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 $J = 17/2 \rightarrow 15/2$ rotational transition that originates from the
$^2\Pi_{1/2}$ fine structure level of its ground state, and calculate an
abundance of $n/n_{H_2}$ = $1.7\times 10^{-10}$, 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.
