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Title: The atomic and molecular content of disks around very low-mass stars and brown dwarfs

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{sup +}, H{sub 2}, CO{sub 2}, and tentative detections of H{sub 2}O toward these faint and low-mass disks. Two of our [Ne II] 12.81 μm emission lines likely trace the hot (≥5000 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 [Ne II] 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 ≥600 K disk gas. These trends suggest that the inner regions ofmore » 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.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4]
  1. Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721 (United States)
  2. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China)
  3. Naval Research Laboratory, Code 7211, Washington, DC 20375 (United States)
  4. Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, D-85748 Garching (Germany)
Publication Date:
OSTI Identifier:
22348392
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 779; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; CARBON DIOXIDE; DENSITY; DWARF STARS; EMISSION; HYDROCYANIC ACID; HYDROGEN; LTE; MASS; NEON IONS; PROTOPLANETS; RESOLUTION; SIMULATION; SPECTRA; STARS; WATER; X RADIATION