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