The molecular condensations ahead of Herbig-Haro objects

II. A theoretical investigation of the HH 2 condensation

¹ Department of Physics and Astronomy, UCL, Gower St., London, WC1E 6BT, UK
² CNR-Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ Department d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Catalunya, Spain

Corresponding author: S. Viti, sv@star.ucl.ac.uk

Received: 22 October 2002
Accepted: 22 November 2002

Abstract

Clumps of enhanced molecular emission are present close to a number of Herbig-Haro (HH) objects. These enhancements may be the consequence of an active photochemistry driven by the UV radiation originating from the shock front of the HH object. On the basis of this picture and as a follow up to a molecular line survey toward the quiescent molecular clump ahead of the HH object, HH 2 (Girart et al. [CITE]), we present a detailed time and depth dependent chemical model of the observed clump. Despite several difficulties in matching the observations, we constrain some of the physical and chemical parameters of the clump ahead of HH 2. In particular, we find that the clump is best described by more than one density component with a peak density of cm^-3 and a visual extinction of ≤3.5 mag; its lifetime can not be much higher than 100 years and the impinging radiation is enhanced with respect to the ambient one by probably no more than 3 orders of magnitude. Our models also indicate that carbon-bearing species should not completely hydrogenate as methane when freezing out on grains during the formation of the clump.