THE ASTRONOMICAL JOURNAL
SEPTEMBER 1993
VOLUME 106, NUMBER 3, PAGE 1059
THE EVOLUTION OF THE LITHIUM ABUNDANCES OF SOLAR-TYPE STARS. III. THE
PLEIADES
DAVID R. SODERBLOM
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
Maryland 21218
Electronic mail: soderblom@stsci.edu
BURTON F. JONES
Lick Observatory, Board of Studies in Astronomy and Astrophysics,
University of
California, Santa Cruz, California 95064
Electronic mail: jones@helios.ucsc.edu
SUCHITRA BALACHANDRAN
Department of Physics and Astronomy, University of North Carloina, Chapel
Hill,
North Carolina 27599-3255
Electronic mail: suchitra@sloth.astro.unc.edu
JOHN R. STAUFFER
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge,
Massachusetts 02138
Electronic mail: stauffer@cfa.harvard.edu
DOUGLAS K. DUNCAN
Department of Astronomy and Astrophysics, University of Chicago, 5640
Ellis
Ave., Chicago, Illinois 60637
Electronic mail: duncan@oddjob.uchicago.edu
STEPHEN B. FEDELE
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
Maryland 21218
J. DANIEL HUDON
Department of Astronomy, University of Toronto, Toronto, Ontario M5S 1A7
Canada
ABSTRACT
We report new measurements of lithium in more than 100 Pleiades F, G,
and K dwarfs.
Abundances were determined from spectrum synthesis
fits
to the data as well as from use of new curves of growth for the Li
6708 A
feature (presented in an Appendix). We confirm the intrinsic spread
in
lithium abundance within the Pleiades seen by Duncan & Jones [ApJ,
271,
663 (1983)], but we establish more observational constraints on Li in
this cluster: First, for stars near 1.0 M(sun) [about 0.60 to 0.75 in
(B-V)o], the scatter in the relation between log N(Li) (defined as
N(Li)) and T(eff) is consistent with our observational uncertainty.
That means that most late-F and early-G dwarfs in the Pleiades are
consistent with the tight N(Li) versus mass relation seen in the
Hyades
in the same mass range. Second, at (B-V)o approx. 0.8 (M approx. 0.9
M(sun)), large and real star-to-star differences in N(Li) appear.
The
range in N(Li) at (B-V)o approx. 0.8 is about 1 dex, and grows to as
much as 1.5 dex for less massive stars. Third, the most Li-rich
stars
have abundances at or near the primordial level for Population I
(N(Li)
approx. 3.2), and none exceed that level by a significant amount.
Fourth, at any given color the stars that rotate fastest have the
most
Li and have the strongest chromospheric activity. We consider the
ways
in which an apparent spread in N(Li) could arise from an
intrinsically
tight N(Li)-mass relation and conclude that the spread is probably
real
and is not an artifact of line formation conditions or inhomogeneous
atmospheres on the stars. It is possible to produce large apparent
changes in N(Li) by covering a significant fraction of a star's
surface
with cooler regions ("spots"), but doing so has other ramifications
that conflict with the observations. Some current models lead to a
spread in N(Li) in which the fastest rotators (those that have lost
the
least angular momentum) have the most Li, and that mechanism may
account for what is seen. A comparison of the Pleiades to the Alpha
Persei cluster shows that most Alpha Persei stars have Li abundances
comparable to their Pleiades counterparts, but there is a significant
fraction (about 30%) of Alpha Persei stars that lie below the
Pleiades
in N(Li) by 1 dex or more. Some of these anomolous stars have even
less Li than Hyades stars of the same T(eff). If these stars are
bona
fide Alpha Persei members (and they probably are), their Li
abundances
strain our understanding of Li depletion. The Pleiades, considered
together with Alpha Persei and the Hyades, shows that stars with
[Fe/H]
>= 0.0 and which are more massive than about 1.25 M(sun) do not
deplete
Li prior to reaching main the sequence. Moreover, solar-abundance
stars
([Fe/H] approx. 0.0) with M >~ 1.1 M(sun) do not experience
pre-main-sequence depletion either. Pleiades dwarfs near T(eff) =
6700 K
show evidence of being depleted in Li, indicating that an incipient
Li
"chasm" is present even at an age of 70 Myr.