THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 85:315-346, 1993 April

 ROTATION AND CHROMOSPHERIC EMISSION AMONG F, G, AND K DWARFS OF THE PLEIADES

                              DAVID R. SODERBLOM
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218

                               JOHN R. STAUFFER
   Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,
                                   MA 02138

                                J. DANIEL HUDON
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218

                                      AND
                                BURTON F. JONES

Lick Observatory, Board of Studies in Astronomy and Astrophysics, University of
                       California, Santa Cruz, CA 95064


                                   ABSTRACT

       We report and discuss high-resolution echelle spectra of more than
    100 F, G, and K dwarfs in the Pleiades.  Comparisons of the profiles of
    Halpha and the Ca II infrared triplet to chromospherically inactive
    field stars allow us to measure chromospheric activity in these stars,
    and we have determined v sin i values to a limit of 7 km/s.  We have
    combined these new observations with published data and have conducted
    a census to assess the state of our knowledge of Pleiades solar-type
    stars.  After allowing for a patch of heavy reddening in the southwest
    part of the cluster, consistent dereddened colors are determined from
    the available photometry and temperatures are derived.
       Rotation is fully resolved in the Pleiades for stars more massive
    than the Sun, but for less massive stars there are still too many upper
    limits to v sin i to determine the underlying, intrinsic distribution
    of rotation.  Over the entire mass range we consider (1.4 >~ M/M(sun)
    >~ 0.6), no single value can characterize rotation at any one mass
    because the spread is too large.  Most G and K dwarfs in the Pleiades
    rotate slowly (v sin i <~ 10 km/s), but ~20% of the stars are ultrafast
    rotators (UFRs) with v sin i >~ 30 km/s.  That fraction of UFRs is
    independent of color, and the highest rotation rates are found among
    the K dwarfs.
       The surfaces of the many slowly rotating solar-type stars in the
    Pleiades are already rotating at or near the rate of stars of the same
    mass in the Hyades.  That means that the dominant aspect of the
    evolution of rotation from the age of the Pleiades to the Hyades is
    convergence on rotation.  Among the F dwarfs, the degree of convergence
    is about a factor of 5, but among the K stars it is at least a factor
    of 20.  We argue that this convergence, as well as the large number of
    slow rotators at 1.0 M(sun), suggests that the convective envelopes and
    radiative cores of these Pleiads are rotationally decoupled, with later
    replenishment of angular momentum to the envelope.
       The Pleiades also shows a broad range in the strength of
    chromospheric emission (CE) at any one color.  CE depends on both mass
    and rotation, but, once the rotation is normalized to an index akin to
    the Rossby number, rotation and activity are well correlated.  Most G
    and K dwarfs in the Pleiades show Halpha and the infrared triplet in
    absorption, with filling in of the line cores.  Some stars show overt
    Halpha emission comparable in strength and profile to the weaker T
    Tauri stars, and a few even show overt emission at the infrared
    triplet.  We have also found three G dwarfs with peculiar Halpha
    profiles whose rotation rates substantially exceed those seen
    previously in the Pleiades among stars of their color.  These stars
    show very broad wings to their Halpha emission profiles, suggesting
    that the emission arises over a broad range of height in the stars'
    atmospheres.

    Subject headings:  open clusters and associations: individual (Pleiades)
                       -- stars: chromospheres -- stars: late-type --
                       stars:rotation