Wednesday 14 May 2014: CANCELLED - The stability of isothermal dense cores - CANCELLED
Dr Eva Ntormousi - CEA, Saclay (Paris)
Physics, 4th floor 14:00-15:00
This seminar is CANCELLED due to flight problems.
The issue of pre-stellar core fragmentation is tightly correlated to the understanding of binary and multiple system formation. At the same time, numerical experiments of gravitational collapse of turbulent cores indicate that the results are strongly determined by the choice of initial conditions, and especially by the shape of the radial density profile of the core. In this work this dependence in explored by studying the fate of linear perturbations of a homogeneous gas sphere or to the Larson-Penston (LP) collapse flow, both analytically and numerically.
The analytical problem for the LP flow has been studied in the past, however for perturbations with only a real part. Here it is considered in its full form, taking into account also the oscillatory component of the linear disturbances. We find a number of stable and some unstable modes, which however have very low growth rates. The spectrum of eigenvalues, composed by a discrete series of oscillatory modes, can be interpreted as sonic-like waves propagating in the flow. We thus confirm the overall stability of the LP flow, with the exception of a marginal instability to disturbances of l = 2. The previously unknown oscillatory components can act to remove energy from the core at the first stages of collapse.
On the other hand, a homogeneous sphere is found unstable to perturbations. A shock boundary condition is used in the analytic treatment in order to connect the subsonic and the supersonic regimes. The growth rates thus calculated are compared to Adaptive Mesh Refinement (AMR) numerical simulations of the linear stages of the collapse. These numerical simulations allow us to explore non-axisymmetric perturbations and derive the linear growth rates as a function of the core's initial Virial parameter and as a function of the azimuthal wave number of the perturbation.