Death of a cluster: the destruction of M67 as seen by the SDSS

TL;DR

This study maps the spatial and dynamical structure of the old open cluster M67 using SDSS data, revealing an extended halo, a larger core radius, and evidence of mass segregation influenced by the galactic tidal field.

Contribution

It introduces a matched filter algorithm applied to SDSS and 2MASS data to analyze M67's structure, binary fraction, and mass segregation, providing new insights into its spatial extent and stellar distribution.

Findings

Extended halo of M67 up to 60' radius

Core radius measured at 8.24 arcminutes, larger than previous estimates

Binary fraction in M67 is at least 45%

Abstract

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.