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

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.

Generalized sqrt(epsilon)-law. The role of unphysical source terms in resonance line polarization transfer and its importance as an additional test of NLTE radiative transfer codes

Context. A derivation of a generalized sqrt(epsilon)-law for nonthermal collisional rates of excitation by charged perturbers is presented. Aims. Aim of this paper is to find a more general analytical expression for a surface value of the source function which can be used as an addtional tool for verification of the non-LTE radiative transfer codes. Methods. Under the impact approximation hypothesis, static, one-dimensional, plane-parallel atmosphere, constant magnetic field of arbitrary strength and direction, two-level atom model with unpolarized lower level and stimulated emission neglected, we introduce the unphysical terms into the equations of statistical equilibrium and solve the appropriate non-LTE integral equations. Results. We derive a new analytical condition for the surface values of the source function components expressed in the basis of irreducible spherical tensors.Comment: 5 pages, 1 figure, accepted for publication in A&

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.

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

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

A Mixed Solar Core, Solar Neutrinos and Helioseismology

We consider a wide class of solar models with mixed core. Most of these models can be excluded as the predicted sound speed profile is in sharp disagreement with helioseismic constraints. All the remaining models predict $^7$Be and/or $^7$B neutrino fluxes at least as large as those of SSMs. In conclusion, helioseismology shows that a mixed solar core cannot account for the neutrino deficit implied by the current solar neutrino experiments.Comment: 6 pages, RevTeX, plus 5 postscript figure