Mass inventory of the giant-planet formation zone in a solar nebula analogue

Abstract

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today's Solar System. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks, but this approach suffers from large uncertainties in dust opacity and gas-to-dust ratio. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir. Here we present the first partially spatially resolved observations of the ^(13)C^(18)O J = 3–2 line emission in the closest protoplanetary disk, TW Hydrae, a gas tracer that probes the bulk mass distribution. Combining it with the C^(18)O J = 3–2 emission and the previously detected HD J = 1–0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution with radius, R, of 13^(+8)_(–5)×(R/20.5au)^(–0.9^(+0.4)_(–0.3)) g cm^(−2) in the expected formation zone of gas and ice giants (5–21 au). We find that the mass ratio of total gas to millimetre-sized dust is 140 in this region, suggesting that at least 2.4M_⊕ of dust aggregates have grown to centimetre sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.

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