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

• Main Authors: Nabizadeh, Armin, Zackrisson, Erik, Lundqvist, Emma, Ricotti, Massimo, Park, Seyong, Welch, Brian, Diego, Jose M
• Format: Journal Article
• Language: English
• Published: 18-06-2024
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.2406.12607

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.