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 (TiO2)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 Gaussian 09 to derive updated thermodynamical data for (TiO2)N clusters as input for our TiO2 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 Drift-Phoenixatmosphere structure. Results. Updated Gibbs free energies for the (TiO2)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 TiO2 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 TiO2 in the gas phase, TiO2 remains considerably more efficient at forming condensation seeds by homogeneous nucleation. The paper discusses the effect on the cloud structure in more detail.
Dynamic mineral clouds on HD 189733b Lee, G. K. H.; Wood, K.; Dobbs-Dixon, I. ...
Astronomy and astrophysics (Berlin),
05/2017, Letnik:
601
Journal Article
Recenzirano
Odprti dostop
Context. As the 3D spatial properties of exoplanet atmospheres are being observed in increasing detail by current and new generations of telescopes, the modelling of the 3D scattering effects of ...cloud forming atmospheres with inhomogeneous opacity structures becomes increasingly important to interpret observational data. Aims. We model the scattering and emission properties of a simulated cloud forming, inhomogeneous opacity, hot Jupiter atmosphere of HD 189733b. We compare our results to available Hubble Space Telescope (HST) and Spitzer data and quantify the effects of 3D multiple scattering on observable properties of the atmosphere. We discuss potential observational properties of HD 189733b for the upcoming Transiting Exoplanet Survey Satellite (TESS) and CHaracterising ExOPlanet Satellite (CHEOPS) missions. Methods. We developed a Monte Carlo radiative transfer code and applied it to post-process output of our 3D radiative-hydrodynamic, cloud formation simulation of HD 189733b. We employed three variance reduction techniques, i.e. next event estimation, survival biasing, and composite emission biasing, to improve signal to noise of the output. For cloud particle scattering events, we constructed a log-normal area distribution from the 3D cloud formation radiative-hydrodynamic results, which is stochastically sampled in order to model the Rayleigh and Mie scattering behaviour of a mixture of grain sizes. Results. Stellar photon packets incident on the eastern dayside hemisphere show predominantly Rayleigh, single-scattering behaviour, while multiple scattering occurs on the western hemisphere. Combined scattered and thermal emitted light predictions are consistent with published HST and Spitzer secondary transit observations. Our model predictions are also consistent with geometric albedo constraints from optical wavelength ground-based polarimetry and HST B band measurements. We predict an apparent geometric albedo for HD 189733b of 0.205 and 0.229, in the TESS and CHEOPS photometric bands respectively. Conclusions. Modelling the 3D geometric scattering effects of clouds on observables of exoplanet atmospheres provides an important contribution to the attempt to determine the cloud properties of these objects. Comparisons between TESS and CHEOPS photometry may provide qualitative information on the cloud properties of nearby hot Jupiter exoplanets.
Aims. In our ongoing search for close and faint companions around T Tauri stars in the Chamaeleon star-forming region, we here present observations of a new common proper motion companion to the ...young T-Tauri star and Chamaeleon member CT Cha and discuss its properties in comparison to other young, low-mass objects and to synthetic model spectra from different origins. Methods. Common proper motion of the companion and CT Cha was confirmed by direct $K\rm s$-band imaging data taken with the VLT Adaptive Optics (AO) instrument NACO in February 2006 and March 2007, together with a Hipparcos binary for astrometric calibration. An additional J-band image was taken in March 2007 to obtain color information for a first classification of the companion. Moreover, AO integral field spectroscopy with SINFONI in J, and $H+K$ bands was obtained to deduce physical parameters of the companion, such as temperature and extinction. Relative flux calibration of the bands was achieved using photometry from the NACO imaging data. Results. We found a very faint ($K\rm s$ = 14.9 mag, $K{\rm s}_{0}$ = 14.4 mag) object, just ~2.67´´ northwest of CT Cha corresponding to a projected separation of ~440 AU at 165 ± 30 pc. We show that CT Cha A and this faint object form a common proper motion pair and that the companion is by ≥4σ significance not a stationary background object. The near-infrared spectroscopy yields a temperature of 2600 ± 250 K for the companion and an optical extinction of AV = 5.2 ± 0.8 mag, when compared to spectra calculated from Drift-Phoenix model atmospheres. We demonstrate the validity of the model fits by comparison to several other well-known young sub-stellar objects. Conclusions. We conclude that the CT Cha companion is a very low-mass member of Chamaeleon and very likely a physical companion to CT Cha, as the probability for a by chance alignment is ≤0.01. Due to a prominent Pa-β emission in the J-band, accretion is probably still ongoing onto the CT Cha companion. From temperature and luminosity (log$(L_{\rm bol}/L_{\odot})$ = –2.68 ± 0.21), we derive a radius of R = $2.20_{\rm -0.60}^{+0.81}$ $R_{\mathrm{Jup}}$. We find a consistent mass of M = 17 ± 6 $M_{\mathrm{Jup}}$ for the CT Cha companion from both its luminosity and temperature when placed on evolutionary tracks. Hence, the CT Cha companion is most likely a wide brown dwarf companion or possibly even a planetary mass object.
Equilibrium chemistry down to 100 K Woitke, P.; Helling, Ch; Hunter, G. H. ...
Astronomy and astrophysics (Berlin),
06/2018, Letnik:
614
Journal Article
Recenzirano
Odprti dostop
We have introduced 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 have reviewed the data for molecular equilibrium constants, kp(T), from several sources and discussed which functional fits are most suitable for low temperatures. We benchmarked our results against another chemical equilibrium code. We collected Gibbs free energies, ΔGf⊖, for about 200 solid and liquid species from the NIST-JANAF database and the geophysical database SUPCRTBL. We discussed the condensation sequence of the elements with solar abundances in phase equilibrium down to 100 K. Once the major magnesium silicates Mg2SiO4s and MgSiO3s have formed, the dust to 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 to 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 such as zirconium dioxide (ZrO2) or corundum (Al2O3) 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 atmospheres of rocky exoplanets Herbort, O.; Woitke, P.; Helling, Ch ...
Astronomy and astrophysics (Berlin),
04/2020, Letnik:
636
Journal Article
Recenzirano
Odprti dostop
Context.
Little is known about the interaction between atmospheres and crusts of exoplanets so far, but future space missions and ground-based instruments are expected to detect molecular features in ...the spectra of hot rocky exoplanets.
Aims.
We aim to understand the composition of the gas in an exoplanet atmosphere which is in equilibrium with a planetary crust.
Methods.
The molecular composition of the gas above a surface made of a mixture of solid and liquid materials was determined by assuming phase equilibrium for given pressure, temperature, and element abundances. We study total element abundances that represent different parts of the Earth’s crust (continental crust, bulk silicate Earth, mid oceanic ridge basalt), CI chondrites and abundances measured in polluted white dwarfs.
Results.
For temperatures between ~600 and ~3500 K, the near-crust atmospheres of all considered total element abundances are mainly composed of H
2
O, CO
2
, and SO
2
and in some cases of O
2
and H
2
. For temperatures ≲500 K, only N
2
-rich or CH
4
-rich atmospheres remain. For ≳3500 K, the atmospheric gas is mainly composed of atoms (O, Na, Mg, and Fe), metal oxides (SiO, NaO, MgO, CaO, AlO, and FeO), and some metal hydroxides (KOH and NaOH). The inclusion of phyllosilicates as potential condensed species is crucial for lower temperatures, as they can remove water from the gas phase below about 700 K and inhibit the presence of liquid water.
Conclusions.
Measurements of the atmospheric composition could, in principle, characterise the rock composition of exoplanet crusts. H
2
O, O
2
and CH
4
are natural products from the outgassing of different kinds of rocks that had time to equilibrate. These are discussed as biomarkers, but they do emerge naturally as a result of the thermodynamic interaction between the crust and atmosphere. Only the simultaneous detection of all three molecules might be a sufficient biosignature, as it is inconsistent with chemical equilibrium.
Ultra-hot Jupiters orbit very close to their host star and consequently receive strong irradiation, causing their atmospheric chemistry to be different from the common gas giants. Here, we have ...studied the atmosphere of one of these particular hot planets, MASCARA-2b/KELT-20b, using four transit observations with high resolution spectroscopy facilities. Three of these observations were performed with HARPS-N and one with CARMENES. Additionally, we simultaneously observed one of the transits with MuSCAT2 to monitor possible spots in the stellar surface. At high resolution, the transmission residuals show the effects of Rossiter-McLaughlin and centre-to-limb variations from the stellar lines profiles, which we have corrected to finally extract the transmission spectra of the planet. We clearly observe the absorption features of CaII, FeII, NaI, Hα, and Hβ in the atmosphere of MASCARA-2b, and indications of Hγ and MgI at low signal-to-noise ratio. In the case of NaI, the true absorption is difficult to disentangle from the strong telluric and interstellar contamination. The results obtained with CARMENES and HARPS-N are consistent, measuring an Hα absorption depth of 0.68 ± 0.05 and 0.59 ± 0.07%, and NaI absorption of 0.11 ± 0.04 and 0.09 ± 0.05% for a 0.75 Å passband, in the two instruments respectively. The Hα absorption corresponds to ~1.2 Rp, which implies an expanded atmosphere, as a result of the gas heating caused by the irradiation received from the host star. For Hβ and Hγ only HARPS-N covers this wavelength range, measuring an absorption depth of 0.28 ± 0.06 and 0.21 ± 0.07%, respectively. For CaII, only CARMENES covers this wavelength range measuring an absorption depth of 0.28 ± 0.05, 0.41 ± 0.05 and 0.27 ± 0.06% for CaII λ8498Å, λ8542Å and λ8662Å lines, respectively. Three additional absorption lines of FeII are observed in the transmission spectrum by HARPS-N (partially covered by CARMENES), measuring an average absorption depth of 0.08 ± 0.04% (0.75 Å passband). The results presented here are consistent with theoretical models of ultra-hot Jupiters atmospheres, suggesting the emergence of an ionised gas on the day-side of such planets. Calcium and iron, together with other elements, are expected to be singly ionised at these temperatures and be more numerous than its neutral state. The Calcium triplet lines are detected here for the first time in transmission in an exoplanet atmosphere.
ABSTRACT
Pluto’s icy surface has changed colour and its atmosphere has swelled since its last closest approach to the Sun in 1989. The thin atmosphere is produced by evaporating ices, and so can also ...change rapidly, and in particular carbon monoxide should be present as an active thermostat. Here we report the discovery of gaseous CO via the 1.3‐mm wavelength J=2–1 rotational transition, and find that the line‐centre signal is more than twice as bright as a tentative result obtained in 2000 by Bockélee‐Morvan et al. Greater surface ice evaporation over the last decade could explain this, or increased pressure could have caused the atmosphere to expand. The gas must be cold, with a narrow line‐width consistent with temperatures around 50 K, as predicted for the very high atmosphere, and the line brightness implies that CO molecules extend up to ≈3 Pluto radii above the surface. The upper atmosphere must have changed markedly over only a decade since the prior search, and more alterations could occur by the arrival of the New Horizons mission in 2015.
Context.
Exoplanet atmosphere characterisation has become an important tool in understanding exoplanet formation, evolution, and it also is a window into potential habitability. However, clouds ...remain a key challenge for characterisation: upcoming space telescopes (e.g. the
James Webb
Space Telescope, JWST, and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey) and ground-based high-resolution spectrographs (e.g. the next-generation CRyogenic high-resolution InfraRed Echelle Spectrograph) will produce data requiring detailed understanding of cloud formation and cloud effects for a variety of exoplanets and brown dwarfs.
Aims.
We aim to understand how the micro-porosity of cloud particles affects the cloud structure, particle size, and material composition on exoplanets and brown dwarfs. We further examine the spectroscopic effects of micro-porous particles, the particle size distribution, and non-spherical cloud particles.
Methods.
We expanded our kinetic non-equilibrium cloud formation model to study the effect of micro-porosity on the cloud structure using prescribed 1D (
T
gas
–
p
gas
) profiles from the D
RIFT
-P
HOENIX
model atmosphere grid. We applied the effective medium theory and the Mie theory to model the spectroscopic properties of cloud particles with micro-porosity and a derived particle size distribution. In addition, we used a statistical distribution of hollow spheres to represent the effects of non-spherical cloud particles.
Results.
Highly micro-porous cloud particles (90% vacuum) have a larger surface area, enabling efficient bulk growth higher in the atmosphere than for compact particles. Increases in single scattering albedo and cross-sectional area for these mineral snowflakes cause the cloud deck to become optically thin only at a wavelength of ~100
μ
m instead of at the ~20
μ
m for compact cloud particles. A significant enhancement in albedo is also seen when cloud particles occur with a locally changing Gaussian size distribution. Non-spherical particles increase the opacity of silicate spectral features, which further increases the wavelength at which the clouds become optically thin.
Conclusions.
Retrievals of cloud properties, particularly particle size and mass of clouds, are biased by the assumption of compact spherical particles. The JWST mid-infrared instrument will be sensitive to signatures of micro-porous and non-spherical cloud particles based on the wavelength at which clouds are optically thin. Details of spectral features are also dependent on particle shape, and greater care must be taken in modelling clouds as observational data improves.
Context.
Brown dwarfs and exoplanets provide unique atmospheric regimes that hold information about their formation routes and evolutionary states. Cloud particles form through nucleation, ...condensation, evaporation, and collisions, which affect the distribution of cloud particles in size and throughout these atmospheres. Cloud modelling plays a decisive role in understanding these regimes.
Aims.
Modelling mineral cloud particle formation in the atmospheres of brown dwarfs and exoplanets is a key element in preparing for missions and instruments like CRIRES+, JWST, and ARIEL, as well as possible polarimetry missions like P
ol
S
tar
. The aim is to support the increasingly detailed observations that demand greater understanding of the microphysical cloud processes.
Methods.
We extend our kinetic cloud formation model that treats nucleation, condensation, evaporation, and settling of mixed material cloud particles to consistently model cloud particle-particle collisions. The new hybrid code Hybrid moments (Ls) and Size (H
y
L
and
S) is then applied to a grid of
Drift-Phoenix
(
T
gas
,
p
gas
) profiles. Effective medium theory and Mie theory are used to investigate the optical properties.
Results.
Turbulence proves to be the main driving process of particle-particle collisions, with collisions becoming the dominant process in the lower atmosphere (
p >
10
−4
bar) at the cloud base. Particle-particle collisions produce one of three outcomes for brown dwarf and gas-giant atmospheres: fragmenting atmospheres (log
10
(
g
cms
−2
)=3.0) coagulating atmospheres (log
10
(
g
)=5.0),
T
eff
≤1800K) or condensational growth dominated atmospheres (log
10
(
g
) = 5.0,
T
eff
>
1800 K). Cloud particle opacity slope at optical wavelengths
(Hubble)
is increased with fragmentation, as are the silicate features at JWST NIRSpec, JWST MIRI, and ARIEL AIRS wavelengths.
Conclusions.
The hybrid moment-bin method H
y
L
and
S demonstrates the feasibility of combining a moment and a bin method for cloud modelling, whilst assuring element conservation. It provides a powerful and fast tool for capturing general trends of particle collisions, consistently with other microphysical growth processes. Collisions are an important process in exoplanet and brown dwarf atmospheres, but cannot be assumed to be hit-and-stick only. The spectral effects of cloud particle collisions in both optical and mid-infrared wavelengths complicate inferences of cloud particle size and material composition from observational data.
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