JWST’s PEARLS: Temperatures of Nine Highly Magnified Stars in a Galaxy at Redshift 0.94 and Simulated Stellar Population Dependence on Stellar Metallicity and the Initial Mass Function

We present stellar atmosphere modeling of JWST NIRCam photometry of nine highly magnified individual stars in a single galaxy at redshift z = 0.94 known as the Warhol arc, which is strongly lensed by the galaxy cluster MACS J0416. Seven of these transients were identified by Yan et al. The nine sources are likely red supergiants with temperatures T_eff ≈ 4000 K. We present new long-slit spectroscopy of the Warhol arc acquired with Keck I telescope and the Large Binocular Telescope, and use these data to help constrain the arc’s oxygen abundance to be 12 + log (O / H) = 8.45 ± 0.08. A microlensing simulation is performed on synthetic stellar populations using a range of stellar metallicities and initial mass function (IMF) slopes. The temperature distribution of the simulated detectable stars is sensitive to the choice of stellar metallicity, and setting the stellar metallicity equal to the arc’s nebular metallicity (log ( Z * / Z ⊙) = − 0.24) produces a simulated temperature distribution that is consistent with the observations, while lower stellar metallicities (log ( Z * / Z ⊙) < – 0.75) produce simulated temperatures that are inconsistent with the observations. The expected detection rate is strongly anticorrelated with the IMF slope for α > 1.2. For the canonical IMF slope α = 2.35, the simulation yields expected transient detection rates that agree with the observed detection rates in the Hubble Space Telescope Flashlights filters, but overpredicts the detection rate by a factor of ∼3–12 (<2σ tension) in the JWST filters. The simulated detection rate is sensitive to the choice of stellar metallicity, with lower metallicities (log ( Z * / Z ⊙) < – 0.75) yielding a significantly lower simulated detection rate that further reduces the modest tension with the observations.