H2O Distribution In The Disc Of HD 100546 And HD 163296: The Role Of Dust Dynamics And Planet–Disc Interaction

By Keith Cowing
July 25, 2022
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H2O Distribution In The Disc Of HD 100546 And HD 163296: The Role Of Dust Dynamics And Planet–Disc Interaction
DALI disc structure for HD 163296. The top panels show the gas and dust density structure, the gas temperature, and the H2O abundance structure. The three main H2O reservoirs are indicated by the labels s1, s2, and s3. The bottom panels show the model predictions of the SED (left), CO ladder (middle), and H2O fluxes (right). The observed fluxes and 3σ upper limits are shown as (grey) circles and open triangles, respectively. The representative model shown as a red line and red squares) reproduces the SED (left) and the CO rotational ladder (middle) well but is not able to reproduce the H2O line fluxes and upper limits. We note that the H2O abundances shown here are those of the representative disc model based on the full gas-grain chemistry calculation.

Far-infrared observations with Herschel revealed a surprisingly low abundance of cold-water reservoirs in protoplanetary discs. On the other hand, a handful of discs show emission of hot water transitions excited at temperatures above a few hundred Kelvin.

In particular, the protoplanetary discs around the Herbig Ae stars HD 100546 and HD 163296 show opposite trends in terms of cold versus hot water emission: in the first case, the ground-state transitions are detected and the high-J lines are undetected, while the trend is opposite in HD 163296. We performed a spectral analysis using the thermo-chemical model DALI. We find that HD 163296 is characterised by a water-rich (abundance ≳10−5) hot inner disc (within the snowline) and a water-poor (<10−10) outer disc: the relative abundance may be due to the thermal desorption of icy grains that have migrated inward. Remarkably, the size of the H2O emitting region corresponds to a narrow dust gap visible in the millimeter continuum at r=10au with ALMA. The low-J lines detected in HD 100546 instead imply an abundance of a few 10−9 in the cold outer disc (>40 au). The emitting region of the cold H2O transitions is spatially coincident with that of the H2O ice previously seen in the near-infrared. Notably, millimetre observations with ALMA reveal the presence of a large dust gap between nearly 40 and 150 au, likely opened by a massive embedded protoplanet. In both discs, we find that the warm molecular layer in the outer region (beyond the snow line) is highly depleted of water molecules, implying an oxygen-poor chemical composition of the gas. We speculate that gas-phase oxygen in the outer disc is readily depleted and its distribution in the disc is tightly coupled to the dynamics of the dust grains.

L.M. Pirovano, D. Fedele, E.F. van Dishoeck, M.R. Hogerheijde, G. Lodato, S. Bruderer

Comments: Accepted for publication on A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2207.10744 [astro-ph.EP] (or arXiv:2207.10744v1 [astro-ph.EP] for this version)
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Submission history
From: Davide Fedele
[v1] Thu, 21 Jul 2022 20:38:36 UTC (10,259 KB)
Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻