599 research outputs found
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 = 695.99 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
-neutrino flux . Without using SSM calculations, from the
solar luminosity constraint we derive that cannot exceed
the SSM estimate by more than a factor three. If one makes the additional
hypothesis that neutrino production occurs where the He concentration
is at equilibrium, helioseismology gives an upper bound which is (less then)
two times the SSM prediction. We argue that the anomalous -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) 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 and accretion rate M
yr -- 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 and percent, while at larger masses it increases
up to and percent. The radius uncertainty gets larger and age
dependent if isochrones are considered, reaching for Msun about
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
percent to and () 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
and percent on ZAMS models with fixed mass and about
and percent at a fixed luminosity.Comment: 18 pages, 20 figures, 1 table; accepted for publication in MNRA
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