The minimum mass for star formation, and the origin of binary brown dwarfs

Our first aim is to calculate the minimum mass for Primary Fragmentation in a variety of potential star-formation scenarios, i.e. (i) hierarchical fragmentation of a 3-D medium; (ii) one-shot, 2-D fragmentation of a shock-compressed layer; (iii) fragmentation of a circumstellar disc. Our second aim is to evaluate the role of H2 dissociation in facilitating Secondary Fragmentation and thereby producing close, low-mass binaries. Results: (i)For contemporary, local star formation, the minimum mass for Primary Fragmentation is in the range 0.001-0.004Msun, irrespective of the scenario considered. (ii)Circumstellar discs are only able to radiate fast enough to undergo Primary Fragmentation in their cool outer parts (R>100AU). Therefore brown dwarfs (BDs) should have difficulty forming by Primary Fragmentation at R100AU could be the source of brown dwarfs in wide orbits, and could explain why massive discs with Rd>100AU are rarely seen.(iii)H2 dissociation can lead to collapse and Secondary Fragmentation, thereby converting primary fragments into close, low-mass binaries, with semi-major axes a~5AU(Msystem/0.1Msun), in good agreement with observation; in this case, the minimum mass for Primary Fragmentation becomes a minimum system mass, rather than a minimum stellar mass.(iv)Any primary fragment can undergo Secondary Fragmentation, producing a close low-mass binary, provided only that the fragment is spinning. Secondary Fragmentation is therefore most likely in fragments formed in the outer parts of discs, and this could explain why a BD in a wide orbit about a Sun-like star has a greater likelihood of having a BD companion than a BD in the field -as seems to be observed