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

¹ Observational Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516 SE-751 20 Uppsala, Sweden
² Swedish Collegium for Advanced Study, Linneanum, Thunbergsvägen 2, SE-752 38 Uppsala, Sweden
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Observational Cosmology Lab., NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Instituto de Física de Cantabria (CSIC-UC), Avda. Los Castros s/n., 39005 Santander, Spain

^⋆ Corresponding author; armin.nabizade@gmail.com

Received: 18 June 2024
Accepted: 6 November 2024

Abstract

Observations of gravitationally lensed, high-mass stars at redshifts ≳1 occasionally reveal spectral energy distributions that contain two components with different effective temperatures. Given that two separate stars are involved, this 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 million years 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 supergiant and hypergiant stars. Hence, we argue that age differences between the two stars are the most likely scenario, given the current data.