Herbig Ae/Be Stars in nearby OB associations

We have carried out a study of the early type stars in nearby OB associations spanning an age range of ∼ 3 to 16 Myr, with the aim of determining the fraction of stars which belong to the Herbig Ae/Be class. We studied the B, A, and F stars in the nearby (≤ 500 pc) OB associations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1, with membership determined from Hipparcos data. We also included in our study the early stars in the Trumpler 37 cluster, part of the Cep OB2 association. We obtained spectra for 440 Hipparcos stars in these associations, from which we determined accurate spectral types, visual extinctions, effective temperatures, luminosities and masses, using Hipparcos photometry. Using colors corrected for reddening, we find that the Herbig Ae/Be stars and the Classical Be stars (CBe) occupy clearly different regions in the JHK diagram. Thus, we use the location on the JHK diagram, as well as the presence of emission lines and of strong 12 μm flux relative to the visual to identify the Herbig Ae/Be stars in the associations. We find that the Herbig Ae/Be stars constitute a small fraction of the early type stellar population even in the younger associations. Comparing the data from associations with different ages and assuming that the nearinfrared excess in the Herbig Ae/Be stars arises from optically thick dusty inner disks, we determined the evolution of the inner disk frequency with age. We find that the inner disk frequency in the age range 3 10 Myr in intermediate mass stars is lower than that in the low mass stars (< 1 M⊙); in particular, it is a factor of ∼ 10 lower at ∼ 3 Myr. This indicates that the time-scales for disk evolution are much shorter in the intermediate mass stars, which could be a consequence of more efficient mechanisms of inner disk dispersal (viscous evolution, dust growth and settling toward the midplane). Subject headings: stars: emission-line — stars: pre–main-sequence — stars: HertzsprungRussell diagram — open clusters and associations: general Centro de Investigaciones de Astronomı́a (CIDA), Apartado Postal 264, Mérida 5101-A, Venezuela; Electronic mail: [email protected], [email protected] Postgrado de F́ısica Fundamental, Universidad de Los Andes (ULA), Mérida 5101-A, Venezuela Visiting Student, Harvard-Smithsonian Center for Astrophysics Harvard-Smithsonian Center for Astrophysics, 60 Cambridge, MA 02138, USA, Electronic mail: [email protected], [email protected],[email protected]