Dust in brown dwarfs and extra-solar planets

Helling, Christiane; Woitke, P.; Thi, W. F.

doi:10.1051/0004-6361:20078220

Cited by 158 publications

(217 citation statements)

References 43 publications

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“…The general-purpose model atmosphere code Phoenix (Hauschildt & Baron 1999;Baron et al 2003) solves the gas-phase equation of state, provides the atmosphere structure to Drift, determines the gas opacities and solves the radiative transfer. On the other hand, the Drift code by Helling et al (2008c) returns a consistent dust cloud structure with corresponding opacities and the altitude-dependent depletion and redistribution of gas phase abundances, which feed back on both the thermodynamical structures and the radiation field. An iteration of this method allows the determination of stationary atmosphere and dust cloud properties and yields the respective synthetic spectra.…”

Section: Methodsmentioning

confidence: 99%

“…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%

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Dust in brown dwarfs and extra-solar planets

Witte

Helling

Barman

et al. 2011

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108

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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%

Section: Methodsmentioning

confidence: 99%

Dust in brown dwarfs and extra-solar planets

Witte

Helling

Barman

et al. 2011

A&A

Self Cite

108

123

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show abstract

“…The general-purpose model atmosphere code Phoenix (Hauschildt & Baron 1999;Baron et al 2003) solves the gas-phase equation of state, provides the atmosphere structure to Drift, determines the gas opacities and solves the radiative transfer. The Drift code by Helling et al (2008c) is included as a module in order to calculate the dust clouds, which feed back on both the thermodynamical structures and the radiation field. An iteration of this method allows the determination of the atmosphere and dust cloud properties and yields the corresponding synthetic spectra.…”

Section: Methodsmentioning

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%

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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“…For T eff below 3000 K, del Burgo et al (2009b) used the Drift-PHOENIX code to produce theoretical spectra of late M-and T-dwarfs, which satisfactorily reproduced the observed spectra. That code is a merger of the PHOENIX code and the dust model Drift (Helling et al 2008a;Witte et al 2009). The dust grains are composites and yield improved opacities in contrast to the grains in earlier models, and the use of a kinematic, phase-nonequilibrium dust-formation model avoids an overestimated condensation/evaporation (Helling et al 2008b).…”

Section: Theoretical Models For M-and L-dwarfsmentioning

confidence: 99%

Detecting planets around very cool dwarfs at near infrared wavelengths with the radial velocity technique

Rodler

Burgo

Witte

et al. 2011

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Context. Radial velocity monitoring of very cool dwarfs such as late M-and hot L-dwarfs has become a promising tool in the search for rocky planets as well as follow-up planetary candidates around dwarfs detected by transit surveys. These stars are faint at optical wavelengths, as their spectral flux distribution peaks at near-infrared (NIR) wavelengths. For this reason, it is desirable to measure the radial velocities in this wavelength regime. However, in the NIR very few medium-and high-resolution spectrographs are available at large telescopes. In the near future, high-resolution spectrographs for the NIR will be built, which will allow us to search for rocky planets around cool M-dwarfs and L-dwarfs from radial velocities monitoring. Aims. We investigate the precision that can be attained in radial velocity measurements of very cool dwarfs in the NIR. The goal is to determine in which atmospheric window of the Earth's atmosphere the highest radial velocity precision can be achieved to help in designing the next generation of NIR high-resolution spectrographs. Methods. We use stellar atmosphere synthetic models for an M-and an L-dwarf with temperatures of 2200 K and 1800 K, respectively, and a theoretical spectrum of the Earth's transmission in the spectral range from 0.9 to 2.5 μm. We simulate a series of Doppler-shifted spectra observed with different resolving powers and signal-to-noise ratios, and for different rotational broadenings of the dwarf. For different combinations of the input parameters, we recover the radial velocity by means of cross-correlation with a high signal-to-noise ratio template and determine the associate uncertainties. Results. The highest precision in radial velocity measurements for the cool M-dwarf is found in the Y band around 1.0 μm, while for the L-dwarf it is determined in the J band around 1.25 μm. We note that synthetic models may lack some faint absorption features or underestimate their abundances. In addition, some instrumental/calibration aspects that are not taken into account in our estimations would increase the uncertainties.

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Dust in brown dwarfs and extra-solar planets

Cited by 158 publications

References 43 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

Detecting planets around very cool dwarfs at near infrared wavelengths with the radial velocity technique

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