The BINCOS Project

The Binary INformation from Open Clusters using SEDS (BINOCS) Project

Current cluster binary studies are carried out using one of two methods, two-band photometry, and time-baseline radial velocity studies, each of which experience issues which limit their effectiveness in answering the above science questions. However, to deeply understand the binary populations of open clusters, we have created a new method which can determine accurate masses for all members of a cluster within a reasonable amount of telescope time. This new binary detection method is nicknamed binocs: Binary INformation from Open Clusters using SEDs. By imaging a star using multiple filters across the spectrum (e.g., UBV RIJHKS[3.6][4.5][5.8][8.0]), one should be able to “re- build” spectral energy distribution (SED) of a star given its parameters: age, metallicity, mass. Similarly, a binary system could not be accurately modelled by a single SED curve, but instead by two SEDs added together. By matching stars to these models, mass can be determined, similar to how temperature could be determined in the idealized black- body case. Since the star is a member of a cluster with known parameters, so age and metallicity are given. By matching stars to models of a library of synthetic single stars SEDs, mass can be determined.

This research formed the core of the Ph.D. Thesis of Dr. Ben Thompson and continues with Taylor Spoo.

BINOCS I: Reliable Photometric Mass Determinations of Binary Star Systems in Clusters (Thompson et al. 2021)

The BINOCS Method and Method Verification

A majority of stars are formed in open clusters, and then ejected into the Galactic field population through tidal effects from external masses, as well as internal gravitational interactions. Therefore, understanding the internal dynamics of open clusters, through N-Body simulations, will inform the growth of the Galactic stellar population. A major input into these N-Body simulations is the frequency and mass distribution of binary star systems, which are currently based on statistics derived from the field population, but the distributions of binaries in clusters may be different. Current binary detection techniques, such as radial velocity surveys, have drawbacks which limit their usefulness for detailed studies over large mass ranges. As presented in the literature, different mass ranges may produce different interpretations of the observed binary population, e.g, as published recently for NGC 1818. A clearer picture of the binary population, covering a wide mass range, is needed to improve the understanding of cluster binary populations, which will inform cluster simulations. We introduce a new binary detection method, Binary INformation from Open Clusters Using SEDs (BINOCS). Using newly-observed multi-wavelength photometric catalogs (0.3 - 8 micron) of the key open clusters M35, M36, M37, M67 and NGC 2420, the BINOCS method is able to determine accurate component masses for unresolved cluster binaries. We showed how binary fraction decrease as a function of cluster age.

This part of the OCCAM project is led by Dr. Ben Thompson and Dr. Peter Frinchaboy.

The BINOCS code can be obtained from the Github Repository

The BINOCS Survey

Completing an analysis large scale binary analysis using only RV surveys could take centuries to build up enough analysis clusters to produce any useful insights. Two-band analysis, though fast, is dominated by degeneracies, and is limited to small magnitude ranges across the main sequence. For this project, we will create a unprecedented systematic cluster binary stars survey that will provide significant empirical constraints, key for verifying and improving the input physics for cluster N-body codes. Analyzing hundreds of star clusters, in a uniform way allows us to yield significant insights into the true distribution of cluster binary fractions.

We will create a new multi-wavelength (optical, near infrared, and mid-infrared) photometric catalog and analysis of over 100 star clusters to answer three primary questions about open clusters and their stellar/binary populations: 1) How do binary population properties change as a function of time in the cluster environment? 2) What is the binary fraction of cluster stars and how does it vary as a function of primary star mass? 3) Does the evolution and impact of the binary population vary with the cluster mass, concentration, location, and/or chemical composition? As an integral component in addressing the above questions, we will produce a membership cleaned catalog of single and binary stars, including component masses, which will be combined with upcoming Gaia kinematic data and chemical data from other studies (SDSS/APOGEE, GaiaESO, Hermes/Galah) to create a statistically significant sample to test the overall dynamical evolution of cluster in detail. In addition to providing a definitive answer on the effects of metallicity and Galactic location, this expanded dataset will allow detailed analysis in a number of axes including mass and central concentration.

This part of the OCCAM project is being led by Taylor Spoo and Dr. Peter Frinchaboy.