JWST photometry and astrometry of 47 Tucanæ

Discontinuity in the stellar sequence at the star–brown dwarf transition

¹ Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Giuseppe Saragat 1, Ferrara I-44122, Italy
² Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova I-35122, Italy
³ Department of Physics and Astronomy, University of Notre Dame, Nieuwland Science Hall, Notre Dame, Indiana 46556, USA
⁴ Department of Astronomy, Indiana University, Swain West, 727 E. 3rd Street, Bloomington, IN 47405, USA
⁵ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monteporzio Catone, Rome, Italy
⁶ Agenzia Spaziale Italiana – Space Science Data Center, Via del Politecnico, I-00133 Rome, Italy
⁷ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, Padova I-35122, Italy
⁸ Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, California 92093, USA
⁹ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
¹⁰ Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria
¹¹ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^⋆ Corresponding author; michele.scalco@inaf.it

Received: 6 November 2024
Accepted: 7 January 2025

Abstract

Using JWST Near Infrared Camera (NIRCam) images of the globular cluster 47 Tucanæ (or NGC 104), taken at two epochs just 7 months apart, we derived proper-motion membership down to m_F322W2 ∼ 27. We identified an intriguing feature at the very low-mass end of the main sequence, around ∼0.08 M_⋅, at magnitudes m_F322W2 ∼ 24 and m_F150W2 ∼ 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 CH₄ 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.