The history of mass dispersal around Herbig Ae/Be stars

We present a systematic study of the material surrounding intermediate-mass stars. Our sample includes 34 Herbig Ae/Be (HAEBE) stars of different ages and luminosities. This is a quite complete representation of the whole class of HAEBE stars and consequently, our conclusions should have a solid statistical meaning. In addition, we have observed 2 intermediate-mass protostars and included published data on 15 protostellar objects in order to determine the evolution of the circumstellar material in the early stages of stellar evolution. All the HAEBE stars have been classified according with the three Types already defined in Fuente et al. (1998): Type I stars are immersed in a dense clump and have associated bipolar outflows, their ages are similar to0.1 Myr; Type II stars are still immersed in the molecular cloud though not in a dense clump, their ages are between similar toa few 0.1 to similar toa few Myr; Type III stars have completely dispersed the surrounding material and are located in a cavity of the molecular cloud, their ages are >1 Myr. Our observations are used to reconstruct the evolution of the circumstellar material around intermediate-mass stars and investigate the mass dispersal mechanisms at the different stages of the stellar evolution. Our results can be summarized as follows: intermediate-mass stars disperse greater than or equal to90% of the mass of the parent clump during the protostellar phase. During this phase, the energetic outflows sweep out the gas and dust forming a biconical cavity while the equatorial material is infalling to feed the circumstellar disk and eventually the protostar. In this way, the density structure of the parent clump remains well described by a density law n proportional to r(beta) with -2 < beta < -1 although a large fraction of the mass is dispersed. In similar toa few 0.1 Myr, the star becomes visible and the outflow fades. Some material is dispersed from similar toa few 0.1 to greater than or equal to1 Myr. Since the outflow declines and the stars are still too cold to generate UV photons, stellar winds are expected to be the only dispersal mechanism at work. In 1 Myr an early-type star (B0-B5) and in greater than or equal to1 to 10 Myr a late-type star (later than B6) meets the ZAMS. Now the star is hot enough to produce UV photons and starts excavating the molecular cloud. Significant differences exist between early-type and late-type stars at this evolutionary stage. Only early-type stars are able to create large (R > 0.08 pc) cavities in the molecular cloud, producing a dramatic change in the morphology of the region. This difference is easily understood if photodissociation plays an important role in the mass dispersal around these objects.