Clues on the presence and segregation of very massive stars in the Sunburst Lyman-continuum cluster at z=2.37

International audienceWe report on the identification of very massive stars (VMS, mass $> 100$ \msun) possibly segregated in the center of the young massive star cluster at $z$=2.37 hosted in the {\tt Sunburst} lensed galaxy. Such a result is based on two pieces of evidence: (1) the VLT/MUSE spectra of several multiple images of the same star cluster show key spectral signatures of VMS, like the \heii broad emission, \nivblue emission and \niv P-Cygni profile. In particular, \heii is broad ($\sim1610\pm300$\kms) with an equivalent width of 3Å and shows an asymmetric profile. Such features require an extremely young ($\sim2.5$ Myr) stellar population component with masses of the stars exceeding 100~\msun. Assuming a Salpeter IMF and BPASS models for normal massive stars, the observed spectral features require $\sim$400 VMS; (2) the same star cluster is detected at S/N~$\sim100$ in the LyC domain ($\lambda < 900$Å). The LyC emission emerges from a region with a radius at least 2 times smaller than what is observed at 1700Å~(independently from magnification) and is located in the center of the cluster. In absolute scales, after de-lensing, the effective radii are R$_{\tt eff}[{\tt LyC}]\sim4.7 \pm 1.5$ pc and R$_{\tt eff}[1700]= 7.8 \pm 1.4$ pc. The LyC radiation is mainly produced by hot and massive stars, implying that their spatial distribution (including VMS) is preferentially more confined in the central parts of the cluster. Approximately 400 VMS hosted by a cluster of $\sim 10^7$\msun are producing $\sim$15% of the escaping LyC photons, while the rest is produced from other massive early-type stars