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.

arXiv commentAccepted for publication in MNRAS

References in corpus (60)

• The Infrared Spectrograph on the Spitzer Space Telescope
• A Non-Gray Theory of Extrasolar Giant Planets and Brown Dwarfs
• Precipitating Condensation Clouds in Substellar Atmospheres
• The Theory of Brown Dwarfs and Extrasolar Giant Planets
• The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars
• Sixty-seven Additional L Dwarfs Discovered by the Two Micron All Sky Survey (2MASS)
• The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
• Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Temperature, Clouds, and Gravity
• BANYAN. XI. The BANYAN $\Sigma$ multivariate Bayesian algorithm to identify members of young associations within 150 pc
• A Unified Near Infrared Spectral Classification Scheme for T Dwarfs
• Characterization of M,L and T dwarfs in the Sloan Digital Sky Survey
• Meeting the Cool New Neighbors V: A 2MASS-Selected Sample of Ultracool Dwarfs
• The 0.8-14.5 micron Spectra of Mid-L to Mid-T Dwarfs: Diagnostics of Effective Temperature, Grain Sedimentation, Gas Transport, and Surface Gravity
• Fundamental Parameters and Spectral Energy Distributions of Young and Field Age Objects with Masses Spanning the Stellar to Planetary Regime
• Brown Dwarf Companions to G-type Stars. I: Gliese 417B and Gliese 584C
• Atmospheric Parameters of Field L and T Dwarfs
• Discoveries from a Near-infrared Proper Motion Survey using Multi-epoch 2MASS Data
• Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5
• Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field
• Masses, Radii, and Cloud Properties of the HR 8799 Planets
• Weather on Other Worlds. II. Survey Results: Spots Are Ubiquitous on L and T Dwarfs
• Population Properties of Brown Dwarf Analogs to Exoplanets
• Evidence for Cloud Disruption in the L/T Dwarf Transition
• Photometric Variability of the T2.5 Brown Dwarf SIMP J013656.5+093347; Evidence for Evolving Weather Patterns
• Brown Dwarfs and the TW Hya Association
• Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs
• A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages
• Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574
• BANYAN. VII. A New Population of Young Substellar Candidate Members of Nearby Moving Groups from the BASS Survey
• Individual Dynamical Masses of Ultracool Dwarfs
• Four nearby L dwarfs
• A self-consistent cloud model for brown dwarfs and young giant exoplanets: comparison with photometric and spectroscopic observations
• The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b
• Model atmospheres for massive gas giants with thick clouds: Application to the HR 8799 planets and predictions for future detections
• Discovery of Two Nearby, Peculiar L Dwarfs from the 2MASS Proper Motion Survey: Young or Metal-Rich?
• The Age of AB Dor
• Clouds, Gravity and Metallicity in Blue L dwarfs: The Case of 2MASS J11263991-5003550
• Young Nearby Loose Associations
• A Flaring L5 Dwarf: The Nature of HαEmission in Very Low Mass (Sub)Stellar Objects
• Optical Spectroscopy of 2MASS Color-Selected Ultracool Subdwarfs
• Cloud busting: enstatite and quartz clouds in the atmosphere of 2M2224-0158
• Discovery of the Young L Dwarf WISE J1741102.78-464225.5
• Discovery of a Second L Subdwarf in the Two Micron All Sky Survey
• The Nearby, Young, Argus Association: Membership, Age, and Dusty Debris Disks
• A Kine-Chemical Investigation of the AB Dor Moving Group "Stream"
• A large spectroscopic sample of L and T dwarfs from UKIDSS LAS: peculiar objects, binaries, and space density
• Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. II. Emergence and Sedimentation of Silicate Clouds in L Dwarfs, and Analysis of the Full M5--T9 Field Dwarf Spectroscopic Sample
• Variability in a Young, L/T Transition Planetary-Mass Object
• Parallaxes of Southern Extremely Cool objects (PARSEC) II: Spectroscopic Follow-up and Parallaxes of 52 Targets
• Patchy Forsterite Clouds in the Atmospheres of Two Highly Variable Exoplanet Analogs
• The Ultracool SpeXtroscopic Survey. I. Volume-Limited Spectroscopic Sample and Luminosity Function of M7$-$L5 Ultracool Dwarfs
• A Search for Variability in Exoplanet Analogues and Low-Gravity Brown Dwarfs
• Variability of the Lowest Mass Objects in the AB Doradus Moving Group
• Strong Near-Infrared Spectral Variability of the Young Cloudy L Dwarf Companion VHS J1256-1257 b
• Empirically Determining Substellar Cloud Compositions in the era of JWST
• Cloud Atlas: Hubble Space Telescope Near-Infrared Spectral Library of Brown Dwarfs, Planetary-mass companions, and hot Jupiters
• Dust in brown dwarfs and extra-solar planets. VII. Cloud formation in diffusive atmospheres
• Serendipitous Discovery and Parallax of a Nearby L Dwarf
• Infrared Parallaxes of Young Field Brown Dwarfs and Connections to Directly Imaged Gas-Giant Exoplanets
• Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. I. An Accurate Look at the L-to-T Transition at $\sim$300 Myr from Optical through Mid-infrared Spectrophotometry

Cited by in corpus (7)

• Four-of-a-kind? Comprehensive atmospheric characterisation of the HR 8799 planets with VLTI/GRAVITY
• Evidence for SiO cloud nucleation in the rogue planet PSO J318
• VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B
• The JWST Weather Report: retrieving temperature variations, auroral heating, and static cloud coverage on SIMP-0136
• Vanadium oxide clusters in substellar atmospheres: A quantum chemical study
• Large-amplitude Variability Driven by Giant Dust Storms on a Planetary-mass Companion
• Silicate Clouds, SiO, and Molecular Trends across the L-T Sequence: A JWST/MIRI LRS Spectral Library of Brown Dwarfs