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

Abstract: 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)e11 cm-2 and (1.5+/-0.2)e10 cm-2, respectively. We have also analysed the data for all the molecular species of the families HCnO and CnO within our QUIJOTE 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, HC2n+1N, which shows a continuous decrease in the abundances with increasing n, the CnO and HCnO 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 CnH 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 CnHm+ ions with CO and reactions of Cn- and CnH- 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 Cn and CnH carbon chains with O, OH, or HCO, could be behind the formation of these species.