Hierarchical Fragmentation


A cartoon display of hierarchical structures in a molecular cloud. For analysis of fragmentation scenario in all these scales for the Perseus molecular cloud.

In this paper I provide the first ever observational evidence of hierarchical fragmentation of a molecular cloud over five scales that are separated by five orders of magnitude. A molecular cloud of over 10 pc size does not collapse as a whole in a monolithic pattern to make a group of stars, rather a wave of supersonic turbulence sets density enhancements in certain regimes, and each over-dense region later contracts by gravitational instability. Turns out, each such region further produces other over-dense regions, and this process is repeated in a hierarchical fashion until stars are born in the densest regions. An outward pressure in clouds acts opposing the inward gravitational collapse of a cloud. If the outward pressure is the same as the inward pull, the cloud attains a balanced state called hydrostatic equilibrium. If outward pressure is more, the cloud becomes unbound and expands, but if outward pressure is less, the cloud contracts gravitationally. I test the possibility of thermal and nonthermal pressure as candidates for outward pressure in each level of hierarchy, and find that the role of thermal pressure increases sharply when going from large to small scales in the Perseus molecular cloud.

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