Dust in brown dwarfs

Helling, Christiane; Woitke, P.

doi:10.1051/0004-6361:20054598

Cited by 178 publications

(226 citation statements)

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“…The modified classical nucleation theory of Gail et al (1984) is applied to determine the nucleation rate (for more details see Woitke & Helling 2004). The dust growth of dirty particles is modeled according to Woitke & Helling (2003), Helling & Woitke (2006) and Helling et al (2008c The growth of the grains is governed by the supersaturation ratio of the gas and is triggered by the actual collision rates between grains and gas molecules and, therefore, depends on the grain surface. Thus, the model does not enforce a phase equilibrium.…”

Section: Methodsmentioning

confidence: 99%

Dust in brown dwarfs and extra-solar planets

Witte

Helling

Barman

et al. 2011

A&A

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112

123

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Context. This work is concerned with dust formation in ultra-cool atmospheres, encompassing the latest type stars, brown dwarfs, and hot giant exoplanets. Dust represents one of the most important and yet least understood sources of opacity in these types of objects. Aims. We compare our model spectra with SpeX data in order to draw conclusions about the dust cloud structure and related quantities in ultra-cool atmospheres. Methods. We use the self-consistent Drift-Phoenix atmosphere code, which features a kinetic dust formation mechanism and accounts for the dust cloud influence on the spectra. Results. We present fits of our latest model spectra to observations that cover a wide range of our model grid. The results are remarkably good, yielding significant improvement over the older Cond-/Dusty-Phoenix models, especially in the L-dwarf regime. The new models are able to properly reproduce observed spectra, including complicated features such as the molecular band strengths. This raises confidence in the reliability of our dust-modeling approach. Conclusions. We demonstrate that our code produces excellent results concerning the fitting with observations. This suggests that our dust cloud and atmosphere structures are reasonably accurate. Like all other current cloud models, ours is not able to produce satisfying results for spectral types later than L6 without manually tuning down the amount of dust. Our results show the formation of convective cells within the cloud, which are able to destroy the lower cloud parts. The dust opacity is reduced significantly without the need to tune the dust cloud thickness. There are indications that the cycle of dust accumulation and cloud destruction by convection is time-dependent on rather long timescales. Considering a statistical distribution of locally variable dust clouds over a dwarf's surface can result in a large number of spectral configurations for the same model atmosphere parameters, hence introducing an additional and more or less random degree of freedom to those atmospheres. Without resorting to the model atmosphere parameters, this alone can account for the unusually red and blue objects that have been discovered.

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Section: Methodsmentioning

confidence: 99%

Dust in brown dwarfs and extra-solar planets

Witte

Helling

Barman

et al. 2011

A&A

Self Cite

112

123

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“…(34) in Helling & Woitke (2006), applying the modified classical nucleation theory of Gail et al (1984). We use the value of the surface tension σ fitted to small cluster data by Jeong et al (2000) as outlined in Woitke & Helling (2004).…”

Section: Dust Seed Formationmentioning

confidence: 99%

“…The dust growth of dirty particles is modelled according to Woitke & Helling (2003), Helling & Woitke (2006) and Helling et al (2008c Nuth & Ferguson (2006); (3) Sharp & Huebner (1990). Data sources for the pure solid refractive indices: (A) Posch (2008); (B) Palik (1985Palik ( , 1991; (C) Jäger et al (2003); (D) Palik (1991), Begemann et al (1997). 15 (3 × Eq.…”

Section: Dust Growth/evaporation and Refractive Indicesmentioning

confidence: 99%

“…This function is not a direct result of the dust moment method. Only the total dust particle number density n d = ρL 0 and the mean particle size a = 3 √ 3/(4π) L 1 /L 0 are direct results that have been used in Helling & Woitke (2006). In this paper, we reconstruct f (a) from the calculated dust moments L j ( j = 1 ... 4) in an approximate way as described in Appendix A in Helling et al (2008c).…”

Section: Grain Size Distribution Functionmentioning

confidence: 99%

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Dust in brown dwarfs and extra-solar planets

Witte¹,

Helling²,

Hauschildt³

2009

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Context. Substellar objects have extremely long life spans. The cosmological consequence for older objects are low abundances for heavy elements, which in turn results in a wide distribution of objects over metallicity, hence over age. Within their cool atmosphere, dust clouds become a dominant feature, affecting the opacity and the remaining gas phase abundance of heavy elements. Aims. We investigate the influence of the stellar metallicity on the dust formation in substellar atmospheres and on the dust cloud structure and its feedback on the atmosphere. This work has implications for the general questions of star formation and of dust formation in the early universe. Methods. We utilise numerical simulations to solve a set of moment equations to determine the quasi-static dust cloud structure (Drift). These equations model the nucleation, the kinetic growth of composite particles, their evaporation, and the gravitational settling as a stationary dust formation process. Element conservation equations augment this system of equations by including the element replenishment by convective overshooting. The integration with an atmosphere code (Phoenix) allows determination of a consistent (T, p, v conv )-structure (T -local temperature, p -local pressure, v conv -convective velocity), hence, to calculate synthetic spectra. Results. A grid of Drift-Phoenix model atmospheres was calculated for a wide range of metallicity, [M/H] ∈ [+0.5, −0.0, −0.5, ..., −6.0], to allow for systematic study of atmospheric cloud structures throughout the evolution of the universe. We find dust clouds in even the most metal-poor ([M/H] = −6.0) atmosphere of brown dwarfs. Only the most massive among the youngest brown dwarfs and giant gas planets can resist dust formation. For very low heavy element abundances, a temperature inversion develops that has a drastic impact on the dust cloud structure. Conclusions. The combination of metal depletion by dust formation and the uncertainty of interior element abundances makes the modelling of substellar atmospheres an intricate problem in particular for old substellar objects. We furthermore show that the dust-togas ratio does not scale linearly with the object's [M/H] for a given effective temperature. The mean grain sizes and the composition of the grains change depending on [M/H], which influences the dust opacity that determines radiative heating and cooling, as well as the spectral appearance.

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“…These are compared with the turbulent mixing timescale to predict grain densities and sizes. Woitke & Helling (2003 and Helling & Woitke (2006) have developed a kinetic (non-equilibrium) model for the nucleation, accretion, gravitational settling and evaporation of dust grains. A version of this cloud model has been integrated with the PHOENIX stellar atmosphere code (Hauschildt & Baron 1999) to provide the DRIFT-PHOENIX models for substellar atmospheres (Helling et al 2008a).…”

Section: Clouds In Brown Dwarfsmentioning

confidence: 99%

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

Bailey

2014

Publ. Astron. Soc. Aust.

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The last few years has seen a dramatic increase in the number of exoplanets known and in the range of methods for characterising their atmospheric properties. At the same time, new discoveries of increasingly cooler brown dwarfs have pushed down their temperature range which now extends down to Y-dwarfs of < 300 K. Modelling of these atmospheres has required the development of new techniques to deal with the molecular chemistry and clouds in these objects. The atmospheres of brown dwarfs are relatively well understood, but some problems remain, in particular the behavior of clouds at the L/T transition. Observational data for exoplanet atmosphere characterisation is largely limited to giant exoplanets that are hot because they are near to their star (hot Jupiters) or because they are young and still cooling. For these planets there is good evidence for the presence of CO and H2O absorptions in the IR. Sodium absorption is observed in a number of objects. Reflected light measurements show that some giant exoplanets are very dark, indicating a cloud free atmosphere. However, there is also good evidence for clouds and haze in some other planets. It is also well established that some highly irradiated planets have inflated radii, though the mechanism for this inflation is not yet clear. Some other issues in the composition and structure of giant exoplanet atmospheres such as the occurrence of inverted temperature structures, the presence or absence of CO2and CH4, and the occurrence of high C/O ratios are still the subject of investigation and debate.

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Dust in brown dwarfs

Cited by 178 publications

References 42 publications

Dust in brown dwarfs and extra-solar planets

Dust in brown dwarfs and extra-solar planets

Dust in brown dwarfs and extra-solar planets

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

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