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

TL;DR

This study uses JWST imaging to explore the lowest-mass objects in M4 and the Galactic bulge, revealing potential brown dwarfs and a bottom-heavy mass function, with implications for stellar composition and cluster evolution.

Contribution

First JWST observations of M4's lower Main Sequence down to the hydrogen-burning limit, revealing extremely red objects and insights into cluster composition.

Findings

Detection of extremely red, cool objects consistent with brown dwarfs.

Evidence of oxygen deficiency in M4's lower Main Sequence.

Mass function of the Galactic bulge is bottom-heavy with a slope of 0.88.

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.