Ultracool dwarfs observed with the Spitzer Infrared Spectrograph - III. Dust grains in young L dwarf atmospheres are heavier
Abstract
Analysis of all archival 5-14 micron spectra of field ultracool dwarfs from the Infrared Spectrograph on the Spitzer Space Telescope has shown that absorption by silicates in the 8-11 micron region is seen in most L-type (1300 to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the atmospheres of L dwarfs and is strongest at L4-L6 spectral types. Herein we compare averages of the mid-infrared silicate absorption signatures of L3-L7 dwarfs that have low (≲104.5 cm s-2) versus high (≳105 cm s-2) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity, with young atmospheres having a broader, redder, and more asymmetric absorption profile. This indicates a difference in grain size and composition between dust condensates in young and old mid-L dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs matches expectations for ~1 micron-sized amorphous iron- and magnesium-bearing pyroxene (MgxFe1 - xSiO3) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller (≲0.1 μm) and more volatile amorphous enstatite (MgSiO3) or SiO grains. This is the first direct spectroscopic evidence for gravity-dependent sedimentation of dust condensates in ultracool atmospheres. It confirms theoretical expectations for lower sedimentation efficiencies in low-gravity atmospheres and independently confirms their increased dustiness.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2023
- DOI:
- 10.1093/mnras/stad1711
- arXiv:
- arXiv:2306.01119
- Bibcode:
- 2023MNRAS.523.4739S
- Keywords:
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- stars: atmospheres;
- brown dwarfs;
- infrared: stars;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted for publication in MNRAS