Reaching for the stars – JWST/NIRSpec spectroscopy of a lensed star candidate at z = 4.76

Abstract

We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of z phot 4.8, previously detected in JWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647 + 7015 ( z = 0.591).The spectroscopic observ ation allo ws us to precisely measure the redshift of the host arc at z spec = 4.758 0.004, and the star's spectrum displays clear Lyman-and Balmer-breaks commensurate with this redshift.A fit to the spectrum suggests a B-type super-giant star of surface temperature T eff, B 15 000 K with either a redder F-type companion ( T eff, F 6 250 K) or significant dust attenuation ( A V 0.82) along the line of sight.We also investigate the possibility that this object is a magnified young globular cluster rather than a single star.We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs.Ho we ver, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, r 0.5 pc, could indicate that this scenario is somewhat less likely -albeit not completely ruled out by the current data.The presented spectrum seen at a time when the Universe was only 1.2 Gyr old showcases the ability of JWST to study early stars through extreme lensing.