697 research outputs found
Pulsation-Initiated Mass Loss in Luminous Blue Variables: A Parameter Study
Luminous blue variables (LBVs) are characterized by semi-periodic episodes of
enhanced mass-loss, or outburst. The cause of these outbursts has thus far been
a mystery. One explanation is that they are initiated by kappa-effect
pulsations in the atmosphere caused by an increase in luminosity at
temperatures near the so-called ``iron bump'' (T ~ 200,000 K), where the Fe
opacity suddenly increases. Due to a lag in the onset of convection, this
luminosity can build until it exceeds the Eddington limit locally, seeding
pulsations and possibly driving some mass from the star. We present some
preliminary results from a parameter study focusing on the conditions necessary
to trigger normal S-Dor type (as opposed to extreme eta-Car type) outbursts. We
find that as Y increases or Z decreases, the pulsational amplitude decreases
and outburst-like behavior, indicated by a large, sudden increase in
photospheric velocity, becomes likes likely.Comment: 6 pages, 4 figures, to be published in the Proceedings of Massive
Stars as Cosmic Engines, IAU Symp 250, ed. F. Bresolin, P. A. Crowther, & J.
Puls (Cambridge Univ. Press
Radiation-hydrodynamics simulations of surface convection in low-mass stars: connections to stellar structure and asteroseismology
Radiation-hydrodynamical simulations of surface convection in low-mass stars
can be exploited to derive estimates of i) the efficiency of the convective
energy transport in the stellar surface layers; ii) the convection-related
photometric micro-variability. We comment on the universality of the
mixing-length parameter, and point out potential pitfalls in the process of its
calibration which may be in part responsible for the contradictory findings
about its variability across the Hertzsprung-Russell digramme. We further
comment on the modelling of the photometric micro-variability in HD49933 - one
of the first main COROT targets.Comment: 6 pages, 5 figures, Proceedings paper of IAU Symposium 25
Excitation of stellar p-modes by turbulent convection: 1. Theoretical formulation
Stochatic excitation of stellar oscillations by turbulent convection is
investigated and an expression for the power injected into the oscillations by
the turbulent convection of the outer layers is derived which takes into
account excitation through turbulent Reynolds stresses and turbulent entropy
fluctuations. This formulation generalizes results from previous works and is
built so as to enable investigations of various possible spatial and temporal
spectra of stellar turbulent convection. For the Reynolds stress contribution
and assuming the Kolmogorov spectrum we obtain a similar formulation than those
derived by previous authors. The entropy contribution to excitation is found to
originate from the advection of the Eulerian entropy fluctuations by the
turbulent velocity field. Numerical computations in the solar case in a
companion paper indicate that the entropy source term is dominant over Reynold
stress contribution to mode excitation, except at high frequencies.Comment: 14 pages, accepted for publication in A&
On the DB gap of white dwarf evolution: effects of hydrogen mass fraction and convective overshooting
We investigate the spectral evolution of white dwarfs by considering the
effects of hydrogen mass in the atmosphere and convective overshooting above
the convection zone. We notice the importance of the convective overshooting
and suggest that the overshooting length should be proportional to the
thickness of the convection zone to better fit the observations.Comment: 13 figure
Effects of Rotation and Input Energy Flux on Convective Overshooting
We study convective overshooting by means of local 3D convection
calculations. Using a mixing length model of the solar convection zone (CZ) as
a guide, we determine the Coriolis number (Co), which is the inverse of the
Rossby number, to be of the order of ten or larger at the base of the solar CZ.
Therefore we perform convection calculations in the range Co = 0...10 and
interpret the value of Co realised in the calculation to represent a depth in
the solar CZ. In order to study the dependence on rotation, we compute the
mixing length parameters alpha_T and alpha_u relating the temperature and
velocity fluctuations, respectively, to the mean thermal stratification. We
find that the mixing length parameters for the rapid rotation case,
corresponding to the base of the solar CZ, are 3-5 times smaller than in the
nonrotating case. Introducing such depth-dependent alpha into a solar structure
model employing a non-local mixing length formalism results in overshooting
which is approximately proportional to alpha at the base of the CZ. Although
overshooting is reduced due to the reduced alpha, a discrepancy with
helioseismology remains due to the steep transition to the radiative
temperature gradient. In comparison to the mixing length models the transition
at the base of the CZ is much gentler in the 3D models. It was suggested
recently (Rempel 2004) that this discrepancy is due to the significantly larger
(up to seven orders of magnitude) input energy flux in the 3D models in
comparison to the Sun and solar models, and that the 3D calculations should be
able to approach the mixing length regime if the input energy flux is decreased
by a moderate amount. We present results from local convection calculations
which support this conjecture.Comment: 6 pages, 3 figures, to appear in Convection in Astrophysics, Proc.
IAUS 239, edited by F. Kupka, I.W. Roxburgh, K.L. Cha
Line formation in convective stellar atmospheres. I. Granulation corrections for solar photospheric abundances
In an effort to estimate the largely unknown effects of photospheric
temperature fluctuations on spectroscopic abundance determinations, we have
studied the problem of LTE line formation in the inhomogeneous solar
photosphere based on detailed 2-dimensional radiation hydrodynamics simulations
of the convective surface layers of the Sun. By means of a strictly
differential 1D/2D comparison of the emergent equivalent widths, we have
derived "granulation abundance corrections" for individual lines, which have to
be applied to standard abundance determinations based on homogeneous 1D model
atmospheres in order to correct for the influence of the photospheric
temperature fluctuations. In general, we find a line strengthening in the
presence of temperature inhomogeneities as a consequence of the non-linear
temperature dependence of the line opacity. For many lines of practical
relevance, the magnitude of the abundance correction may be estimated from
interpolation in the tables and graphs provided with this paper. The
application of abundance corrections may often be an acceptable alternative to
a detailed fitting of individual line profiles based on hydrodynamical
simulations. The present study should be helpful in providing upper bounds for
possible errors of spectroscopic abundance analyses, and for identifying
spectral lines which are least sensitive to the influence of photospheric
temperature inhomogeneities.Comment: Accepted by A&
Time-dependent Turbulence in Stars
Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning
(Meakin and Arnett 2007) exhibit bursty, recurrent fluctuations in turbulent
kinetic energy. These are shown to be due to a global instability in the
convective region, which has been suppressed in calculations of stellar
evolution which use mixing-length theory (MLT). Quantitatively similar behavior
occurs in the model of a convective roll (cell) of Lorenz (1963), which is
known to have a strange attractor that gives rise to random fluctuations in
time.An extension of the Lorenz model, which includes Kolmogorov damping and
nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those
seen in the 3D simulations. A simple model of a convective layer (composed of
multiple Lorenz cells) gives luminosity fluctuations which are suggestive of
irregular variables (red giants and supergiants, Schwarzschild 1975).
Apparent inconsistencies between Arnett, Meakin, and Young (2009) and
Nordlund, Stein, and Asplund (2009) on the nature of convective driving have
been resolved, and are discussed.Comment: 8 pages, 2 figures, IAU Symposium 271 "Astrophysical Dynamics: From
Galaxies to Stars", Nice, FR, 201
A test of time-dependent theories of stellar convection
Context: In Cepheids close to the red edge of the classical instability
strip, a coupling occurs between the acoustic oscillations and the convective
motions close to the surface.The best topical models that account for this
coupling rely on 1-D time-dependent convection (TDC) formulations. However,
their intrinsic weakness comes from the large number of unconstrained free
parameters entering in the description of turbulent convection. Aims: We
compare two widely used TDC models with the first two-dimensional nonlinear
direct numerical simulations (DNS) of the convection-pulsation coupling in
which the acoustic oscillations are self-sustained by the kappa-mechanism.
Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC
recipes are constrained using a chi2-test with the time-dependent convective
flux that evolves in nonlinear simulations of highly-compressible convection
with kappa-mechanism. Results: This work emphasises some inherent limits of TDC
models, that is, the temporal variability and non-universality of their free
parameters. More importantly, within these limits, Stellingwerf's formalism is
found to give better spatial and temporal agreements with the nonlinear
simulation than Kuhfuss's one. It may therefore be preferred in 1-D TDC
hydrocodes or stellar evolution codes.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in A&
Ruthenium and hafnium abundances in giant and dwarf barium stars
We present abundances for Ru and Hf, compare them to abundances of other
heavy elements, and discuss the problems found in determining Ru and Hf
abundances with laboratory gf-values in the spectra of barium stars. We
determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by
the spectral synthesis of two RuI (4080.574A and 4757.856A) and two HfII
(4080.437A and 4093.155A) transitions. The stellar spectra were observed with
FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 <
Teff/K < 6500, -1.2 < [Fe/H] <= 0 and 1.4 <= log g < 4.6. The HfII 4080A and
the RuI 4758A observed transitions result in a unreasonably high solar
abundance, given certain known uncertainties, when fitted with laboratory
gf-values. For these two transitions we determined empirical gf-values by
fitting the observed line profiles of the spectra of the Sun and Arcturus. For
the sample stars, this procedure resulted in a good agreement of Ru and Hf
abundances given by the two available lines. The resulting Ru and Hf abundances
were compared to those of Y, Nd, Sm and Eu. In the solar system Ru, Sm and Eu
are dominated by the r-process and Hf, Nd and Y by the s-process, and all of
these elements are enhanced in barium stars since they lie inside the s-process
path. Ru abundances show large scatter when compared to other heavy elements,
whereas Hf abundances show less scatter and closely follow the abundances of Sm
and Nd, in good agreement with theoretical expectations. We also suggest a
possible, unexpected, correlation of Ru and Sm abundances. The observed
behaviour in abundances is probably due to variations in the 13C pocket
efficiency in AGB stars, and, though masked by high uncertainties, hint at a
more complex scenario than proposed by theory.Comment: 11 pages, 7 figures and 7 tables. accepted to A&
Asteroseismic signatures of helium gradients in late F-type stars
Element diffusion is expected to occur in all kinds of stars : according to
the relative effect of gravitation and radiative acceleration, they can fall or
be pushed up in the atmospheres. Helium sinks in all cases, thereby creating a
gradient at the bottom of the convective zones. This can have important
consequences for the sound velocity, as has been proved in the sun with
helioseismology.
We investigate signatures of helium diffusion in late F-type stars by
asteroseismology.
Stellar models were computed with different physical inputs (with or without
element diffusion) and iterated in order to fit close-by evolutionary tracks
for each mass. The theoretical oscillation frequencies were computed and
compared for pairs of models along the tracks. Various asteroseismic tests
(large separations, small separations, second differences) were used and
studied for the comparisons.
The results show that element diffusion leads to changes in the frequencies
for masses larger than 1.2 Msun. In particular the helium gradient below the
convective zone should be detectable through the second differences.Comment: 8 pages, 11 figures, 2 tables Accepted for publication in Astronomy
and Astrophysics. The official date of acceptance is 03/05/200
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