JWST Imaging of Earendel, the Extremely Magnified Star at Redshift $z=6.2$ (2208.09007v2)

Abstract: The gravitationally lensed star WHL0137-LS, nicknamed Earendel, was identified with a photometric redshift $z_{phot} = 6.2 \pm 0.1$ 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.

• The paper presents high-resolution JWST NIRCam imaging of Earendel, demonstrating extreme gravitational lensing with a magnification exceeding 4000.
• The study constrains Earendel's physical parameters using multi-filter observations, estimating an effective temperature of 13000–16000 K and bolometric luminosity log(L/L⊙) between 5.8 and 6.6.
• The work highlights the potential of combining gravitational lensing and planned JWST spectroscopy to advance our understanding of massive star evolution and early cosmic star formation.

Analysis of JWST Imaging of Earendel, the Extremely Magnified Star at Redshift $z = 6.2$

This paper by Welch et al. presents observations conducted with the James Webb Space Telescope (JWST) focusing on Earendel, a star at a redshift of $z = 6.2$ that is extremely magnified by the gravitational lensing effects of the WHL J013719.8-082841 galaxy cluster. The significance of this paper lies in the opportunity to observe and analyze a stellar object from the early universe, significantly advancing our understanding of massive star formation and evolution in the first billion years after the Big Bang.

Observational Insights

The researchers utilized the Near Infrared Camera (NIRCam) on JWST, capturing images in eight filters ranging from 0.8 to 5.0 $\mu$m. These images demonstrated that Earendel remains an unresolved point source situated on the lensing critical curve, implying a lensing magnification exceeding $\mu > 4000$. Remarkably, they constrain the radius in the source plane to be less than 0.02 pc ($\sim 4000$ AU). This high-resolution image confirms previous suggestions that Earendel is either an individual star or a compact multiple star system, supporting earlier photometric redshift estimates determined with the Hubble Space Telescope (HST).

Physical Parameters and Theoretical Implications

Using stellar atmosphere models, the paper estimates the effective temperature of Earendel to be in the range of 13000-16000 K if the light originates from a single star. The deduced delensed bolometric luminosity ranges from $\log(L/L_\odot) = 5.8$ to 6.6, consistent with expectations for luminous blue variable (LBV) stars or evolved massive stars in the local universe. These constraints are significant when placed on an HR diagram with theoretical stellar evolution tracks, showcasing possible evolutionary phases of such stars under varying metallicities and ages.

Variability and Multi-Star Hypothesis

The observations over multiple epochs show no significant time variation in Earendel's brightness, which might be indicative of microlensing variability or intrinsic luminosity variation usually seen in massive stars. The analyses also entertain a multi-star hypothesis due to the complex spectral energy distribution (SED), considering the presence of potentially two distinct components: a cooler star accounting for the Balmer break and a hotter companion explaining the UV continuum slope.

Future Implications and Developments

The continued monitoring and planned follow-up spectroscopy with JWST NIRSpec can provide deeper insights not just into the precise nature of Earendel, but more broadly into the physics governing massive stars in the quantum early universe. This will aid in addressing open questions such as the initial mass function of early stars, the role of metallicity in star formation at high redshifts, and potentially observing Population III stars. Moreover, the combined capabilities of gravitational lensing and JWST imaging allow for unprecedented paper of distant and luminous stars, directly informing models of stellar evolution and contributing to a more comprehensive cosmological framework.

Such observations have profound implications for theoretical models of star formation and evolution and are pivotal for understanding the assembly of galaxies at critical phases of the universe's history. Continued advancements in observational techniques and data analysis methods, as demonstrated in this robust collaboration, will further enrich our understanding of these distant and elusive cosmic phenomena.