The effect of diffusion on the Red Giant luminosity function 'bump'

This paper investigates the effect of microscopic diffusion of helium and heavy elements on the location of the Red Giant Branch Luminosity Function Bump in Population II stellar models. To this aim updated evolutionary models taking into account diffusion from the Main Sequence until the Zero Age Horizontal Branch have been computed. The observational luminosity difference between the RGB bump and the ZAHB, as collected for a sample of galactic globular clusters, has been compared with the corresponding theoretical values obtained by adopting both canonical and diffusive models. We find that the effect of diffusion, even if slightly improving the agreement between observations and theory, is negligible with respect to the observational uncertainties. In any case the theoretical predictions in models with and without diffusion appear in agreement with the observational results within the estimated errors. Thus canonical models can be still safely adopted, at least until much more accurate observational data will be available.Comment: TeX, 6 pages, uses mnrass.sty (included), 2 postscript figures, in publication on MNRA

Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure

The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS) and, they respond to hG magnetic field strengths differently from the lines used in solar magnetometry. This peculiarity has been employed to measure magnetic field strengths in quiet Sun regions. However, the methods applied so far assume the magnetic field to be constant in the resolution element. The assumption is clearly insufficient to describe the complex quiet Sun magnetic fields, biasing the results of the measurements. We present the first syntheses of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how the MnI lines weaken with increasing field strength. In particular, kG magnetic concentrations produce NnI 5538 circular polarization signals (Stokes V) which can be up to two orders of magnitude smaller than the weak magnetic field approximation prediction. Consequently, (1) the polarization emerging from an atmosphere having weak and strong fields is biased towards the weak fields, and (2) HFS features characteristic of weak fields show up even when the magnetic flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to disappear the filling factor of kG fields has to be larger than the filling factor of sub-kG fields. Stokes V depends on magnetic field inclination according to the simple consine law. Atmospheres with unresolved velocities produce asymmetric line profiles, which cannot be reproduced by simple one-component model atmospheres. The uncertainty of the HFS constants do not limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure

The Hot End of Evolutionary Horizontal Branches

In this paper we investigate the hot end of the HB, presenting evolutionary constraints concerning the CM diagram location and the gravity of hot HB stars. According to the adopted evolutionary scenario, we predict an upper limit for HB temperatures of about logTe = 4.45, remarkably cooler than previous estimates. We find that such a theoretical prescription appears in good agreement with available observational data concerning both stellar temperatures and gravities.Comment: postscript file of 10 pages plus 1 tables,rep.1 5 figures will be added later as postscript file The tex file and the other two not postscript figures are available upon request at [email protected], rep.

Which radius for the Sun?

The high accuracy reached by solar limb observations, by helioseismic measurements and by Standard Solar Models (SSMs) calculations suggests that general relativity corrections are included when discussing the solar radius. The Allen value (R$_{\odot}$ = 695.99 $\pm$ 0.07 Mm) has to be reduced by 1.5 Km. This correction, which is small as compared with present accuracy, should be kept in mind for future more precise measurements and/or calculations.Comment: Latex, 3 page

Bounds on hep neutrinos

The excess of highest energy solar-neutrino events recently observed by Superkamiokande can be in principle explained by anomalously high $hep$-neutrino flux $\Phi_{\nu}(hep)$. Without using SSM calculations, from the solar luminosity constraint we derive that $\Phi_\nu(hep)/S_{13}$ cannot exceed the SSM estimate by more than a factor three. If one makes the additional hypothesis that $hep$ neutrino production occurs where the $^3$He concentration is at equilibrium, helioseismology gives an upper bound which is (less then) two times the SSM prediction. We argue that the anomalous $hep$-neutrino flux of order of that observed by Superkamiokande cannot be explained by astrophysics, but rather by a large production cross-section.Comment: 7 pages, RevTeX fil

Accreting pre-main sequence models and abundance anomalies in globular clusters

We investigated the possibility of producing helium enhanced stars in globular clusters by accreting polluted matter during the pre-main sequence phase. We followed the evolution of two different classes of pre-main sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the protostellar evolution. The later is the beginning of the late accretion and the lower are both the central helium and the surface lithium abundances at the end of the accretion phase and in ZAMS (Zero Age Main Sequence). In order to produce a relevant increase of the central helium content the accretion of polluted matter should start at ages lower than 1 Myr. The inclusion of the protostellar evolution has a strong impact on the ZAMS models too. The adoption of a very low seed mass (i.e. 0.001 M$_{\odot}$) results in models with the lowest central helium and surface lithium abundances. The higher is the accretion rate and the lower is the final helium content in the core and the residual surface lithium. In the worst case -- i.e. seed mass 0.001 M$_\odot$ and accretion rate $\ge 10^{-5}$ M$_\odot$ yr$^{-1}$ -- the central helium is not increased at all and the surface lithium is fully depleted in the first few million years.Comment: Accepted for pubblication in MNRAS. 19 pages, 15 figures, 2 table

Helioseismology can test the Maxwell-Boltzmann distribution

Nuclear reactions in stars occur between nuclei in the high-energy tail of the energy distribution and are sensitive to possible deviations from the standard equilibrium thermal-energy distribution. We are able to derive strong constraints on such deviations by using the detailed helioseismic information of the solar structure. If a small deviation is parameterized with a factor exp{-delta*(E/kT)^2}, we find that delta should lie between -0.005 and +0.002. However, even values of delta as small as 0.003 would still give important effects on the neutrino fluxes.Comment: 10 pages in ReVTeX + 1 postscript figure. Submitted to Phys. Lett.

Lithium-7 surface abundance in pre-MS stars. Testing theory against clusters and binary systems

The disagreement between theoretical predictions and observations for surface lithium abundance in stars is a long-standing problem, which indicates that the adopted physical treatment is still lacking in some points. However, thanks to the recent improvements in both models and observations, it is interesting to analyse the situation to evaluate present uncertainties. We present a consistent and quantitative analysis of the theoretical uncertainties affecting surface lithium abundance in the current generation of models. By means of an up-to-date and well tested evolutionary code, FRANEC, theoretical errors on surface 7Li abundance predictions, during the pre-main sequence (pre-MS) and main sequence (MS) phases, are discussed in detail. Then, the predicted surface 7Li abundance was tested against observational data for five open clusters, namely Ic 2602, \alpha Per, Blanco1, Pleiades, and Ngc 2516, and for four detached double-lined eclipsing binary systems. Stellar models for the aforementioned clusters were computed by adopting suitable chemical composition, age, and mixing length parameter for MS stars determined from the analysis of the colour-magnitude diagram of each cluster. We restricted our analysis to young clusters, to avoid additional uncertainty sources such as diffusion and/or radiative levitation efficiency. We confirm the disagreement, within present uncertainties, between theoretical predictions and 7Li observations for standard models. However, we notice that a satisfactory agreement with observations for 7Li abundance in both young open clusters and binary systems can be achieved if a lower convection efficiency is adopted during the pre-MS phase with respect to the MS one.Comment: 10 pages, 5 figures. Accepted for publication in A&

Theoretical uncertainties on the radius of low- and very-low mass stars

We performed an analysis of the main theoretical uncertainties that affect the radius of low- and very-low mass-stars predicted by current stellar models. We focused on stars in the mass range 0.1-1Msun, on both the zero-age main-sequence (ZAMS) and on 1, 2 and 5 Gyr isochrones. First, we quantified the impact on the radius of the uncertainty of several quantities, namely the equation of state, radiative opacity, atmospheric models, convection efficiency and initial chemical composition. Then, we computed the cumulative radius error stripe obtained by adding the radius variation due to all the analysed quantities. As a general trend, the radius uncertainty increases with the stellar mass. For ZAMS structures the cumulative error stripe of very-low mass stars is about $\pm 2$ and $\pm 3$ percent, while at larger masses it increases up to $\pm 4$ and $\pm 5$ percent. The radius uncertainty gets larger and age dependent if isochrones are considered, reaching for $M\sim 1$Msun about $+12(-15)$ percent at an age of 5 Gyr. We also investigated the radius uncertainty at a fixed luminosity. In this case, the cumulative error stripe is the same for both ZAMS and isochrone models and it ranges from about $\pm 4$ percent to $+7$ and $+9$($-5$) percent. We also showed that the sole uncertainty on the chemical composition plays an important role in determining the radius error stripe, producing a radius variation that ranges between about $\pm 1$ and $\pm 2$ percent on ZAMS models with fixed mass and about $\pm 3$ and $\pm 5$ percent at a fixed luminosity.Comment: 18 pages, 20 figures, 1 table; accepted for publication in MNRA