EPoS Contribution
EPoS Contribution
Hierarchical Gravitational Fragmentation

Enrique Vazquez-Semadeni
CRyA, Morelia, MX
I will discuss the scenario of Hierarchical Gravitational Fragmentation, consisting of collapses within collapses, and argue that it corresponds to the physical state of molecular clouds (MCs). This scenario is similar to Hoyle fragmentation but seeded with nonlinear density fluctuations produced by turbulence. In it, a MC is formed from cold, gravitationally unstable density fluctuations in the atomic medium, which may form themselves from larger-scale instabilities or turbulent compressions. The formation mechanism induces transonic or moderately (though not strongly) supersonic turbulence in the cloud that does not by itself induce collapse initially. As a cold cloud contracts, its mean density increases, reducing its mean Jeans mass, so that the turbulent density fluctuations begin collapsing when the mean Jeans mass becomes smaller than their own mass. At this point, star formation begins, long before the global collapse is completed. The collapsing clumps are therefore immersed in an environment that is also collapsing, albeit on a longer timescale. The collapsing fluctuations have shorter collapse times than the free-fall time and may be considered to be confined by ram pressure of the surrounding material. Strongly supersonic velocities are a result of the collapse, and not viceversa, as is often considered, and do not provide support against the clumps' self-gravity.
Collaborators:
M. Zamora-Aviles, CRyA, Mexico
J. Toala, IAA-CSIC, Spain
R. Naranjo-Romero, CRyA, Mexico
J. Ballesteros-Paredes, CRyA, Mexico
G. Gomez, CRyA, Mexico
P. Colin, CRyA, Mexico
Suggested Session: Molecular Clouds