200 research outputs found
A standard stellar library for evolutionary synthesis: I. Calibration of theoretical spectra
A comprehensive hybrid library of synthetic stellar spectra based on three
original grids of model atmosphere spectra by Kurucz (1995), Fluks et al.
(1994), and Bessell et al. (1989, 1991) is presented. The combined library is
intended for multiple-purpose synthetic photometry applications and is
constructed in order (i) to cover the largest possible ranges in Teff, log g,
and [M/H]), (ii) to provide flux spectra with useful resolution on the uniform
grid of wavelengths, and (iii) to provide realistic synthetic broad-band colors
for the largest possible parameter and wavelength ranges. For each value of the
effective temperature and for each wavelength, we calculate the correction
function that must be applied to a (theoretical) solar-abundance model flux
spectrum which yields synthetic UBVRIJHKL colors matching the (empirical)
color-temperature calibrations derived from observations. The calibration
algorithm is designed to preserve the original differential grid properties
implied by metallicity and/or luminosity changes in the new library. The
corresponding color calibration is described in some detail.Comment: 17 pages, Latex, 19 Figures (postscript), accepted for publication in
A&A. Also available with figures as ps-file at
http://www.astro.unibas.ch/structure/papers.htm
A standard stellar library for evolutionary synthesis: II. The M dwarf extension
A standard library of theoretical stellar spectra intended for multiple
synthetic photometry applications including spectral evolutionary synthesis is
presented. The grid includes M dwarf model spectra, hence complementing the
first library version established in Paper I (Lejeune, Cuisinier & Buser 1997).
It covers wide ranges of fundamental parameters: Teff : 50,000 K to 2000 K, log
g : 5.5 to -1.02, and [M/H] : +1.0 to -5.0. A correction procedure is also
applied to the theoretical spectra in order to provide color-calibrated flux
distributions over a large domain of effective temperatures. For this purpose,
empirical Teff-color calibrations are constructed between 11500 K and 2000 K,
and semi-empirical calibrations for non-solar abundances ([M/H] = -3.5 to +1.0)
are established. Model colors and bolometric corrections for both the original
and the corrected spectra, synthesized in the UBVRcIcJHKLL'M system, are given
for the full range of stellar parameters. We find that the corrected spectra
provide a more realistic representation of empirical stellar colors, though the
method employed is not completely adapted to the lowest temperature models. In
particular the original differential colors of the grid implied by metallicity
and/or luminosity changes are not preserved below 2500 K. Limitations of the
correction method used are also discussed.Comment: Accepted for publication in A&A Supplement Series (LaTex); 22 pages,
including 8 postscript figures and 10 tables. Also available at
http://www.astro.unibas.ch/~lejeune
Gas and stellar metallicities in H ii galaxies
We examine the gas and stellar metallicities in a sample of H ii galaxies from the Sloan Digital Sky Survey, which possibly contains the largest homogeneous sample of H ii galaxy spectra to date. We eliminated all spectra with an insufficient signal-to-noise ratio, without strong emission lines and without the [O ii] λ3727 Å line, which is necessary for the determination of the gas metallicity. This excludes galaxies with redshift ≲ 0.033. Our final sample contains ∼700 spectra of H ii galaxies. Through emission line strength calibrations and a detailed stellar population analysis employing evolutionary stellar synthesis methods, which we already used in previous works, we determined the metallicities of both the gas and the stellar content of these galaxies. We find that in H ii galaxies up to stellar masses of 5 × 109 M⊙, enrichment mechanisms do not vary with galactic mass, being the same for low- and high-mass galaxies on average. They do seem to present a greater variety at the high-mass end, though, indicating a more complex assembly history for high-mass galaxies. In around 23 per cent of our H ii galaxies, we find a metallicity decrease over the last few Gyr. Our results favour galaxy evolution models featuring constantly infalling low-metallicity clouds that retain part of the galactic winds. Above 5 × 109 M⊙ stellar mass, the retention of high-metallicity gas by the galaxies' gravitational potential dominate
Exploration of the BaSeL stellar library for 9 F-type stars COROT potential targets
The Basel Stellar Library (BaSeL models) is constituted of the merging of
various synthetic stellar spectra libraries, with the purpose of giving the
most comprehensive coverage of stellar parameters. It has been corrected for
systematic deviations detected in respect to UBVRIJHKLM photometry at solar
metallicity, and can then be considered as the state-of-the-art knowledge of
the broad band content of stellar spectra. In this paper, we consider a sample
of 9 F-type stars with detailed spectroscopic analysis to investigate the Basel
Stellar Library in two photometric systems simultaneously, Johnson (B-V, U-B)
and Stromgren (b-y, m_1, and c_1). The sample corresponds to potential targets
of the central seismology programme of the COROT space experiment, which have
been recently observed at OHP. The atmospheric parameters T_eff, [Fe/H], and
log g obtained from the BaSeL models are compared with spectroscopic
determinations as well as with results of other photometric calibrations. For a
careful interpretation of the BaSeL solutions, we computed confidence regions
around the best ^2-estimates and projected them on T_eff-[Fe/H],
T_eff-log g, and log g-[Fe/H] diagrams. (Abridged)Comment: 16 pages, LaTeX2e; version accepted for publication in the new A&A
Journal: minor changes + figures in black and white for better readabilit
Mechanical Properties of Calcareous Fly Ash Stabilised Soil
Construction of any type of structure on clayey ground could be problematic due to the high swelling potential and/or low strength characteristics of the soil. This can lead to low stability or differential settlement of the ground. Many soil stabilization techniques have been proposed to prevent the uneven settlement and failure of the soil. Stabilization of soil with class C fly ash offers many advantages such as improving engineering caracteristics, being cost-effective and being environmentally friendly. Class C fly ash chemically reacts with clay which results in a more durable and stronger soil. It has been shown by various researchers that fly ash-stabilized soil is typically stiff and strong even though there is no available standard or guidelines for the use of fly ash in construction industry. This paper presents the results from a program of experimental research on stabilization of a fine-grained soil with fly ash. Laboratory experiments, including Atterberg limits, compaction, uniaxial, and consolidation tests, were conducted on samples of a clay soil with different percentages of fly ash. The results show that adding fly ash decreased the plasticity index, increased compressive strength, and decreased the swelling and compressibility index. The maximum dry density increased and optimum moisture content decreased with addition of over 5 % fly ash by dry weight of the soil.Turkish governmentEuropean Union Horizon 202
Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79
northern galactic planetary nebulae whose morphological classes have been
uniformly determined. Ionic abundances and plasma diagnostics were derived from
selected optical line strengths in the literature, and elemental abundances
were estimated with the Ionization Correction Factor developed by Kingsbourgh &
Barlow (1994). We compare the elemental abundances to the final yields obtained
from stellar evolution models of low-and intermediate-mass stars, and we
confirm that most Bipolar planetary nebulae have high nitrogen and helium
abundance, and are the likely progeny of stars with main-sequence mass larger
than 3 solar masses. We derive =0.27, and discuss the implication of such
a high ratio in connection with the solar neon abundance. We determine the
galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01
dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do
not reconcile with galactic metallicity gradients flattening with time.Comment: The Astrophysical Journal, in pres
Thermo-mechanical behaviour of a compacted swelling clay
Compacted unsaturated swelling clay is often considered as a possible buffer
material for deep nuclear waste disposal. An isotropic cell permitting
simultaneous control of suction, temperature and pressure was used to study the
thermo-mechanical behaviour of this clay. Tests were performed at total
suctions ranging from 9 to 110 MPa, temperature from 25 to 80 degrees C,
isotropic pressure from 0.1 to 60 MPa. It was observed that heating at constant
suction and pressure induces either swelling or contraction. The results from
compression tests at constant suction and temperature evidenced that at lower
suction, the yield pressure was lower, the elastic compressibility parameter
and the plastic compressibility parameter were higher. On the other hand, at a
similar suction, the yield pressure was slightly influenced by the temperature;
and the compressibility parameters were insensitive to temperature changes. The
thermal hardening phenomenon was equally evidenced by following a
thermo-mechanical path of loading-heating-cooling-reloading
Abundances of planetary nebulae in the Galactic bulge
Context. Planetary nebulae (PNe) abundances are poorly known for those nebulae in the Galactic bulge. This is because of the high and uneven extinction in the bulge which makes visual spectral measurements difficult. In addition, the extinction corrections may be unreliable. Elements considered are O, N, Ne, S, Ar, and Cl.
Aims. We determine the abundances in 19 PNe, 18 of which are located in the bulge. This doubles the number of PNe abundance determinations in the bulge. The Galactic abundance gradient is discussed for five elements.
Methods. The mid-infrared spectra measured by the Spitzer Space Telescope are used to determine the abundances. This part of the spectrum is little affected by extinction for which an uncertain correction is no longer necessary. In addition the connection with the visible and ultraviolet spectrum becomes simpler because hydrogen lines are observed both in the infrared and in the visible spectra. In this way we more than double the number of PNe with reliable abundances.
Results. Reliable abundances are obtained for O, N, Ne, S, and Ar for Galactic bulge PNe.
Conclusions. The Galactic abundance gradient is less steep than previously thought. This is especially true for oxygen. The sulfur abundance is reliable because all stages of ionization expected have been measured. It is not systematically low compared to oxygen as has been found for some Galactic PNe
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