44 research outputs found
New input data for synthetic AGB evolution
Analytic formulae are presented to construct detailed secular lightcurves of
both early asymptotic giant branch (AGB) and thermally pulsing AGB stars. They
are based on an extensive grid of evolutionary calculations, performed with an
updated stellar evolution code. Basic input parameters are the initial mass
between 0.8 and 7 solar mass, metallicity of 0.0001, 0.008, 0.02, and the
mixing length theory (MLT) parameter. The formulae allow for two important
effects, namely that the first pulses do not reach the full amplitude, and hot
bottom burning (HBB) in massive stars, which are both not accounted for by core
mass - luminosity relations of the usual type.
Furthermore, the dependence of the effective temperature and a few other
quantities characterizing the conditions at the base of the convective
envelope, which are relevant for HBB, are investigated as functions of
luminosity, total and core mass for different formulations of the convection
theory applied, MLT or Canuto & Mazzitelli's theory.Comment: Accepted for A&
On the Metallicity-Color Relations and Bimodal Color Distributions in Extragalactic Globular Cluster Systems
We perform a series of numerical experiments to study how the nonlinear
metallicity--color relations predicted by different stellar population models
affect the color distributions observed in extragalactic globular cluster
systems. % We present simulations in the bandpasses based on five
different sets of simple stellar population (SSP) models. The presence of
photometric scatter in the colors is included as well. % We find that unimodal
metallicity distributions frequently ``project'' into bimodal color
distributions. The likelihood of this effect depends on both the mean and
dispersion of the metallicity distribution, as well as of course on the SSP
model used for the transformation. % Adopting the Teramo-SPoT SSP models for
reference, we find that optical--to--near-IR colors should be favored with
respect to other colors to avoid the bias effect in globular cluster color
distributions discussed by \citet{yoon06}. In particular, colors such as \vh\
or \vk are more robust against nonlinearity of the metallicity--color relation,
and an observed bimodal distribution in such colors is more likely to indicate
a true underlying bimodal metallicity distribution. Similar conclusions come
from the simulations based on different SSP models, although we also identify
exceptions to this result.Comment: ApJ accepte
On the helium flash in low-mass Population III Red Giant stars
We investigate the evolution of initially metal-free, low-mass Red Giant
stars through the He core flash at the tip of the Red Giant Branch. The low
entropy barrier between the helium- and hydrogen-rich layers enables a
penetration of the helium flash driven convective zone into the inner tail of
the extinguishing H-burning shell. As a consequence, protons are mixed into
high-temperature regions triggering a H-burning runaway. The subsequent
dredge-up of matter processed by He and H burning enriches the stellar surface
with large amounts of helium, carbon and nitrogen. Extending previous results
by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the
H-burning runaway is an intrinsic property of extremely metal-poor low-mass
stars, we found that its occurrence depends on additional parameters like the
initial composition and the treatment of various physical processes.
We perform some comparisons between predicted surface chemical abundances and
observational measurements for extremely metal-deficient stars. As in previous
investigations, our results disclose that although the described scenario
provides a good qualitative agreement with observations, considerable
discrepancies still remain. They may be due to a more complex evolutionary path
of `real' stars, and/or some shortcomings in current evolutionary models.
In addition, we analyze the evolutionary properties after the He core flash,
during both the central and shell He-burning phases, allowing us to deduce some
interesting differences between models whose Red Giant Branch progenitor has
experienced the H-flash and canonical models. In particular, the Asymptotic
Giant Branch evolution of extremely metal-deficient stars and the occurrence of
thermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap
A Large Stellar Evolution Database for Population Synthesis Studies. III. Inclusion of the full Asymptotic Giant Branch phase and Web tools for stellar population analyses
Stellar evolution tracks and isochrones are key inputs for a wide range of
astrophysical studies; in particular, they are essential to the interpretation
of photometric and spectroscopic observations of resolved and unresolved
stellar populations. We have made available to the astrophysical community a
large, homogenous database of up-to-date stellar tracks and isochrones, and a
set of programs useful in population synthesis studies.
In this paper we first summarize the main properties of our stellar model
database (BaSTI) already introduced in Pietrinferni et al. (2004) and
Pietrinferni et al. (2006). We then discuss an important update of the
database, i.e., the extension of all stellar models and isochrones until the
end of the thermal pulses along the Asymptotic Giant Branch. This extension of
the library is particularly relevant for stellar population analyses in the
near-infrared, or longer wavelengths, where the contribution to the integrated
photometric properties by cool and bright Asymptotic Giant Branch stars is
significant. A few comparisons with empirical data are also presentend and
briefly discussed. We then present three web-tools that allow an interactive
access to the database, and make possible to compute user-specified
evolutionary tracks, isochrones, stellar luminosity functions, plus synthetic
Color-Magnitude-Diagrams and integrated magnitudes for arbitrary Star Formation
Histories. All these web tools are available at the BaSTI database official
site: http://www.oa-teramo.inaf.it/BASTI.Comment: 18 pages, 5 tables, 7 figures, accepted in The Astronomical Journal.
The BaSTI database and its web-tools are available on the web
http://www.oa-teramo.inaf.it/BASTI. The web-tools are also mirrored in the
BaSTI section of http://astro.ensc-rennes.f
Frequency-dependent magnetotransport and particle dynamics in magnetic modulation systems
We analyze the dynamics of a charged particle moving in the presence of
spatially-modulated magnetic fields. From Poincare surfaces of section and
Liapunov exponents for characteristic trajectories we find that the fraction of
pinned and runaway quasiperiodic orbits {\em vs}. chaotic orbits depends
strongly on the ratio of cyclotron radius to the structure parameters, as well
as on the amplitude of the modulated field. We present a complete
characterization of the dynamical behavior of such structures, and investigate
the contribution to the magnetoconductivity from all different orbits using a
classical Kubo formula. Although the DC conductivity of the system depends
strongly on the pinned and runaway trajectories, the frequency response
reflects the topology of all different orbits, and even their unusual temporal
behavior.Comment: Submitted to PRB - 14 figure files - REVTEX tex
Bloch Electrons in a Magnetic Field - Why Does Chaos Send Electrons the Hard Way?
We find that a 2D periodic potential with different modulation amplitudes in
x- and y-direction and a perpendicular magnetic field may lead to a transition
to electron transport along the direction of stronger modulation and to
localization in the direction of weaker modulation. In the experimentally
accessible regime we relate this new quantum transport phenomenon to avoided
band crossing due to classical chaos.Comment: 4 pages, 3 figures, minor modifications, PRL to appea
The Nature of the Radiative Hydrodynamic Instabilities in Radiatively Supported Thomson Atmospheres
Atmospheres having a significant radiative support are shown to be
intrinsically unstable at luminosities above a critical fraction Gamma_crit ~
0.5-0.85 of the Eddington limit, with the exact value depending on the boundary
conditions. Two different types of absolute radiation-hydrodynamic
instabilities of acoustic waves are found to take place even in the electron
scattering dominated limit. Both instabilities grow over dynamical time scales
and both operate on non radial modes. One is stationary and arises only after
the effects of the boundary conditions are taken into account, while the second
is a propagating wave and is insensitive to the boundary conditions. Although a
significant wind can be generated by these instabilities even below the
classical Eddington luminosity limit, quasi-stable configurations can exist
beyond the Eddington limit due to the generally reduced effective opacity.
The study is done using a rigorous numerical linear analysis of a gray plane
parallel atmosphere under the Eddington approximation. We also present more
simplified analytical explanations.Comment: 18 Pages, 7 figures, uses emulateapj5.sty, accepted to Ap
Parameterising the third dredge-up in asymptotic giant branch stars
We present new evolutionary sequences for low and intermediate mass stars for
three different metallicities, Z = 0.02,0.008 and 0.004. We evolve the models
from the pre-main sequence to the thermally-pulsing asymptotic giant branch
phase. We have two sequences of models for each mass, one which includes mass
loss and one without mass loss. Typically 20 or more pulses have been followed
for each model, allowing us to calculate the third dredge-up parameter for each
case. Using the results from this large and homogeneous set of models, we
present an approximate fit for the core mass at the first thermal pulse, as
well as for the third dredge-up efficiency parameter, and the core mass at the
first third dredge-up episode as a function of metallicity and total mass. We
also examine the effect of a reduced envelope mass on the value of the third
dredge-up efficiency parameter.Comment: 23 pages, 19 figures, accepted for publication in PASA (Publications
of the Astronomical Society of Australia
Optimization of Starburst99 for Intermediate-Age and Old Stellar Populations
We have incorporated the latest release of the Padova models into the
evolutionary synthesis code Starburst99. The Padova tracks were extended to
include the full asymptotic giant branch (AGB) evolution until the final
thermal pulse over the mass range 0.9 to 5 solar mass. With this addition,
Starburst99 accounts for all stellar phases that contribute to the integrated
light of a stellar population with arbitrary age from the extreme ultraviolet
to the near-infrared. AGB stars are important for ages between 0.1 and 2 Gyr,
with their contribution increasing at longer wavelengths. We investigate
similarities and differences between the model predictions by the Geneva and
the Padova tracks. The differences are particularly pronounced at ages > 1 Gyr,
when incompleteness sets in for the Geneva models. We also perform detailed
comparisons with the predictions of other major synthesis codes and found
excellent agreement. Our synthesized optical colors are compared to
observations of old, intermediate-age, and young populations. Excellent
agreement is found for the old globular cluster system of NGC 5128 and for old
and intermediate-age clusters in NGC 4038/39. In contrast, the models fail for
red supergiant dominated populations with sub-solar abundances. This failure
can be traced back to incorrect red supergiant parameters in the stellar
evolutionary tracks. Our models and the synthesis code are publicly available
as version 5.0 of Starburst99 at http://www.stsci.edu/science/starburst99/.Comment: The revised Starburst99 code discussed in this paper will replace the
current version 4.0 on our Starburst99 website by December 31, 2004. Accepted
for publication in ApJ; 39 pages, 23 figures, 5 table