Early stars - Properties of lensed stars at z 7
Résumé
Recent years have seen the advent of a new method to directly observe individual stars at cosmological distances through the micro-lensing effect that occurs when a star in a distant lensed galaxy crosses the caustic of a foreground lens. Its unprecedented sensitivity and spatial resolution make JWST the perfect observatory to detect and study these ultra-faint sources and indeed, every JWST observation of a lensing cluster to date has yielded at least one lensed star candidate. We propose here a small pioneering JWST/NIRCam imaging program specifically designed to monitor the highest-redshift caustic-crossing arc known to date in order to detect extremely magnified stars. The target is the spectroscopically confirmed z=6.7 arc in the cluster MS 0451.6-0305. Several characteristics of this arc make it especially well suited for detecting lensed stars: its configuration that makes it cross the caustic not once but twice and therefore yields a very large causic-crossing area in which micro-lensed stars can become observable, and its high-redshift nature which enhances the attainable magnifications. The lensing magnification pushes single stars to observed magnitudes brighter than 29 AB, depths easily achievable with JWST. Our program will not only crown the new highest-redshfit stars observed to date, but also for the first time directly constrain the radii of individual stars on such cosmological distances.
