Multiple stellar populations in globular clusters with JWST: an NIRCam view of 47 Tucanae

Abstract

We use images collected with the near-infrared camera (NIRCam) on board the James Webb Space Telescope and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main sequence (MS) of 47 Tucanae. The mF115W vs. mF115W − mF322W2 CMD from NIRCam shows that, below the knee, the MS stars span a wide color range, where the majority of M-dwarfs exhibit blue colors, and a tail of stars are distributed toward the red. A similar pattern is observed from the mF160W vs. mF110W − mF160W color-magnitude diagram (CMD) from HST, and multiple populations of M-dwarfs are also visible in the optical mF606W vs. mF606W − mF814W CMD. The NIRCam CMD shows a narrow sequence of faint MS stars with masses smaller than $0.1\, \mathcal {M}_{\odot }$. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colors and magnitudes of different stellar populations in the NIRCam filters (at metallicities [Fe/H]=-1.5 and [Fe/H]=-0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs’ chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colors of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, constraining the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.

Multiple Stellar Populations in Metal-poor Globular Clusters with JWST: A NIRCam View of M92

“…The magnitude difference is primarily due to the effect of H 2 O and CO molecules in the atmospheres of these stars. The 2G stars, which are O-depleted with respect to the 1G, exhibit weaker lines of these molecules and hence brighter fluxes when compared to 1G stars (Salaris et al 2019;Milone et al 2023b).…”

“…The difference is that we derived a spatially variable PSF model for each image based on the available unsaturated, bright, and isolated stars. To do this, we used the computer program img2psf, originally developed by for images collected with the Wide Field Imager of the 2.2 m telescope in La Silla and adapted by Milone et al (2023b) for NIRCam (see Milone et al 2023b for details).…”

“…Recently, Milone et al (2023b) computed isochrones of different stellar populations in the James Webb Space Telescope (JWST)/NIRCam filters for metal-rich ([Fe/H] = −0.75) and metal-intermediate ([Fe/H] = −1.5) GCs. The results allowed them to identify photometric diagrams suitable for identifying and characterizing multiple stellar populations.…”

“…NIRCam observations confirm these theoretical results. Milone et al (2023b) showed that the M dwarfs of 47 Tucanae span a wide F115W-F322W2 color interval. The color broadening is due to the different amounts of blanketing from molecules composed of oxygen (mostly water vapor) in the F322W2 filter, as 2G stars have lower oxygen and hence low H 2 O content, compared to 1G stars that are oxygen-rich and hence H 2 O-rich.…”

“…The color broadening is due to the different amounts of blanketing from molecules composed of oxygen (mostly water vapor) in the F322W2 filter, as 2G stars have lower oxygen and hence low H 2 O content, compared to 1G stars that are oxygen-rich and hence H 2 O-rich. Based on a pseudo-two-color diagram or "chromosome map" (ChM), Milone et al (2023b) unveiled an extended first population and three main groups of secondpopulation stars among the M dwarfs of 47 Tucanae with different oxygen abundances. Isochrones indicate that the diagrams composed of NIRCam photometry alone have poor sensitivity to multiple populations among stars that are brighter than the MS knee, with the exception of stars close to the tip of the red giant branch (RGB), where we expect that 1G and 2G stars exhibit different fluxes in certain NIRCam filters, like F277W and F460M.…”