2013A&A...559A.131B

Spatial structure in molecular material has a strong impact on its physical and chemical evolution and is still poorly known, especially on very small scales. To better characterize the small-scale structure in diffuse molecular gas and in particular to investigate the CH⁺ production mechanism, we study the spatial distribution of CH⁺, CH, and CN towards the bright star ζPer on scales in the range 1-20AU. We use ζPer's proper motion and the implied drift of the line of sight through the foreground gas at a rate of about 2AU/yr to probe absorption line variations between adjacent lines of sight. The good S/N, high or intermediate resolution spectra of ζPer, obtained in the interval 2003-2011, allow us to search for low column-density and line width variations for CH⁺, CH, and CN. CH and CN lines appear remarkably stable in time, implying an upper limit δN/N≤6 % for CH and CN (3σ limit). The weak CH⁺ λ4232 line shows a possible increase of 11% during the interval 2004-2007, which appears to be correlated with a comparable increase in the CH⁺ velocity dispersion over the same period. The excellent stability of CH and CN lines implies that these species are distributed uniformly to good accuracy within the cloud. The small size implied for the regions associated with the CH⁺ excess is consistent with scenarios in which this species is produced in very small (a few AU) localized active regions, possibly weakly magnetized shocks or turbulent vortices.