[Paper Review] Photometric binaries in fifty Globular Clusters

A. P. Milone, G. Piotto+2 more|ArXiv.org|Jan 21, 2008

Astrophysics and Star Formation Studies3 citations

TL;DR

This study analyzes photometric binaries in 50 Galactic globular clusters using high-precision HST/ACS photometry from the Anderson et al. (2008) catalog. By identifying stars redward of the main sequence ridge line and correcting for completeness and field contamination via artificial star tests, it finds a significant anti-correlation between binary fraction and cluster absolute luminosity, with fainter clusters hosting higher binary fractions.

ABSTRACT

The HST/ACS Survey of Galactic globular clusters (GGCs) is a HST Treasury project aimed at obtaining high precision photometry in a large sample of globular clusters. The homogeneous photometric catalogs that has been obtained from these data by Anderson et al. (2008) represents a golden mine for a lot of astrophysical studies. In this paper we used the catalog to analyse the properties of MS-MS binary systems from a sample of fifty GGCs. We measured the fraction of binaries (divided in different groups), studied their radial distribution and constrained the mass ratio distribution. We investigated possible relations between the fraction of binaries and the main parameters of their host GGCs. We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass).

Motivation & Objective

• To measure the fraction of main-sequence binary systems in a large, homogeneous sample of 50 Galactic globular clusters (GGCs).
• To investigate the radial distribution of binaries within cluster cores and outer regions to assess dynamical segregation.
• To constrain the mass ratio distribution of binaries and assess its dependence on cluster properties.
• To explore correlations between binary fraction and key cluster parameters such as luminosity, metallicity, and relaxation time.
• To correct for photometric systematics including differential reddening, completeness, and field star contamination using artificial star simulations.

Proposed method

• Utilized high-precision photometry from the HST/ACS Survey of Galactic Globular Clusters, specifically the homogeneous catalog by Anderson et al. (2008).
• Identified potential binaries by selecting stars located on the red side of the main sequence ridge line (MSRL) in color-magnitude diagrams (CMDs).
• Corrected for photometric completeness and field star contamination using artificial star tests, applying a completeness-weighted correction: $ N = \sum{1/c_i} $.
• Applied a color correction technique from Milone et al. (2008) to mitigate spread in color due to differential reddening and zero-point errors.
• Calculated high mass ratio binary fractions using the formula: $ f_{bin} = \frac{N_{OBS}^{B} - N_{FIELD}^{B}}{N_{OBS}^{A} - N_{FIELD}^{A}} - \frac{N_{ART}^{B}}{N_{ART}^{A}} $, where A and B are color-magnitude regions.
• Estimated the global binary fraction using a statistical method comparing observed CMDs with 10,000 simulated CMDs enriched with varying binary fractions and mass ratio distributions $ f(q) $.

Experimental results

Research questions

• RQ1 Is there a significant correlation between the binary fraction in a globular cluster and its absolute luminosity?
• RQ2 How does the radial distribution of binaries compare to that of single main-sequence stars across different cluster regions?
• RQ3 What is the distribution of mass ratios $ q = M_2/M_1 $ among MS-MS binaries, and how does it vary with cluster properties?
• RQ4 To what extent do cluster parameters such as metallicity, core relaxation time, and central density influence the observed binary fraction?
• RQ5 How do photometric systematics like differential reddening and completeness affect the measured binary fraction, and how are they corrected?

Key findings

• A highly significant anti-correlation was found between the binary fraction in the core (I_CORE sample) and the absolute visual magnitude of the cluster, with fainter clusters exhibiting higher binary fractions.
• For the I_CORE sample, the fraction of binaries with mass ratio $ q > 0.5 $ increases as cluster luminosity decreases, as shown in Figure 2.
• The anti-correlation between binary fraction and luminosity persists in the HM and O_HM samples, indicating that the trend holds across radial regions, though with reduced significance in outer regions.
• The radial distribution of binaries is more centrally concentrated than that of single main-sequence stars in approximately 60% of the 37 clusters with good inner-field photometry.
• No strong correlation was found between binary fraction and the cluster collisional parameter, suggesting that binary evolution may not be primarily driven by two-body relaxation in the core.
• The global binary fraction estimation, based on statistical comparison with simulated CMDs, is sensitive to photometric inaccuracies and is therefore considered an approximate estimate.