150 research outputs found
Magnetic Bubbles and Extramixing in Stars
The possible role of magnetic flux tubes in transporting matter from near the H shell through the radiative zone and into the convective envelope is explored. It is shown that the required rates of mass transport necessary to provide nuclear processed material to the envelope can be achieved if large magnetic fields are present just above the H shell in AGB and RGB stars. The required fields in this zone reach 5 MG for the AGB case and 0.5-0.05 MG for the RGB case. It may thus be possible that magnetic bouyancy play a major role in providing the extra mixing needed for these stars
Isotope Anomalies in the Fe-group Elements in Meteorites and Connection to Nucleosynthesis in AGB Stars
We study the effects of neutron captures in AGB stars on \oq Fe-group\cqb
elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in
Cr, Fe, and Ni in solar-system materials, which are
commonly attributed to SNe. However, as large fractions of the interstellar
medium (ISM) were reprocessed in AGB stars, these elements were reprocessed,
too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through
1.5\msb and 3\msb AGB models, adopting solar and 1/3 solar metallicities. All
cases produce excesses of Cr, Fe, and Ni, while the other
isotopes are little altered; hence, the observations may be explained by AGB
processing. The results are robust and not dependent on the detailed initial
isotopic composition. Consequences for other \oq Fe group\cqb elements are then
explored. They include Ti excesses, and some production of
Ti. In many circumstellar condensates, Ti quantitatively reflects
these effects of AGB neutron captures. Scatter in the data results from small
variations (granularity) in the isotopic composition of the local ISM. For Si,
the main effects are instead due to variations in the local ISM from different
SNe sources. The problem of Ca is discussed, particularly with regard to
Ca. The measured data are usually represented assuming terrestrial
values for Ca/Ca. Materials processed in AGB stars or sources
with variable initial Ca/Ca ratios can give apparent Ca
excesses/deficiencies, attributed to SNe. The broader issue of Galactic
Chemical Evolution is also discussed in view of the isotopic granularity in the
ISM. \end{abstract
MS, S and C Stars in the Infrared. Luminosities and Mass Loss Rates
In this note I present an outline of infrared (IR) photometric AGB
properties, based on two samples of Galactic Long Period Variables (C- and
S-type respectively). I show the various selection criteria used during the
choice of the sources and describe the motivations of observing them at near-
and mid-IR wavelengths. I discuss the problems encountered in estimating their
luminosity and distance and motivate the methods I choose for this purpose.
Properties of the luminosity functions and of the Hertzsprung-Russell (HR)
diagrams obtained from the analysis are discussed. Finally, the choices made
for estimating of the mass loss rates are described and preliminary results
concerning them are shown.Comment: 10 pages, 6 figures, contribution from the IX Torino Workshop, to be
published by AI
Theoretical Estimates of Stellar e-Captures. I. The half-life of 7Be in Evolved Stars
The Li enrichment in the Universe still presents various puzzles to
astrophysics. One open issue is that of obtaining estimates for the rate of
e-captures on 7Be, for T and rho conditions different from solar. This is
important to model the Galactic nucleosynthesis of Li. In this framework, we
present a new theoretical method for calculating the e-capture rate in
conditions typical of evolved stars. We show how our approach compares with
state-of-the-art techniques for solar conditions, where various estimates are
available. Our computations include: i) "traditional" calculations of the
electronic density at the nucleus, to which the e-capture rate for 7Be is
proportional, for different theoretical approaches including the Thomas--Fermi,
Poisson--Boltzmann and Debye--Hueckel (DH) models of screening, ii) a new
computation, based on a formalism that goes beyond the previous ones, adopting
a mean-field "adiabatic" approximation to the scattering process. The results
obtained with our approach as well as with the traditional ones and their
differences are discussed in some detail, starting from solar conditions, where
our method and the DH model converge to the same solution. We then analyze the
applicability of the various models to a rather broad range of T and rho
values, embracing those typical of red giant stars. We find that, over a wide
region of the parameter space explored, the DH approximation does not stand,
and the more general method we suggest is preferable. We then briefly reanalyze
the 7Li abundances in RGB and AGB stars of the Galactic Disk using the new
Be-decay rate. We also underline that the different values of the electron
density at the nucleus we find should induce effects on electron screening (for
p-captures on Li itself, as well as for other nuclei) so that our new approach
might have wide astrophysical consequences.Comment: Astrophts. Journal Feb. 1, 201
Production of n-rich nuclei in red giant stars
We outline a partial historical summary of the steps through which the
nucleosynthesis phenomena induced by {\it slow} neutron captures (the {\it
s-process}) were clarified, a scientific achievement in which Franz K\"appeler
played a major role. We start by recalling the early phenomenological approach,
which yielded a basic understanding of the subject even before models for the
parent stellar evolutionary stages were developed. Through such a tool, rough
limits for the neutron density and exposure were set, and the crucial fact was
understood that more than one nucleosynthesis component is required to account
for solar abundances of -process nuclei up to the Pb-Bi region. We then
summarize the gradual understanding of the stellar processes actually involved
in the production of nuclei from Sr to Pb (the so-called {\it Main Component},
achieved in the last decade of the past century and occurring in red giants of
low and intermediate mass, ( 8 ), populating, in the
{\it HR} diagram, the {\it Asymptotic Giant Branch} or {\it AGB} region. We
conclude by giving some details on more recent research concerning mixing
mechanisms inducing the activation of the main neutron source,
C(,n)O.Comment: 35 page, 8 figure
Dynamos and Chemical Mixing in Evolved Stars
In low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars,
anomalous mixing must transport material near the hydrogen-burning shell to the
convective envelope. Recently, it was suggested that buoyant magnetic flux
tubes could supply the necessary transport rate (Busso et al. 2007). The fields
are assumed to originate from a dynamo operating in the stellar interior. Here,
we show what is required of an dynamo in the envelope of an AGB
star to maintain these fields. Differential rotation and rotation drain via
turbulent dissipation and Poynting flux, so if shear can be resupplied by
convection, then large-scale toroidal field strengths of
\left\simeq3\times10^4 G can be sustained at the base of the
convection zone.Comment: 7 pages, 3 figures. To appear in AIP Proceedings of the IXth Torino
Workshop on AGB Nucleosynthesi
Low temperature mean opacities for the carbon-rich regime
Asymptotic Giant Branch (AGB) stars undergo a change in their chemical
composition during their evolution. This in turn leads to an alteration of the
radiative opacities, especially in the cool layers of the envelope and the
atmosphere, where molecules are the dominant opacity sources. A key parameter
in this respect is the number ratio of carbon to oxygen atoms (C/O). In terms
of low temperature mean opacities, a variation of this parameter usually cannot
be followed in stellar evolution models, because up to now tabulated values
were only available for scaled solar metal mixtures (with C/O ~ 0.5). We thus
present a set of newly generated tables containing Rosseland mean opacity
coefficients covering both the oxygen-rich (C/O
1) regime. We compare our values to existing tabular data and investigate the
relevant molecular opacity contributors.Comment: 8 pages, 5 figures. To appear in the AIP Proceedings of the IXth
Torino Workshop on AGB Nucleosynthesi
Spitzer/IRAC Observations of AGB stars
We present here the first observation of galactic AGB stars with the InfraRed
Array Camera (IRAC) onboard the Spitzer Space Telescope. Our sample consists of
48 AGB stars of different chemical signature, mass loss rate and variability
class. For each star we have measured IRAC photometry and colors. Preliminary
results shows that IRAC colors are sensitive to spectroscopic features
associated to molecules and dust in the AGB wind. Period is only loosely
correlated to the brightness of the stars in the IRAC bands. We do find,
however, a tight period-color relation for sources classified as semiregular
variables. This may be interpreted as the lack of warm dust in the wind of the
sources in this class, as opposed to Mira variables that show higher infrared
excess in all IRAC bands.Comment: 8 pages, to be published in proceedings "IX Torino Workshop on
Evolution and Nucleosynthesis in AGB Stars", 22-26 October 2007, Perugia,
Ital
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