Progressive dispersal of the dense gas in the environment of early-type and late-type Herbig Ae-Be stars

A.Fuente , J.Martin-Pıntado , R.Bachiller , R.Neri , F.Palla
Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800 Alcalá de Henares, Spain
IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Hères, France
Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy

Abstract: We have carried out a systematic study of the environment of 14 Herbig Ae/Be (HAEBE) stars at millimeter wavelenghts. Our data show that there is a progressive dispersal of the dense gas associated with these stars in their evolution to the main sequence. The efficiency of this dispersal 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 radius of 0.08pc decreases by almost two orders of magnitude during their 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 Hillenbrand's infrared (IR) groups. Early-type stars evolve from the Hillenbrand's Group I to Group III in their way to the main sequence, while ``late-type'' stars evolve from Group II to Group I.

Since the morphology of the parent molecular cloud seems to be strongy dependent on the age of the stars, we propose a new classification for both, early-type and 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 have ages yrs. We call Type III stars those that have completely dispersed the surrounding dense gas and are located in a cavity of the molecular cloud. Bipolar outflows are not associated with them and their ages are yrs. Type II stars represent the intermediate case, they are immersed in the molecular cloud but they are not at the peak of a dense clump. The advantage of this new classification is that it allows a simple and easy estimate of the evolutionary stage and age of HAEBE stars.

To appear in Astronomy & Astrophysics.