A 20-year follow-up of CH+, CH, and CN absorption lines. I. AE Aur and zeta Per sightlines

The runaway star AE Aur (HD 34078) is currently crossing the molecular cloud associated with the IC 405 nebula. Previous observations have revealed large time variations of CH+$ and CH absorption lines in AE Aur spectra, as the line of sight drifts through the foreground gas at a rate of about 20 au/year. We wish to extend the study of these changes over a longer interval, 2003--2023, and determine their properties (e.g. amplitude, timescales) with greater accuracy to obtain constraints on the processes involved in the production of CH$^+ and CH. We have obtained new high resolution spectra for AE Aur and for ζ Per, a bright reddened star that we used as a reference to check the instrumental stability. We focused not only on the strength of the lines (equivalent widths) but also on line widths, line centroids, and full velocity distributions in order to more thoroughly characterise the time variations of absorption lines. We also include CN in our analysis of variations in addition to CH+ and CH. The large and complex CH+$ and CH column density variations (by up to -35 and -20% respectively, in our measurements) on scales ranging from a few au to about 400 au are confirmed and appear consistent with those observed by other authors. A significant decrease of CH^+ and CH line widths (by -19 and -13%) was also detected, together with a change in the CH^+ centroid velocity of ≈ -1 $ ̊m km s ^ -1 $. Finally, wings extending ± 12 $ ̊m km s ^ -1 $ around CH$^+ and CH line centre are clearly present. We show that most of the H_2 gas towards AE Aur (about 70%) is associated with two dense globulettes previously imaged in CO and located close to the star. Despite the large amount of highly excited H_2, CH+$ production in the photodissociation region (PDR) facing the star is far too small to account for the observed CH$^+ column density. In line with a recently published model, we propose that CH+$ is produced in a thin layer at the interface between a dense photoevaporating globulette and the ambient warm neutral medium (WNM) gas. The observed time variations were used to set constraints on the thickness of this CH^+-rich shell (≈ 7 pc) and on its density structure (sharp decrease of n(CH$^+) outwards). Such a picture appears to provide a simple and coherent description of both the CH+ and CH line profiles and their time variations towards AE Aur. 10^ -3