A census of massive eclipsing binaries in the Whirlpool Galaxy

Abstract

We present a census of massive eclipsing binary (EB) candidates in the Whirlpool Galaxy (M51), a face-on spiral galaxy at 7.5 Mpc with $Z\sim Z_{\odot }$. Using 34 epochs of red-optical Hubble Space Telescope (HST) photometry and a single epoch of broadband UV-optical photometry, we identify 173 massive EB candidates ($>5{\mathrm{M}}_{\odot }$) with orbital periods $P_{\mathrm{orb}}$=1-30 days. The sample traces young stellar populations across the spiral arms of M51. Joint light-curve and spectral energy distribution fits for 136 systems yield binaries spanning $T_{\mathrm{eff}}=10,000$-$50,000$ K, $R=7$-$66{\mathrm{R}}_{\odot }$, and masses of $5$-$120{\mathrm{M}}_{\odot }$. We use detailed injection-recovery simulations to quantify the survey's completeness and infer intrinsic properties of the massive binary population. At the population level, we infer a close-binary fraction of $f_{\mathrm{close}}=27.4\pm 7.1\%$ for $2\le P_{\mathrm{orb}}/\mathrm{d}<20$ and $0.1\le q\le 1$. Over the same period range, the intrinsic period distribution is consistent with being log-uniform, $\mathrm{d}N/\mathrm{d}\log P_{\mathrm{orb}}\propto P_{\mathrm{orb}}^{\pi }$ with $\pi =0.29\pm 0.32$. These demographics broadly match those of massive-star populations in the Milky Way, LMC, and SMC, with tentative evidence for a lower close-binary fraction at lower metallicity. Across the $\sim 0.2$-$1.1Z_{\odot }$ range spanned by these galaxies, the overall similarities suggest that the formation of close massive binaries depends only weakly on metallicity. Our survey establishes EBs as a powerful tool for measuring massive-binary demographics beyond the Local Group and provides a rich panchromatic census of $\sim$100,000 luminous stars and variables in M51.

41 pages, 25 figures, 6 tables, including appendices. Submitted to PASP; comments are welcome. Catalogs and light curves are publicly available on Zenodo: https://doi.org/10.5281/zenodo.22727123