ABSTRACT
We perform radiation hydrodynamical simulations in spherical symmetry in order to investigate the formation of very low mass objects, i.e. brown dwarfs, by external compression. According to the Jeans stability criterion, a very low mass molecular cloud core must reach a very high density in order to become gravitationally unstable. One possibility to create such a high density is the compression by turbulent flows within the larger molecular cloud. Using our self-developed radiation hydrodynamics code, we aim to test the validity of this scenario, and to constrain the strength of the turbulence that is needed. We find that the probability for sufficiently strong and long-lived turbulence is very low under typical conditions even when using very optimistic assumptions, and therefore conclude that turbulent compression is unlikely to be the dominant mechanism for creating brown dwarfs. We also investigate the properties of objects formed by this turbulent compression process. Specifically, we compare the lifetime of the first core stage for the cases with and without external compression. We confirm our previous findings that the first core lifetime increases by about an order of magnitude at the extremely low-mass end, but this increase is somewhat less dramatic and occurs at even lower masses than in our previous work, in which no external compression was present.
A U-band survey of brown dwarfs in the Taurus molecular
cloud with the XMM-Newton optical/UV monitor