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 M 4—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 $T_{\mathrm{eff}}\sim 1000K$. 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 $T_{\mathrm{eff}}\sim 3000K$. The observed color distribution indicate that the lower Main Sequence of M 4 is likely deficient in oxygen compared to its higher-mass post-Main Sequence members by $\sim 0.5\mathrm{dex}$. This feature has now been observed in three different globular clusters (M 4, NGC 6397 and 47 Tuc), suggesting a general trend. Finally, we derive the mass function of the Galactic bulge in the background of M 4. The mass function was found to have the bottom-heavy slope of $\alpha =0.88\pm 0.36$ and appears to terminate at $\sim 0.15M_{\odot }$, although the latter value may be overestimated due to the limited sample size.