  The final composition of giant planets formed as a result of gravitational
instability in the disk gas depends on their ability to capture solid material
(planetesimals) during their 'pre-collapse' stage, when they are extended and
cold, and contracting quasi-statically. The duration of the pre-collapse stage
is inversely proportional roughly to the square of the planetary mass, so
massive protoplanets have shorter pre-collapse timescales and therefore limited
opportunity for planetesimal capture. The available accretion time for
protoplanets with masses of 3, 5, 7, and 10 Jupiter masses is found to be
7.82E4, 2.62E4, 1.17E4 and 5.67E3 years, respectively. The total mass that can
be captured by the protoplanets depends on the planetary mass, planetesimal
size, the radial distance of the protoplanet from the parent star, and the
local solid surface density. We consider three radial distances, 24, 38, and 68
AU, similar to the radial distances of the planets in the system HR 8799, and
estimate the mass of heavy elements that can be accreted. We find that for the
planetary masses usually adopted for the HR 8799 system, the amount of heavy
elements accreted by the planets is small, leaving them with nearly stellar
compositions.
