96 research outputs found
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
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
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
Lithium abundances in AGB stars and a new estimate for the7Be life-time
In most cases RGB and AGB stars with M <= 2M(circle dot) destroy Li (which is instead synthesized trough electron-captures on Be-7). This occurs through the combined operation of mixing processes and proton captures, when H-burning operates close to the envelope. Observed Li abundances are however difficult to explain, as they cover a wide spread. Various uncertainties affect model attempts, but so far the largest one concerns the processes of bound and free e-captures on Be-7, hence its life-time, whose known estimates are valid only for solar conditions. RGB and AGB stages have temperatures and densities below the envelope covering a wide range and differing from solar by up to a factor of five for T and up to five orders of magnitudes for rho, hence extrapolations are unreliable. Recently, we presented an estimate of the Be-7 half-life based on a fully quantistic method that goes beyond the Debye-Huckel approximation. Here we discuss its consequences on Li nucleosynthesis in low mass AGB stars
The connection between mass loss and nucleosynthesis
I discuss the relationship between mass loss and nucleosynthesis on the
Asymptotic Giant Branch (AGB). Because of thermal pulses and possibly other
mixing processes, products of nucleosynthesis can be brought to the surface of
AGB stars, increasingly so as the star becomes more luminous, cooler, and
unstable against pulsation of its tenuous mantle. As a result, mass loss is at
its most extreme when dredge-up is too. As the high rate of mass loss truncates
AGB evolution, it determines the enrichment of interstellar space with the AGB
nucleosynthesis products. The changing composition of the stellar atmosphere
also affects the mass-loss process, most obviously in the formation of dust
grains - which play an important role in driving the wind of AGB stars.Comment: 8 pages, including 3 colour figures. To appear in the AIP proceedings
of "The IXth Torino Workshop on Evolution and Nucleosynthesis in AGB Stars".
Also available from http://www.astro.keele.ac.uk/~jacco/research/ecology.htm
Oxygen and Aluminum-Magnesium Isotopic Systematics of Presolar Nanospinel Grains from CI Chondrite Orgueil
Presolar oxide grains have been previously divided into several groups (Group
1 to 4) based on their isotopic compositions, which can be tied to several
stellar sources. Much of available data was acquired on large grains, which may
not be fully representative of the presolar grain population present in
meteorites. We present here new O isotopic data for 74 small presolar oxide
grains (~200 nm in diameter on average) from Orgueil and Al-Mg isotopic
systematics for 25 of the grains. Based on data-model comparisons, we show that
(i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent
(red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red
giant stars and (ii) Group 1 grains with (26Al/27Al)0 >= 5x10^-3 and Group 2
grains with (26Al/27Al)0 <= 1x10^-2 all likely experienced extra circulation
processes in their parent low-mass stars but under different conditions,
resulting in proton-capture reactions occurring at enhanced temperatures. We do
not find any large 25Mg excess in Group 1 oxide grains with large 17O
enrichments, which provides evidence that 25Mg is not abundantly produced in
low-mass stars. We also find that our samples contain a larger proportion of
Group 4 grains than so far suggested in the literature for larger presolar
oxide grains (~400 nm). We also discuss our observations in the light of
stellar dust production mechanisms
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