THE 0.8–14.5 μm SPECTRA OF MID-L TO MID-T DWARFS: DIAGNOSTICS OF EFFECTIVE TEMPERATURE, GRAIN SEDIMENTATION, GAS TRANSPORT, AND SURFACE GRAVITY

By D. Stephens, S. Leggett, M. Cushing, M. Marley, D. Saumon, T. Geballe, D. Golimowski, Xiaohui Fan, Keith S. Noll. 2009

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We present new 5.2–14.5 μm low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0–4.1 μm spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (∼0.6–2.5 μm), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing et al. We compare these spectra to those generated from the model atmospheres of Saumon & Marley. The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381−0158521. The broad wavelength coverage allows us to constrain almost independently the four parameters used to describe these photospheres in our models: effective temperature (Teff), surface gravity, grain sedimentation efficiency (fsed), and vertical gas transport efficiency (Kzz). The CH4 bands centered at 2.2, 3.3, and 7.65 μm and the CO band at 2.3 μm are sensitive to Kzz, and indicates that chemical mixing is important in all L and T dwarf atmospheres. The sample of L3.5 to T5.5 dwarfs spans the range 1800 K≳ Teff ≳1000 K, with an L–T transition (spectral types L7 to T4) that lies between 1400 and 1100 K for dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K warmer or cooler, respectively, when using infrared spectral types. When using optical spectral types, the bluer dwarfs have more typical Teff values as they tend to have earlier optical spectral types. In this model analysis, fsed increases rapidly between types T0 and T4, indicating that increased sedimentation can explain the rapid disappearance of clouds at this stage of brown dwarf evolution. There is a suggestion that the transition to dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/34b8a047edea49b75f6ec51c02e97249ab489e45

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Citations in the corpus

GASEOUS MEAN OPACITIES FOR GIANT PLANET AND ULTRACOOL DWARF ATMOSPHERES OVER A RANGE OF METALLICITIES AND TEMPERATURES (2014, background): “The generally good agreement between models of brown dwarf atmospheres and observations (Stephens et al. 2009; Saumon et al. 2012; Morley et al. 2012) suggests that we are on the right track in understanding the chemical abundances and opacity of cool atmospheres.”
THE HAWAII INFRARED PARALLAX PROGRAM. I. ULTRACOOL BINARIES AND THE L/T TRANSITION, (2012, background): “…of the late-L and early-T dwarfs, including non-equilibrium (dynamic) processes such rapid particle growth/sedimentation (Knapp et al. 2004; Stephens et al. 2009) and cloud disruption leading to spatially inhomogeneous photospheres (Ackerman & Marley 2001; Burgasser et al. 2002; Marley et…”
MID-INFRARED PHOTOMETRY OF COLD BROWN DWARFS: DIVERSITY IN AGE, MASS, AND METALLICITY (2010, methodology): “We generally find that the colors and spectra of L dwarfs are reproduced with fsed = 1 – 2, while those of later T dwarfs are reproduced with fsed = 4 or cloud-free (fsed → ∞) models (e.g. Knapp et al. 2004; Stephens et al. 2009).”
An Accurate, Extensive, and Practical Line List of Methane for the HITEMP Database (2020, background): “For mid-tolate L dwarfs, CH4 absorption can remain observable near 3.3 µm for higher temperatures (Noll et al. 2000; Stephens et al. 2009).”
CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b (2011, background): “Stephens et al. (2009), fitting their model atmospheres to available near-IR and mid-IR spectra of SDSS 1516+30, found Teff = 1000 to 1100 K, log(g) = 4.5 and a fairly cloudy atmosphere for such a low Teff (fsed = 1 to 2, in the Ackerman & Marley (2001) parlance).”
MASSES, RADII, AND CLOUD PROPERTIES OF THE HR 8799 PLANETS (2012, background): “In Cushing et al. (2008) and Stephens et al. (2009), we compared our group’s models to observed far-red to mid-infrared spectra of L and T dwarfs, including L dwarfs with IR spectral types as late as L9 (with seven objects in the range L7 to L9.”
LARGE-AMPLITUDE VARIATIONS OF AN L/T TRANSITION BROWN DWARF: MULTI-WAVELENGTH OBSERVATIONS OF PATCHY, HIGH-CONTRAST CLOUD FEATURES (2012, methodology): “…of dust condensation and settling can be used to explore atmospheric properties of field BDs including effective temperature, gravity, metallicity, vertical mixing (via departures from chemical equilibrium), and condensate clouds (e.g. Cushing et al. 2008; Stephens et al. 2009; Witte et al. 2011).”
A STUDY OF THE DIVERSE T DWARF POPULATION REVEALED BY WISE (2013, background): “T dwarfs are classified by increased methane, water, and molecular hydrogen collisioninduced absorption (CIA) in the near-infrared (Burgasser et al. 2002, 2006b; Geballe et al. 2002), and have inferred temperatures between ∼500 and ∼1300 K (Cushing et al. 2008, 2011; Stephens et al. 2009).”
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS (2016, background): “For objects of spectral type L (Teff ≈ 1300−2300 K; Golimowski et al. 2004; Stephens et al. 2009), a field object of typical age (≈1–5 Gyr) has a mass of ≈40–80 MJup according to evolutionary models (Chabrier et al. 2000; Saumon & Marley 2008).”
BANYAN. VII. A NEW POPULATION OF YOUNG SUBSTELLAR CANDIDATE MEMBERS OF NEARBY MOVING GROUPS FROM THE BASS SURVEY (2015, methodology): “2MASS J08575849+5708514 has been discovered by Geballe et al. (2002) as an L8±1 BD. Stephens et al. (2009) used atmosphere model fitting to determine that this object is unusually cloudy and seems to have a low surface gravity (logg = 4.5).”
The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations (2012, background): “It is thought that the colours of L dwarfs are explained by the appearance of clouds, which then sink below the photosphere and become invisible in T dwarfs (Burgasser et al. 2006; Saumon & Marley 2008; Stephens et al. 2009).”
THE EXTREMELY RED, YOUNG L DWARF PSO J318.5338−22.8603: A FREE-FLOATING PLANETARY-MASS ANALOG TO DIRECTLY IMAGED YOUNG GAS-GIANT PLANETS (2013, methodology): “…with the resulting temperatures being comparable to previous fitting of field L dwarfs but with both young objects having lower surface gravities (log(g)=4.0–4.5 dex) than field objects (Cushing et al. 2008; Stephens et al. 2009; Testi 2009, though the number of fitted field objects is small).”
SpeX SPECTROSCOPY OF UNRESOLVED VERY LOW MASS BINARIES. I. IDENTIFICATION OF 17 CANDIDATE BINARIES STRADDLING THE L DWARF/T DWARF TRANSITION (2009, background): “On the other hand, SDSS J1750+1759 and 2MASS J05591404 have both been suggested as unresolved multiples (Golimowski et al. 2004; Liu et al. 2006; Stephens et al. 2009), although neither satisfy any of our selection criteria.”
A PATCHY CLOUD MODEL FOR THE L TO T DWARF TRANSITION (2010, background): “Cushing et al. (2008) and Stephens et al. (2009) have demonstrated that progressively later T dwarfs can be fit by increasing fsed across the transition.”
A new set of atmosphere and evolution models for cool T–Y brown dwarfs and giant exoplanets (2020, background): “…have been developed (Allard et al. 2001; Ackerman & Marley 2001; Helling et al. 2008a) and invoked to explain the reddening L dwarf spectral sequence (Chabrier et al. 2000; Cushing et al. 2008; Stephens et al. 2009; Witte et al. 2011), the sharp change to bluer near-infrared colours at the L–”
On the Cool Side: Modeling the Atmospheres of Brown Dwarfs and Giant Planets (2014, methodology): “54 Marley & Robinson www.annualreviews.org • Brown Dwarf and Giant Planet Atmospheres 55 Spectral comparison between observations and best-fit models (gray), after Stephens et al. (2009).”