The binary fraction of red supergiants in the Magellanic Clouds

ABSTRACT Red supergiants (RSGs), as the descendants of OB-type stars and the progenitors of supernovae, provide crucial insights into the evolution of massive stars, particularly in binary systems. Previous studies show that the binary fraction of RSGs ($\approx 15\!-\!40~{{\ \rm per\ cent}}$) is significantly lower than that of their predecessors ($\approx 50\!-\!70~{{\ \rm per\ cent}}$). In this work, we investigate the binary fraction of RSGs with the recently selected largest samples of 4695 and 2097 RSGs in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively. Binary systems with a hot companion (O-, B-, or A-type star) are identified by detecting the ultraviolet (UV) excess in the observed spectral energy distribution (SED) ranging from the UV to mid-infrared after subtracting the model SED of the RSG, because RSGs are very weak in the UV band. It is found that the lower limit of binarity is 30.2 $\pm$ 0.7 per cent and 32.2 $\pm$ 1 per cent in the LMC and SMC, respectively. If the sample is limited to luminous RSGs with log $L/{\rm L}_{\odot } > 4.0$, the binary fraction becomes 26.6 $\pm$ 1.1 per cent and 26.4 $\pm$ 1.7 per cent in the LMC and SMC, respectively. The derived binary fraction is valid in the range of $\sim$2.3 < $\log P / [\text{d}]$ < $\sim$8. Our study suggests that roughly one-third of massive stars host a third companion within $\sim$ 30 000 au. In addition, 15 RSGs are identified as binaries via HST/STIS spectra, and a handful of the binaries identified by the SED fitting are confirmed by their light curve and radial velocity dispersion. The stellar parameters of the companions, namely $T_{\mathrm{eff}}$, R, L, and log g, are calculated by model fitting.