Early results from GLASS-JWST VIII: An Extremely Magnified Blue Supergiant Star at Redshift 2.65 in the Abell 2744 Cluster Field

TL;DR

This paper reports the discovery of a highly magnified blue supergiant star at redshift 2.65 in the Abell 2744 cluster, observed through gravitational lensing and microlensing effects with JWST, providing insights into distant stellar populations.

Contribution

First detection of an extremely magnified blue supergiant star at high redshift using JWST and gravitational lensing techniques, demonstrating the potential to study individual stars in the early universe.

Findings

Star's redshift is 2.65, indicating a distant galaxy.

Magnification suggests strong gravitational lensing and microlensing effects.

Star's temperature estimated between 7,000 and 12,000 K.

Abstract

We report the discovery of an extremely magnified star at redshift $z=2.65$ in James Webb Space Telescope (JWST) NIRISS pre-imaging of the Abell 2744 galaxy-cluster field. The star's background host galaxy lies on a fold caustic of the foreground lens, and the cluster creates a pair of images of the region close to the lensed star. We identified the bright transient in one of the merging images at a distance of $\sim 0.15"$ from the critical curve, by subtracting the JWST F115W and F150W imaging from coadditions of archival Hubble Space Telescope (HST) F105W and F125W images and F140W and F160W images, respectively. Since the time delay between the two images should be only hours, the transient must be the microlensing event of an individual star, as opposed to a luminous stellar explosion which would persist for days to months. Analysis of individual exposures suggests that the star's magnification is not changing rapidly during the observations. From photometry of the point source through the F115W, F150W, and F200W filters, we identify a strong Balmer break, and modeling allows us to constrain the star's temperature to be approximately 7,000--12,000 K.