Star clusters are superb astrophysical laboratories containing co-spatial and coeval samples of stars with similar composition. Open clusters are particularly valuable as they span a wide range of age, metallicity, richness, and galactic radius. As such open clusters are the observational foundation for stellar astrophysics, provide essential tracers of galactic structure and evolution, and are unique stellar dynamical environments. Indeed there are few fields in astrophysics that do not in some way rely on results derived from open cluster studies.

Recent advances in instrumentation are driving a renaissance in the study of open clusters. Members of the WIYN collaboration have initiated the WIYN Open Cluster Study, or ”WOCS”, a project dedicated to comprehensive photometric, astrometric, and spectroscopic studies of a select set of open clusters spanning the range of age and metallicity. The goals of WOCS are two:

Comprehensive and definitive photometric, spectroscopic, and astrometric databases for new fundamental clusters.

A handful of thoroughly studied open clusters – for example, the Hyades, the Pleiades, NGC 752, and M67 – have repeatedly provided the foundation for numerous fields of stellar astrophysics. The WIYN telescope furnishes an opportunity to expand this set of fundamental clusters, and in particular to provide extensive spectroscopic analyses of superb clusters at larger apparent distance moduli. A goal of WOCS is to expand by a factor of several the ranks of fundamental clusters which provide the foundation of stellar astrophysics.

A body of investigations which address critical astrophysical problems through study of open clusters.

Subjects under active investigation within WOCS include: detailed testing of core convective overshoot and implications for stellar lifetimes; photometric monitoring of periods for study of angular momentum evolution; delineation of faint main sequences to test stellar evolution theory of very low mass stars; discovery of white dwarf sequences as independent dating mechanisms; Fe, CNO and Li abundance analyses for studies of internal stellar processes (mixing, diffusion, mass loss, etc), Galactic chemical evolution, and primordial abundances; binary populations; stellar evolution in close binary environments; initial and present-day mass functions; and stellar dynamics in clusters with rich binary populations.

WOCS looks specifically at the three projects below.

Blue Stragglers

The classical fields of stellar evolution and stellar dynamics each have relied heavily on the stars of open clusters. Yet for most of their histories the fields have developed largely independently. Today, investigations at the interface of stellar evolution and stellar dynamics are expanding our understanding of both fields. Stellar dynamicists have long recognized that encounters involving binary stars supply the inevitable energy flow out of the cores of star clusters.  These complex dynamical dances lead to stellar exchanges, binary orbit evolution, close stellar passages, mass transfer, mergers, and stellar collisions, all of which lead to a rich array of new stellar evolution paths and products.

Open clusters are uniquely valuable for study of the interface of stellar dynamics and evolution due to their large primordial binary populations and their accessibility to observation. Detailed observational studies of blue stragglers, sub-subgiants, X-ray sources, and binary populations are defining the products of binary encounters and mapping their evolutionary paths. At the same time, a vigorous theoretical effort is exploring formation and evolution of the entire array of evolutionary products in open clusters.

We have underway an integrated program of observational and theoretical study of open clusters in order to define and understand the interface of stellar evolution and stellar dynamics.Specifically, our work seeks to i) comprehensively define the dynamical states of rich, evolved open clusters with ages from 0.1 to 7 Gyr, with particular attention to their binary populations, ii) characterize the populations of blue stragglers and other anomalous stars in the older clusters, focusing on their binary, rotation and activity distributions; iii) evolve N-body cluster simulations with realistic stellar and binary populations to ages of 7 Gyr, providing distributions of observed diagnostics as a function of age to compare with the observed clusters; and iv) model the rotational evolution of collision and mass transfer products in the cluster simulations to compare with the observed populations of anomalous stars.

Current student work:

  • N. Gosnell, XMM-Newton, “An XMM-Newton Survey of Rich Open Clusters” – Search for X-ray sources in 7 open cluster spanning a range of ages.
  • N. Gosnell, Hubble Space Telescope, 41 orbits, “The Nature of the Binary Companions to the Blue Stragglers in the Old Open Cluster NGC 188” – A search for UV emission from white dwarf companions to blue straggler binaries.

Stellar Dynamics

Stellar dynamicists have long recognized that encounters involving binary stars supply the inevitable energy flow out of the cores of star clusters. Specifically, “hard binaries” – binaries with orbital velocities greater than the internal motions of cluster stars – act as energy sources for star clusters.

Such binaries are also the ones discoverable with precise radial-velocity measurements, which reveal both the variations of their orbital motions and – with enough measurements – their orbital parameters. The WIYN Hydra Multi-Object-Spectrograph was perfectly designed for surveys for short-period binary stars among thousands of stars spread over one-degree fields – a perfect match for open clusters.

UW students have obtained more than 60,000 radial velocity measurements of more than 12,000 solar-type stars in 10 open clusters. This database is a remarkable treasure trove for studying binary populations and thereafter the dynamical evolution of star clusters through detailed N-body simulations.

Current student work:

  • A. Geller (Lindheimer Fellow, Northwestern University) – N-body simulations of the very old cluster NGC 188
  • K. Milliman, WIYN Observatory – Comprehensive survey of the intermediate-age cluster NGC 7789

Angular Momentum in Solar-Type Stars

Binary Populations and Stellar Rotation: Angular momentum is one of the key physical outcomes of the star formation process. Indeed one of the challenges of the star formation problem is the decrease in angular momentum from molecular clouds to single stars, a challenge that is likely solved largely by distributing excess angular momentum in binaries and clusters.

Thereafter the life history of a star is marked by angular momentum loss from winds … except in binary stars where angular momentum can be transferred in both directions between orbits and the rotation of stars. One component of our research studies the evolution of stellar angular momentum, both within and outside of binaries, with special emphasis on the evaluation of the powerful Eccentricity – log Period diagrams.

Our binary searches inevitably discover eclipsing binaries, which allow fundamental measurements of masses, radii and distances. Each of these make excellent student projects.

Both our studies of angular momentum and eclipsing binaries have entered a new regime with the advent of the Kepler satellite, and members of our team are co-investigators on several GTO and GO programs with Kepler.

Current student work:

  • K. Milliman, WIYN Observatory – Orbit solutions for binary population of the Kepler cluster NGC 6819.
  • J. Penn, WIYN Observatory – Synthetic spectra analyses of eclipsing binaries