2017ApJ...843L..28M

We report the detection of the carbon-chain radical HC₅O for the first time in the interstellar medium toward the cold core TMC-1 using the 100 m Green Bank Telescope. We observe four hyperfine components of this radical in the J=17/2→15/2 rotational transition that originates from the ²Π_1/2 fine structure level of its ground state and calculate an abundance of n/n_H2_=1.7×10^–10, assuming an excitation temperature of T_ex=7 K. No indication of HC₃O, HC₄O, or HC₆O, is found in these or archival observations of the source, while we report tentative evidence for HC₇O. We compare calculated upper limits and the abundance of HC₅O to predictions based on (1) the abundance trend of the analogous HC_nN 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 HC₅O, as well as the upper limits of HC₃O, HC₆O, and HC₇O, but HC₄O is overproduced. The prospects for astronomical detection of both shorter and longer HC_nO chains are discussed.