Dust in brown dwarfs

Helling, Christiane; Oevermann, Michael; Lüttke, M.; Klein, Rupert; Sedlmayr, E.

doi:10.1051/0004-6361:20010937

Cited by 86 publications

(110 citation statements)

References 35 publications

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“…Dynamical processes in the atmospheres of individual objects ultimately control the growth and sedimentation of condensates and the optical properties of the various cloud decks expected in these atmospheres (e.g., Helling et al 2001Helling et al , 2004Woitke & Helling 2003). Although our modeling parameter, f sed , aims to self-consistently capture these processes in a tractable way, the global properties of each individual object must ultimately define the true properties of the clouds.…”

Section: The Modelsmentioning

confidence: 99%

Atmospheric Parameters of Field L and T Dwarfs1

Cushing

Marley

Saumon

et al. 2008

ApJ

313

448

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We present an analysis of the 0.95Y14.5 m 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. Overall, the model spectra fit the data well, although the agreement at some wavelengths remains poor due to remaining inadequacies in the models. Derived effective temperatures decrease steadily through the L1YT4.5 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 condensate cloud properties vary significantly among the L dwarfs in our sample, ranging from very thick clouds to relatively thin clouds with no particular trend with spectral type. The two objects in our sample with very red J À K s colors are, however, 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 CH 4 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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Section: The Modelsmentioning

confidence: 99%

Atmospheric Parameters of Field L and T Dwarfs1

Cushing

Marley

Saumon

et al. 2008

ApJ

313

448

View full text Add to dashboard Cite

show abstract

“…For more details, please refer to Bilger et al (2013) and Helling et al (2000) for the thermodynamical data used. The Grevesse et al (2007) solar composition is used to calculate the gas-phase chemistry outside the metal depleted cloud layers and before cloud formation.…”

Section: Chemical Equilibrium Routinementioning

confidence: 99%

Dust in brown dwarfs and extra-solar planets

Lee

Helling

Giles

et al. 2015

A&A

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Context. Clouds form in atmospheres of brown dwarfs and planets. The cloud particle formation processes, seed formation and growth/evaporation are very similar to the dust formation process studied in circumstellar shells of AGB stars and in supernovae. Cloud formation modelling in substellar objects requires gravitational settling and element replenishment in addition to element depletion. All processes depend on the local conditions, and a simultaneous treatment is required. Aims. We apply new material data in order to assess our cloud formation model results regarding the treatment of the formation of condensation seeds. We look again at the question of the primary nucleation species in view of new (TiO 2 ) N -cluster data and new SiO vapour pressure data. Methods. We applied the density functional theory (B3LYP, 6-311G(d)) using the computational chemistry package G 09 to derive updated thermodynamical data for (TiO 2 ) N clusters as input for our TiO 2 seed formation model. We tested different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation for a prescribed D-P atmosphere structure. Results. Updated Gibbs free energies for the (TiO 2 ) N clusters are presented, as well as a slightly temperature dependent surface tension for T = 500 . . . 2000 K with an average value of σ ∞ = 480.6 erg cm −2 . The TiO 2 seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO nucleation. The question about the most efficient nucleation species can only be answered if all dust/cloud formation processes and their feedback are taken into account. Despite the higher abundance of SiO over TiO 2 in the gas phase, TiO 2 remains considerably more efficient at forming condensation seeds by homogeneous nucleation. The paper discusses the effect on the cloud structure in more detail.

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“…It is impossible to account for all microphysical processes that might play a role in dust formation (Helling et al 2001;Woitke & Helling 2003) in current time-dependent multi-dimensional simulations. We instead chose a treatment of dust that includes only the most important physical processes.…”

Section: Dust Modelmentioning

confidence: 99%

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Freytag¹,

Allard

Ludwig

et al. 2010

A&A

218

297

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Context. Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. Aims. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. Methods. We performed two-dimensional radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for a sequence of effective temperatures between 900 K and 2800 K, all with log g = 5 assuming solar chemical composition.Results. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in transporting and mixing material than gravity waves. The presence of clouds often leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales (presently inaccessible to observations). Conclusions. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.

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Dust in brown dwarfs

Cited by 86 publications

References 35 publications

Atmospheric Parameters of Field L and T Dwarfs1

Atmospheric Parameters of Field L and T Dwarfs1

Dust in brown dwarfs and extra-solar planets

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

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