HIPPARCOS PHOTOMETRY OF HERBIG AE BE STARS

The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe) candi date stars was studied using a uniform set of optical photometry obtai ned by the Hipparcos mission. Astrophysical parameters (distance, temp erature, luminosity, mass, age) of this sample of stars were derived a s well by combining the astrometric data provided by Hipparcos with da ta from literature. Our main conclusions can be summarized as follows: (1) More than 65% of all HAeBe stars show photometric variations with an amplitude larger than 0.(m)05; (2) HAeBes with a spectral type ear lier than AO only show moderate (amplitude < 0.(m)5) variations, where as those of later spectral type can (but not necessarily have to) show variations of more than 2.(m)5. We explain this behaviour as being du e to the fact that stars with lower masses become optically visible, a nd hence recognizable as Herbig Ae stars, while still contracting towa rds the zero-age main sequence (ZAMS), whereas their more massive coun terparts only become optically visible after having reached the ZAMS; (3) The Herbig stars with the smallest infrared excesses do not show l arge photometric variations. This can be understood by identifying the stars with lower infrared excesses with the more evolved objects in o ur sample; (4) No correlation between the level of photometric variabi lity and the stellar v sin i could be found. If the large photometric variations are due to variable amounts of extinction by dust clouds in the equatorial plane of the system, the evolutionary effects probably disturb the expected correlation between the two.