438 research outputs found
Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models
Context. Copper is an element whose interesting evolution with metallicity is
not fully understood. Observations of copper abundances rely on a very limited
number of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and
327.3 nm which can be measured even at extremely low metallicities. Aims. We
investigate the quality of these lines as abundance indicators. Method. We
measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752
and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D
hydrodynamical model atmospheres computed with the CO5BOLD code. These
abundances are compared to the Cu abundances measured in giant stars of the
same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results.
The abundances derived from the lines of Mult. 1 in TO stars differ from the
abundances of giants of the same clusters. This is true both using CO5BOLD
models and using traditional 1D model atmospheres. The LTE 3D corrections for
TO stars are large, while they are small for giant stars. Conclusions. The Cu I
resonance lines of Mult. 1 are not reliable abundance indicators. It is likely
that departures from LTE should be taken into account to properly describe
these lines, although it is not clear if these alone can account for the
observations. An investigation of these departures is indeed encouraged for
both dwarfs and giants. Our recommendation to those interested in the study of
the evolution of copper abundances is to rely on the measurements in giants,
based on the lines of Mult. 2. We caution, however, that NLTE studies may imply
a revision in all the Cu abundances, both in dwarfs and giants.Comment: to be published on A\&
Prospects of using simulations to study the photospheres of brown dwarfs
We discuss prospects of using multi-dimensional time-dependent simulations to
study the atmospheres of brown dwarfs and extrasolar giant planets, including
the processes of convection, radiation, dust formation, and rotation. We argue
that reasonably realistic simulations are feasible, however, separated into two
classes of local and global models. Numerical challenges are related to
potentially large dynamic ranges, and the treatment of scattering of radiation
in multi-D geometries.Comment: 6 pages, 3 figures, to appear in the Proceedings of the IAU Symposium
239 "Convection in Astrophysics", eds. F. Kupka, I.W. Roxburgh, and K.L. Cha
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
Spatially resolved spectroscopy across stellar surfaces. I. Using exoplanet transits to analyze 3-D stellar atmospheres
CONTEXT: High-precision stellar analyses require hydrodynamic modeling to
interpret chemical abundances or oscillation modes. Exoplanet atmosphere
studies require stellar background spectra to be known along the transit path
while detection of Earth analogs require stellar microvariability to be
understood. Hydrodynamic 3-D models can be computed for widely different stars
but have been tested in detail only for the Sun with its resolved surface
features. Model predictions include spectral line shapes, asymmetries, and
wavelength shifts, and their center-to-limb changes across stellar disks. AIMS:
To observe high-resolution spectral line profiles across spatially highly
resolved stellar surfaces, which are free from the effects of spatial smearing
and rotational broadening present in full-disk spectra, enabling comparisons to
synthetic profiles from 3-D models. METHODS: During exoplanet transits,
successive stellar surface portions become hidden and differential spectroscopy
between various transit phases provides spectra of small surface segments
temporarily hidden behind the planet. Planets cover no more than about 1% of
any main-sequence star, enabling high spatial resolution but demanding very
precise observations. Realistically measurable quantities are identified
through simulated observations of synthetic spectral lines. RESULTS: In normal
stars, line profile ratios between various transit phases may vary by some
0.5%, requiring S/N ratios of 5,000 or more for meaningful spectral
reconstruction. While not yet realistic for individual spectral lines, this is
achievable for cool stars by averaging over numerous lines with similar
parameters. CONCLUSIONS: For bright host stars of large transiting planets,
spatially resolved spectroscopy is currently practical. More observable targets
are likely to be found in the near future by ongoing photometric searches.Comment: Accepted by Astronomy & Astrophysics; 14 pages, 12 figure
Solar Photospheric Spectrum Microvariability I. Theoretical searches for proxies of radial-velocity jittering
Extreme precision radial-velocity spectrometers enable extreme precision
stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars
are limited by the physical variability in stellar spectra, such as the
short-term jittering of apparent radial velocities. To understand the physical
origins of such jittering, the solar spectrum is assembled, as far as possible,
from basic principles. Surface convection is modeled with time-dependent 3D
hydrodynamics, followed by the computation of hyper-high resolution spectra
during numerous instances of the simulation sequences. The behavior of
different classes of photospheric absorption lines is monitored to identify
commonalities or differences between different classes of lines: weak or
strong, neutral or ionized, high- or low-excitation, atomic or molecular. For
Fe I and Fe II lines, the radial-velocity jittering over the small simulation
area typically amounts to +-150 m/s, scaling to about 2 m/s for the full solar
disk. Most photospheric lines vary in phase but with different amplitudes among
different classes of lines. Radial-velocity excursions are greater for stronger
and for ionized lines, decreasing at longer wavelengths. The differences
between various line-groups are about one order of magnitude less than the full
jittering amplitudes. By matching very precisely measured radial velocities to
the characteristic jittering patterns between different line-groups should
enable to identify and to remove a significant component of the stellar noise
originating in granulation. To verify the modeling toward such a filter,
predictions of solar center-to-limb dependences of jittering amplitudes are
presented for different classes of lines, testable with spatially resolving
solar telescopes connected to existing radial-velocity instruments.Comment: 18 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
Overview of the lithium problem in metal-poor stars and new results on 6Li
Two problems are discussed here. The first one is the 0.4 dex discrepancy
between the 7Li abundance derived from the spectra of metal-poor halo stars on
the one hand, and from Big Bang nucleosynthesis, based on the cosmological
parameters constrained by the WMAP measurements, on the other hand. Lithium,
indeed, can be depleted in the convection zone of unevolved stars. The
understanding of the hydrodynamics of the crucial zone near the bottom of the
convective envelope in dwarfs or turn-off stars of solar metallicity has
recently made enormous progress with the inclusion of internal gravity waves.
However, similar work for metal-poor stars is still lacking. Therefore it is
not yet clear whether the depletion occurring in the metal-poor stars
themselves is adequate to produce a 7Li plateau. The second problem pertains to
the large amount of 6Li recently found in metal-poor halo stars. The
convection-related asymmetry of the 7Li line could mimic the signal attributed
so far to the weak blend of 6Li in the red wing of the 7Li line. Theoretical
computations show that the signal generated by the asymmetry of 7Li is 2.0,
2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively (Teff =6250 K
and log g=4.0 [cgs]). In addition we re-investigate the statistical properties
of the 6Li plateau and show that previous analyses were biased. Our conclusion
is that the 6Li plateau can be reinterpreted in terms of intrinsic line
asymmetry, without the need to invoke a contribution of 6Li. (abridged)Comment: Invited talk at the 10th Symposium on Nuclei in the Cosmos - July 27
- August 1 2008 - Mackinac Island, Michigan, USA, Accepted version. Minor
changes following referee's suggestion
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
Spatially resolved spectroscopy across stellar surfaces. III. Photospheric Fe I lines across HD189733A (K1 V)
Spectroscopy across spatially resolved stellar surfaces reveals spectral line
profiles free from rotational broadening, whose gradual changes from disk
center toward the stellar limb reflect an atmospheric fine structure that is
possible to model by 3-D hydrodynamics. Previous studies of photospheric
spectral lines across stellar disks exist for the Sun and HD209458 (G0 V) and
are now extended to the planet-hosting HD189733A to sample a cooler K-type star
and explore the future potential of the method. During exoplanet transit,
stellar surface portions successively become hidden and differential
spectroscopy between various transit phases uncovers spectra of small surface
segments temporarily hidden behind the planet. In Paper I, observable
signatures were predicted quantitatively from hydrodynamic simulations. From
observations of HD189733A with the ESO HARPS spectrometer at R=115,000,
profiles for stronger and weaker Fe I lines are retrieved at several
center-to-limb positions, reaching adequate S/N after averaging over numerous
similar lines. Retrieved line profile widths and depths are compared to
synthetic ones from models with parameters bracketing those of the target star
and are found to be consistent with 3-D simulations. Center-to-limb changes
strongly depend on the surface granulation structure and much greater
line-width variation is predicted in hotter F-type stars with vigorous
granulation than in cooler K-types. Such parameters, obtained from fits to full
line profiles, are realistic to retrieve for brighter planet-hosting stars,
while their hydrodynamic modeling offers previously unexplored diagnostics for
stellar atmospheric fine structure and 3-D line formation. Precise modeling may
be required in searches for Earth-analog exoplanets around K-type stars, whose
more tranquil surface granulation and lower ensuing microvariability may enable
such detections.Comment: 14 pages, 12 figures, accepted by Astronomy & Astrophysic
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