LARGE HCO-0528+134( AND HCN ABUNDANCES IN THE TRANSLUCENT CLOUD TOWARD PKS)

Observations of (CO)-C-12 J = 1-0, (CO)-C-13 1-0, HCO+ 1-0, HCN 1-0, a nd CS 2-1 absorption against the quasar PKS 0528 + 134 obtained with t he Owens Valley Millimeter Array are presented. The line of sight to t his standard phase calibrator crosses the diffuse outer edge of the da rk cloud B30, located in the lambda Ori ring of clouds. An excitation analysis of the absorption lines and (CO)-C-12 1-0 emission yields an estimated H-2 number density of n(H2) = 100-500 cm(-3), which is much lower than found for lines of sight studied previously by millimeter a bsorption lines. Surprisingly, HCO+ and HCN are found to have abundanc es relative to (CO)-C-12 in this diffuse cloud which are comparable to or larger than those found in dark clouds. Significant differences in line width are found between (CO)-C-12, HCN, and HCO+ which cannot be attributed to line saturation. A possible explanation for the observe d abundances and line widths involving cloud turbulence and the chemis try of CH+ is proposed. It is concluded that HCO+ absorption lines sam ple low-density regions which cannot be observed in molecular emission nor in CO millimeter absorption.