A new formation scenario of a counter-rotating circumstellar disk:
  spiral-arm accretion from a circumbinary disk in a triple protostar system
  We present the evolution of rotational directions of circumstellar disks in a
triple protostar system simulated from a turbulent molecular cloud core with no
magnetic field. We find a new formation pathway of a counter-rotating
circumstellar disk in such triple systems. The tertiary protostar forms via the
circumbinary disk fragmentation and the initial rotational directions of all
the three circumstellar disks are almost parallel to that of the orbital motion
of the binary system. Their mutual gravito-hydrodynamical interaction for the
subsequent $\sim10^4\thinspace\rm{yr}$ greatly disturbs the orbit of the
tertiary, and the rotational directions of the tertiary disk and star are
reversed due to the spiral-arm accretion of the circumbinary disk. The
counter-rotation of the tertiary circumstellar disk continues to the end of the
simulation ($\sim6.4\times10^4\thinspace\rm{yr}$ after its formation), implying
that the counter-rotating disk is long-lived. This new formation pathway during
the disk evolution in Class 0/I Young Stellar Objects possibly explains the
counter-rotating disks recently discovered by ALMA.
