Ultracool dwarfs observed with the Spitzer Infrared Spectrograph – III. Dust grains in young L dwarf atmospheres are heavier

Abstract

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.