THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 99:135-172, 1995 July

     THE RELATION BETWEEN ROTATIONAL VELOCITIES AND SPECTRAL PECULIARITIES
                              AMONG A-TYPE STARS

                                 HELMUT A. ABT
    Kitt Peak National Observatory, NOAO, Box 26732, Tucson, AZ 85726-6732;
                                 apj@noao.edu

                               NIDIA I. MORRELL
     Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional
                       de La Plata, La Plata, Argentina


                                   ABSTRACT

        We obtained new data to determine whether the spectral appearance
     of A-type stars is entirely determined by their rotational velocities.
     For this purpose we derived rotational velocities for 1700 northern
     A-type stars from CCD coude spectra, calibrated with the new Slettebak
     et al. system, and new MK classifications based on wide photographic
     Cassegrain spectra for 2000 northern and some southern stars in the
     Bright Star Catalogue.  In addition we determined the equivalent
     widths of the lambda 4481 Mg II lines in the coude spectra.  Tables
     and graphs show variations of rotational velocities and lambda 4481
     line strengths as functions of type and luminosity, and frequencies of
     the normal and abnormal stars.
        After deconvolutions of the rotational velocities, assuming random
     orientations of rotational axes, we find that all rapid rotators have
     normal spectra and nearly all slow rotators have abnormal spectra (Ap
     or Am).  Those abnormalities are generally attributed to diffusion and
     can occur only with little rotational mixing.  However at all types
     there are overlaps of these distributions, implying that a given
     intermediate rotational velocity is insufficient to determine whether
     the star should have a normal or abnormal spectrum.  However, we
     realized that (1) some of our "standards," such as Vega and Alpha Dra,
     are really abnormal, causing us to classify similar peculiar stars as
     "normal," (2) many of the "normal" stars near A2 IV have the
     characteristics of peculiar stars such as low rotational velocities
     and weak 4481 Mg II and K lines, and (3) the mean rotational
     velocities of "normal" stars are depressed just at those types where
     the Ap and Am stars are most frequent.  Therefore we conclude that the
     overlaps are due to our failure to detect all the abnormal stars and
     that a specific rotational velocity is probably enough to determine
     whether a star will have a normal or abnormal spectrum.

     Subject headings: stars: chemically peculiar -- stars: early-type --
                       stars: fundamental parameters -- stars: rotation