A SURVEY OF C4H, C6H, AND C6H–WITH THE GREEN BANK TELESCOPE

Abstract

A survey of the carbon chain negative ion C6H− and the structurally similar radicals C4H and C6H has been done with the 100 m Green Bank Telescope toward 24 galactic molecular sources. The most readily observed molecular anion to date, C6H− was detected in two new sources: the dark clouds L1544 and L1521F, where it was found to be present at the level of 2.5% ± 0.8% and 4% ± 1%, respectively relative to neutral C6H. C4H was detected in nearly all dark clouds surveyed (in six for the first time), and C6H was detected in five dark clouds and one translucent cloud. The observed C6H−/C6H (1%–4%) and C6H/C4H (0.2%–1%) ratios in these sources suggest that C6H− may be close to detection in many other dark clouds. The fractional ionization derived from the C6H−/C6H ratio in L1544 and L1521F of 10−8 to 10−7 is comparable to earlier estimates from positive molecular ions.