Substructure formation in starless cores
Abstract
Motivated by recent observational searches of substructure in starless molecular cloud cores, we investigate the evolution of density perturbations on scales smaller than the Jeans length embedded in contracting isothermal clouds, adopting the same formalism developed for the expanding Universe and the solar wind. We find that initially small amplitude, Jeansstable perturbations (propagating as sound waves in the absence of a magnetic field) are amplified adiabatically during the contraction, approximately conserving the wave action density, until they either become nonlinear and steepen into shocks at a time t_{nl}, or become gravitationally unstable when the Jeans length decreases below the scale of the perturbations at a time t_{gr}. We evaluate analytically the time t_{nl} at which the perturbations enter the nonlinear stage using a Burgers' equation approach, and we verify numerically that this time marks the beginning of the phase of rapid dissipation of the kinetic energy of the perturbations. We then show that for typical values of the rms Mach number in molecular cloud cores, t_{nl} is smaller than t_{gr}, and therefore density perturbations likely dissipate before becoming gravitational unstable. Solenoidal modes grow at a faster rate than compressible modes, and may eventually promote fragmentation through the formation of vortical structures.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 February 2018
 DOI:
 10.1093/mnras/stx2790
 arXiv:
 arXiv:1710.09124
 Bibcode:
 2018MNRAS.474.1288T
 Keywords:

 hydrodynamics;
 ISM: clouds;
 ISM: kinematics and dynamics;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 8 pages, 4 figures