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Contact: F.X.Timmes
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JWST Imaging of Earendel, the Extremely Magnified Star at Redshift z = 6.2 (2022)

The gravitationally lensed star WHL0137-LS, nicknamed Earendel, was identified with a photometric redshift $z_{\text{phot}} = 6.2 \pm 0.1$ based on images taken with the Hubble Space Telescope. Here we present James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) images of Earendel in 8 filters spanning 0.8$-$5.0$\mu$m. In these higher resolution images, Earendel remains a single unresolved point source on the lensing critical curve, increasing the lower limit on the lensing magnification to $\mu > 4000$ and restricting the source plane radius further to $r < 0.02$ pc, or $\sim 4000$ AU. These new observations strengthen the conclusion that Earendel is best explained by an individual star or multiple star system, and support the previous photometric redshift estimate. Fitting grids of stellar spectra to our photometry yields a stellar temperature of $T_{\mathrm{eff}} \simeq$ 13000$-$16000 K assuming the light is dominated by a single star. The delensed bolometric luminosity in this case ranges from $\log(L)$ = 5.8$-$6.6 $L_{\odot}$, which is in the range where one expects luminous blue variable stars. Follow-up observations, including JWST NIRSpec scheduled for late 2022, are needed to further unravel the nature of this object, which presents a unique opportunity to study massive stars in the first billion years of the universe.

SunriseArc and Earendel

A highly magnified star at redshift 6.2 (2022)

Galaxy clusters magnify background objects through strong gravitational lensing. Typical magnifications for lensed galaxies are factors of a few but can also be as high as tens or hundreds, stretching galaxies into giant arcs. Individual stars can attain even higher magnifications given fortuitous alignment with the lensing cluster. Recently, several individual stars at redshifts between approximately 1 and 1.5 have been discovered, magnified by factors of thousands, temporarily boosted by microlensing. In this article, we report observations of a more distant and persistent magnified star at a redshift of 6.2 ± 0.1, 900 million years after the Big Bang. This star is magnified by a factor of thousands by the foreground galaxy cluster lens WHL0137-08 (redshift 0.566), as estimated by four independent lens models. Unlike previous lensed stars, the magnification and observed brightness (AB magnitude, 27.2) have remained roughly constant over 3.5 years of imaging and follow-up. The delensed absolute UV magnitude, -10 ± 2, is consistent with a star of mass greater than 50 times the mass of the Sun. Confirmation and spectral classification are forthcoming from approved observations with the James Webb Space Telescope.

This article made some news and some more news.

SunriseArc and Earendel
Caustic curve and Earendel

On The Observability Of Individual Population III Stars And Their Stellar-mass Black Hole Accretion Disks Through Cluster Caustic Transits (2018)

In this article we summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-infrared surface brightness that may come from Population III stars and possible accretion disks around their stellar-mass black holes in the epoch of First Light, broadly taken from z $\simeq$ 7 - 17.

Theoretical predictions and recent near-infrared power-spectra provide tighter constraints on their sky-signal. We outline the physical properties of zero metallicity Population III stars from MESA stellar evolution models through helium-depletion, and black hole accretion disks at z $\gtrsim$ 7. We assume that second-generation non-zero metallicity stars can form at higher multiplicity, so that black hole accretion disks may be fed by Roche-lobe overflow from lower-mass companions. We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next generation ground-based telescopes may observe for both Population III stars and their black hole accretion disks.

Typical caustic magnifications can be $\mu$ $\simeq$ 10$^4$ - 10$^5$, with rise times of hours and decline times of $\lesssim$ 1 year for cluster transverse velocities of $v_T$ $\lesssim$ 1000 km/s. Microlensing by intracluster medium objects can modify transit magnifications, but lengthen visibility times. Depending on black hole masses, accretion-disk radii and feeding efficiencies, stellar-mass black hole accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require to monitor 3 - 30 lensing clusters to AB $\lesssim$ 29 mag over a decade.

panchromatic backgrounds

Pop III HR diagram

cluster caustics and magnification map
redshift space distribution