THE INCIDENCE OF j STARS VIA AN EXTENSION OF THE MK SPECTRAL BOO TIS CLASSIFICATION SYSTEM TO VERY YOUNG A-TYPE STARS

In this paper we introduce an extension to the MK system of spectral classiÐcation that allows the precise classiÐcation of preÈmain-sequence (PMS) A-type stars, including most Herbig Ae stars. This classiÐcation scheme characterizes the star by a standard MK type, the presence and strength of emission and/or shell lines, and the strength of the Balmer decrement. It can be used to summarize temporal spectral changes in PMS A-type stars, and to search for peculiar types. We have used this scheme to classify 38 Herbig Ae stars as well as 22 PMS stars in the young open clusters NGC 2264, NGC 7160, and IC 348. We have also used this extended system to search for j BooŽ tis stars among PMS A-type stars. We have found one deÐnite j BooŽ tis star among the Ae stars, and one marginal j BooŽ tis star in NGC 2264, yielding statistics not signiÐcantly di†erent from those of the j BooŽ tis stars in the Ðeld. This, in addition to other considerations from previous studies, leads us to conclude that the j BooŽ tis mechanism is operable from very early ages (PMS) to well into the main-sequence life (a few times 108 yr) of only 2%È3% of A-type stars.