The need for small-scale turbulence in atmospheres of substellar objects (original) (raw)
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Dust Formation in Substellar Atmospheres: A Multi-Scale Problem
2005
The large scales of substellar atmospheres are determined by an interplay between convection, dust formation and gravitational settling. The consequence is an element depletion of the upper dust forming regions and an element enrichment of the dust evaporating sites. The formation of dust cloud structures in substellar atmospheres is investigated based on a consistent theoretical description of hydrodynamics, dust formation and destruction, gravitational settling, and element depletion including the effect of mixing by convective overshoot. Results of the calculation are, e.g., the mean size of the dust particles and the element depletion which both vary with atmospheric height. Every convectively unstable gas may be turbulent if its viscosity is small. Therefore, the classical turbulent closure problem needs to be tackled in connection with dust formation in substellar atmospheres because a complete three-dimensional and time-dependent solution of the model equations on all involved scale length is simply not feasible. A description has to be found which represents governing effects and mechanisms of the unresolved scale regime in the substellar atmosphere. In order to understand the interaction of turbulence and dust formation, we have performed investigations of the smallest scale regimes in 1D and 2D. We deduce important demands on subgrid models from our results as important ingredients for a new generation of atmosphere simulations.
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] in [ +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-to-gas 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.
Dust in brown dwarfs. III. Formation and structure of quasi-static cloud layers
Astronomy and Astrophysics
In this paper, first solutions of the dust moment equations developed in for the description of dust formation and precipitation in brown dwarf and giant gas planet atmospheres are presented. We consider the special case of a static brown dwarf atmosphere, where dust particles continuously nucleate from the gas phase, grow by the accretion of molecules, settle gravitationally and re-evaporate thermally. Mixing by convective overshoot is assumed to replenish the atmosphere with condensable elements, which is necessary to counterbalance the loss of condensable elements by dust formation and gravitational settling (no dust without mixing). Applying a kinetic description of the relevant microphysical and chemical processes for TiO 2 -grains, the model makes predictions about the large-scale stratification of dust in the atmosphere, the depletion of molecules from the gas phase, the supersaturation of the gas in the atmosphere as well as the mean size and the mass fraction of dust grains as function of depth. Our results suggest that the presence of relevant amounts of dust is restricted to a layer, where the upper boundary (cloud deck) is related to the requirement of a minimum mixing activity (mixing time-scale τ mix ≈ 10 6 s) and the lower boundary (cloud base) is determined by the thermodynamical stability of the grains. The nucleation occurs around the cloud deck where the gas is cool, strongly depleted, but nevertheless highly supersaturated (S 1). These particles settle gravitationally and populate the warmer layers below, where the in situ formation (nucleation) is ineffective or even not possible. During their descent, the particles grow and reach mean radii of ≈30 µm ... 400 µm at the cloud base, but the majority of the particles in the cloud layer remains much smaller. Finally, the dust grains sink into layers which are sufficiently hot to cause their thermal evaporation. Hence, an effective transport mechanism for condensable elements exists in brown dwarfs, which depletes the gas above and enriches the gas below the cloud base of a considered solid/liquid material. The dust-to-gas mass fraction in the cloud layer results to be approximately given by the mass fraction of condensable elements in the gas being mixed up. Only for artificially reduced mixing we find a self-regulation mechanism that approximately installs phase equilibrium (S ≈ 1) in a limited region around the cloud base.
Consistent Simulations of Substellar Atmospheres and Nonequilibrium Dust Cloud Formation
We aim to understand cloud formation in substellar objects. We combined our nonequilibrium, stationary cloud model DRIFT (seed formation, growth, evaporation, gravitational settling, element conservation) with the general-purpose model atmosphere code PHOENIX (radiative transfer, hydrostatic equilibrium, mixing-length theory, chemical equilibrium) in order to consistently calculate cloud formation and radiative transfer with their feedback on convection and gas-phase depletion. We calculate the complete 1D model atmosphere structure and the chemical details of the cloud layers. The DRIFT-PHOENIX models enable the first stellar atmosphere simulation that is based on the actual cloud formation process. The resulting (T, p)-profiles differ considerably from the previous limiting PHOENIX cases DUSTY and COND. A tentative comparison with observations demonstrates that the determination of effective temperatures based on simple cloud models has to be applied with care. Based on our new models, we suggest a mean Teff = 1800 K for the L dwarf twin-binary system DENIS J0205-1159, which is up to 500 K hotter than suggested in the literature. We show transition spectra for gas-giant planets which form dust clouds in their atmospheres and evaluate photometric fluxes for a WASP-1 type system.
Dust and gas distribution in molecular clouds: an observational approach
Journal of Physics: Conference Series, 2005
The interstellar medium (ISM), gas and dust, appears to be arranged in clouds, whose dimensions, masses and densities span a large range of scales: from giant molecular clouds to small isolated globules. The structure of these objects show a high degree of complexity appearing, in the range of the observed scales, as a non-homogeneous ("clumpy") distribution of matter. The arrangement of the ISM is clearly relevant for the study of the fragmentation of the clouds and then of the star formation processes. To quantify observationally the ISM structure, many methods have been developed and our study is focused on some of them, exploiting multiwavelength observations of IS objects. The investigations presented here have been carried out by considering both the dust absorption (in optical and near IR wavelengths) and the gas emission (in the submm-radio spectral range). We present the maps obtained from the reduction of raw data and a first tentative analysis by means of methods as the structure function, the autocorrelation, and the ∆-variance. These are appropriate tools to highlight the complex structure of the ISM with reference to the paradigm given by the supersonic turbulence. Three observational cases are briefly discussed. In order to analyse the structure of objects characterized by different sizes, we applied the above-mentioned algorithms to the extinction map of the dark globule CB 107 and to the CO(J=1-0) integrated intensity map of Vela Molecular Ridge, D Cloud. Finally we compare the results obtained with synthetic fractal maps known as "fractional Brownian motion" fBm images. .
Clustering and dynamic decoupling of dust grains in turbulent molecular clouds
Monthly Notices of the Royal Astronomical Society, 2018
We present high-resolution (1024 3) simulations of super-/hypersonic isothermal hydrodynamic turbulence inside an interstellar molecular cloud (resolving scales of typically 20-100 au), including a multidisperse population of dust grains, i.e. a range of grain sizes is considered. Due to inertia, large grains (typical radius a 1.0 μm) will decouple from the gas flow, while small grains (a 0.1 μm) will tend to better trace the motions of the gas. We note that simulations with purely solenoidal forcing show somewhat more pronounced decoupling and less clustering compared to simulations with purely compressive forcing. Overall, small and large grains tend to cluster, while intermediate-size grains show essentially a random isotropic distribution. As a consequence of increased clustering, the grain-grain interaction rate is locally elevated; but since small and large grains are often not spatially correlated, it is unclear what effect this clustering would have on the coagulation rate. Due to spatial separation of dust and gas, a diffuse upper limit to the grain sizes obtained by condensational growth is also expected, since large (decoupled) grains are not necessarily located where the growth species in the molecular gas is.
Dust in brown dwarfs and extra-solar planets
Astronomy and Astrophysics, 2009
Aims. Substellar objects have extremely long life-spans. The cosmological consequence for older objects are low abundances of heavy elements, which 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. 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 question of star formation and of dust formation in the early universe. Methods. We utilize numerical simulations in which we solve a set of moment equations in order 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 including the element replenishment by convective overshooting. The integration with an atmosphere code (Phoenix) allows to determine a consistent (T, p, v conv )-structure (T -local temperature, p -local pressure, v conv -convective velocity), and, hence, also 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 a 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 which has a drastic impact on the dust cloud structure.
The Astrophysical Journal, 2012
In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J = 1-0 and 3-2 lines have been carried out in an interacting region of a molecular cloud with an H II region. As a result, several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a few km s −1 . It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small scale structures with line width of ∼ 0.6 km s −1 have a relatively high CO line ratio of J =3-2 to 1-0, 1 R 3−2/1−0 2. The large velocity gradient analysis implies that the 0.6 km s −1 width component cloudlets have an average density of 10 3−4 cm −3 , which is relatively high at cloud edges, but their masses are only 0.05 M .
Rain and clouds in brown dwarf atmospheres: A coupled problem from small to large
The large scale structure of a brown dwarf atmosphere is determined by an interplay of convection, radiation, dust formation, and gravitational settling, which possibly provides an explanation for the observed variability. The result is an element depletion of the dust forming regions and an element enrichment of the dust evaporating sites. The formation of dust cloud structures in substellar atmospheres is demonstrated based on a consistent theoretical description of dust formation and destruction, gravitational settling, and element depletion including the effect of convective overshoot.