192 research outputs found
Overtures to the pulsational instability of ZZ Ceti variables
Results of nonradial, nonadiabatic pulsation calculations on hydrogen-rich
white dwarf models are presented. In contrast to earlier attempts, the modeling
builds on hydrodynamically simulated convective surface layers supplemented
with standard interior models. Based on our stellar models and despite of
various simple attempts to couple convection and pulsation we could not
reproduce theoretically the presently adopted location of the observed blue
edge of the ZZ Ceti variables. When the convective efficiency is high enough we
found a sensitive dependence of the stability properties of the g-modes on the
pulsational treatment of shear within the convection zone.Comment: 13 pages, postscript figures included in text, uuencoded gzipped
ps-file. Submitted for publication in Astron.&Astrophy
Solar abundances and 3D model atmospheres
We present solar photospheric abundances for 12 elements from optical and
near-infrared spectroscopy. The abundance analysis was conducted employing 3D
hydrodynamical (CO5BOLD) as well as standard 1D hydrostatic model atmospheres.
We compare our results to others with emphasis on discrepancies and still
lingering problems, in particular exemplified by the pivotal abundance of
oxygen. We argue that the thermal structure of the lower solar photosphere is
very well represented by our 3D model. We obtain an excellent match of the
observed center-to-limb variation of the line-blanketed continuum intensity,
also at wavelengths shortward of the Balmer jump.Comment: Contributed paper, to be published in the proceedings of IAU
Symposium 265, eds. K. Cunha, M. Spite, and B. Barbuy, Cambridge University
Press (CUP). 2 figures, 4 page
Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere
Three-dimensional numerical simulations with CO5BOLD, a new radiation
hydrodynamics code, result in a dynamic, thermally bifurcated model of the
non-magnetic chromosphere of the quiet Sun. The 3-D model includes the middle
and low chromosphere, the photosphere, and the top of the convection zone,
where acoustic waves are excited by convective motions. While the waves
propagate upwards, they steepen into shocks, dissipate, and deposit their
mechanical energy as heat in the chromosphere. Our numerical simulations show
for the first time a complex 3-D structure of the chromospheric layers, formed
by the interaction of shock waves. Horizontal temperature cross-sections of the
model chromosphere exhibit a network of hot filaments and enclosed cool
regions. The horizontal pattern evolves on short time-scales of the order of
typically 20 - 25 seconds, and has spatial scales comparable to those of the
underlying granulation. The resulting thermal bifurcation, i.e., the
co-existence of cold and hot regions, provides temperatures high enough to
produce the observed chromospheric UV emission and -- at the same time --
temperatures cold enough to allow the formation of molecules (e.g., carbon
monoxide). Our 3-D model corroborates the finding by Carlsson & Stein (1994)
that the chromospheric temperature rise of semi-empirical models does not
necessarily imply an increase in the average gas temperature but can be
explained by the presence of substantial spatial and temporal temperature
inhomogeneities.Comment: 18 pages, 13 figures, accepted by Astronomy & Astrophysics (30/10/03
The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres
Observationally, spectra of brown dwarfs indicate the presence of dust in
their atmospheres while theoretically it is not clear what prevents the dust
from settling and disappearing from the regions of spectrum formation.
Consequently, standard models have to rely on ad hoc assumptions about the
mechanism that keeps dust grains aloft in the atmosphere. We apply
hydrodynamical simulations to develop an improved physical understanding of the
mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres,
in particular of the influence of the underlying convection zone. We performed
2D radiation hydrodynamics simulations including a description of dust grain
formation and transport with the CO5BOLD code. The simulations cover the very
top of the convection zone and the photosphere including the dust layers for
effective temperatures between 900K and 2800K, all with logg=5 assuming solar
chemical composition. Convective overshoot occurs in the form of exponentially
declining velocities with small scale heights, so that it affects only the
region immediately above the almost adiabatic convective layers. From there on,
mixing is provided by gravity waves that are strong enough to maintain thin
dust clouds in the hotter models. With decreasing effective temperature, the
amplitudes of the waves become smaller but the clouds become thicker and
develop internal convective flows that are more efficient in mixing material
than gravity waves. The presence of clouds leads to a highly structured
appearance of the stellar surface on short temporal and small spatial scales.
We identify convectively excited gravity waves as an essential mixing process
in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud
formation, dust convection is the dominant self-sustaining mixing component
Spatially resolved spectroscopy across stellar surfaces. IV. F, G, & K-stars: Synthetic 3D spectra at hyper-high resolution
High-precision stellar analyses require hydrodynamic 3D modeling. Such models
predict changes across stellar disks of spectral line shapes, asymmetries, and
wavelength shifts. For testing models in stars other than the Sun, spatially
resolved observations are feasible from differential spectroscopy during
exoplanet transits, retrieving spectra of stellar surface segments that
successively become hidden behind the transiting planet, as shown in Papers I,
II, and III. Synthetic high-resolution spectra over extended spectral regions
are now available from 3D models. Similar to other ab initio simulations, these
data contain patterns that have not been specifically modeled but may be
revealed after analyses analogous to those of a large volume of observations.
From five 3D models spanning T=3964-6726K (approx. spectral types K8V-F3V),
synthetic spectra at hyper-high resolution (R>1,000,000) were analyzed.
Selected FeI and FeII lines at various positions across stellar disks were
searched for patterns between different lines in the same star and for similar
lines between different stars. Such patterns are identified for representative
photospheric lines of different strengths, excitation potential, and ionization
level, encoding the hydrodynamic 3D structure. Line profiles and bisectors are
shown for various stars at different positions across stellar disks. Absolute
convective wavelength shifts are obtained as differences to 1D models, where
such shifts do not occur. Observable relationships for line properties are
retrieved from realistically complex synthetic spectra. Such patterns may also
test very detailed 3D modeling, including non-LTE effects. While present
results are obtained at hyper-high spectral resolution, the subsequent Paper V
examines their practical observability at realistically lower resolutions, and
in the presence of noise.Comment: 17 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Radiation hydrodynamics simulations of dust clouds in the atmospheres of substellar objects
Molecules in the Atmospheres of Extrasolar Planets, proceedings of a conference held at Observatoire de Paris, Paris, France 19-21 November, 2008. ASP Conference Series, Vol. 450. Edited by J.P. Beaulieu, S. Dieteres, and G. Tinetti. San Francisco: Astronomical Society of the Pacific, 2011., p.125The temperature structure and the motions in the atmospheres of cool stars are affected by the underlying convection zone. The radiation hy- drodynamics code CO5BOLD has been developed to simulate (small patches of the) convective surface layers of these stars. Updated opacity tables based on PHOENIX data and a description for the formation, destruction, advective transport, and settling of dust have made the code fit to handle the conditions in brown dwarf atmospheres. Currently, objects from 8500K down to about 900K have been simulated. Recently, incident radiation has been included, allow- ing simulations with conditions found on hot planets. In non-irradiated brown dwarf models we encounter mixing by gravity waves and in the cooler models convection within the clouds. The qualitative effects of incident radiation are surprisingly small, as long as the effective temperature of the object stays well below the dust condensation temperature. Beyond that point, there are no layers where dust could form, anymore
Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere
to be published in Solar PhysicsIn the last decade, the photospheric solar metallicity as determined from spectroscopy experienced a remarkable downward revision. Part of this effect can be attributed to an improvement of atomic data and the inclusion of NLTE computations, but also the use of hydrodynamical model atmospheres seemed to play a role. This ``decrease'' with time of the metallicity of the solar photosphere increased the disagreement with the results from helioseismology. With a CO5BOLD 3D model of the solar atmosphere, the CIFIST team at the Paris Observatory re-determined the photospheric solar abundances of several elements, among them C, N, and O. The spectroscopic abundances are obtained by fitting the equivalent width and/or the profile of observed spectral lines with synthetic spectra computed from the 3D model atmosphere. We conclude that the effects of granular fluctuations depend on the characteristics of the individual lines, but are found to be relevant only in a few particular cases. 3D effects are not reponsible for the systematic lowering of the solar abundances in recent years. The solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209
Imaging simulations of selected science with the Magdalena Ridge Observatory Interferometer
We present simulated observations of surface features on Red Supergiant (RSG) stars and clumpy dust structures surrounding Active Galactic Nuclei (AGN) with the Magdalena Ridge Observatory Interferometer (MROI). These represent two of the classes of astrophysical targets enumerated in the MROI Key Science Mission that are typical of the types of complex astrophysical phenomena that the MROI has been designed to image. The simulations are based on source structures derived from recent theoretical models and include both random and systematic noise on the measured Fourier data (visibility amplitudes and closure phases) consistent with our expectations for typical such targets observed with the MROI. Image reconstructions, obtained using the BSMEM imaging package, are presented for 4-, 6- and 8- telescope implementations of the array. Although a rudimentary imaging capability is demonstrated with only 4 telescopes, the detailed features of targets are only reliably determined when at least 6 telescopes are present. By the tine 8 telescope are used, the reconstructed images are sufficiently faithful to allow the discrimination between competing models, confirming the design goal of the MROI, i.e. to offer model-independent near-infrared imaging on sub-milliarcsecond scales
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