Large HCO+ and HCN abundances in the translucent cloud toward PKS 0528+134

M.R. Hogerheijde, E.J. de Geus, M. Spaans, H.J. van Langevelde, & E.F. van Dishoeck

1995, The Astrophysical Journal, Letters, vol. 441, p. L93

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

gzipped postscript of paper


| Home page | Publications | Send e-mail |