In 1970, Buhl & Snyder
reported a radio line at 89.190 GHZ that they observed in a number of objects, including W3(OH),
Orion, L134, W51, and an ammonia cloud near Sgr A. With no solid spectroscopic data available for
an assignment, they dubbed the carrier X-ogen.
suggested that protonated carbon monoxide, HCO+, might be the carrier, confirmation was
delayed until 1975, when Woods et al.
observed the line in the laboratory. While the subsequent observation of the 13C isotope by
Snyder et al. provided further
evidence, another rotational line of HCO+ was not reported until the 1979 work of
Huggins et al.. HCO+
and two of its isotopomers have also been detected in
HCO+ (also known as the formyl cation) can be formed by the proton
transfer from H3+ to CO and by other pathways. While HCO+ is
more stable, the proton can also be added to the other end of CO to form HOC+, which has also been