Basic Stellar Physics Palmerini, S.
EPJ Web of conferences,
2023, Letnik:
275
Journal Article
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Odprti dostop
Polytropes, Virial theorem, evolutionary time scales, degrees of freedom, radiative transport, molecular weight, degeneracy, Jeans mass, and Eddington luminosity are basic ingredients to describe the ...physics of stars. In the present paper they will be presented in details as long with their role in the stellar evolution.
PACS.
PACS-key stellar evoluion - PACS-key basic physics
Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While observations and theory agree at close-to-solar ...metallicity, stellar models at lower metallicities overestimate the fluorine production with respect to that of heavy elements. We present
19
F nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the
13
C neutron source (the so-called
13
C pocket). We find that
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F is mainly produced as a result of nucleosynthesis involving secondary
14
N during convective thermal pulses, with a negligible contribution from the
14
N present in the
13
C pocket region. A large
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F production is thus prevented, resulting in lower fluorine surface abundances. As a consequence, AGB stellar models with mixing induced by magnetic buoyancy at the base of the convective envelope agree well with available fluorine spectroscopic measurements at low and close-to-solar metallicity.
We present computations of nucleosynthesis in low-mass (LM) red giant branch (RGB) and asymptotic giant branch (AGB) stars of Population delta experiencing extended mixing. We adopt the updated ...version of the FRANEC evolutionary model, a new post-process code for non-convective mixing and the most recent revisions for solar abundances. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass (IM) nuclei (from carbon to aluminum). For each nucleus, we briefly discuss the new choices and their motivations. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that the new rates (and notably the one for the 14N(p, Delta *g)15O reaction) imply considerable modifications in the composition of post-main-sequence stars. In particular, the slight temperature changes due to the reduced efficiency of proton captures on 14N induce abundance variations at the first dredge-up (especially for 17O, whose equilibrium ratio to 16O is very sensitive to the temperature). In this new scenario, presolar oxide grains of AGB origin turn out to be produced almost exclusively by very low mass stars (M <= 1.5-1.7 M ), never becoming C-rich. The whole population of grains with 18O/16O below 0.0015 (the limit permitted by first dredge-up) is now explained. Also, there is now no forbidden area for very low values of 17O/16O (below 0.0005), contrary to previous findings. A rather shallow type of transport seems to be sufficient for the CNO changes in RGB stages. Both thermohaline diffusion and magnetic-buoyancy-induced mixing might provide a suitable physical mechanism for this. Thermohaline mixing is in any case certainly inadequate to account for the production of 26Al on the AGB. Other transport mechanisms must therefore be at play. In general, observational constraints from RGB and AGB stars, as well as from presolar grains, are well reproduced by our approach. The nitrogen isotopic ratio in mainstream SiC grains remains an exception. For the low values measured in them (i.e., for 14N/15N <=2000), we have no explanation. Actually, for the several grains with subsolar nitrogen isotopic ratios, no known stellar process acting in LM stars can provide a clue. This might be an evidence that some form of contamination from cosmic ray spallation occurs in the interstellar medium, adding fresh 15N to the grains.
In a recent study based on homogeneous barium abundance measurements in open clusters (OCs), a trend of increasing Ba/Fe ratios for decreasing cluster age was reported. We present here further ...abundance determinations, relative to four other elements having important s-process contributions, with the aim of investigating whether or not the growth found for Ba/Fe is indicative of a general property, shared also by the other heavy elements formed by slow neutron captures. In particular, we derived abundances for yttrium, zirconium, lanthanum, and cerium, using equivalent width measurements and the MOOG code. Our sample includes 19 OCs of different ages, for which the spectra were obtained by the ESO Very Large Telescope using the UVES spectrometer. The growth previously suggested for Ba is confirmed for all the elements analyzed in our study. This fact implies significant changes in our views of the Galactic chemical evolution for elements beyond iron. Our results necessarily require that very low mass asymptotic giant branch stars (M1.5 M ) produce larger amounts of s-process elements (and hence activate the 13C-neutron source more effectively) than previously expected. Their role in producing neutron-rich elements in the Galactic disk has been so far underestimated, and their evolution and neutron-capture nucleosynthesis should now be reconsidered.
Recent improvements in stellar models for intermediate-mass stars and massive stars (MSs) are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetimes τ 25 Myr, ...in order to re-examine the origins of now extinct radioactivities that were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if r-process nuclei are produced by neutron star merging, according to recent models. Instead, 26Al, 41Ca, 135Cs, and possibly 60Fe require nucleosynthetic events close to the solar formation. We outline the persisting difficulties to account for these nuclei by intermediate-mass stars (2 M/M 7-8). Models of their final stages now predict the ubiquitous formation of a 13C reservoir as a neutron capture source; hence, even in the presence of 26Al production from deep mixing or hot bottom burning, the ratio 26Al/107Pd remains incompatible with measured data, with a large excess in 107Pd. This is shown for two recent approaches to deep mixing. Even a late contamination by an MS encounters problems. In fact, the inhomogeneous addition of supernova debris predicts nonmeasured excesses on stable isotopes. Revisions invoking specific low-mass supernovae and/or the sequential contamination of the presolar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space that remains to be explored for solving this puzzle is discussed.
We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After outlining the problems affecting our knowledge of the Li content in low-mass stars (M <= 3 M ), we discuss ...deep-mixing models for the red giant branch stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M . Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich asymptotic giant branch (AGB) stars. Here, the constraints on extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) = log (Li) 1.5 (and sometimes more). Also, their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, and the C/O and 12C/13C ratios) can be explained. This requires generally moderate efficiencies ( yr--1) for non-convective mass transport. At such rates, slow extra mixing does not remarkably modify Li abundances in early AGB phases; on the other hand, faster mixing encounters a physical limit in destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be produced; therefore, its destruction on the AGB is modest. Li is then significantly produced by the third dredge up. We also show that effective circulation episodes, while not destroying Li, would easily bring the 12C/13C ratios to equilibrium, contrary to the evidence in most AGB stars, and would burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the common idea that efficient extra mixing drastically reduces the Li content of C stars with respect to K-M giants. This misleading appearance is induced by biases in the data, namely: (1) the difficulty of measuring very low Li abundances in O-rich AGB stars due to the presence of TiO bands and (2) the fact that many, relatively massive (M > 3 M ) K- and M-type giants may remain Li-rich, not evolving to the C-rich stages. Efficient extra mixing on the AGB is instead typical of very low masses (M 1.5 M ). It also characterizes CJ stars, where it produces Li and reduces F and the carbon isotope ratio, as observed in these peculiar objects.
The abundance of 26Al carries a special role in astrophysics, since it probes active nucleosynthesis in the Milky Way and constrains the Galactic core-collapse supernovae rate. It is estimated ...through the detection of the 1809 keV γ-line and from the superabundance of 26Mg in comparison with the most abundant Mg isotope (A = 24) in meteorites. For this reason, high precision is necessary also in the investigation of the stable 27Al and 24Mg isotopes. Moreover, these nuclei enter the so-called MgAl cycle, playing an important role in the production of Al and Mg. Recently, high-resolution stellar surveys have shown that the Mg–Al anticorrelation in red-giant stars in globular clusters may hide the existence of multiple stellar populations, and that the relative abundances of Mg isotopes may not be correlated with Al. The common thread running through these astrophysical scenarios is the 27Al(p,α)24Mg reaction, which is the main 27Al destruction channel and directly correlates its abundance with the 24Mg one. Since available reaction rates show large uncertainties owing to the vanishingly small cross section at astrophysical energies, we have applied the Trojan Horse Method to deduce the reaction rate with no need of extrapolation. The indirect measurement made it possible to assess the contribution of the 84 keV resonance and to lower upper limits on the strength of nearby resonances. In intermediate-mass AGB stars experiencing hot bottom burning, a sizeable increase in surface aluminum abundance is observed at the lowest masses, while 24Mg is essentially unaffected by the change in the reaction rate.
Low-mass asymptotic giant branch stars are among the most important polluters of the interstellar medium. In their interiors, the main component (A 90) of the slow neutron capture process (the ...s-process) is synthesized, the most important neutron source being the 13C( ,n)16O reaction. In this paper, we review its current experimental status, discussing possible future synergies between some experiments currently focused on the determination of its rate. Moreover, in order to determine the level of precision needed to fully characterize this reaction, we present a theoretical sensitivity study, carried out with the FUNS evolutionary stellar code and the NEWTON post-process code. We modify the rate up to a factor of 2 with respect to a reference case. We find that variations of the 13C( ,n)16O rate do not appreciably affect s-process distributions for masses above 3 M at any metallicity. Apart from a few isotopes, in fact, the differences are always below 5%. The situation is completely different if some 13C burns in a convective environment: this occurs in FUNS models with M < 3 M at solar-like metallicities. In this case, a change of the 13C( ,n)16O reaction rate leads to nonnegligible variations of the element surface distribution (10% on average), with larger peaks for some elements (such as rubidium) and neutron-rich isotopes (such as 86Kr and 96Zr). Larger variations are found in low-mass, low-metallicity models if protons are mixed and burned at very high temperatures. In this case, the surface abundances of the heavier elements may vary by more than a factor of 50.
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