2023MNRAS.523.4739S

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 (≲10^4.5 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 (Mg_xFe_1 - x_SiO3_) 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.