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Pascucci, I; Herczeg, G; Carr, J S; Bruderer, S
Astrophysical journal/The Astrophysical journal, 12/2013, Volume: 779, Issue: 2Journal Article
There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne super(+), H sub(2), CO sub(2), and tentative detections of H sub(2)O toward these faint and low-mass disks. Two of our NeII 12.81 mu m emission lines likely trace the hot (> or =, slanted5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H sub(2) S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the NeII emission in gas at ~600 K. We confirm the higher C sub(2)H sub(2)/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H sub(2)O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H sub(2)O lines trace warm > or =, slanted600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.
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