Title: Possible formation pathways for the low density Neptune-mass planet HAT-P-26b,
Abstract: We investigate possible pathways for the formation of the low density
Neptune-mass planet HAT-P-26b. We use two formation different models based on
pebbles and planetesimals accretion, and includes gas accretion, disk migration
and simple photoevaporation. The models tracks the atmospheric oxygen
abundance, in addition to the orbital period, and mass of the forming planets,
that we compare to HAT-P-26b. We find that pebbles accretion can explain this
planet more naturally than planetesimals accretion that fails completely unless
we artificially enhance the disk metallicity significantly. Pebble accretion
models can reproduce HAT-P-26b with either a high initial core mass and low
amount of envelope enrichment through core erosion or pebbles dissolution, or
the opposite, with both scenarios being possible. Assuming a low envelope
enrichment factor as expected from convection theory and comparable to the
values we can infer from the D/H measurements in Uranus and Neptune, our most
probable formation pathway for HAT-P-26b is through pebble accretion starting
around 10 AU early in the disk's lifetime.