The slow neutron capture process (s-process) is responsible for the production of about half the elements beyond the Fe-peak. The production sites and the conditions under which the different ...components of s-process occur are relatively well established. A detailed quantitative understanding of s-process nucleosynthesis may yield light in physical processes, e.g. convection and mixing, taking place in the production sites. For this, it is important that the impact of uncertainties in the nuclear physics is well understood. In this work we perform a study of the sensitivity of s-process nucleosynthesis, with particular emphasis in the main component, on the nuclear reaction rates. Our aims are: to quantify the current uncertainties in the production factors of s-process elements originating from nuclear physics and, to identify key nuclear reactions that require more precise experimental determinations. In this work we studied two different production sites in which s-process occurs with very different neutron exposures: 1) a low-mass extremely metal-poor star during the He-core flash (nn reaching up to values of ∼ 1014cm-3); 2) the TP-AGB phase of a M , Z=0.01 model, the typical site of the main s-process component (nn up to 108 - 109cm-3). In the first case, the main variation in the production of s-process elements comes from the neutron poisons and with relative variations around 30%-50%. In the second, the neutron poison are not as important because of the higher metallicity of the star that actually acts as a seed and therefore, the final error of the abundances are much lower around 10%-25%.
We investigate different methods used to construct (zero-age) horizontal branch models and compare the resulting horizontal branch evolution with that of models resulting from the calculation of the ...complete stellar evolution from the main sequence and through the core helium flash. We show that the approximate methods may lead to small but discernible effects, but that some methods, which are as simple, reproduce the complete evolution very well.
What Is the Neon Abundance of the Sun? Bahcall, John N; Basu, Sarbani; Serenelli, Aldo M
The Astrophysical journal,
10/2005, Letnik:
631, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have evolved a series of 13 complete solar models that utilize different assumed heavy-element compositions. Models that are based upon the heavy-element abundances recently determined by Asplund ...and coworkers are inconsistent with helioseismological measurements. However, models in which the neon abundance is increased by 0.4 -0.5 dex to log N(Ne) = 8.29 c 0.05 on the scale in which log N(H) = 12 are consistent with the helioseismological measurements even though the other heavy-element abundances are in agreement with the determinations of Asplund et al. These results sharpen and strengthen an earlier study by Antia & Basu. The predicted solar neutrino fluxes are affected by the uncertainties in the composition by less than their 1 s theoretical uncertainties.
Neutrinos provide us with a novel probe in which to explore the deep interior of astrophysical objects that otherwise would not be accessible with optical observations; among the notable examples are ...solar and supernova neutrinos. We show that there is a new class of strong neutrino emission from helium burning, super(14)N + a 1 super(18)F + g followed by beta decay super(18)F 1 super(18)O + e super(+) + u sub(e), that gives a maximum neutrino luminosity of 10 super(8) times the solar bolometric luminosity at the helium-core flash of a 1 M sub( )star, whereas the flash is not observable by optical means. This means that the neutrino flux, with an average energy of 0.382 MeV, will be 10% the solar CNO neutrino flux on Earth if the star is located at 10 pc.
In this paper we investigate the effects of element diffusion on the structure and evolution of low-mass helium white dwarfs. Attention is focused mainly on the occurrence of hydrogen-shell flashes ...induced by diffusion processes during cooling phases. Physically sound initial models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196, 0.169 and 0.161 M⊙ are constructed by applying mass-loss rates at different stages of the red giant branch evolution of a solar model up to the moment the model begins to evolve to the blue part of the HR diagram. The multicomponent flow equations describing gravitational settling, and chemical and thermal diffusion are solved and the diffusion calculations are coupled to an evolutionary code. In addition, the same sequences are computed but neglecting diffusion. Results without diffusion are similar to recent results of Driebe, Schönberner, Blöcker and Herwig. We find that element diffusion strongly affects the structure and cooling history of helium white dwarfs. In particular, diffusion induces the occurrence of hydrogen-shell flashes in models with masses ranging from 0.18 to 0.41 M⊙, which is in sharp contrast with the situation when diffusion is neglected. In connection with further evolution, these diffusion-induced flashes lead to much thinner hydrogen envelopes, preventing stable nuclear burning from being a sizeable energy source at advanced stages of evolution. This implies much shorter cooling ages than in the case when diffusion is neglected. These new evolutionary models are discussed in light of recent observational data on some millisecond pulsar systems with white dwarf companions. In this context, we find that discrepancies between spin-down ages and the predictions of standard evolutionary models appear to be the result of ignoring element diffusion in such evolutionary models. Indeed, such discrepancies vanish when account is taken of diffusion.
Solar Neutrinos: Status and Prospects Haxton, W.C; Hamish Robertson, R.G; Serenelli, Aldo M
Annual review of astronomy and astrophysics,
08/2013, Letnik:
51, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We describe the current status of solar neutrino measurements and of the theory-both neutrino physics and solar astrophysics-employed in interpreting measurements. Important recent developments ...include Super-Kamiokande's determination of the ν−e elastic scattering rate for
8
B neutrinos to 3%; the latest Sudbury Neutrino Observatory (SNO) global analysis in which the inclusion of low-energy data from SNO I and II significantly narrowed the range of allowed values for the neutrino mixing angle
θ
12
; Borexino results for both the
7
Be and proton-electron-proton (pep) neutrino fluxes, the first direct measurements constraining the rate of proton-proton (pp) I and pp II burning in the Sun; global reanalyses of solar neutrino data that take into account new reactor results on
θ
13
; a new decadal evaluation of the nuclear physics of the pp chain and CNO cycle defining best values and uncertainties in the nuclear microphysics input to solar models; recognition of an emerging discrepancy between two tests of solar metallicity, helioseismological mappings of the sound speed in the solar interior, and analyses of the metal photoabsorption lines based on our best current description of the Sun's photosphere; a new round of standard solar model calculations optimized to agree either with helioseismology or with the new photospheric analysis; and, motivated by the solar abundance problem, the development of nonstandard, accreting solar models, in order to investigate possible consequences of the metal segregation that occurred in the proto-solar disk. We review this progress and describe how new experiments such as SNO+ could help us further exploit neutrinos as a unique probe of stellar interiors.
For studies of Galactic evolution, the accurate characterization of stars in terms of their evolutionary stage and population membership is of fundamental importance. A standard approach relies on ...extracting this information from stellar evolution models but requires the effective temperature, surface gravity and metallicity of a star obtained by independent means. In previous work, we determined accurate effective temperatures and non-local thermodynamic equilibrium log g and Fe/H (NLTE-Opt) for a large sample of metal-poor stars, −3 < Fe/H < −0.5, selected from the Radial Velocity Experiment (RAVE) survey. As a continuation of that work, we derive here their masses, ages and distances using a Bayesian scheme and garstec stellar tracks. For comparison, we also use stellar parameters determined from the widely used 1D LTE excitation-ionization balance of Fe (LTE-Fe). We find that the latter leads to systematically underestimated stellar ages, by 10-30 per cent, but overestimated masses and distances. Metal-poor giants suffer from the largest fractional distance biases of 70 per cent. Furthermore, we compare our results with those released by the RAVE collaboration (DR3) for the stars in common. This reveals −400 to +400 K offsets in effective temperature, −0.5 to 1 dex offsets in surface gravity and 10 to 70 per cent in distances. The systematic trends strongly resemble the correlation we find between the NLTE-Opt and LTE-Fe parameters, indicating that the RAVE DR3 data may be affected by the physical limitations of the 1D LTE synthetic spectra. Our results bear on any study, where spectrophotometric distances underlie stellar kinematics. In particular, they shed new light on the debated controversy about the Galactic halo origin raised by the SDSS/SEGUE observations.
We evaluate the logarithmic derivative of the depth of the solar convective zone (CZ) with respect to the logarithm of the radiative opacity, ln R sub(CZ)/ ln . We use this expression to show that ...the radiative opacity near the base of the solar CZ must be known to an accuracy of c1% in order to calculate the CZ depth to the accuracy of the helioseismological measurement, R sub(CZ) = 0.713 c 0.001 R sub( ). The radiative opacity near the base of the CZ that is obtained from OPAL tables must be increased by 621% in the 2004 Bahcall-Pinsonneault solar model if one wants to invoke opacity errors in order to reconcile recent solar heavy abundance determinations with the helioseismological measurement of R sub(CZ). We show that the radiative opacity near the base of the CZ depends sensitively on the assumed heavy-element mass fraction, Z. The uncertainty in the measured value of Z is currently the limiting factor in our ability to calculate the depth of the CZ. Different state-of-the-art interpolation schemes using the existing OPAL tables yield opacity values that differ by 64%. We describe the finer grid spacings that are necessary to interpolate the radiative opacity to c1%. Uncertainties due to the equation of state do not significantly affect the calculated depth of the CZ.
PDF
