Hi All

Recent study of a disk-star encounter and whether it produces planets/brown-dwarfs through gravitational instability indicates that shocks and similar compressional processes keep the disk heated up too much for direct collapse of planets from the disk...

The role of the energy equation in the fragmentation of protostellar discs during stellar encounters

...which challenges earlier work of Boffin et. al. and Woolfson & Dormand on this process and its role in making planets. The current debate in cosmogony is mainly between core-accretion and disk-instability. In the former a 10 Earth-mass core accretes like a souped up terrestrial planet and then it rapidly captures gas from the nebula. In the latter regions of the disk become dense enough to collapse directly into planets by being denser/heavier than the Jeans Mass.

What has been found by this study, and others, is that the disk forms travelling spiral shockwaves too easily and these keep the disk energised. Unless the disk can efficiently cool within a short timespan any over-density in the disk will be destroyed. What the new study indicates is that close encounters between stars and stars-with-disks are insufficient to overcome this difficulty, unlike earlier theoretical and simulation work which indicated it was quite easy to make brown-dwarfs/Jovians via tidal disruptions of protostars/protostellar disks.

As more exoplanets are found with heavy, high-Z cores the harder it is to argue for disk instability to play a major role in making planets. Only core accretion seems to do the trick in sufficient numbers.

Adam