Explanations for the two-component spectral energy distributions of gravitationally lensed stars at high redshifts

Abstract: Observations of gravitationally lensed, high-mass stars at redshifts $\gtrsim1$ occasionally reveal spectral energy distributions that contain two components with different effective temperatures. Given that two separate stars are involved, it suggests that both stars have simultaneously reached very high magnification, as expected for two stars in a binary system close to the caustic curve of the foreground galaxy-cluster lens. The inferred effective temperatures and luminosities of these stars are, however, difficult to reconcile with known binaries, or even with isolated stars of the same age. Here, we explore three alternative explanations for these cases: circumstellar dust around the cooler of the two stars; age differences of a few Myr among stars in the same star cluster, and a scenario in which the stars originate in two separate star clusters of different age along the lensing caustic. While all of these scenarios are deemed plausible in principle, dust solutions would require more circumstellar extinction than seen in local observations of the relevant super/hypergiant stars. Hence, we argue that age differences between the two stars are the most likely scenario, given the current data.