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

We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of $z_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}\simeq 4.8$, previously detected in JWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647+7015 ($z=0.591$). The spectroscopic observation allows us to precisely measure the redshift of the host arc at $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=4.758\pm 0.004$, 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 $T_{\mathrm{e}\mathrm{f}\mathrm{f},\mathrm{B}}\simeq 15000$ K with either a redder F-type companion ($T_{\mathrm{e}\mathrm{f}\mathrm{f},\mathrm{F}}\simeq 6250$K) or significant dust attenuation ($A_V\simeq 0.82$) 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. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, $r\lesssim 0.5$ 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 $\sim 1.2$ Gyr old showcases the ability of JWST to study early stars through extreme lensing.