Issue |
A&A
Volume 386, Number 1, April IV 2002
|
|
---|---|---|
Page(s) | 187 - 203 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20020183 | |
Published online | 15 April 2002 |
Critical tests of stellar evolution in open clusters*,**
III. Stellar population and dynamical evolution of IC 4651
1
Astronomical Observatory, Niels Bohr Institute for Astronomy, Physics, and Geophysics, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: ja@astro.ku.dk
2
Astronomy Department, University of Wisconsin, Madison, WI 53706, USA
3
Lund Observatory, PO Box 43, 221 00 Lund, Sweden e-mail: birgitta@astro.lu.se
Corresponding author: S. Meibom, meibom@astro.wisc.edu
Received:
28
November
2001
Accepted:
30
January
2002
We present multiple-epoch radial-velocity observations for 104 stars
in a field of the intermediate-age open
cluster IC 4651 to
. Only 13 stars (13%) of the full sample
are field stars. From the 44 single member stars we find a mean radial
velocity of -1, and
the 12 single red-giant members yield a true radial-velocity dispersion
of -1. Of the 19 giant members, 7 (37%) are
spectroscopic binaries with periods up to 5000 days, while 35 (52%) of
the 67 main-sequence and turnoff members are binaries with periods less
than ~1000 days. Combined with our deep, accurate CCD Strömgren
photometry in a ~
field of IC 4651
[CITE], these data substantially improve the definition of the
cluster locus in the colour-magnitude diagram and the spatial structure
of the cluster, although the photometry shows that IC 4651 contains at
least twice as many stars on the upper main sequence as was believed when
the radial-velocity survey was initiated.
The single cluster members define a very tight sequence in the CMD, and two
sets of isochrones from stellar models with convective overshooting (
= 0.2) have been fit to it. Our best estimate for the age of IC 4651 is
Gyr, assuming
(Hyades) and
.
Including the ~650 stars newly discovered from the photometry,
we estimate the present total mass of IC 4651 to be ~
,
excluding any undetected stellar remnants. The corresponding tidal cutoff
radius is ~
. IC 4651 shows evidence of moderate mass
segregation: Most of the turn-off stars and nearly all the red giants are
located at radii smaller than ~
, while the lower main-sequence
stars are less centrally concentrated. The spatial distributions of cluster
and field stars indicate that additional cluster stars are probably still
to be found outside the fields studied so far.
Comparison of the present mass function of IC 4651 with plausible initial
mass functions indicates that the cluster initially contained at least
~8300 stars with a total mass of ~
. Thus, of the
original cluster stars only ~7%, containing ~12% of the initial
mass, remain today. Of the initial cluster mass, ~35% has been lost due
to evolution of the most massive stars into white dwarfs or other remnants
while the remaining ~53%, comprising ~93% of the original
low-mass stars, appear to have migrated out of the observed field or been
lost from the cluster altogether. IC 4651 is currently 1 kpc closer to the
Galactic center than its “sister” cluster NGC 3680 [CITE],
but their Galactic orbital parameters indicate that the mean orbital radius
of IC 4651 is in fact larger by 0.7 kpc, providing a plausible reason why
it is much less advanced in its dynamical evolution than the coeval cluster
NGC 3680.
Key words: open clusters and associations: IC 4651 / stars: HR diagram / stars: evolution / stars: kinematics / stars: fundamental parameters
© ESO, 2002
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