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 1 . Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks 2, 3 , but this approach suffers from large uncertainties in grain growth and gas-to-dust ratio 2 . On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir 4 .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 Hya, 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 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 the total gas/millimetersized dust mass ratio is 140 in this region, suggesting that at least 2.4 M ⊕ of dust aggregates have grown to >centimeter 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.The primary theory for the formation of giant planets is the so-called core accretion scenario, where a rock+ice core forms through the coagulation of planetesimals until it becomes sufficiently massive to accrete a gaseous envelope 1 . In this theory, the spatial distribution of gas in the primitive nebula is not only critical to the later accretion of the atmosphere of giant planets, but also plays an 1

CO and Dust Properties in the TW Hya Disk from High-resolution ALMA Observations

“…The peak brightness temperatures between radii of 25 and 90 AU range from ∼70 to 35 K, which are about twice the value of the gas temperatures derived by Schwarz et al (2016) from multiple transitions of the rarer 13 CO and C 18 O isotopologues. Zhang et al (2017) estimate that the C 18 O J = 3 − 2 flux contribution largely originates from within two gas scale heights of the midplane, in contrast to three to four scale heights for 12 CO J = 3 − 2. The difference in inferred gas temperatures for the isotopologues implies a steep vertical temperature gradient.…”

“…Like the 12 CO channel maps, the 13 CO channel maps in Figure C2 also show a steep dropoff in intensity at a radius of ≈ 90 AU, followed by very faint emission extending out to ≈ 200 AU, which is slightly less extended than the 12 CO emission. C 18 O exhibits an outer emission ring that sharply drops off within a radius of 100 AU (Zhang et al 2017), but no emission is observed beyond this radius. The apparent differences between the isotopologues is likely primarily due to sensitivity limits, but may also be partially due to selective photodissociation in the more tenuous outer disk (e.g.…”

“…The 13 CO and C 18 O data in Schwarz et al (2016), Nomura et al (2016), and Zhang et al (2017) show a bright core of emission, an annular gap at ≈ 40 AU, and an outer emission ring peaking at ≈ 65 AU. In the channel maps (see Figure C2 for 13 CO and Zhang et al (2017) Temperatures between 17 and 27 K are shaded gray to show CO freezeout temperatures that have been estimated for TW Hya (e.g.…”

“…ALMA observations of the C 18 O and 13 CO J = 3 − 2 transitions in the TW Hya disk have been presented in Schwarz et al (2016), Nomura et al (2016), andZhang et al (2017) at spatial resolutions ranging from 0. 3 to 0.…”

“…In the channel maps (see Figure C2 for 13 CO and Zhang et al (2017) Temperatures between 17 and 27 K are shaded gray to show CO freezeout temperatures that have been estimated for TW Hya (e.g. Qi et al 2013 andZhang et al 2017). …”