Detection of Interstellar HC${}_5$O in TMC-1 with the Green Bank Telescope

We report the detection of the carbon-chain radical HC${}_5$O for the first time in the interstellar medium toward the dark cloud TMC-1 using the 100 m Green Bank Telescope. We observe four hyperfine components of this radical in the $J=17\mathrm{/}2\to 15\mathrm{/}2$ rotational transition that originates from the ${}^2{\mathrm{\Pi }}_{1\mathrm{/}2}$ fine structure level of its ground state, and calculate an abundance of $n\mathrm{/}{n}_{H_2}$ = $1.7\times {10}^{-10}$, assuming an excitation temperature of $T_{ex}=7$~K. No indication of HC${}_3$O, HC${}_4$O, HC${}_6$O, is found in these or archival observations of the source, while we report tentative evidence for HC${}_7$O. We compare calculated upper limits, and the abundance of HC${}_5$O to predictions based on (1) the abundance trend of the analogous HC${}_n$N family in TMC-1 and (2) a gas-grain chemical model. We find that the gas-grain chemical model well reproduces the observed abundance of HC${}_5$O, as well as the upper limits of HC${}_3$O, HC${}_6$O, and HC${}_7$O, but HC${}_4$O is over produced. The prospects for astronomical detection of both shorter and longer HC${}_n$O chains are discussed.