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We present a constraint on the abundance of supergiant (SG) stars at redshiftz approx. 1, based on recent observations of a strongly lensed arc at thisredshift. First we derive a free-form model of MACS J0416.1-2403 using datafrom the BUFFALO program. The new lens model is based on 72 multiply lensedgalaxies that produce 214 multiple images, making it the largest sample ofspectroscopically confirmed lensed galaxies on this cluster. The largercoverage in BUFFALO allows us to measure the shear up to the outskirts of thecluster, and extend the range of lensing constraints up to ~ 1 Mpc from thecentral region, providing a mass estimate up to this radius. As an application,we make predictions for the number of high-redshift multiply-lensed galaxiesdetected in future observations with JWST. Then we focus on a previously knownlensed galaxy at z=1.0054, nicknamed Spock, which contains four previouslyreported transients. We interpret these transients as microcaustic crossings ofSG stars and compute the probability of such events. Based on simplificationsregarding the stellar evolution, we find that microlensing (by stars in theintracluster medium) of SG stars at z=1.0054 can fully explain these events.The inferred abundance of SG stars is consistent with either (1) a numberdensity of stars with bolometric luminosities beyond the Humphreys-Davidson(HD) limit (L ~ $6\times10^5 L_{\odot}$) that is below 400 stars per sq. kpc,or (2) the absence of stars beyond the HD limit but with a SG number density of~ 9000 per sq. kpc for stars with luminosities between $10^5$ and$6\times10^5$. This is equivalent to one SG star per 10x10 pc$^2$. We finallymake predictions for future observations with JWST's NIRcam. We find that inobservations made with the F200W filter that reach 29 mag AB, if cool red SGstars exist at z~1 beyond the HD limit, they should be easily detected in thisarc

BUFFALO/Flashlights: Constraints on the abundance of lensed supergiant stars in the Spock galaxy at redshift 1