The Coupled Physical Structure of Gas and Dust in the IM Lup
  Protoplanetary Disk
  The spatial distribution of gas and solids in protoplanetary disks determines
the composition and formation efficiency of planetary systems. A number of
disks show starkly different distributions for the gas and small grains
compared to millimeter-centimeter sized dust. We present new Atacama Large
Millimeter/Submillimeter Array (ALMA) observations of the dust continuum, CO,
$^{13}$CO, and C$^{18}$O in the IM Lup protoplanetary disk, one of the first
systems where this dust-gas dichotomy was clearly seen. The $^{12}$CO is
detected out to a radius of 970 AU, while the millimeter continuum emission is
truncated at just 313 AU. Based upon this data, we have built a comprehensive
physical and chemical model for the disk structure, which takes into account
the complex, coupled nature of the gas and dust and the interplay between the
local and external environment. We constrain the distributions of gas and dust,
the gas temperatures, the CO abundances, the CO optical depths, and the
incident external radiation field. We find that the reduction/removal of dust
from the outer disk exposes this region to higher stellar and external
radiation and decreases the rate of freeze-out, allowing CO to remain in the
gas out to large radial distances. We estimate a gas-phase CO abundance of
$5\%$ of the ISM value and a low external radiation field ($G_0\lesssim4$). The
latter is consistent with that expected from the local stellar population. We
additionally find tentative evidence for ring-like continuum substructure,
suggestions of isotope-selective photodissociation, and a diffuse gas halo.
