78 research outputs found
The Distribution of Stellar Mass in the Pleiades
As part of an effort to understand the origin of open clusters, we present a
statistical analysis of the currently observed Pleiades. Starting with a
photometric catalog of the cluster, we employ a maximum likelihood technique to
determine the mass distribution of its members, including single stars and both
components of binary systems. We find that the overall binary fraction for
unresolved pairs is 68%. Extrapolating to include resolved systems, this
fraction climbs to about 76%, significantly higher than the accepted field-star
result. Both figures are sensitive to the cluster age, for which we have used
the currently favored value of 125 Myr. The primary and secondary masses within
binaries are correlated, in the sense that their ratios are closer to unity
than under the hypothesis of random pairing. We map out the spatial variation
of the cluster's projected and three-dimensional mass and number densities.
Finally, we revisit the issue of mass segregation in the Pleiades. We find
unambiguous evidence of segregation, and introduce a new method for quantifying
it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa
The Dynamical Evolution of the Pleiades
We present the results of a numerical simulation of the history and future
development of the Pleiades. This study builds on our previous one that
established statistically the present-day structure of this system. Our
simulation begins just after molecular cloud gas has been expelled by the
embedded stars. We then follow, using an N body code, the stellar dynamical
evolution of the cluster to the present and beyond. Our initial state is that
which evolves, over the 125 Myr age of the cluster, to a configuration most
closely matching the current one.
We find that the original cluster, newly stripped of gas, already had a
virial radius of 4 pc. This configuration was larger than most observed,
embedded clusters. Over time, the cluster expanded further and the central
surface density fell by about a factor of two. We attribute both effects to the
liberation of energy from tightening binaries of short period. Indeed, the
original binary fraction was close to unity. The ancient Pleiades also had
significant mass segregation, which persists in the cluster today.
In the future, the central density of the Pleiades will continue to fall. For
the first few hundred Myr, the cluster as a whole will expand because of
dynamical heating by binaries. The expansion process is aided by mass loss
through stellar evolution, which weakens the system's gravitational binding. At
later times, the Galactic tidal field begins to heavily deplete the cluster
mass. It is believed that most open clusters are eventually destroyed by close
passage of a giant molecular cloud. Barring that eventuality, the density
falloff will continue for as long as 1 Gyr, by which time most of the cluster
mass will have been tidally stripped away by the Galactic field.Comment: 45 pages, 13 figures, 2 tables; Accepted for publication in MNRA
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