JWST's PEARLS: A Candidate Massive Binary Star System in a Lensed Galaxy at Redshift 0.94

TL;DR

This paper reports the discovery of a candidate massive binary star system at redshift 0.94 through gravitational microlensing events observed by JWST, providing insights into distant stellar populations and microlensing phenomena.

Contribution

First detection of a candidate massive binary star system at cosmological distance via microlensing, with detailed modeling of the system's properties and dynamics.

Findings

Microlensing events consistent with a binary system of a red supergiant and a B-type star.

Estimated binary star masses: approximately 24 and 20 solar masses.

Small transverse velocity of about 50 km/s inferred from light curve modeling.

Abstract

Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceed ~1000. Nine such sources were identified in JWST NIRCam imaging of a single galaxy at redshift z=0.94 known as the "Warhol arc,'' which is mirror-imaged by the galaxy cluster MACSJ0416.1-2403. Here we present the discovery of two coincident and well-characterized microlensing events at the same location followed by a third event observed in a single filter approximately 18 months later. The events can be explained by microlensing of a binary star system consisting of a red supergiant (T ~ 4000 K) and a B-type (T ~ 13,000 K) companion star. The timescale of the coincident microlensing events constrains the estimated projected source-plane size to tens of AU. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses M1=23.6+5.3-4.3 Msol and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curve in all filters gives a relatively small transverse velocity of 50 km/s. This requires the dominant velocity component of several hundreds of km/s to be roughly parallel to the microcaustic. An alternative possibility would be that the three microlensing events correspond to unrelated stars crossing distinct microcaustics, but this would imply a highly elevated rate of events at their common position, even though no underlying knot is present at the location.