Grand Design Spiral Arms in A Young Forming Circumstellar Disk
  We study formation and long-term evolution of a circumstellar disk in a
collapsing molecular cloud core using a resistive magnetohydrodynamic
simulation. While the formed circumstellar disk is initially small, it grows as
accretion continues and its radius becomes as large as 200 AUs toward the end
of the Class-I phase. A pair of grand-design spiral arms form due to
gravitational instability in the disk, and they transfer angular momentum in
the highly resistive disk. Although the spiral arms disappear in a few
rotations as expected in a classical theory, new spiral arms form recurrently
as the disk soon becomes unstable again by gas accretion. Such recurrent spiral
arms persist throughout the Class-0 and I phase. We then perform synthetic
observations and compare our model with a recent high-resolution observation of
a young stellar object Elias 2-27, whose circumstellar disk has grand design
spiral arms. We find good agreement between our theoretical model and the
observation. Our model suggests that the grand design spiral arms around Elias
2-27 are consistent with material arms formed by gravitational instability. If
such spiral arms commonly exist in young circumstellar disks, it implies that
young circumstellar disks are considerably massive and gravitational
instability is the key process of angular momentum transport.
