Discovery of HCCCO and C<sub>5</sub>O in TMC-1 with the QUIJOTE line survey
J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, R. Fuentetaja, J. R. Pardo, P. de Vicente
Abstract
We report on the detection, for the first time in space, of the radical HCCCO and of pentacarbon monoxide, C 5 O. The derived column densities are (1.6 ± 0.2) × 10 11 cm −2 and (1.5 ± 0.2) × 10 10 cm −2 , respectively. We have also analysed the data for all the molecular species of the families HC n O and C n O within our QUIJOTE’s line survey. Upper limits are obtained for HC 4 O, HC 6 O, C 4 O, and C 6 O. We report a robust detection of HC 5 O and HC 7 O based on 14 and 12 rotational lines detected with a signal-to-noise ratio ≥30 and ≥5, respectively. The derived N(HC 3 O)/N(HC 5 O) abundance ratio is 0.09 ± 0.03, while N(C 3 O)/N(C 5 O) is 80 ± 2, and N(HC 5 O)/N(HC 7 O) is 2.2 ± 0.3. As opposed to the cyanopolyyne family, HC 2 n + 1 N, which shows a continuous decrease in the abundances with increasing n , the C n O and HC n O 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 n H 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 n H m + ions with CO and reactions of C n̄ and C n H − anions with O atoms, and we found that several species, such as C 5 O, HC 4 O, and HC 6 O, 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 n H carbon chains with O, OH, or HCO, could be behind the formation of these species.