Star-disk alignment in the protoplanetary disks: SPH simulation of the
  collapse of turbulent molecular cloud cores
  We perform a series of three-dimensional smoothed particle hydrodynamics
(SPH) simulations to study the evolution of the angle between the protostellar
spin and the protoplanetary disk rotation axes (the star-disk angle $\psi_{\rm
sd}$) in turbulent molecular cloud cores. While $\psi_{\rm sd}$ at the
protostar formation epoch exhibits broad distribution up to $\sim 130^{\circ}$,
$\psi_{\rm sd}$ decreases ($\lesssim 20^{\circ}$) in a timescale of $\sim
10^{4}$ yr. This timescale of the star-disk alignment, $t_{\rm alignment}$,
corresponds basically to the mass doubling time of the central protostar, in
which the protostar forgets its initial spin direction due to the mass
accretion from the disk. Values of $\psi_{\rm sd}$ both at $t=10^2$ yr and
$t=10^5$ yr after the protostar formation are independent of the ratios of
thermal and turbulent energies to gravitational energy of the initial cloud
cores: $\alpha=E_{\rm thermal}/|E_{\rm gravity}|$ and $\gamma_{\rm turb}=E_{\rm
turbulence}/|E_{\rm gravity}|$. We also find that a warped disk is possibly
formed by the turbulent accretion flow from the circumstellar envelope.
