PROGRESSIVE DISPERSAL OF THE DENSE GAS IN THE ENVIRONMENT OF EARLY-TYPE AND LATE-TYPE HERBIG AE-BE STARS

We have carried out a systematic study of the environment of 14 Herbig Ae/Be (HAEBE) stars at millimeter wavelengths. Our data show that the re is a progressive dispersal of the dense gas associated with these s tars in their evolution to the main sequence. The efficiency of this d ispersal is very different for ''early-type'' (B0-B5) and ''late-type' ' (B5-A5) stars. While in early-type stars the mean gas density in a r adius of 0.08 pc decreases by almost two orders of magnitude during th eir evolution to the main sequence, in late-type stars it decreases by less than an order of magnitude. Because of this different efficiency , there is no correlation between the ages of the stars and the Hillen brands' infrared (IR) groups. Early-type stars evolve from the Hillenb rand's Group I to Group III in their way to the main sequence, while l ate-type stars evolve from Group II to Group I. Since the morphology o f the parent molecular cloud seems to be strongly dependent on the age of the stars, we propose a new classification for both, early-type an d late-type HAEBE stars. We refer as Type I stars to those immersed in a dense clump. These stars are associated with bipolar outflows and h ave ages similar to 10(5) yrs. We call Type III stars those that have completely dispersed the surrounding dense gas and are located in a ca vity of the molecular cloud. Bipolar outflows are not associated with them and their ages are > 10(6) yrs. Type II stars represent the inter mediate case, they are immersed in the molecular cloud but they are no t at the peak of a dense clump. The advantage of this new classificati on is that it allows a simple and easy estimate of the evolutionary st age and age of HAEBE stars.