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 .