A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres

By Ch. Helling, A. Ackerman, F. Allard, M. Dehn, Peter Hauschildt, Derek Homeier, K. Lodders, M. Marley, F. Rietmeijer, T. Tsuji and 1 more. 2008

180 citations recorded by the source corpus.

The atmospheres of substellar objects contain clouds of oxides, iron, silicates and other refractory condensates. Water clouds are expected in the coolest objects. The opacity of these 'dust' clouds strongly affects both the atmospheric temperature-pressure profile and the emergent flux. Thus, any attempt to model the spectra of these atmospheres must incorporate a cloud model. However, the diversity of cloud models in atmospheric simulations is large and it is not always clear how the underlying physics of the various models compare. Likewise, the observational consequences of different modelling approaches can be masked by other model differences, making objective comparisons challenging. In order to clarify the current state of the modelling approaches, this paper compares five different cloud models in two sets of tests. Test case 1 tests the dust cloud models for a prescribed L-, L-T and T-dwarf atmospheric (temperature T, pressure p, convective velocity υ conv ) structures. Test case 2 compares complete model atmosphere results for given (effective temperature T eff , surface gravity log g). All models agree on the global cloud structure but differ in opacity relevant details such as grain size, amount of dust, dust and gas-phase composition. These models can loosely be grouped into high- and low-altitude cloud models whereas the first appears generally redder in near-infrared colours than the latter. Comparisons of synthetic photometric fluxes translate into a modelling uncertainty in apparent magnitudes for our L-dwarf (T-dwarf) test case of 0.25 ≤ Am ≤ 0.875 (0.1 ≤ Δm ≤ 1.375), taking into account the Two-Micron All Sky Survey, the UKIRT WFCAM, the Spitzer IRAC and VLT VISIR filters with UKIRT WFCAM being the most challenging for the models. Future developments will need closer links with laboratory astrophysics, and a consistent treatment of the cloud chemistry and turbulence.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ccb5a5125e6e7da7663e093293ad62035dc0b8a9

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In Astronomy and Astrophysics

1. Jon Fortney
2. M. Marley
3. N. Madhusudhan
4. Michael C. Liu
5. B. Bowler
6. T. Dupuy
7. Adam Burrows
8. A. Showman

Citations in the corpus

• Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e (2020, background): “51 Pegasi b fellow. cloud models are described in Woitke & Helling (2004); Helling et al. (2008a); Woitke et al. (2020), Gao et al. (2018), and Powell et al. (2018).”
• THE 0.8–14.5 μm SPECTRA OF MID-L TO MID-T DWARFS: DIAGNOSTICS OF EFFECTIVE TEMPERATURE, GRAIN SEDIMENTATION, GAS TRANSPORT, AND SURFACE GRAVITY (2009, methodology): “Woitke & Helling (2003) and Helling et al. (2008a) use seed particles and nucleation to model grain growth and then allow the grains to drift into regions where they may continue growing before settling and evaporating deep in the atmosphere.”
• THE FLAT TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ1214b FROM WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE (2011, background): “The condensation and complicated evolution of clouds has been studied within the context of cool stars and hot Jupiters (e.g Lodders & Fegley 2006; Helling et al. 2008), but further study into the theoretical landscape for equilibrium clouds on planets in GJ1214b’s gravity and temperature regime is…”
• NEGLECTED CLOUDS IN T AND Y DWARF ATMOSPHERES (2012, background): “Helling et al. (2008) review various cloud modeling techniques and compare model predictions for various cases.”
• HST SPECTRAL MAPPING OF L/T TRANSITION BROWN DWARFS REVEALS CLOUD THICKNESS VARIATIONS (2013, methodology): “…that the brown dwarf spectral mapping technique introduced in this paper will lead to a major step in testing and refining physically realistic models of cloud formation and cloud structure (e.g. Helling et al. 2008a,b) and atmospheric dynamics (Freytag et al. 2010; Showman & Kaspi 2012b) 4.6.”
• Equilibrium chemistry down to 100 K (2017, methodology): “…& Dominik 1995; Helling et al. 1996; Woitke 2001; Jeong et al. 2003), or the formation of clouds in brown dwarfs and planetary atmospheres (Lunine et al. 1986; Tsuji et al. 1996; Marley et al. 1999; Allard et al. 2001; Burrows et al. 2002; Tsuji 2005; Marley et al. 2002; Helling et al. 2008).”
• QUANTITATIVELY ASSESSING THE ROLE OF CLOUDS IN THE TRANSMISSION SPECTRUM OF GJ 1214b (2013, background): “For a much more extensive comparison, Helling et al. (2008b) review various cloud modeling techniques and compare model predictions for various cases.”
• On the Cool Side: Modeling the Atmospheres of Brown Dwarfs and Giant Planets (2014, methodology): “A more detailed review can be found in Marley et al. (2013), and detailed comparisons of cloud treatments can be found in Helling et al. (2008c) (which is now slightly dated).”
• NEAR-INFRARED SPECTROSCOPY OF THE EXTRASOLAR PLANET HR 8799 b (2010, background): “…in the cloud models that can influence the emergent spectra, such as particle size and cloud model prescription (see, e.g., Burrows et al. 2006 and Helling et al. 2008 for detailed discussions), and thus the best-fitting vertical mixing and/or metallicity value may change with a broader set of…”
• ATMOSPHERIC CHEMISTRY IN GIANT PLANETS, BROWN DWARFS, AND LOW-MASS DWARF STARS. III. IRON, MAGNESIUM, AND SILICON (2010, result): “In contrast to the models of Helling et al. (2006, 2008), we find that quartz will not condense in the atmospheres of substellar objects unless enstatite condensation is suppressed.”
• THE HAWAII INFRARED PARALLAX PROGRAM. I. ULTRACOOL BINARIES AND THE L/T TRANSITION, (2012, background): “However, given the difficulty of modeling clouds (e.g., Helling et al. 2008), a robust physical theory is still lacking.”
• Imaging Extrasolar Giant Planets (2016, background): “…planet formation (Chabrier et al. 2007; Helled et al. 2013; Chabrier et al. 2014; Helling et al. 2014), atmospheric models (Marley et al. 2007a; Helling et al. 2008, Allard et al. 2012; Marley & Robinson 2015), evolutionary models, (Burrows et al. 2001; Fortney & Nettelmann 2009),…”
• Models of very-low-mass stars, brown dwarfs and exoplanets (2011, methodology): “However,this model made several explicit assumptions concerning the efficiency of supersaturation, the coagulation, etc. Helling et al. (2008a) have compared different cloud models and their impact on model atmospheres.”
• CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b (2011, background): “For an extensive review and comparison of theoretical cloud modeling in substellar atmospheric conditions, see Helling et al. (2008a).”
• TRANSITIONS IN THE CLOUD COMPOSITION OF HOT JUPITERS (2016, background): “Numerous mechanisms can promote or inhibit the formation of a given cloud species, leading to a final condensate composition that is not the one predicted by thermochemical equilibrium calculations (see Helling et al. 2008; Marley et al. 2013, for reviews).”