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Rubin Data Preview 1: Extending the View of Unresolved Binary Stars in 47 Tucanae

Giacomo Cordoni, Luca Casagrande, Helmut Jerjen

TL;DR

The paper presents the first detection of unresolved main-sequence binaries in the outskirts of the globular cluster 47 Tucanae using Rubin DP1, extending observations beyond the half-light radius toward the tidal radius. It employs deep multi-band CMDs and a mass-ratio threshold of $q>0.7$ to identify binaries, finding $f_{\mathrm{bin}}(q>0.7)=0.016\pm0.005$, with cross-checks in alternative CMDs yielding consistent results. After accounting for field contamination and blending, the authors extrapolate to all mass ratios to obtain $f_{\mathrm{bin}}(q>0)=0.053\pm0.017$, in line with core measurements and suggesting a primordial binary population persists in the outskirts. Overall, the work demonstrates Rubin’s wide-field, high-precision photometric capabilities for probing binary populations and cluster dynamics across the Milky Way.

Abstract

Until now, the study of unresolved main sequence binary stars in globular clusters has been possible almost exclusively in their central regions with deep Hubble Space Telescope (HST) observations. We present the first detection of unresolved main-sequence binary stars in the outer field of 47 Tucanae using Rubin Observatory's Data Preview 1 (DP1). Our analysis exploits deep $i$ vs. $g-i$ colour-magnitude diagrams beyond the cluster's half-light radius, reaching almost to the tidal radius. The high-quality photometry allowed to identify unresolved binaries with mass ratios $q$ larger than 0.7. The derived binary fraction of $f_\mathrm{bin}(q>0.7) = 0.016 \pm 0.005$ stands in contrast to the significantly lower values in the cluster innermost regions, as measured from HST photometry. This result provides new empirical input for testing physical processes that drive the formation and evolution of binary stars in globular clusters. It also demonstrates Rubin's unique wide-field and high-precision photometric capabilities to address a broader range of outstanding questions in star cluster research. Future full data releases will enable to significantly expand the study of dense stellar systems across the Milky Way.

Rubin Data Preview 1: Extending the View of Unresolved Binary Stars in 47 Tucanae

TL;DR

The paper presents the first detection of unresolved main-sequence binaries in the outskirts of the globular cluster 47 Tucanae using Rubin DP1, extending observations beyond the half-light radius toward the tidal radius. It employs deep multi-band CMDs and a mass-ratio threshold of to identify binaries, finding , with cross-checks in alternative CMDs yielding consistent results. After accounting for field contamination and blending, the authors extrapolate to all mass ratios to obtain , in line with core measurements and suggesting a primordial binary population persists in the outskirts. Overall, the work demonstrates Rubin’s wide-field, high-precision photometric capabilities for probing binary populations and cluster dynamics across the Milky Way.

Abstract

Until now, the study of unresolved main sequence binary stars in globular clusters has been possible almost exclusively in their central regions with deep Hubble Space Telescope (HST) observations. We present the first detection of unresolved main-sequence binary stars in the outer field of 47 Tucanae using Rubin Observatory's Data Preview 1 (DP1). Our analysis exploits deep vs. colour-magnitude diagrams beyond the cluster's half-light radius, reaching almost to the tidal radius. The high-quality photometry allowed to identify unresolved binaries with mass ratios larger than 0.7. The derived binary fraction of stands in contrast to the significantly lower values in the cluster innermost regions, as measured from HST photometry. This result provides new empirical input for testing physical processes that drive the formation and evolution of binary stars in globular clusters. It also demonstrates Rubin's unique wide-field and high-precision photometric capabilities to address a broader range of outstanding questions in star cluster research. Future full data releases will enable to significantly expand the study of dense stellar systems across the Milky Way.

Paper Structure

This paper contains 6 sections, 2 figures.

Table of Contents

  1. INTRODUCTION
  2. DATA
  3. RESULTS
  4. Estimating Field Star Contamination
  5. Effect of photometric blends
  6. SUMMARY AND DISCUSSION

Figures (2)

  • Figure 1: Data selection procedure. Panel a. Distribution of stars in the whole sample of Rubin DP1 coadd object catalogue (grey), stars with a Gaia matched source (black) and 47 Tuc selected cluster members (azure). X and Y have been determined projecting ra and dec and are displayed in degrees, while the solid grey and black circles represent the half-light and tidal radii, respectively. The dashed grey circle indicate the 18 arcmin radius utilised in the selection of the final sample of stars. Panels b-d. Uncertainties on $gri$ photometry as a function of $i$ magnitude. The azure lines indicate the threshold adopted to select stars with low uncertainties. Panel e-f.$i$ vs $r-i$ and $g-i$ CMDs. Colour-coding as for previous panels. The best‐fit PARSEC isochrone marigo2017a as in choi2025 is plotted as a solid black line, and the equal‐mass MS binary sequence as a dashed black line.
  • Figure 2: Binary fraction determination. Panel a.$i$ vs. $g-i$ CMD of selected cluster members. The black line represent the fiducial line of the MS, while the grey lines are shifted by $\pm 3\sigma$, where $\sigma$ is the spread in colour at each $i$ magnitude. The fiducial line of MS-MS binary with mass-ratio of $q=0.6;0.7;1$ are shown in light-blue, orange and purple, respectively. Photometric uncertainties are displayed on the left side with black errorbars. Panel b. Same CMD as panel a, with the regions of single and binary stars shown in purple (region A) and red (region B), respectively. Identified single and binary stars are marked with purple circles and red crosses. Panel c. Recovered binary fraction in this work, compared with the value determined in milone2012a from HST photometry of the cluster centre. Dotted and dashed vertical line indicate the core and half-light radii, respectively. Panel d. Binary fraction for different high and low mass primary mass stars, determined in the two regions depicted in the CMD on panel b.