Hierarchical gravitational fragmentation. I. Collapsing cores within collapsing clouds

The Hierarchical Gravitational Fragmentation scenario is investigated 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^−2 density profile. However, these structures are never hydrostatic. In this case, the central flat region is characterized by an infall speed linear with radius, while the envelope is characterized by a uniform infall speed. This implies that the hydrostatic SIS initial condition leading to Shuʼs 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 time-variable mass and radius, and evolves along the locus of the ensemble of observed prestellar cores in a plot of Mcore/MBE versus Mcore, 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.