The effect of unresolved binaries on globular cluster proper-motion dispersion profiles

Abstract

High-precision kinematic studies of globular clusters require an accurate knowledge of all possible sources of contamination. Amongst other sources, binary stars can introduce systematic biases in the kinematics. Using a set of Monte Carlo cluster simulations with different concentrations and binary fractions, we investigate the effect of unresolved binaries on proper-motion dispersion profiles, treating the simulations like HST proper-motion samples. Since globular clusters evolve towards a state of partial energy equipartition, more massive stars lose energy and decrease their velocity dispersion. As a consequence, on average, binaries have a lower velocity dispersion, since they are more massive kinematic tracers. We show that, in the case of clusters with high binary fraction (initial binary fraction of 50%) and high concentration (i.e., closer to energy equipartition), unresolved binaries introduce a color-dependent bias in the velocity dispersion of main-sequence stars of the order of 0.1-0.3 km s${}^{-1}$ (corresponding to $1-6$% of the velocity dispersion), with the reddest stars having a lower velocity dispersion, due to the higher fraction of contaminating binaries. This bias depends on the ability to distinguish binaries from single stars, on the details of the color-magnitude diagram and the photometric errors. We apply our analysis to the HSTPROMO data set of NGC 7078 (M15) and show that no effect ascribable to binaries is observed, consistent with the low binary fraction of the cluster. Our work indicates that binaries do not significantly bias proper-motion velocity-dispersion profiles, but should be taken into account in the error budget of kinematic analyses.

arXiv comment7 pages, 3 figure, 2 tables. Accepted for publication in ApJ Letters

References in corpus (21)

• On the variation of the Initial Mass Function
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations
• The ACS Survey of Galactic Globular Clusters. XII. Photometric Binaries along the Main-Sequence
• An ACS Survey of Globular Clusters V: Star Catalog for Each Cluster
• Inverse Dynamical Population Synthesis and Star Formation
• New Limits on an Intermediate Mass Black Hole in Omega Centauri: I. Hubble Space Telescope Photometry and Proper Motions
• Compact Binaries in Star Clusters I - Black Hole Binaries Inside Globular Clusters
• HST Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae
• Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results
• The fraction of binary systems in the core of thirteen low-density Galactic globular clusters
• Hubble Space Telescope proper motion (HSTPROMO) catalogs of Galactic globular clusters. II. Kinematic profiles and maps
• No Energy Equipartition in Globular Clusters
• A Dynamical Signature of Multiple Stellar Populations in 47 Tucanae
• Revisiting the influence of unidentified binaries on velocity dispersion measurements in ultra-faint stellar systems
• A novel look at energy equipartition in globular clusters
• The Hubble Space Telescope UV Legacy Survey of galactic globular clusters. VI. The internal kinematics of the multiple stellar populations in NGC 2808
• Hubble Space Telescope proper motion (HSTPROMO) catalogs of Galactic globular clusters. III. Dynamical distances and mass-to-light ratios
• Multiple populations in globular clusters: the distinct kinematic imprints of different formation scenarios
• Structure and Dynamics of the Globular Cluster Palomar 13
• Correcting velocity dispersions of dwarf spheroidal galaxies for binary orbital motion
• The Binary Populations of Eight Globular Clusters in the Outer Halo of the Milky Way

Cited by in corpus (15)

• A novel look at energy equipartition in globular clusters
• Systematic errors in Gaia DR2 astrometry and their impact on measurements of internal kinematics of star clusters
• Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?
• Hubble Space Telescope proper motion (HSTPROMO) catalogs of Galactic globular clusters. IV. Kinematic profiles and average masses of blue straggler stars
• Stellar graveyards: Clustering of compact objects in globular clusters NGC 3201 and NGC 6397
• Characterization of the velocity anisotropy of accreted globular clusters
• Stellar Spins in the Open Cluster NGC 2516
• Dynamical modelling of globular clusters: challenges for the robust determination of IMBH candidates
• The relation between the mass-to-light ratio and the relaxation state of globular clusters
• Prospects for detection of intermediate-mass black holes in globular clusters using integrated-light spectroscopy
• HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco
• Unresolved binaries and Galactic clusters' mass estimates
• Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)
• Rubin Data Preview 1: Extending the View of Unresolved Binary Stars in 47 Tucanae
• Using Binaries in Globular Clusters to Catch Sight of Intermediate-Mass Black Holes