Detection of the Carbon Chain Negative Ion C ₈ H ^- in TMC-1

Abstract

The negative molecular ion C8H- has been detected in the Galactic molecular source TMC-1. Four rotational transitions have been observed in the centimeter-wave band with the NRAO 100 m Green Bank Telescope (GBT) at precisely the frequencies calculated from the recent laboratory spectroscopy of this large carbon chain anion. C8H- is about 5% as abundant as C8H, or somewhat more than C6H- relative to C6H (1.6%). Improved values of the column densities of C6H- and C6H, and an upper limit for the abundance of the smaller carbon chain C4H- of 0.014% with respect to C4H, have also been determined.