  During a common envelope episode in a binary system, the engulfed companion
spirals to tighter orbital separations under the influence of drag from the
surrounding envelope material. As this object sweeps through material with a
steep radial gradient of density, net angular momentum is introduced into the
flow, potentially leading to the formation of an accretion disk. The presence
of a disk would have dramatic consequences for the outcome of the interaction
because accretion might be accompanied by strong, polar outflows with enough
energy to unbind the entire envelope. Without a detailed understanding of the
necessary conditions for disk formation during common envelope, therefore, it
is difficult to accurately predict the population of merging compact binaries.
This paper examines the conditions for disk formation around objects embedded
within common envelopes using the `wind tunnel' formalism developed by MacLeod
et al. (2017). We find that the formation of disks is highly dependent on the
compressibility of the envelope material. Disks form only in the most
compressible of stellar envelope gas, found in envelopes' outer layers in zones
of partial ionization. These zones are largest in low-mass stellar envelopes,
but comprise small portions of the envelope mass and radius in all cases. We
conclude that disk formation and associated accretion feedback in common
envelope is rare, and if it occurs, transitory. The implication for LIGO black
hole binary assembly is that by avoiding strong accretion feedback, common
envelope interactions should still result in the substantial orbital tightening
needed to produce merging binaries.
