Deep near-IR observations of the Globular Cluster M: Hunting for Brown Dwarfs
Dieball et al
We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD) clearly shows the main sequence extending towards the expected end of the Hydrogen burning limit and going beyond this point towards fainter sources. The white dwarf sequence can be identified. As such, this is the deepect NIR CMD of a globular cluster to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the WDs from BD candidates. Detection limits in the NIR are around F110W approx. 26.5 mag and F160W approx. 27 mag, and in the optical around F775W approx. 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical-NIR CMDs. Thus, faint NIR sources that have no optical counterpart are potential Brown Dwarf candidates. We visually inspected the positions of NIR sources fainter than H-burning limit in F110W and without optical counterparts in the optical images. We found one such source which is therefore a very good BD candidate. For a further four faint NIR sources upper optical magnitude limits of a possible optical counterpart are estimated, resulting in three additional good BD candidates and one WD candidate.