40 research outputs found
Automated spectroscopic abundances of A and F-type stars using echelle spectrographs I. Reduction of ELODIE spectra and method of abundance determination
This paper presents an automated method to determine detailed abundances for
A and F-type stars. This method is applied on spectra taken with the ELODIE
spectrograph. Since the standard reduction procedure of ELODIE is optimized to
obtain accurate radial velocities but not abundances, we present a more
appropriate reduction procedure based on IRAF. We describe an improvement of
the method of Hill & Landstreet (1993) for obtaining Vsini, microturbulence and
abundances by fitting a synthetic spectrum to the observed one. In particular,
the method of minimization is presented and tested with Vega and the Sun. We
show that it is possible, in the case of the Sun, to recover the abundances of
27 elements well within 0.1 dex of the commonly accepted values.Comment: 12 pages, 10 figures, accepted for publication in A&
Amorphous alumina in the extended atmosphere of Alpha Orionis
In this paper we study the extended atmosphere of the late-type supergiant
Alpha Orionis. Infrared spectroscopy of red supergiants reveals strong
molecular bands, some of which do not originate in the photosphere but in a
cooler layer of molecular material above it. Lately, these layers have been
spatially resolved by near and mid-IR interferometry. In this paper, we try to
reconcile the IR interferometric and ISO-SWS spectroscopic results on Alpha
Orionis with a thorough modelling of the photosphere, molecular layer(s) and
dust shell. From the ISO and near-IR interferometric observations, we find that
Alpha Orionis has only a very low density water layer close above the
photosphere. However, mid-IR interferometric observations and a narrow-slit
N-band spectrum suggest much larger extra-photospheric opacity close to the
photosphere at those wavelengths, even when taking into account the detached
dust shell. We argue that this cannot be due to the water layer, and that
another source of mid-IR opacity must be present. We show that this opacity
source is probably neither molecular nor chromospheric. Rather, we present
amorphous alumina (Al2O3) as the best candidate and discuss this hypothesis in
the framework of dust-condensation scenarios.Comment: 15 pages, 18 figures, accepted for publication in A&
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
Low-temperature gas opacity - AESOPUS: a versatile and quick computational tool
We introduce a new tool - AESOPUS: Accurate Equation of State and OPacity
Utility Software - for computing the equation of state and the Rosseland mean
(RM) opacities of matter in the ideal gas phase. Results are given as a
function of one pair of state variables, (i.e. temperature T in the range 3.2
<= log(T) <= 4.5, and parameter R= rho/(T/10^6 K)^3 in the range -8 <= log(R)
<= 1), and arbitrary chemical mixture. The chemistry is presently solved for
about 800 species, consisting of almost 300 atomic and 500 molecular species.
The gas opacities account for many continuum and discrete sources, including
atomic opacities, molecular absorption bands, and collision-induced absorption.
Several tests made on AESOPUS have proved that the new opacity tool is accurate
in the results,flexible in the management of the input prescriptions, and agile
in terms of computational time requirement. We set up a web-interface
(http://stev.oapd.inaf.it/aesopus) which enables the user to compute and
shortly retrieve RM opacity tables according to his/her specific needs,
allowing a full degree of freedom in specifying the chemical composition of the
gas. Useful applications may regard RM opacities of gas mixtures with i)
scaled-solar abundances of metals, choosing among various solar mixture
compilations available in the literature; ii) varying CNO abundances, suitable
for evolutionary models of red and asymptotic giant branch stars and massive
stars in the Wolf-Rayet stages; iii) various degrees of enhancement in
alpha-elements, and C-N, Na-O and Mg-Al abundance anti-correlations, necessary
to properly describe the properties of stars in early-type galaxies and
Galactic globular clusters; iv) zero-metal abundances appropriate for studies
of gas opacity in primordial conditions.Comment: 32 pages, 34 postscript figures, A&A in press; new section 4.1.2
showing first tests with stellar models, sections 2.2, 2.2.2 and 5 expanded;
interactive web-page at http://stev.oapd.inaf.it/aesopu
Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities
We investigate the effects of molecular opacities on the evolution of TP-AGB
stars that experience the third dredge-up, i.e. with surface abundances of
carbon and oxygen varying with time. To this aim, a routine is constructed to
derive the molecular concentrations through dissociation equilibrium
calculations, and estimate the opacities due to H2, H2O, OH, C2, CN, and CO for
any given density, temperature, and chemical composition of the gas. Then,
synthetic TP-AGB models with dredge-up are calculated by either 1) adopting the
newly developed routine, or 2) interpolating between fixed opacity tables for
solar chemical composition, which is the usual prescription in AGB evolution
calculations. The comparison between the two cases shows that the change in the
dominant opacity sources, as the C/O ratio grows from below to above unity,
crucially affects the evolution of the effective temperature, i.e. causing a
notable cooling of the carbon-rich models (with C/O>1). From the comparison
with observational data, it turns out that TP-AGB models with variable
molecular opacities are able to reproduce the observed range of effective
temperatures, mass-loss rates, and wind expansion velocities of C-type giants
in the solar neighbourhood, otherwise failed if assuming fixed molecular
opacities for solar-scaled mixtures. Finally, we mention other important
evolutionary and observational effects that result from the adoption of the
variable opacities, such as: i) significant shortening of the C-star phase due
to the earlier onset of the super-wind; ii) consequent reduction of the carbon
yields; iii) reproduction of the observed range of near-infrared colours of
C-stars.Comment: 13 pages, 10 postscript figures, to appear in A&
Uncertainties of Cosmic Ray Spectra and Detectability of Antiproton mSUGRA Contributions With PAMELA
We studied the variation of and top of the atmosphere spectra
due to the parameters uncertainties of the Milky Way geometry, propagation
models and cross sections. We used the B/C data and Galprop code for the
propagation analysis. We also derived the uncertainty bands for subFe/Fe ratio,
H and He. Finally, we considered a neutralino induced component in the
antiproton flux in the mSUGRA framework. PAMELA expectations for positrons and
antiprotons are calculated. We studied in details the possibility of
disentanglement of an eventual signal component in the antiproton spectra in a
clumpy halo scenario: minimal values of clumpiness factors necessary to
disentangle the signal from the background without violating the quality of the
antiproton data fit are found. There are also given examples of total spectra
in comparison with existing experimental data and an example of PAMELA
prediction for the total spectra. The main result of this work is that for the
diffusion and convection background model PAMELA will be able to disentangle an
eventual supersymmetric signal even for small clumpiness factors.Comment: 26 pages, 27 eps figures. Final JCAP accepted versio
Derivation and performance of an entirely autologous injectable hydrogel delivery system for cell-based therapies
The solar chemical composition is an important ingredient in our
understanding of the formation, structure and evolution of both the Sun and our
solar system. Furthermore, it is an essential reference standard against which
the elemental contents of other astronomical objects are compared. In this
review we evaluate the current understanding of the solar photospheric
composition. In particular, we present a re-determination of the abundances of
nearly all available elements, using a realistic new 3-dimensional (3D),
time-dependent hydrodynamical model of the solar atmosphere. We have carefully
considered the atomic input data and selection of spectral lines, and accounted
for departures from LTE whenever possible. The end result is a comprehensive
and homogeneous compilation of the solar elemental abundances. Particularly
noteworthy findings are significantly lower abundances of carbon, nitrogen,
oxygen and neon compared with the widely-used values of a decade ago. The new
solar chemical composition is supported by a high degree of internal
consistency between available abundance indicators, and by agreement with
values obtained in the solar neighborhood and from the most pristine
meteorites. There is, however, a stark conflict with standard models of the
solar interior according to helioseismology, a discrepancy that has yet to find
a satisfactory resolution.Comment: Due to ARAA rules, the actual published version is not allowed to be
placed on arxiv.org. We recommend the interested reader to download the
article from ARAA at
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.astro.46.060407.145222
rather than using the arxiv.org versio
Why GN93 should not be used anymore
We show why the solar chemical composition of [1] (GN93) and the similar values of [2] (AG89) and [3] (GS98), characterized by values of the metallicity of the order of 0.017 to 0.020, largely used in solar and stellar modeling, are now obsolete. They should be replaced by the new and more precise results of [4] (AGSS09), with a much lower metallicity of 0.0134
Why GN93 should not be used anymore
We show why the solar chemical composition of [1] (GN93) and the similar values of [2] (AG89) and [3] (GS98), characterized by values of the metallicity of the order of 0.017 to 0.020, largely used in solar and stellar modeling, are now obsolete. They should be replaced by the new and more precise results of [4] (AGSS09), with a much lower metallicity of 0.0134
Paris-midi : seul journal quotidien paraissant à midi / dir. Maurice de Waleffe
18 décembre 19231923/12/18 (A13,N4240)