THE INCIDENCE OF LAMBDA-BOOTIS STARS VIA AN EXTENSION OF THE MK SPECTRAL CLASSIFICATION-SYSTEM TO VERY YOUNG A-TYPE STARS

In this paper we introduce an extension to the MK system of spectral c lassification that allows the precise classification of pre-main-seque nce (PMS) A-type stars, including most Herbig Ae stars. This classific ation scheme characterizes the star by a standard MK type, the presenc e and strength of emission and/or shell lines, and the strength of the Balmer decrement. It can be used to summarize temporal spectral chang es 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 lambda Bootis stars among PMS A-type stars. We have found one definite lambda Bootis star among the Ae stars, and one marginal lambda Bootis star in NGC 2264, yielding st atistics not significantly different from those of the lambda Bootis s tars in the field. This, in addition to other considerations from prev ious studies, leads us to conclude that the lambda Bootis mechanism is operable from very early ages (PMS) to well into the main-sequence li fe (a few times 10(8) yr) of only 2%-3% of A-type stars.