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

TL;DR

This study uses JWST/NIRSpec spectroscopy to analyze a highly magnified star candidate at redshift 4.76, providing insights into early stars and the potential of JWST for high-redshift stellar observations.

Contribution

First spectroscopic confirmation of a lensed star candidate at high redshift, demonstrating JWST's capability to study early stars and distinguish them from star clusters.

Findings

Redshift precisely measured at 4.758

Star likely a B-type super-giant with possible dust attenuation

Spectrum consistent with a star cluster, but less likely due to lensing constraints

Abstract

We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of $z_{\mathrm{phot}}\simeq4.8$, 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 $z_{\mathrm{spec}}=4.758\pm0.004$, 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 $T_{\mathrm{eff,B}}\simeq15000$ K with either a redder F-type companion ($T_{\mathrm{eff,F}}\simeq6250$K) or significant dust attenuation ($A_V\simeq0.82$) 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, $r\lesssim0.5$ 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 $\sim1.2$ Gyr old showcases the ability of JWST to study early stars through extreme lensing.