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Discovery of linear propadienone: Study of the chemistry of linear and cyclic H$_2$C$_3$O and H$_2$C$_3$S in TMC-1

G. Esplugues, J. C. Loison, M. Agúndez, G. Molpeceres, N. Marcelino, B. Tercero, J. Cernicharo

TL;DR

This study reports the first interstellar detection of the linear isomer propadienone (l-H$_2$C$_3$O) in TMC-1, along with isotopologues and deuterated forms of related cyclic isomers, using the QUIJOTE survey with Yebes-40 m and IRAM-30 m data. Rotational analyses yield $T_{rot}$ mostly around $4.8$–$6.5$ K and reveal that l-H$_2$C$_3$O is about eight times less abundant than c-H$_2$C$_3$O, while l-H$_2$C$_3$S is more than an order of magnitude more abundant than l-H$_2$C$_3$O, and c-H$_2$C$_3$O dominates over c-H$_2$C$_3$S. Isotopic fractionation shows enhanced $^{13}$C in certain positions and an overall $^{12}$C/$^{13}$C ratio of $23\pm9$ for c-H$_2$$^{13}$CCCO, with deuteration levels ($\sim$12) comparable to other TMC-1 species. Chemical modelling with Nautilus indicates distinct formation pathways for the O- and S-isomers, with DR and OH–driven routes important for O-bearing species and ion–neutral chemistry driving the S-bearing species, underscoring kinetics as a key driver of isomer abundances in the ISM.

Abstract

We report the first detection in space of propadienone, the linear isomer (l-H$_2$C$_3$O) of cyclopropenone (c-H$_2$C$_3$O). We also report the first detection of the isotopologue c-H$_2$$^{13}$CCCO, and c-HDCCCO of c-H$_2$C$_3$O. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0-50.3 GHz, complemented with data between 71.6-116.0 GHz, and carried out with Yebes-40m and IRAM-30m telescopes, respectively. We obtain a total column density of 3.7$\times$10$^{10}$ cm$^{-2}$ for l-H$_2$C$_3$O at an excitation temperature of 4.8 K. We find that the isomer is about eight times less abundant than the cyclic one. We also report a detailed line-by-line study of cyclopropenethione (c-H$_2$C$_3$S) to compare the abundance of the O and S isomers. We find that cyclic O-isomers are more abundant than cyclic S-isomers; however, the opposite trend is found for the most stable linear isomers, with l-H$_2$C$_3$S being more than one order of magnitude more abundant than l-H$_2$C$_3$O. A comprehensive theoretical chemical analysis shows that the abundances of the H$_2$C$_3$O and H$_2$C$_3$S isomers are controlled by different formation pathways. In particular, while l-H$_2$C$_3$O is potentially produced by dissociative electron recombination reactions, ion-neutral chemistry is more effective at producing l-H$_2$C$_3$S and c-H$_2$C$_3$S.

Discovery of linear propadienone: Study of the chemistry of linear and cyclic H$_2$C$_3$O and H$_2$C$_3$S in TMC-1

TL;DR

This study reports the first interstellar detection of the linear isomer propadienone (l-HCO) in TMC-1, along with isotopologues and deuterated forms of related cyclic isomers, using the QUIJOTE survey with Yebes-40 m and IRAM-30 m data. Rotational analyses yield mostly around – K and reveal that l-HCO is about eight times less abundant than c-HCO, while l-HCS is more than an order of magnitude more abundant than l-HCO, and c-HCO dominates over c-HCS. Isotopic fractionation shows enhanced C in certain positions and an overall C/C ratio of for c-HCCCO, with deuteration levels (12) comparable to other TMC-1 species. Chemical modelling with Nautilus indicates distinct formation pathways for the O- and S-isomers, with DR and OH–driven routes important for O-bearing species and ion–neutral chemistry driving the S-bearing species, underscoring kinetics as a key driver of isomer abundances in the ISM.

Abstract

We report the first detection in space of propadienone, the linear isomer (l-HCO) of cyclopropenone (c-HCO). We also report the first detection of the isotopologue c-HCCCO, and c-HDCCCO of c-HCO. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0-50.3 GHz, complemented with data between 71.6-116.0 GHz, and carried out with Yebes-40m and IRAM-30m telescopes, respectively. We obtain a total column density of 3.710 cm for l-HCO at an excitation temperature of 4.8 K. We find that the isomer is about eight times less abundant than the cyclic one. We also report a detailed line-by-line study of cyclopropenethione (c-HCS) to compare the abundance of the O and S isomers. We find that cyclic O-isomers are more abundant than cyclic S-isomers; however, the opposite trend is found for the most stable linear isomers, with l-HCS being more than one order of magnitude more abundant than l-HCO. A comprehensive theoretical chemical analysis shows that the abundances of the HCO and HCS isomers are controlled by different formation pathways. In particular, while l-HCO is potentially produced by dissociative electron recombination reactions, ion-neutral chemistry is more effective at producing l-HCS and c-HCS.

Paper Structure

This paper contains 11 sections, 1 equation, 15 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Results
  4. Rotational diagrams
  5. Column densities and abundances
  6. Discussion
  7. Isotopic fractionation
  8. The ortho-to-para ratio
  9. Chemical modelling
  10. Conclusions
  11. Additional tables and figures

Figures (15)

  • Figure 1: Simplified diagram (without HCCHCO) of potential energy for H$_2$C$_3$O isomers calculated at the M06-2X/AVTZ level.
  • Figure 2: Diagram of potential energy for H$_2$C$_3$S isomers calculated at the M06-2X/AVTZ level.
  • Figure 3: Observed lines of o-c-H$_2$C$_3$O in TMC-1 at 7 and 3 mm for $E$$_{\mathrm{upp}}$$\lesssim$17 K.
  • Figure 4: Observed lines of p-c-H$_2$C$_3$O in TMC-1 in the 31.0-50.4 GHz range.
  • Figure 5: Observed lines of c-HDC$_3$O for $E$$_{\mathrm{upp}}$$<$9 K and $A$$_{\mathrm{ul}}$$>$10$^{-6}$ s$^{-1}$ in TMC-1 in the 31.0-50.4 GHz range.
  • ...and 10 more figures