The 4D-structure of a dark cloud, B164 under the influence of a bright star, azelfafage

Abstract

We present the 3D image and temporal evolution of a remarkable dark cloud: B164. This large Bok globule (Bok & Cordwell 1973) possesses a seldom shared property: it rotates. The rotation curve presents a clear ‘S’ shape but this cloud is unique today because a third of the rotation curve is missing. Indeed all the observations show that the cloud ends up abruptly on one side suggesting that a part of the cloud is missing. Taking the present rotational axis as the original symmetry axis we can deduce some important information such as the original size and mass of the cloud and an upper limit on the time at which the perturbation took place. The rotation curve also allows to give a 3D picture of the cloud which we explore with a Monte-Carlo model: the closest solution indicates a large envelope of increasing density from 100 cm⁻³ to 1000 cm⁻³ and a relatively small core of density of the order of 5000 cm⁻³. CO isotopomers have a relatively low abundance in this model with the ¹³CO abundance peaking at 2–3×10⁻⁶ for Av=0.5 to 1 mag. and C¹⁸O being one to two orders of magnitude less abundant, as a function of depth. Next, we show that if Azelfafage (HD 206672), a Be3-III star is at the same distance from us and thus only at 3 pc from the cloud, it could be the progenitor of the observed perturbation. Using a chemico- hydro-dynamical model (CHD) (Breart de Boisanger et al. 1992) we show that the most probable explanation is that the star has just arrived and that its main action is to compress the cloud via the heating of the diffuse interstellar medium and/or the increase in radiation pressure rather than by photodissociating the molecular gas. Preliminary HI data and ¹²CO observations show some evidence that the general contraction of the molecular envelope would be due to a direct pressure of the HI halo. It would seem that under the action of the star the whole molecular cloud is moving backwards and is getting compressed against its HI halo. The HD model to describe this effect is being currently developed and also more HI observations are underway to confirm this idea.

A detailed version of this poster has been submitted to A&A.