Hierarchical Gravitational Fragmentation. I. Collapsing Cores within Collapsing Clouds
Astrophys. J., (Nov 19, 2015)DOI: 10.1088/0004-637x/814/1/48
Abstract
We investigate the Hierarchical Gravitational Fragmentation scenario
through numerical simulations of the prestellar stages of the collapse
of a marginally gravitationally unstable isothermal sphere immersed in a
strongly gravitationally unstable, uniform background medium. The core
developes a Bonnor-Ebert (BE)-like density profile, while at the
time of singularity (the protostar) formation the envelope approaches a
singular-isothermal-sphere (SIS)-like r<SUP>-2</SUP> density
profile. However, these structures are never hydrostatic. In this case,
the central flat region is characterized by an infall speed, while the
envelope is characterized by a uniform speed. This implies that the
hydrostatic SIS initial condition leading to Shu's classical inside-out
solution is not expected to occur, and therefore neither should the
inside-out solution. Instead, the solution collapses from the
outside-in, naturally explaining the observation of extended infall
velocities. The core, defined by the radius at which it merges with the
background, has a time-variable mass, and evolves along the locus of the
ensemble of observed prestellar cores in a plot of M/M<SUB>BE</SUB>
versus M, where M is the core's mass and M<SUB>BE</SUB> is the critical
BE mass, spanning the range from the ``stable'' to the
``unstable'' regimes, even though it is collapsing at all
times. We conclude that the presence of an unstable background allows a
core to evolve dynamically from the time when it first appears, even
when it resembles a pressure-confined, stable BE-sphere. The core can be
thought of as a ram-pressure confined BE-sphere, with an increasing mass
due to the accretion from the unstable background.