Death of a cluster: the destruction of M67 as seen by the SDSS
  We probe the spatial and dynamical structure of the old open cluster M67
using photometric data from the Sloan Digital Sky Survey's sixth data release.
Making use of an optimal contrast, or matched filter, algorithm, we map the
distribution of high probability members of M67. We find an extended and
elongated halo of likely members to a radius of nearly 60'. Our measured core
radius of Rcore = 8.'24+/-0.'60 is somewhat larger than that of previous
estimates. We attribute the larger core radius measurement to the SDSS probing
lower mass main sequence stars than has been done before for similar studies of
M67, and the exclusion of post main sequence M67 members in the SDSS sample. We
estimate the number of M67 members in our SDSS sample to be 1385+/-67 stars. A
lower limit on the binary fraction in M67 is measured to be 45%. A higher
fraction of binary stars is measured in the core as compared to the halo, and
the luminosity function of the core is found to be more depleted of low-mass
stars. Thus the halo is consistent with mass segregation within the cluster.
The galactic orbit of M67 is calculated from recent proper motion and radial
velocity determinations. The elongated halo is roughly aligned to the proper
motion of the cluster. This appears to be a result of mass segregation due to
the galactic tidal field. Our algorithm is run on 2MASS photometry to directly
compare to previous studies of M67. Decreasing core radii are found for stars
with greater masses. We test the accuracy of our algorithm using 1000
artificial cluster Monte Carlo simulations. It is found that the matched filter
technique is suitable for recovering low-density spatial structures, as well as
measuring the binary fraction of the cluster.
