Publications by Graham Lee


Dust in brown dwarfs and extra-solar planets VI. Assessing seed formation across the brown dwarf and exoplanet regimes

ASTRONOMY & ASTROPHYSICS 614 (2018) ARTN A126

GKH Lee, J Blecic, C Helling


Simulating the cloudy atmospheres of HD 209458 b and HD 189733 b with the 3D Met Office Unified Model

ASTRONOMY & ASTROPHYSICS 615 (2018) ARTN A97

S Lines, NJ Mayne, IA Boutle, J Manners, GKH Lee, C Helling, B Drummond, DS Amundsen, J Goyal, DM Acreman, P Tremblin, M Kerslake


Dynamic mineral clouds on HD 189733b II. Monte Carlo radiative transfer for 3D cloudy exoplanet atmospheres: combining scattering and emission spectra

ASTRONOMY & ASTROPHYSICS 601 (2017) ARTN A22

GKH Lee, K Wood, I Dobbs-Dixon, A Rice, C Helling


Dust in brown dwarfs and extrasolar planets V. Cloud formation in carbon- and oxygen-rich environments

ASTRONOMY & ASTROPHYSICS 603 (2017) ARTN A123

C Helling, D Tootill, P Woitke, G Lee


Dynamic mineral clouds on HD 189733b: I. 3D RHD with kinetic, non-equilibrium cloud formation

Astronomy and Astrophysics 594 (2016)

G Lee, I Dobbs-Dixon, C Helling, K Bognar, P Woitke

© ESO, 2016. Context. Observations of exoplanet atmospheres have revealed the presence of cloud particles in their atmospheres. 3D modelling of cloud formation in atmospheres of extrasolar planets coupled to the atmospheric dynamics has long been a challenge. Aims. We investigate the thermo-hydrodynamic properties of cloud formation processes in the atmospheres of hot Jupiter exoplanets. Methods. We simulate the dynamic atmosphere of HD 189733b with a 3D model that couples 3D radiative-hydrodynamics with a kinetic, microphysical mineral cloud formation module designed for RHD/GCM exoplanet atmosphere simulations. Our simulation includes the feedback effects of cloud advection and settling, gas phase element advection and depletion/replenishment and the radiative effects of cloud opacity. We model the cloud particles as a mix of mineral materials which change in size and composition as they travel through atmospheric thermo-chemical environments. All local cloud properties such as number density, grain size and material composition are time-dependently calculated. Gas phase element depletion as a result of cloud formation is included in the model. In situ effective medium theory and Mie theory is applied to calculate the wavelength dependent opacity of the cloud component. Results. We present a 3D cloud structure of a chemically complex, gaseous atmosphere of the hot Jupiter HD 189733b. Mean cloud particle sizes are typically sub-micron (0.01-0.5 μm) at pressures less than 1 bar with hotter equatorial regions containing the smallest grains. Denser cloud structures occur near terminator regions and deeper (~1 bar) atmospheric layers. Silicate materials such as MgSiO3[s] are found to be abundant at mid-high latitudes, while TiO2[s] and SiO2[s] dominate the equatorial regions. Elements involved in the cloud formation can be depleted by several orders of magnitude. Conclusions. The interplay between radiative-hydrodynamics and cloud kinetics leads to an inhomogeneous, wavelength dependent opacity cloud structure with properties differing in longitude, latitude and depth. This suggests that transit spectroscopy would sample a variety of cloud particles properties (sizes, composition, densities).


The mineral clouds on HD 209458b and HD 189733b

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 460 (2016) 855-883

C Helling, G Lee, I Dobbs-Dixon, N Mayne, DS Amundsen, J Khaimova, AA Unger, J Manners, D Acreman, C Smith


Dynamic mineral clouds on HD 189733b I. 3D RHD with kinetic, non-equilibrium cloud formation

ASTRONOMY & ASTROPHYSICS 594 (2016) ARTN A48

G Lee, I Dobbs-Dixon, C Helling, K Bognar, P Woitke


Modelling the local and global cloud formation on HD 189733b

ASTRONOMY & ASTROPHYSICS 580 (2015) ARTN A12

G Lee, C Helling, I Dobbs-Dixon, D Juncher


EXTENDED BASELINE PHOTOMETRY OF RAPIDLY CHANGING WEATHER PATTERNS ON THE BROWN DWARF BINARY LUHMAN-16

ASTROPHYSICAL JOURNAL 812 (2015) ARTN 161

RA Street, BJ Fulton, A Scholz, K Horne, C Helling, D Juncher, G Lee, S Valenti


Understanding the water emission in the mid- and far-IR from protoplanetary disks around T Tauri stars

ASTRONOMY & ASTROPHYSICS 582 (2015) ARTN A105

S Antonellini, I Kamp, P Riviere-Marichalar, R Meijerink, P Woitke, W-F Thi, M Spaans, G Aresu, G Lee


Dust in brown dwarfs and extra-solar planets: IV. Assessing TiO<inf>2</inf>and SiO nucleation for cloud formation modelling

Astronomy and Astrophysics 575 (2015)

G Lee, C Helling, H Giles, ST Bromley

© ESO, 2015. 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)Nclusters as input for our TiO2seed 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)Nclusters are presented, as well as a slightly temperature dependent surface tension for T = 500... 2000 K with an average value of σ∞= 480.6 ergcm-2. The TiO2seed 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 TiO2in the gas phase, TiO2remains considerably more efficient at forming condensation seeds by homogeneous nucleation. The paper discusses the effect on the cloud structure in more detail.