arXiv subjects

Peter Woitke

Publications and source records attributed to Peter Woitke.

42 records · Page 3Linked to original sources

arXiv2009

Hot and cool water in Herbig Ae protoplanetary disks. A challenge for Herschel

The spatial origin and detectability of rotational H2O emission lines from Herbig Ae type protoplanetary disks beyond 70 micron is discussed. We use the recently developed disk code ProDiMo to calculate the thermo-chemical structure of a Herbig Ae type disk and apply the non-LTE line radiative transfer code Ratran to predict water line profiles and intensity maps. The model shows three spatially distinct regions in the disk where water concentrations are high, related to different chemical pathways to form the water: (1) a big water reservoir in the deep midplane behind the inner rim, (2) a belt of cold water around the distant icy midplane beyond the snow-line r>20AU, and (3) a layer of irradiated hot water at high altitudes z/r=0.1...0.3, extending from about 1AU to 30AU, where the kinetic gas temperature ranges from 200K to 1500K. Although region 3 contains only little amounts of water vapour (~3x10^-5 M_Earth), we find this warm layer to be almost entirely responsible for the rotational water emission lines, execpt for the 3 lowest excitation lines. Thus, Herschel will probe first and foremost the conditions and radial extension of region 3, where water is predominantly formed via neutral-neutral reactions and the gas is thermally decoupled from the dust T_gas>T_dust. The observations do not allow for a determination of the snow-line, because the snow-line truncates the radial extension of region 1, whereas the lines originate from region 3. Different line transfer approximations (LTE, escape probability, Monte Carlo) are discussed. A non-LTE treatment is required in most cases, and the results obtained with the escape probability method are found to underestimate the Monte Carlo results by 2%...45%.

astro-ph.EP↗

arXiv2009

Radiation thermo-chemical models of protoplanetary disks I. Hydrostatic disk structure and inner rim

This paper introduces a new disk code, called ProDiMo, to calculate the thermo-chemical structure of protoplanetary disks and to interpret gas emission lines from UV to sub-mm. We combine frequency-dependent 2D dust continuum radiative transfer, kinetic gas-phase and UV photo-chemistry, ice formation, and detailed non-LTE heating & cooling balance with the consistent calculation of the hydrostatic disk structure. We include FeII and CO ro-vibrational line heating/cooling relevant for the high-density gas close to the star, and apply a modified escape probability treatment. The models are characterized by a high degree of consistency between the various physical, chemical and radiative processes, where the mutual feedbacks are solved iteratively. In application to a T Tauri disk extending from 0.5AU to 500AU, the models are featured by a puffed-up inner rim and show that the dense, shielded and cold midplane (z/r<0.1, Tg~Td) is surrounded by a layer of hot (5000K) and thin (10^7 to 10^8 cm^-3) atomic gas which extends radially to about 10AU, and vertically up to z/r~0.5. This layer is predominantly heated by the stellar UV (e.g. PAH-heating) and cools via FeII semi-forbidden and OI 630nm optical line emission. The dust grains in this "halo" scatter the star light back onto the disk which impacts the photo-chemistry. The more distant regions are characterized by a cooler flaring structure. Beyond 100AU, Tgas decouples from Tdust even in the midplane and reaches values of about Tg~2Td. Our models show that the gas energy balance is the key to understand the vertical disk structure. Models calculated with the assumption Tg=Td show a much flatter disk structure.

astro-ph.EP↗

arXiv2008

Gas Evolution in Protoplanetary Disks

This article summarizes a Splinter Session at the Cool Stars XV conference in St. Andrews with 3 review and 4 contributed talks. The speakers have discussed various approaches to understand the structure and evolution of the gas component in protoplanetary disks. These ranged from observational spectroscopy in the UV, infrared and millimeter, through to chemical and hydrodynamical models. The focus was on disks around low-mass stars, ranging from classical T Tauri stars to transitional disks and debris disks. Emphasis was put on water and organic molecules, the relation to planet formation, and the formation of holes and gaps in the inner regions.

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arXiv2008

Consistent simulations of substellar atmospheres and non-equilibrium dust-cloud formation

We aim to understand cloud formation in substellar objects. We combined the non-equilibrium, stationary cloud model of Helling, Woitke & Thi (2008; 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=1800K for the L-dwarf twin-binary system DENIS J0205-1159 which is up to 500K 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.

astro-ph↗

arXiv2006

Too little radiation pressure on dust in the winds of oxygen-rich AGB stars

New dynamical models for dust-driven winds of oxygen-rich AGB stars are presented which include frequency-dependent Monte Carlo radiative transfer by means of a sparse opacity distribution technique and a time-dependent treatment of the nucleation, growth and evaporation of inhomogeneous dust grains composed of a mixture of Mg2SiO4, SiO2, Al2O3, TiO2, and solid Fe. The frequency-dependent treatment of radiative transfer reveals that the gas is cold close to the star (700-900 K at 1.5-2 R*) which facilitates the nucleation process. The dust temperatures are strongly material-dependent, with differences as large as 1000 K for different pure materials, which has an important influence on the dust formation sequence. Two dust layers are formed in the dynamical models: almost pure glassy Al2O3 close to the star (r > 1.5 R*) and the more opaque Fe-poor Mg-Fe-silicates further out. Solid Fe or Fe-rich silicates are found to be the only condensates that can efficiently absorb the stellar light in the near IR. Consequently, they play a crucial role for the wind driving mechanism and act as thermostat. Only small amounts of Fe can be incorporated into the grains, because otherwise the grains get too hot. Thus, the models reveal almost no mass loss, and no dust shells.

astro-ph↗

arXiv2006

2D Models for Dust-driven AGB Star Winds

New axisymmetric (2D) models for dust-driven winds of C-stars are presented which include hydrodynamics with radiation pressure on dust, equilibrium chemistry and time-dependent dust formation with coupled grey Monte Carlo radiative transfer. Considering the most simple case without stellar pulsation (hydrostatic inner boundary condition) these models reveal a more complex picture of the dust formation and wind acceleration as compared to earlier published spherically symmetric (1D) models. The so-called exterior $κ$-mechanism causes radial oscillations with short phases of active dust formation between longer phases without appreciable dust formation, just like in the 1D models. However, in 2D geometry, the oscillations can be out-of-phase at different places above the stellar atmosphere which result in the formation of dust arcs or smaller caps that only occupy a certain fraction of the total solid angle. These dust structures are accelerated outward by radiation pressure, expanding radially and tangentially like mushroom clouds, while dust-poor matter is falling back towards the star at other places. A highly dynamical and turbulent dust formation zone is created in this way, which again leads to inhomogeneous dust production. Further away from the star, flow instabilities (e.g. Rayleigh-Taylor) have time to fragment the outward moving arcs and shells to produce numerous small-scale cloud-like sub-structures.

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