JWST Imaging of the Closest Globular Clusters -- VI. The Lowest-Mass Objects in M 4 and the Galactic Bulge

Abstract

We present `James Webb Space Telescope' observations of M4 -- the closest globular cluster -- that probe the lower Main Sequence down to the hydrogen-burning limit. The unveiled stellar sequence reaches much fainter luminosities than previously possible, revealing a few extremely red objects that are consistent with brown dwarfs as cool as T_eff~1000K. However, the lack of a second JWST epoch presently prevents us from verifying the cluster membership of these objects. By cross-matching our data with archival `Hubble Space Telescope' images, we are able to verify cluster membership for a subset of objects down to T_eff~3000K. The observed color distribution indicate that the lower Main Sequence of M4 is likely deficient in oxygen compared to its higher-mass post-Main Sequence members by ~0.5dex. This feature has now been observed in three different globular clusters (M4, NGC6397 and 47Tuc), suggesting a general trend. Finally, we derive the mass function of the Galactic bulge in the background of M4. The mass function was found to have the bottom-heavy slope of alpha=0.88+/-0.36 and appears to terminate at ~0.15 M_Sun, although the latter value may be overestimated due to the limited sample size.

15 pages, 7 figures. Accepted for publication on Astronomische Nachrichten on 15th July 2025. Manuscript ID: ASNA.20250054