The au-scale structure in diffuse molecular gas towards ζ Persei^⋆

¹ Institut d’Astrophysique de Paris (IAP), UMR7095 CNRS, Université Pierre et Marie Curie – Paris 6, 98bis boulevard Arago, 75014 Paris, France
e-mail: boisse@iap.fr
² Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA
³ Institut d’Astrophysique Spatiale (IAS), UMR 8617, CNRS, Bâtiment 121, Université Paris Sud 11, 91405 Orsay, France
⁴ LERMA (UMR 8112 du CNRS), Observatoire de Paris, 61 Avenue de l’Observatoire, 75014 Paris, France
⁵ Department of Astronomy, University of Washington, Seattle, WA 98195, USA

Received: 29 August 2013
Accepted: 9 October 2013

Abstract

Context. 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.

Aims. 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 ζ Per on scales in the range 1−20 AU.

Methods. We use ζ Per’s proper motion and the implied drift of the line of sight through the foreground gas at a rate of about 2 AU yr^-1 to probe absorption line variations between adjacent lines of sight. The good S/N, high or intermediate resolution spectra of ζ Per, obtained in the interval 2003−2011, allow us to search for low column-density and line width variations for CH⁺, CH, and CN.

Results. 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.

Conclusions. 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.