The dry and carbon poor inner disk of TW Hya: evidence for a massive icy
  dust trap
  Gas giants accrete their envelopes from the gas and dust of proto-planetary
disks, so it is important to determine the composition of the inner few AU,
where most giant planets are expected to form. We aim to constrain the
elemental carbon and oxygen abundance in the inner disk ($R<$2.3 AU) of TW Hya
and compare with the outer disk ($R>2.3$ AU) where carbon and oxygen appear
underabundant by a factor of $\sim$50. Archival infrared observations of TW Hya
are compared with a detailed thermo-chemical model, DALI. The inner disk gas
mass and elemental C and O abundances are varied to fit the infrared CO, H$_2$
and H$_2$O line fluxes. Best fitting models have an inner disk that has a gas
mass of $ 2 \times 10^{-4} M_\odot$ with C/H $\approx 3 \times 10^{-6}$ and O/H
$\approx 6 \times 10^{-6}$. The elemental oxygen and carbon abundances of the
inner disk are $\sim 50$ times underabundant compared to the ISM and are
consistent with those found in the outer disk. The uniformly low volatile
abundances imply that the inner disk is not enriched by ices on drifting bodies
that evaporate. This indicates that drifting grains are stopped in a dust trap
outside the water ice line. Such a dust trap would also form a cavity as seen
in high resolution sub-millimeter continuum observations. If CO is the major
carbon carrier in the ices, dust needs to be trapped efficiently outside the CO
ice line of $\sim$20 AU. This would imply that the shallow sub-millimeter rings
in the TW Hya disk outside of 20 AU correspond to very efficient dust traps.
The more likely scenario is that more than 98\% of the CO has been converted
into less volatile species, e.g. CO$_2$ and CH$_3$OH. A giant planet forming in
the inner disk would be accreting gas with low carbon and oxygen abundances as
well as very little icy dust, potentially leading to a planet atmosphere with
strongly substellar C/H and O/H ratios.
