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New observations of M4 have studied the binary stars in this cluster.
globular cluster is a roughly spherical ensemble of stars, as many as
several million of them, gravitationally bound together in groups whose
diameters can be as small as only tens of light-years. To sense the
dramatic implications of this dense packing, consider that the nearest
star to the Sun, Proxima Centauri, is about four light-years away.
Messier 4 (M4) is the closest globular cluster to Earth at a distance of
about six thousand light-years, and a puzzle to astronomers. Normal
gravitational effects should, over time, redistribute the stars in a
globular cluster until they are more numerous towards the center, but
while M4 shows a central concentration of stars it does not show
evidence for a steep central cusp even though astronomers think enough
time has passed.
To understand what is going on in this globular cluster, and to help
understand how these clusters evolve in general, CfA astronomer Maureen
van den Berg and her collaborators have undertaken a large and
unprecedented set of deep images of M4 with the Hubble Space Telescope
to look for binary stars, that is stars with companions. The dynamical
interactions between the densely crowded stars in a globular cluster
should disrupt many such binaries, but for reasons that are not
understood about fifteen percent of the stars in M4 are binaries, at
least based on monitoring brightness variations (a more typical number
is two percent). Whether or not this unusual abundance is connected to
the lack of a central cusp in stellar density is also not understood.
The astronomers set out to use Hubble to study the binary star
population in M4 looking at both brightness variations and stellar
wobble (astrometric) variations, in particular due to binaries with a
massive, faint, and evolved companion like a white dwarf or neutron
star. The team was able to find and characterize a much more complete
set of binaries, including thirty-six new variables. They note in
passing that, as part of the search process, any stars with massive "hot
Jupiter" exoplanet companions would probably also have been detected,
but that none were. The extensive results are still being analyzed, but
the improved statistics will make the conclusions much more reliable.
M 4 Core Project with HST – III. Search for Variable Stars in the
Primary Field," V. Nascimbeni, L. R. Bedin, D. C. Heggie, M. van den
Berg, M. Giersz, G. Piotto, K. Brogaard, A. Bellini, A. P. Milone, R. M.
Rich, D. Pooley, J. Anderson,, L. Ubeda, S. Ortolani, L. Malavolta, A.
Cunial1, and A. Pietrinferni, MNRAS 442, 2381, 2014