Hunting for brown dwarfs in the globular cluster M4: second epoch HST NIR observations

We present an analysis of the second epoch HST WFC3 F110W near-Infrared (NIR) imaging data of the globular clusterM4. The new dataset suggests that one of the previously suggested four brown dwarfcandidates 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 dwarfarea. The source is too faint to be a low-mass main sequencestar, 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 dwarfsequence in the optical-NIR CMD and suggests that it might be a very cool (T_eff < 4500 K) white dwarfat the bottom of the white dwarfcooling sequence in M4, or a white dwarf/ brown dwarfbinary. We cannot entirely exclude the possibility that the source is a very massive, bright brown dwarf, or a very low-mass main sequencestar, however, we conclude that we still have not convincingly detected a brown dwarfin a globular cluster, but we expect to be very close to the start of the brown dwarfcooling sequence in this cluster. We also note that the main sequenceends at F110W approx.22.5 mag in the proper-motioncleaned CMDs, where completeness is still high.