Hierarchical gravitational contraction: a unified scenario for the structure, dynamics, and star-forming activity of molecular clouds.

September 20, 2018
Phillips Auditorium
Abstract: 

Diverse numerical and observational evidence suggests that star-forming
molecular clouds (MCs) may be in a process of global gravitational
contraction. As originally proposed by Hoyle (1953), in such a regime, a
sequential destabilization of successively smaller masses should occur,
leading to fragmentation of the cloud and ultimately to the formation of
stellar-mass objects, when the equation of state diverts from
isothermal. After disposing of some early objections to the global
gravitational contraction of MCs, I discuss how this mechanism
naturally explains the observed apparent virialization of clouds and
their substructures, the appearance of Larson's relations when
column-density thresholds are used to define the structures, the
ubiquitous formation of filamentary structures that funnel material to
so-called "hubs", the observed morphology of the magnetic field around
the filaments, the scattered nature of low-mass star-forming regions,
the acceleration of the star formation rate in MCs, the observed
SFR-mass relations at both the local (cloud) level and the global
(galactic) level, and the structure of the embedded stellar
associations, such as their fractal structure and the observed radial
mass and age gradients. I conclude by comparing to the prevailing
"gravo-turbulent" scenario, and note a few common misconceptions about
turbulence.

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