  The concentric gaps and rings commonly observed in protoplanetary disks in
millimeter continuum emission have lent the impression that planet formation
generally proceeds within orderly, isolated systems. While deep observations of
spatially resolved molecular emission have been comparatively limited, they are
increasingly suggesting that some disks interact with their surroundings while
planet formation is underway. We present an analysis of complex features
identified around GM Aur in $^{12}$CO $J=2-1$ images at a spatial resolution of
$\sim40$ au. In addition to a Keplerian disk extending to a radius of $\sim550$
au, the CO emission traces flocculent spiral arms out to radii of $\sim$1200
au, a tail extending $\sim1800$ au southwest of GM Aur, and diffuse structures
extending from the north side of the disk up to radii of $\sim1900$ au. The
diffuse structures coincide with a "dust ribbon" previously identified in
scattered light. The large-scale asymmetric gas features present a striking
contrast with the mostly axisymmetric, multi-ringed millimeter continuum
tracing the pebble disk. We hypothesize that GM Aur's complex gas structures
result from late infall of remnant envelope or cloud material onto the disk.
The morphological similarities to the SU Aur and AB Aur systems, which are also
located in the L1517 cloud, provide additional support to a scenario in which
interactions with the environment are playing a role in regulating the
distribution and transport of material in all three of these Class II disk
systems. This paper is part of the MAPS special issue of the Astrophysical
Journal Supplement.
