Discovery of HCCCO and C5O in TMC-1 with the QUIJOTE line survey

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We report on the detection, for the first time in space, of the radical HCCCO and of pentacarbon monoxide, CO. The derived column densities are (1.6 ± 0.2) × 10cmand (1.5 ± 0.2) × 10cm, respectively. We have also analysed the data for all the molecular species of the families HCO and CnO within our QUIJOTE's line survey. Upper limits are obtained for HCO, HCO, CO, and CO. We report a robust detection of HCO and HCO based on 14 and 12 rotational lines detected with a signal-to-noise ratio ≥30 and ≥5, respectively. The derived N(HCO)/N(HCO) abundance ratio is 0.09 ± 0.03, while N(CO)/N(CO) is 80 ± 2, and N(HCO)/N(HCO) is 2.2 ± 0.3. As opposed to the cyanopolyyne family, HCN, which shows a continuous decrease in the abundances with increasing n, the CO and HCO species show a clear abundance maximum for n = 3 and 5, respectively. They also show an odd and even abundance alternation, with odd values of n being the most abundant, which is reminiscent of the behaviour of CH radicals, where in that case species with even values of n are more abundant. We explored the formation of these species through two mechanisms previously proposed, which are based on radiative associations between CHions with CO and reactions of Cand CHanions with O atoms, and we found that several species, such as CO, HCO, and HCO, are significantly overestimated. Our understanding of how these species are formed is incomplete as of yet. Other routes based on neutral-neutral reactions such as those of Cand CH carbon chains with O, OH, or HCO, could be behind the formation of these species.
We thank Ministerio de Ciencia e Innovación of Spain (MICIU) for funding support through projects PID2019-106110GBI00, 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.