Photometry and astrometry with JWST – I. NIRCam point spread functions and the first JWST colour–magnitude diagrams of a globular cluster. Issue 1 (19th September 2022)

• Record Type: Journal Article
• Authors: Nardiello, D; Bedin, L R; Burgasser, A; Salaris, M; Cassisi, S; Griggio, M; Scalco, M
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 517:Issue 1(2022)
• Page Start: 484
• Page End: 497
• Publication Date: 2022-09-19
• Subjects: techniques: image processing -- techniques: photometric -- stars: Population II -- white dwarfs -- globular clusters: individual: NGC 6341 (M 92); Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2659
• Languages: English
• ISSNs: 0035-8711

Abstract

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 M 92, 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 F 090 W, F 150 W, F 277 W, and F 444 W, and find that these PSFs: (i) are generally undersampled (FWHM ∼ 2 pixel) in F 150 W and F 444 W and severely undersampled (FWHM ∼ 1 pixel) in F 090 W and F 277 W ; (ii) have significant variation across the field of view, up to ∼15–20 per cent; and (iii) have temporal variations of ∼ 3–4 per cent across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M 92, 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 M 92 main sequence (∼0.1 M⊙ ), and reveal 24 white dwarf candidate members of M 92 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 M 92.