The Atomic and Molecular Content of Disks around Very Low-mass Stars and Brown Dwarfs
Abstract
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+, H2, CO2, and tentative detections of H2O 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 H2 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 C2H2/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/H2O 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 H2O lines trace warm >=600 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2013
- DOI:
- 10.1088/0004-637X/779/2/178
- arXiv:
- arXiv:1311.1228
- Bibcode:
- 2013ApJ...779..178P
- Keywords:
-
- accretion;
- accretion disks;
- brown dwarfs;
- protoplanetary disks;
- stars: low-mass;
- stars: pre-main sequence;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted to ApJ