  Hydrodynamical simulations show that a pair of spiral arms can form in the
disk around a rapidly-growing young star and that the arms are crucial in
transporting angular momentum as the disk accretes material from the
surrounding envelope. Here we report the detection of a pair of symmetric
spiral structures in a protostellar disk, supporting the formation of spiral
arms in the disk around a forming star. The HH 111 VLA 1 source is a young
Class I source embedded in a massive infalling protostellar envelope and is
actively accreting, driving the prominent HH 111 jet. Previous observations
showed a ring of shock emission around the disk's outer edge, indicating
accretion of the envelope material onto the disk at a high rate. Now with ALMA
observations of thermal emission from dust particles, we detect a pair of
spiral arms extending from the inner region to the disk's outer edge, similar
to that seen in many simulations. Additionally, the disk is massive, with
Toomre's Q parameter near unity in the outer parts where the spiral structures
are detected, supporting the notion that envelope accretion is driving the
outer disk gravitationally unstable. In our observations, another source, HH
111 VLA 2, is spatially resolved for the first time, showing a disk-like
structure with a diameter of ~ 26 au and an orientation nearly orthogonal to
that of the HH 111 VLA 1 disk.
