Interstellar CN toward CH-forming regions

¹ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: gredel@caha.es
² IAS, Université de Paris-Sud, Bât. 121, 92405 Orsay Cedex, France e-mail: forets@obspm.fr
³ Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA e-mail: sfederm@uoft02.utoledo.edu

Corresponding author: R. Gredel, gredel@mpia.de

Received: 22 January 2002
Accepted: 8 April 2002

Abstract

Measurements on interstellar CN absorption are presented for  stars in three southern OB associations, NGC  2439, Vela OB1, and Cen OB1. CN is detected in 21 out of 31 stars observed. The doublet ratio for the R(1) and P(1) lines of the (0, 0) band of the B –X  violet system and a comparison of violet system data with measurements of the (1, 0) and (2, 0) bands of the A –X  red system are used to derive Doppler parameters and total column densities. Inferred CN column densities vary by more than an order of magnitude for lines of sight with similar CH column densities. Observations of the (0, 0) band of the CH B –X  system are used to revise previously published CH column densities toward the lines of sight studied in CN. Together with earlier results on CH, CH⁺, and C₂, the CN data presented here provide a homogeneous set of column densities and radial velocities of diatomic molecules in three individual translucent clouds. We use these data to study CN production via chemical models. Gas densities are inferred from models based on production via CH and C₂ in cool gas. Most sightlines in our sample test densities typical for diffuse molecular gas (a few hundred cm^-3) when the ultraviolet flux permeating the gas is between 1 and 5 times the average interstellar flux. A few lines of sight indicate that CN is produced under dark cloud conditions because relatively large densities are obtained or because this simple chemical scheme is unable to reproduce the observed CN columns. Low densities are indicated for directions with upper limits on CN. We add an ad hoc component of a number of low-velocity (<10 km s^-1) criss-crossing MHD shocks to explain observed column densities of interstellar CH⁺. These shocks also produce about 10 to 30% of the total CH column along the line of sight.