# Hunting for Brown Dwarfs in the Globular Cluster M4: 2nd epoch HST NIR observations

## TL;DR

This study analyzes second epoch HST NIR data of globular cluster M4, identifying a candidate that could be a white dwarf, white dwarf/brown dwarf binary, or a brown dwarf, but no definitive brown dwarf detection is confirmed.

## Contribution

First second-epoch HST NIR analysis of M4 that refines candidate classifications and explores the potential presence of brown dwarfs in the cluster.

## Key findings

- One candidate is likely a white dwarf or binary, not a brown dwarf.
- The data suggests we are close to detecting the brown dwarf cooling sequence.
- No definitive brown dwarf has been confirmed in the cluster yet.

## Abstract

We present an analysis of the second epoch HST WFC3 F110W near-Infrared (NIR) imaging data of the globular cluster M4. The new dataset suggests that one of the previously suggested four brown dwarf candidates in this cluster is indeed a high-probability cluster member. The position of this object in the NIR colour magnitude diagrams (CMDs) is in the white dwarf/brown dwarf area. The source is too faint to be a low-mass main sequence star, but, according to theoretical considerations, also most likely somewhat too bright to be a bona-fide brown dwarf. Since we know that the source is a cluster member, we determined a new optical magnitude estimate at the position the source should have in the optical image. This new estimate places the source closer to the white dwarf sequence in the optical-NIR CMD and suggests that it might be a very cool (T_eff < 4500 K) white dwarf at the bottom of the white dwarf cooling sequence in M4, or a white dwarf/brown dwarf binary. We cannot entirely exclude the possibility that the source is a very massive, bright brown dwarf, or a very low-mass main sequence star, however, we conclude that we still have not convincingly detected a brown dwarf in a globular cluster, but we expect to be very close to the start of the brown dwarf cooling sequence in this cluster. We also note that the main sequence ends at F110W approx.22.5 mag in the proper-motion cleaned CMDs, where completeness is still high.