340 research outputs found
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.
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Astrophysical implications of the proton-proton cross section updates
The p(p,e^+ \nu_e)^2H reaction rate is an essential ingredient for
theoretical computations of stellar models. In the past several values of the
corresponding S-factor have been made available by different authors. Prompted
by a recent evaluation of S(E), we analysed the effect of the adoption of
different proton-proton reaction rates on stellar models, focusing, in
particular, on the age of mid and old stellar clusters (1-12 Gyr) and on
standard solar model predictions. By comparing different widely adopted p(p,e^+
\nu_e)^2H reaction rates, we found a maximum difference in the temperature
regimes typical of main sequence hydrogen-burning stars (5x10^6 - 3x10^7 K) of
about 3%. Such a variation translates into a change of cluster age
determination lower than 1%. A slightly larger effect is observed in the
predicted solar neutrino fluxes with a maximum difference, in the worst case,
of about 8%. Finally we also notice that the uncertainty evaluation of the
present proton-proton rate is at the level of few \permil, thus the p(p,e^+
\nu_e)^2H reaction rate does not constitute anymore a significant uncertainty
source in stellar models.Comment: accepte
The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism
An attractive diagnostic tool for investigating the magnetism of the solar
chromosphere is the observation and theoretical modeling of the Hanle and
Zeeman effects in spicules, as shown in this letter for the first time. Here we
report on spectropolarimetric observations of solar chromospheric spicules in
the He I 10830 \AA multiplet and on their theoretical modeling accounting for
radiative transfer effects. We find that the magnetic field in the observed
(quiet Sun) spicular material at a height of about 2000 km above the visible
solar surface has a strength of the order of 10 G and is inclined by
approximately with respect to the local vertical direction. Our
empirical finding based on full Stokes-vector spectropolarimetry should be
taken into account in future magnetohydrodynamical simulations of spicules.Comment: 12 pages and 2 figure
On the physical origin of the second solar spectrum of the Sc II line at 4247 A
The peculiar three-peak structure of the linear polarization profile shown in
the second solar spectrum by the Ba II line at 4554 A has been interpreted as
the result of the different contributions coming from the barium isotopes with
and without hyperfine structure (HFS). In the same spectrum, a triple peak
polarization signal is also observed in the Sc II line at 4247 A. Scandium has
a single stable isotope (^{45}Sc), which shows HFS due to a nuclear spin I=7/2.
We investigate the possibility of interpreting the linear polarization profile
shown in the second solar spectrum by this Sc II line in terms of HFS. A
two-level model atom with HFS is assumed. Adopting an optically thin slab
model, the role of atomic polarization and of HFS is investigated, avoiding the
complications caused by radiative transfer effects. The slab is assumed to be
illuminated from below by the photospheric continuum, and the polarization of
the radiation scattered at 90 degrees is investigated. The three-peak structure
of the scattering polarization profile observed in this Sc II line cannot be
fully explained in terms of HFS. Given the similarities between the Sc II line
at 4247 A and the Ba II line at 4554 A, it is not clear why, within the same
modeling assumptions, only the three-peak Q/I profile of the barium line can be
fully interpreted in terms of HFS. The failure to interpret this Sc II
polarization signal raises important questions, whose resolution might lead to
significant improvements in our understanding of the second solar spectrum. In
particular, if the three-peak structure of the Sc II signal is actually
produced by a physical mechanism neglected within the approach considered here,
it will be extremely interesting not only to identify this mechanism, but also
to understand why it seems to be less important in the case of the barium line.Comment: 8 pages, 8 figures, and 1 table. Accepted for publication in
Astronomy and Astrophysic
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