We have carried out a study of the early type stars in nearby OB associations spanning an age range of \sim 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 ( \leq 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 \mu m 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 near-infrared 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 _ { \sun } ) ; in particular , it is a factor of \sim 10 lower at \sim 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 ) .