2021A&A...656L..21C

We report on the detection, for the first time in space, of the radical HCCCO and of pentacarbon monoxide, C₅O. The derived column densities are (1.6 ± 0.2) x 10¹¹ cm^–2 and (1.5 ± 0.2) x 10¹⁰ cm^–2, respectively. We have also analysed the data for all the molecular species of the families HC_nO and C_nO within our QUIJOTE's line survey. Upper limits are obtained for HC₄O, HC₆O, C₄O, and C₆O. We report a robust detection of HC₅O and HC₇O based on 14 and 12 rotational lines detected with a signal-to-noise ratio ≥30 and ≥5, respectively. The derived N(HC₃O)/N(HC₅O) abundance ratio is 0.09 ± 0.03, while N(C₃O)/N(C₅O) is 80 =/- 2, and N(HC₅O)/N(HC₇O) is 2.2 ± 0.3. As opposed to the cyanopolyyne family, HC_2n + 1_N, which shows a continuous decrease in the abundances with increasing n, the C_nO and HC_nO 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 C_nH 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 C_nH_m⁺ ions with CO and reactions of Cn^– and C_nH^– anions with O atoms, and we found that several species, such as C₅O, HC₄O, and HC₆O, 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 C_n and C_nH carbon chains with O, OH, or HCO, could be behind the formation of these species.