  Idealized protoplanetary disk and giant planet formation models have been
interpreted to suggest that a giant planet's atmospheric abundances can be used
to infer its formation location in its parent protoplanetary disk. It has
recently been reported that the hot Jupiter WASP-77 A b has sub-solar
atmospheric carbon and oxygen abundances with a solar C/O abundance ratio.
Assuming solar carbon and oxygen abundances for its host star WASP-77 A,
WASP-77 A b's atmospheric carbon and oxygen abundances possibly indicate that
it accreted its envelope interior to its parent protoplanetary disk's H2O ice
line from carbon-depleted gas with little subsequent planetesimal accretion or
core erosion. We comprehensively model WASP-77 A and use our results to better
characterize WASP-77 A b. We show that the photospheric abundances of carbon
and oxygen in WASP-77 A are super-solar with a sub-solar C/O abundance ratio,
implying that WASP-77 A b's atmosphere has significantly sub-stellar carbon and
oxygen abundances with a super-stellar C/O ratio. Our result possibly indicates
that WASP-77 A b's envelope was accreted by the planet beyond its parent
protoplanetary disk's H2O ice line. While numerous theoretical complications to
these idealized models have now been identified, the possibility of non-solar
protoplanetary disk abundance ratios confound even the most sophisticated
protoplanetary disk and giant planet formation models. We therefore argue that
giant planet atmospheric abundance ratios can only be meaningfully interpreted
relative to the possibly non-solar mean compositions of their parent
protoplanetary disks as recorded in the photospheric abundances of their
solar-type dwarf host stars.
