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Clouds and Clearings in the Atmospheres of the L and T Dwarfs

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 Added by Mark S. Marley
 Publication date 2002
  fields Physics
and research's language is English




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A sophisticated approach to condensate opacity is required to properly model the atmospheres of L and T dwarfs. Here we review different models for the treatment of condensates in brown dwarf atmospheres. We conclude that models which include both particle sedimentation and upwards transport of condensate (both gas and particles) provide the best fit for the L dwarf colors. While a globally uniform cloud model fits the L dwarf data, it turns to the blue in J-K too slowly to fit the T dwarfs. Models which include local clearings in the global cloud deck, similar to Jupiters prominent five-micron hot spots, better reproduce the available photometric data and also account for the observed resurgence of FeH absorption in early type T dwarfs.



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100 - D. Saumon 2008
We present new evolution sequences for very low mass stars, brown dwarfs and giant planets and use them to explore a variety of influences on the evolution of these objects. We compare our results with previous work and discuss the causes of the differences and argue for the importance of the surface boundary condition provided by atmosphere models including clouds. The L- to T-type ultracool dwarf transition can be accommodated within the Ackerman & Marley (2001) cloud model by varying the cloud sedimentation parameter. We develop a simple model for the evolution across the L/T transition. By combining the evolution calculation and our atmosphere models, we generate colors and magnitudes of synthetic populations of ultracool dwarfs in the field and in galactic clusters. We focus on near infrared color- magnitude diagrams (CMDs) and on the nature of the ``second parameter that is responsible for the scatter of colors along the Teff sequence. Variations in metallicity and cloud parameters, unresolved binaries and possibly a relatively young population all play a role in defining the spread of brown dwarfs along the cooling sequence. We find that the transition from cloudy L dwarfs to cloudless T dwarfs slows down the evolution and causes a pile up of substellar objects in the transition region, in contradiction with previous studies. We apply the same model to the Pleiades brown dwarf sequence. Taken at face value, the Pleiades data suggest that the L/T transition occurs at lower Teff for lower gravity objects. The simulated populations of brown dwarfs also reveal that the phase of deuterium burning produces a distinctive feature in CMDs that should be detectable in ~50-100 Myr old clusters.
(Abridged) We have obtained radial velocities of a sample of 18 ultracool dwarfs (M6.5-T8) using high-resolution, near-infrared spectra obtained with NIRSPEC and the Keck II telescope. We have confirmed that the radial velocity of Gl 570 D is coincident with that of the K-type primary star Gl 570 A, thus providing additional support for their true companionship. The presence of planetary-mass companions around 2MASS J05591914-1404488 (T4.5V) has been analyzed using five NIRSPEC radial velocity measurements obtained over a period of 4.37 yr. We have computed UVW space motions for a total of 21 L and T dwarfs within 20 pc of the Sun. This population shows UVW velocities that nicely overlap the typical kinematics of solar to M-type stars within the same spatial volume. However, the mean Galactic (44.2 km/s) and tangential (36.5 km/s) velocities of the L and T dwarfs appear to be smaller than those of G to M stars. A significant fraction (~40%) of the L and T dwarfs lies near the Hyades moving group (0.4-2 Gyr), which contrasts with the 10-12% found for earlier-type stellar neighbors. Additionally, the distributions of all three UVW components (sigma_{UVW} = 30.2, 16.5, 15.8 km/s) and the distributions of the total Galactic (sigma_{v_tot} = 19.1 km/s) and tangential (sigma_{v_t} = 17.6 km/s) velocities derived for the L and T dwarf sample are narrower than those measured for nearby G, K, and M-type stars, but similar to the dispersions obtained for F stars. This suggests that, in the solar neighborhood, the L- and T-type ultracool dwarfs in our sample (including brown dwarfs) is kinematically younger than solar-type to early M stars with likely ages in the interval 0.5-4 Gyr.
We present an analysis of the 0.95-14.5 micron spectral energy distributions of nine field ultracool dwarfs with spectral types ranging from L1 to T4.5. Effective temperatures, gravities, and condensate cloud sedimentation efficiencies are derived by comparing the data to synthetic spectra computed from atmospheric models that self-consistently include the formation of condensate clouds. Derived effective temperatures decrease steadily through the L1 to T4.5 spectral types and we confirm that the effective temperatures of ultracool dwarfs at the L/T transition are nearly constant, decreasing by only ~200 K from spectral types L7.5 to T4.5. The two objects in our sample with very red J-Ks colors are best fitted with synthetic spectra that have thick clouds which hints at a possible correlation between the near-infrared colors of L dwarfs and the condensate cloud properties. The fits to the two T dwarfs in our sample (T2 and T4.5) also suggest that the clouds become thinner in this spectral class, in agreement with previous studies. Restricting the fits to narrower wavelength ranges (i.e., individual photometric bands) almost always yields excellent agreement between the data and models. Limitations in our knowledge of the opacities of key absorbers such as FeH, VO, and CH4 at certain wavelengths remain obvious, however. The effective temperatures obtained by fitting the narrower wavelength ranges can show a large scatter compared to the values derived by fitting the full spectral energy distributions; deviations are typically ~200 K and in the worst cases, up to 700 K.
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