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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 731758.0, MPI für Astronomie / Publikationen_mpia
The effect of unresolved binaries on globular cluster proper-motion dispersion profiles
Authors:Bianchini, P.; Norris, M. A.; van de Ven, G.; Schinnerer, E.; Bellini, A.; van der Marel, R. P.; Watkins, L. L.; Anderson, J.
Date of Publication (YYYY-MM-DD):2016
Title of Journal:The Astrophysical Journal Letters
Issue / Number:1
Start Page:id. L22 (7 pp)
Audience:Not Specified
Abstract / Description:High-precision kinematic studies of globular clusters (GCs) require an accurate knowledge of all possible sources of contamination. Among 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 Hubble Space Telescope proper-motion samples. Since GCs evolve toward 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 fractions (initial binary fractions of 50%) and high concentrations (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.
Free Keywords:binaries: general; globular clusters: general; globular clusters: individual: NGC 7078 (M15; proper motions; stars: kinematics and dynamics
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:ISSN:0004-637X %R 10.3847/2041-8205/820/1/L22
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