Observations of pre- and proto-brown dwarfs in nearby clouds: Paving the way to further constraining theories of brown dwarf formation

Brown Dwarfs (BDs) are crucial objects in our understanding of both star and planet formation, as well as in our understanding of what mechanisms shape the lower end of the Initial Mass Function (IMF). However, since the discovery of the first BD in 1995, there is still an unconcluded debate about which is the dominant formation mechanism of these objects. For this, it is mandatory to study BDs in their earliest evolutionary stages (what we call pre- and proto-BDs), comparable to the ‘pre-stellar’ and ‘Class 0/I’ stages well characterized for the formation of low-mass stars. In this review, the recent efforts aimed at searching, identifying and characterizing pre- and proto-BD candidates in nearby star-forming regions are presented, and revised requirements for an object to be a promising proto-BD or pre-BD candidate are provided, based on a new, unexplored so far, relation between the internal luminosity and the accreted mass. By applying these requirements, a list of 68 promising proto-BD candidates is presented, along with a compilation of possible pre-BDs from the literature. In addition, updated correlations of protostellar properties such as mass infall rate or outflow momentum rate with bolometric luminosity are provided down to the low-mass BD regime, where no significant deviations are apparent. Furthermore, the number proto-BD candidates in different clouds of the Solar Neighborhood seem to follow the known relations of number of protostars with cloud properties. In addition, proto(star-to-BD) ratios for the different clouds are also explored, unveiling a particular underproduction of low-mass proto-BD candidates in Ophiuchus compared to Lupus and Taurus. Possible explanations for this behavior are discussed, including heating of the Ophiuchus cloud by the nearby OB stars. The overall results of this work, along with the possibility that the planetary-mass regime of the IMF is subtly shaped by stellar feedback, tend to favor a Jeans-fragmentation process and therefore a star-like formation scenario down to the planetary boundary, of $\sim 0.01$ $M_{\odot }$, below which other mechanisms might be at work. Future observational constraints, such as those provided by upcoming facilities like the next-generation Very Large Array, or the use of isotope ratios based on James Webb Space Telescope data, will provide definite clues to disentangle the origin of BDs in the planetary-mass regime.