Measuring the atomic composition of planetary building blocks. (arXiv:1910.07345v2 [astro-ph.EP] UPDATED)

Measuring the atomic composition of planetary building blocks. (arXiv:1910.07345v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+McClure_M/0/1/0/all/0/1">M. K. McClure</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dominik_C/0/1/0/all/0/1">C. Dominik</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kama_M/0/1/0/all/0/1">M. Kama</a>

Volatile molecules are critical to terrestrial planetary habitability, yet
difficult to observe directly where planets form at the midplanes of
protoplanetary disks. It is unclear whether the inner 1 AU of disks are
volatile-poor or if this region is resupplied with ice-rich dust from colder
disk regions. Dust traps at radial pressure maxima bounding disk gaps can cut
off the inner disk from such volatile reservoirs. However, the trap retention
efficiency and atomic composition of trapped dust have not been measured.

We present a new technique to measure the absolute atomic abundances in gas
accreting onto T Tauri stars and infer the bulk atomic composition and
distribution of midplane solids retained in the disk around the young star TW
Hya. We identify line emission from gas-phase material inside the dust
sublimation rim of TW Hya. Gaussian decomposition of the strongest H Paschen
lines isolates the inner disk hydrogen emission. We measure several key
elemental abundances, relative to hydrogen, using a chemical photoionization
model and infer dust retention in the disk. With a 1D transport model, we
determine approximate radial locations and retention efficiencies of dust traps
for different elements. Volatile and refractory elements are depleted from TW
Hya’s hot gas by factors of ~10^2 and up to 10^5, respectively. Dust traps
beyond the CO and N2 snowline cumulatively sequester 96% of the total dust,
while the trap at 2 AU retains 3% of the initial dust mass. The high depletions
of Si, Mg, and Ca are explained by a third trap at 0.3 AU. TW Hya has a
significant volatile reservoir rich in C- and N-ices in its outer ring
structure. However, the lack of C resupply may leave the terrestrial
planet-forming region dry and carbon-poor. Any planets that form within the
silicate dust trap at 0.3 AU may resemble Earth in terms of the degree of their
volatile depletion.

Volatile molecules are critical to terrestrial planetary habitability, yet
difficult to observe directly where planets form at the midplanes of
protoplanetary disks. It is unclear whether the inner 1 AU of disks are
volatile-poor or if this region is resupplied with ice-rich dust from colder
disk regions. Dust traps at radial pressure maxima bounding disk gaps can cut
off the inner disk from such volatile reservoirs. However, the trap retention
efficiency and atomic composition of trapped dust have not been measured.

We present a new technique to measure the absolute atomic abundances in gas
accreting onto T Tauri stars and infer the bulk atomic composition and
distribution of midplane solids retained in the disk around the young star TW
Hya. We identify line emission from gas-phase material inside the dust
sublimation rim of TW Hya. Gaussian decomposition of the strongest H Paschen
lines isolates the inner disk hydrogen emission. We measure several key
elemental abundances, relative to hydrogen, using a chemical photoionization
model and infer dust retention in the disk. With a 1D transport model, we
determine approximate radial locations and retention efficiencies of dust traps
for different elements. Volatile and refractory elements are depleted from TW
Hya’s hot gas by factors of ~10^2 and up to 10^5, respectively. Dust traps
beyond the CO and N2 snowline cumulatively sequester 96% of the total dust,
while the trap at 2 AU retains 3% of the initial dust mass. The high depletions
of Si, Mg, and Ca are explained by a third trap at 0.3 AU. TW Hya has a
significant volatile reservoir rich in C- and N-ices in its outer ring
structure. However, the lack of C resupply may leave the terrestrial
planet-forming region dry and carbon-poor. Any planets that form within the
silicate dust trap at 0.3 AU may resemble Earth in terms of the degree of their
volatile depletion.

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