| 1. |
- Allard, F, et al.
(författare)
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Model atmospheres and spectra: The role of dust
- 2003
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Ingår i: Proceedings of the International Astronomical Union (Brown Dwarfs). - 0074-1809. - 158381132X ; 211, s. 325-332
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Konferensbidrag (refereegranskat)abstract
- Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.
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| 2. |
- Aufdenberg, J. P., et al.
(författare)
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Procyon: Constraining Its Temperature Structure with High-Precision Interferometry and 3-D Model Atmospheres
- 2004
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have fit synthetic visibilities from 3-D (CO5BOLD +PHOENIX) and 1-D (PHOENIX, ATLAS12) model stellar atmospheres forProcyon (F5 IV) to high-precision interferometric data from the VINCIinstrument at the VLT Interferometer (K-band) and from the Mark IIIinterferometer (500 nm, 800 nm). These data provide a test oftheoretical wavelength-dependent limb-darkening predictions, andtherefore Procyon's atmospheric temperature structure. Earlier work(Allende Prieto et al. 2002 ApJ 567, 544) has shown that the temperaturestructure from a spatially and temporally averaged 3-D hydrodynamicalmodel produces significantly less limb darkening at 500 nm relative tothe temperature structure from a 1-D MARCS model atmosphere which uses amixing-length approximation for convective flux transport. Our directfits to the interferometric data confirm this prediction, however wefind that not all 1-D models fail to reproduce the observations. The keyto matching the interferometric data is a shallower temperature gradientthan provided by the standard 1-D mixing-length approximation. We findthat in addition to our best fitting 3-D hydrodynamical model, a 1-DATLAS12 model, with an additional free parameter for ``approximateovershooting'', provides the required temperature gradient. We estimatethat an interferometric precision better than 0.1% will be required todistinguish between the 3-D model and the ATLAS12 model. Thisovershooting approximation has been shown to match Solar limb-darkeningobservations reasonably well (Castelli et al 1997 A&A 324, 432), howeverpublished work since using Strömgren photometry of solar-type starshas cast doubt on the importance of overshooting. We have also comparedsynthetic spectral energy distributions for Procyon to ultraviolet,optical and near-infrared spectrophotometry and find differences fromcomparisons to Strömgren photometry alone.This work was performed in part contract with the Jet PropulsionLaboratory (JPL) funded by NASA through the Michelson FellowshipProgram. JPL is managed for NASA by the California Institute ofTechnology.
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| 3. |
- Dorch, S B F, et al.
(författare)
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Small-scale magnetic fields on late-type M-dwarfs
- 2002
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - 0004-6337. ; 323:3-4, s. 402-406
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Tidskriftsartikel (refereegranskat)abstract
- We performed kinematic studies of the evolution of small-scale magneticfields in the surface layers of M-dwarfs. We solved the inductionequation for a prescribed velocity field, magnetic Reynolds number ReM,and boundary conditions in a Cartesian box, representing a volumecomprising the optically thin stellar atmosphere and the uppermost partof the optically thick convective envelope. The velocity field isspatially and temporally variable, and stems from detailedradiation-hydrodynamics simulations of convective flows in aproto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solarchemical composition, spectral type ~M6). We find dynamo action for ReM>= 400. Growth time scales of the magnetic field are comparable tothe convective turn-over time scale (~ 150pun {sec}). The convectivevelocity field concentrates the magnetic field in sheets and tubularstructures in the inter-granular downflows. Scaling from solarconditions suggests that field strengths as high as 20pun{kG} might bereached locally. Perhaps surprisingly, ReM is of order unity in thesurface layers of cooler M-dwarfs, rendering the dynamo inoperative. Inall studied cases we find a rather low spatial filling factor of themagnetic field.
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| 4. |
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| 5. |
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| 6. |
- Ludwig, Hans-Günter, et al.
(författare)
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3D Hydrodynamical Modelling of the Solar Chromosphere
- 2003
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Ingår i: International Astronomical Union Symposium. - 158381163X ; 219, s. 40-40
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Konferensbidrag (refereegranskat)abstract
- We report on 3D radiation-hydrodynamics modelling of the non-magneticsolar atmosphere with emphasis on the problem of the acoustic heating ofthe chromosphere. Our models include the generation of acoustic waves inthe uppermost layers of the solar convection zone their propagationthrough the photospheric layers and their ultimate transformation intoshock fronts which are dissipated in the chromospheric layers. In thechromosphere we find a bimodal temperature distribution consisting of acool non-shocked and hot shocked gas component. The hot gas is locatedin small-scale filamentary structures associated with propagating shockfronts which evolve rapidly in time. The detailed spatial and temporalinformation provided by the models can be exploited to predictobservational diagnostics hitherto unavailable. We present syntheticimages of features in the millimetre and sub-millimetre radio continuumpotentially observable by the Atacama Large Millimetre Array (ALMA).
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| 7. |
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| 8. |
- Ludwig, Hans-Günter, et al.
(författare)
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Convection and Dust in Cool Stellar Atmospheres
- 2003
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Ingår i: Stars as Suns: Activity, Evolution and Planets, International Astronomical Union. Symposium no. 219, held 21-25 July, 2003 in Sydney, Australia, meeting abstract. ; 219, s. 41-41
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Konferensbidrag (refereegranskat)abstract
- We report on recent progress in hydrodynamical modelling of the surfacelayers of late M- and L-type main- as well as pre-main-sequence objects.Despite the complex chemistry encountered in the cool atmospheres ofsuch objects a reasonably accurate representation of the radiativetransfer is possible - even within time-dependent multi-dimensionalmodels. The detailed treatment of the interplay between radiation andconvection in the hydrodynamical models allows us to study processesusually not accessible within the framework of conventional modelatmospheres. In particular we derive the efficiency of the convectiveenergy transport expressed in terms of an equivalent mixing-lengthparameter e.g. suitable to construct global stellar structure models.The models also provide an estimate of convective overshooting into theatmospheric layers which are formally stable according to theSchwarzschild criterion. Preliminary brown dwarf models incorporatingthe atmospheric condensation transport and evaporation of dust cloudswill be presented.
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| 9. |
- Ludwig, Hans-Günter, et al.
(författare)
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Convection and Small-scale Magnetic Fields in M-type Atmospheres
- 2003
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Ingår i: Short Contributions of the Annual Scientific Meeting of the Astronomische Gesellschaft in Freiburg ( Astronomische Nachrichten). - : Wiley. - 0004-6337 .- 1521-3994. ; 324:Suppl. Iss. 3, s. 65-65
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Konferensbidrag (refereegranskat)
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| 10. |
- Ludwig, Hans-Günter, et al.
(författare)
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Energy Transport, Overshoot, and Mixing in the Atmospheres of Very Cool Stars
- 2003
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Ingår i: Modelling of Stellar Atmospheres ; Proceedings of the 210th Symposium of the International Astronomical Union held at Uppsala University, Uppsala, Sweden, 17-21 June, 2002. - 1583811605 ; 210, s. 113-125
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Konferensbidrag (refereegranskat)abstract
- We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence objects. Despite the complex chemistry encountered in such cool atmospheres a reasonably accurate representation of the radiative transfer is possible. The detailed treatment of the interplay between radiation and convection in the hydrodynamical models allows to study processes usually not accessible within the framework conventional model atmospheres. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency expressed in terms of an equivalent mixing-length parameter amounts to values around 2 in the optically thick, and 2.8 in the optically thin regime. The thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e. stays close to radiative equilibrium. Mixing by convective overshoot shows an exponential decline with geometrical distance from the Schwarzschild stability boundary. The scale height of the decline varies with gravitational acceleration roughly as g(-1/2), with 0.5 pressure scale heights at log(g)=5.0.
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