JWST photometry and astrometry of 47 Tucanae. Discontinuity in the stellar sequence at the star/brown dwarf transition

Abstract

Using JWST Near Infrared Camera (NIRCam) images of the globular cluster 47 Tucanae (or NGC 104), taken at two epochs just 7 months apart, we derived proper-motion membership down to $m_{\rm F322W2} \sim 27$. We identified an intriguing feature at the very low-mass end of the main sequence, around $\sim$ 0.08 solar masses, at magnitudes $m_{\rm F322W2} \sim 24$ and $m_{\rm F150W2} \sim 25$. This feature, dubbed "kink", is characterized by a prominent discontinuity in the slope of the main sequence. A similar discontinuity is seen in theoretical isochrones with oxygen-poor chemistries, related to the rapid onset of CH$_4$ absorption. We therefore hypothesize that the cluster hosts disproportionately more oxygen-poor stars near the bottom of the main sequence compared to the upper main sequence and the red giant branch. Our results show no strong or conclusive evidence of a rise in the brown dwarf luminosity function at faint magnitudes, in contrast to previous findings likely affected by faint red background galaxies. In our analysis, we accounted for this contamination by using proper motion membership.