The HCO(+)/HOC(+) abundance ratio in molecular clouds

Abstract

Using the recently determined rest frequency of its first rotational transition, a sensitive search has been made for the J = 1 yields 0 transition of HOC(+) in 14 interstellar sources. A line having a frequency and shape consistent with optically thin emission due to this species was detected toward Sgr B2; this spectral feature is provisionally identified as being due to HOC(+). Comparison with the 1 yields 0 transition of H(C-18)O(+) in this source yields an isomeric abundance ratio HCO(+)/HOC(+) of about 330, if the detected line is in fact due to HOC(+). Independent of, and consistent with, this conclusion, comparisons with isotopically substituted HCO(+) in the other clouds searched show that HCO(+) is at least two orders of magnitude more abundant than HOC(+). This large ratio places interesting constraints on the fractional abundance of related species.