Binary Frequencies in a Sample of Globular Clusters. I. Methodology and Initial Results

Abstract

Binary stars are thought to be a controlling factor in globular cluster evolution, since they can heat the environmental stars by converting their binding energy to kinetic energy during dynamical interactions. Through such interaction, the binaries determine the time until core collapse. To test predictions of this model, we have determined binary fractions for 35 clusters. Here we present our methodology with a representative globular cluster NGC 4590. We use HST archival ACS data in the F606W and F814W bands and apply PSF-fitting photometry to obtain high quality color-magnitude diagrams. We formulate the star superposition effect as a Poisson probability distribution function, with parameters optimized through Monte-Carlo simulations. A model-independent binary fraction of (6.2 +- 0.3)% is obtained by counting stars that extend to the red side of the residual color distribution after accounting for the photometric errors and the star superposition effect. A model-dependent binary fraction is obtained by constructing models with a known binary fraction and an assumed binary mass-ratio distribution function. This leads to a binary fraction range of 6.8% to 10.8%, depending on the assumed shape to the binary mass ratio distribution, with the best fit occurring for a binary distribution that favors low mass ratios (and higher binary fractions). We also represent the method for radial analysis of the binary fraction in the representative case of NGC 6981, which shows a decreasing trend for the binary fraction towards the outside, consistent with theoretical predictions for the dynamical effect on the binary fraction.

Accepted for publication in the Astrophysical Journal

References in corpus (24)

• The Photometric Performance and Calibration of the HST Advanced Camera For Surveys
• WFPC2 Stellar Photometry with HSTphot
• A Triple Main Sequence in the Globular Cluster NGC 2808
• Star counts in the Galaxy. Simulating from very deep to very shallow photometric surveys with the TRILEGAL code
• The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry
• The ACS Survey of Galactic Globular Clusters. XII. Photometric Binaries along the Main-Sequence
• Dynamical Formation of Close Binary Systems in Globular Clusters
• Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
• The Frequency of Binary Stars in the Core of 47 Tucanae
• The fraction of binary systems in the core of thirteen low-density Galactic globular clusters
• Monte Carlo Simulations of Globular Cluster Evolution. III. Primordial Binary Interactions
• Dynamical Formation of Close Binaries in Globular Clusters II: Cataclysmic Variables
• Uncloaking globular clusters in the inner Galaxy
• Deep HST-WFPC2 photometry of NGC 288. I. Binary Systems and Blue Stragglers
• Measuring helium abundance difference in giants of NGC 2808
• A Deep HST Study of the Globular Cluster NGC 6397: Reduction Methods
• The binary fraction in the globular cluster NGC 6397
• Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6397: Evidence for a Bimodal Cataclysmic Variable Population
• The evolution of the binary population in globular clusters: a full analytical computation
• Spectroscopic Binaries in Globular Clusters. II. A Search for Long-Period Binaries in M22
• Mapping differential reddening in the inner Galactic globular cluster system
• A Spectroscopic Survey for Binary Stars in the Globular Cluster NGC5053
• Spectroscopic Binaries in Globular Clusters. I. A Search for Ultra-Hard Binaries on the Main Sequence in M4
• X-ray Sources in Galactic Globular Clusters

Cited by in corpus (10)

• No evidence for intermediate-mass black holes in the globular clusters $\omega$ Cen and NGC 6624
• Lithium-Rich Giants in Globular Clusters
• The incidence of binaries in Globular Cluster stellar populations
• The Binary Populations of Eight Globular Clusters in the Outer Halo of the Milky Way
• Effect of binarity in star cluster dynamical mass determination
• Close Encounters of Tight Binary Stars with Stellar-mass Black Holes
• Binary white dwarfs in the halo of the Milky Way
• Dynamical evolution of Milky Way globular clusters on the cosmological timescale I. Mass loss and interaction with the nuclear star cluster
• The Frequency and Mass-Ratio Distribution of Binaries in Clusters I: Description of the method and application to M67
• Low-mass and sub-stellar eclipsing binaries in stellar clusters