Dust in brown dwarfs and extrasolar planets

We introduce a fast and versatile computer code, GGchem, to determine the chemical composition of gases in thermo-chemical equilibrium down to 100 K, with or without equilibrium condensation. We review the data for molecular equilibrium constants, k p (T ), from several sources and discuss which functional fits are most suitable for low temperatures. We benchmark our results against another chemical equilibrium code. We collect Gibbs free energies, ∆G• − f , for about 200 solid and liquid species from the NIST-JANAF database and the geophysical database SUPCRTBL. We discuss the condensation sequence of the elements with solar abundances in phase equilibrium down to 100 K. Once the major magnesium silicates Mg 2 SiO 4 [s] and MgSiO 3 [s] have formed, the dust/gas mass ratio jumps to a value of about 0.0045 which is significantly lower than the often assumed value of 0.01. Silicate condensation is found to increase the carbon/oxygen ratio (C/O) in the gas from its solar value of ∼ 0.55 up to ∼ 0.71, and, by the additional intake of water and hydroxyl into the solid matrix, the formation of phyllosilicates at temperatures below ∼ 400 K increases the gaseous C/O further to about 0.83. Metallic tungsten (W) is the first condensate found to become thermodynamically stable around 1600 − 2200 K (depending on pressure), several hundreds of Kelvin before subsequent materials like zirconium dioxide (ZrO 2 ) or corundum (Al 2 O 3 ) can condense. We briefly discuss whether tungsten, despite its low abundance of ∼ 2 × 10 −7 times the silicon abundance, could provide the first seed particles for astrophysical dust formation. The GGchem code is publicly available at https://github.com/pw31/GGchem.

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“…Helling et al 2006;Bilger et al 2013;Helling et al 2017). The old data collection had 205 molecules.…”

Section: Molecular Equilibrium Constantsmentioning

confidence: 99%

“…Helling & Lucas 2009;Witte et al 2009;Crossfield et al 2013;Hu & Seager 2014;Morley et al 2017) or by changing the carbon-to-oxygen ratio (e.g. Madhusudhan et al 2011;Madhusudhan 2012;Helling et al 2017;Van Eck et al 2017). Gaidos (2000) was one of the earliest works suggesting the importance of C/O in an exoplanet context.…”

Section: Introductionmentioning

confidence: 99%

Equilibrium chemistry down to 100 K

Woitke

Helling

Hunter

et al. 2018

A&A

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“…AlO is difficult to observe for M or L type stellar objects, first because the optical spectra of these low-mass stars are dominated by TiO and VO absorption features, and second because such spectral feature at ∼ 500 nm is challenging to observe for very late type stars because of their intrinsic faintness at these wavelengths. As for the exoplanet atmospheres, AlO is mentioned in the literature of low mass stellar objects as condensate forming species (e.g., Burrows & Sharp 1999;Allard et al 2011;Helling et al 2017).…”

Section: The Aluminum Oxide Featurementioning

confidence: 99%

An optical transmission spectrum of the ultra-hot Jupiter WASP-33 b

Essen

Mallonn

Welbanks

et al. 2019

A&A

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There has been increasing progress toward detailed characterization of exoplanetary atmospheres, in both observations and theoretical methods. Improvements in observational facilities and data reduction and analysis techniques are enabling increasingly higher quality spectra, especially from ground-based facilities. The high data quality also necessitates concomitant improvements in models required to interpret such data. In particular, the detection of trace species such as metal oxides has been challenging. Extremely irradiated exoplanets (∼3000 K) are expected to show oxides with strong absorption signals in the optical. However, there are only a few hot Jupiters where such signatures have been reported. Here we aim to characterize the atmosphere of the ultra-hot Jupiter WASP-33 b using two primary transits taken 18 orbits apart. Our atmospheric retrieval, performed on the combined data sets, provides initial constraints on the atmospheric composition of WASP-33 b. We report a possible indication of aluminum oxide (AlO) at 3.3-σ significance. The data were obtained with the long slit OSIRIS spectrograph mounted at the 10-meter Gran Telescopio Canarias. We cleaned the brightness variations from the light curves produced by stellar pulsations, and we determined the wavelength-dependent variability of the planetary radius caused by the atmospheric absorption of stellar light. A simultaneous fit to the two transit light curves allowed us to refine the transit parameters, and the common wavelength coverage between the two transits served to contrast our results. Future observations with HST as well as other large ground-based facilities will be able to further constrain the atmospheric chemical composition of the planet.

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“…Helling & Woitke 2006). A carbon rich cloud formation scenario has also been investigated by Helling et al (2017).…”

Section: Introductionmentioning

confidence: 99%

Dust in brown dwarfs and extra-solar planets

Lee

Blecic

Helling

2018

A&A

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Context. The cloud formation process starts with the formation of seed particles, after which, surface chemical reactions grow or erode the cloud particles. If seed particles do not form, or are not available by another means, an atmosphere is unable to form a cloud complex and will remain cloud free. Aims. We aim to investigate which materials may form cloud condensation seeds in the gas temperature and pressure regimes (Tgas = 100–2000 K, pgas = 10−8–100 bar) expected to occur in planetary and brown dwarf atmospheres. Methods. We have applied modified classical nucleation theory which requires surface tensions and vapour pressure data for each solid species, which are taken from the literature. Input gas phase number densities are calculated assuming chemical equilibrium at solar metallicity. Results. We calculated the seed formation rates of TiO2[s] and SiO[s] and find that they efficiently nucleate at high temperatures of Tgas = 1000–1750 K. Cr[s], KCl[s] and NaCl[s] are found to efficiently nucleate across an intermediate temperature range of Tgas = 500–1000 K. We find CsCl[s] may serve as the seed particle for the water cloud layers in cool sub-stellar atmospheres. The nucleation rates of four low temperature ice species (Tgas = 100–250 K), H2O[s/l], NH3[s], H2S[s/l], and CH4[s], are also investigated for the coolest sub-stellar and planetary atmospheres. Conclusions. Our results suggest a possibly (Tgas, pgas) distributed hierarchy of seed particle formation regimes throughout the substellar and planetary atmospheric temperature-pressure space. With TiO2[s] providing seed particles for the most refractory cloud formation species (e.g. Al2O3[s], Fe[s], MgSiO3[s], Mg2SiO4[s]), Cr[s] providing the seed particles for MnS[s], Na2S[s], and ZnS[s] sulfides, and K/Na/Rb/Cs/NH4-Cl binding solid species providing the seed particles for H2O[s/l] and NH4-H2PO4/SH[s] clouds. A detached, high-altitude aerosol layer may form in some sub-stellar atmospheres from the nucleation process, dependent on the upper atmosphere temperature, pressure and availability of volatile elements. In order to improve the accuracy of the nucleation rate calculation, further research into the small cluster thermochemical data for each cloud species is warranted. The validity of these seed particle scenarios will be tested by applying it to more complete cloud models in the future.

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Dust in brown dwarfs and extrasolar planets

Cited by 27 publications

References 39 publications

Equilibrium chemistry down to 100 K

Equilibrium chemistry down to 100 K

An optical transmission spectrum of the ultra-hot Jupiter WASP-33 b

Dust in brown dwarfs and extra-solar planets

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