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 K 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 ($\lesssim$10$^{4.5}$ cm s$^{-2}$) vs.\ high ($\gtrsim$10$^5$ cm s$^{-2}$) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity and 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 $\sim$1 micron-sized amorphous iron- and magnesium-bearing pyroxene (Mg$_x$Fe$_{1-x}$SiO$_3$) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller ($\lesssim$0.1 $\mu$m) and more volatile amorphous enstatite (MgSiO$_3$) 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.