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arXiv · 2209.06547

Photometry and astrometry with JWST -- I. NIRCam Point Spread Functions and the first JWST colour-magnitude diagrams of a globular cluster

D. Nardiello·L. R. Bedin·A. Burgasser·M. Salaris·S. Cassisi·M. Griggio·M. Scalco
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Abstract

As the James Webb Space Telescope (JWST) has become fully operational, early-release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M92, which were collected under Director's Discretionary Early Release Science programme ERS-1334. We derived empirical models of the Point Spread Function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally under-sampled (FWHM~2 pixel) in F150W and F444W and severely under-sampled (FWHM~1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ~15-20 %; and (iii) have temporal variations of ~3-4 % across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M92, measured to be at the ~0.01 mag level. We use these data to construct the first JWST colour-magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M92 main sequence (~0.1 M$_{\odot}$), and reveal 24 white dwarf candidate members of M92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M92.

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D. Nardiello, L. R. Bedin, A. Burgasser, M. Salaris, S. Cassisi, M. Griggio, M. Scalco. 2022-09-14. Photometry and astrometry with JWST -- I. NIRCam Point Spread Functions and the first JWST colour-magnitude diagrams of a globular cluster. https://doi.org/10.1093/mnras%2Fstac2659

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