veyret, B., & Pouchan, C. 1986, J. Am. Chem. Soc., 108, 3872

Loison, J.-C., Agúndez, M., Marcelino, N., et al. 2016, MNRAS, 456, 4101

McCarthy, M. C., Lee, K. L. K., Loomis, R. A., et al. 2021, Nature Astron., 5, 176

McElroy, D., Walsh, C., Markwick, A. J., et al. 2013, A&A, 550, A36

McGuire, B. A., Burkhardt, A. M., Shingledecker, C. N., et al. 2017, ApJ, 843, L28

McGuire, B. A., Burkhardt, A. M., Kalenskii, S., et al. 2018, Science, 359, 202

McGuire, B. A., Loomis, R. A., Burkhardt, A. M., et al. 2021, Science, 371, 1265

Pardo, J. R., Cernicharo, J., & Serabyn, E. 2001, IEEE Trans. Anten. Propag., 49, 12

Semenikhin, A. S., Shubina, E. G., Savchenkova, A. S., et al. 2018, Int. J. Chem. Kinet., 50, 273

Sita, M. L., Changala, P. B., Xue, C., et al. 2022, ApJ, 938, L12

Taylor, S. E., Goddard, A., Blitz, M. A., et al. 2008, PCCP, 10, 422

Tercero, F., López-Pérez, J. A., Gallego, J. D., et al. 2021, A&A, 645, A37

Wakelam, V., Loison, J.-C., Herbst, E., et al. 2015, ApJS, 217, 20

Wang, Y., Tang, Y., & Shao, Y. 2017, Comput. Theor. Chem., 1103, 56

Xie, H.-B., Ding, Y.-H., & Sun, C.-C. 2005a, J. Theor. Comput. Chem., 4, 1029

Xie, H.-B., Ding, Y.-H., & Sun, C.-C. 2005b, J. Phys. Chem. A., 109, 8419
## Appendix A: Observed line parameters

The line parameters derived for this work were obtained by fitting a Gaussian line profile to the observed data, using the software Class (GILDAS package). We use a window of ± 15 km s⁻¹ around the V<sub>LSR</sub> (5.83km s⁻¹) of the source for each transition. The results are given in Table A.1. The observed lines of methyl ketene are shown in Fig. 1 for K<sub>a</sub> = 0,1 and in Fig. 2 for K<sub>a</sub> = 2.

<table><caption>**Table A.1.** Observed line parameters for CH<sub>3</sub>CHCO</caption><thead><tr><th>Transition<sup>1</sup></th><th>ν<sub>obs</sub><sup>a</sup><br>(MHz)</th><th>ν<sub>obs</sub>-ν<sub>cal</sub><br>(MHz)</th><th>∫ T<sub>A</sub><sup>*</sup> dv<sup>b</sup><br>(mK km s<sup>-1</sup>)</th><th>Δν<sup>c</sup><br>(km s<sup>-1</sup>)</th><th>T<sub>A</sub><sup>*</sup> d<br>(mK)</th><th>Notes</th></tr></thead><tbody><tr><td>4<sub>1,4</sub>-3<sub>1,3</sub> A</td><td>33830.302±0.010</td><td>-0.013</td><td>0.51±0.13</td><td>0.98±0.31</td><td>0.49±0.13</td><td>A</td></tr><tr><td>4<sub>1,4</sub>-3<sub>1,3</sub> E</td><td>33834.322±0.010</td><td>0.027</td><td>0.49±0.12</td><td>0.66±0.18</td><td>0.69±0.14</td><td>B</td></tr><tr><td>4<sub>0,4</sub>-3<sub>0,3</sub> E</td><td>34545.403±0.010</td><td>0.004</td><td>0.58±0.10</td><td>0.75±0.14</td><td>0.72±0.12</td><td></td></tr><tr><td>4<sub>0,4</sub>-3<sub>0,3</sub> A</td><td>34547.083±0.010</td><td>0.007</td><td>0.69±0.11</td><td>0.82±0.17</td><td>0.78±0.12</td><td></td></tr><tr><td>4<sub>1,3</sub>-3<sub>1,2</sub> E</td><td>35305.954±0.022</td><td>0.029</td><td>0.38±0.15</td><td>0.76±0.35</td><td>0.46±0.14</td><td></td></tr><tr><td>4<sub>1,3</sub>-3<sub>1,2</sub> A</td><td>35313.007±0.010</td><td>0.026</td><td>0.51±0.10</td><td>0.69±0.15</td><td>0.70±0.11</td><td></td></tr><tr><td>5<sub>1,5</sub>-4<sub>1,4</sub> A</td><td>42282.025±0.011</td><td>0.007</td><td>0.46±0.16</td><td>0.50±0.18</td><td>0.86±0.11</td><td></td></tr><tr><td>5<sub>1,5</sub>-4<sub>1,4</sub> E</td><td>42283.591±0.013</td><td>0.032</td><td>0.40±0.11</td><td>0.64±0.19</td><td>0.59±0.11</td><td></td></tr><tr><td>5<sub>0,5</sub>-4<sub>0,4</sub> E</td><td>43158.875±0.011</td><td>-0.020</td><td>0.88±0.14</td><td>0.93±0.16</td><td>0.89±0.13</td><td></td></tr><tr><td>5<sub>0,5</sub>-4<sub>0,4</sub> A</td><td>43160.982±0.010</td><td>0.011</td><td>0.51±0.11</td><td>0.60±0.13</td><td>0.79±0.13</td><td></td></tr><tr><td>5<sub>1,4</sub>-4<sub>1,3</sub> E</td><td>44129.579±0.012</td><td>0.07</td><td>0.33±0.12</td><td>0.48±0.19</td><td>0.65±0.21</td><td>A</td></tr><tr><td>5<sub>1,4</sub>-4<sub>1,3</sub> A</td><td>44134.955±0.014</td><td>0.07</td><td>0.53±0.14</td><td>0.63±0.18</td><td>0.78±0.13</td><td></td></tr><tr><td>4<sub>2,3</sub>-3<sub>2,2</sub> A</td><td>34577.168±0.011</td><td>-0.012</td><td>0.10±0.08</td><td>0.33±3.42</td><td>0.28±0.16</td><td></td></tr><tr><td>4<sub>2,3</sub>-3<sub>2,2</sub> E + 4<sub>2,2</sub>-3<sub>2,1</sub> E</td><td>34590.976±0.015</td><td>0.043 & -0.021</td><td>0.42±0.13</td><td>0.91±0.26</td><td>0.43±0.12</td><td>C</td></tr><tr><td>4<sub>2,2</sub>-3<sub>2,1</sub> A</td><td>34607.086±0.010</td><td>-0.035</td><td>0.11±0.07</td><td>0.35±1.80</td><td>0.28±0.10</td><td></td></tr><tr><td>5<sub>2,4</sub>-4<sub>2,3</sub> A</td><td>43217.244±0.028</td><td>-0.026</td><td>0.13±0.10</td><td>0.44±0.34</td><td>0.26±0.15</td><td></td></tr><tr><td>5<sub>2,4</sub>-4<sub>2,3</sub> E</td><td>43242.864±0.019</td><td>0.026</td><td>0.19±0.09</td><td>0.58±0.29</td><td>0.31±0.16</td><td></td></tr><tr><td>5<sub>2,3</sub>-4<sub>2,2</sub> E</td><td>43248.485±0.033</td><td>-0.074</td><td>0.21±0.13</td><td>0.65±0.35</td><td>0.31±0.22</td><td></td></tr><tr><td>5<sub>2,3</sub>-4<sub>2,2</sub> A</td><td>43277.034±0.025</td><td>-0.071</td><td>0.12±0.09</td><td>0.32±0.39</td><td>0.36±0.12</td><td></td></tr></tbody></table>

**Notes.** <sup>1</sup>Quantum numbers are J<sub>K<sub>a</sub> K<sub>c</sub></sub> - J<sub>K<sub>a</sub> K<sub>c</sub></sub>. <sup>a</sup>Observed frequency of the transition assuming a LSR velocity of 5.83 km s⁻¹. <sup>b</sup>Integrated line intensity in mK km s⁻¹. <sup>c</sup>Linewidth at half intensity derived by fitting a Gaussian function to the observed line profile (in km s⁻¹). <sup>d</sup>Antenna temperature in millikelvin. <sup>e</sup>Frequency switching data with a throw of 10 MHz only. Negative feature present in the data with a 8 MHz throw. <sup>f</sup>Frequency switching data with a throw of 8 MHz only. Negative feature present in the data with a 10 MHz throw. <sup>g</sup>The line is blended and corresponds to two transitions