1,318 research outputs found
Homogenization of sound hard metamaterials in the time domain
International audienceWe present a homogenization method based on a matched asymptotic expansion technique for sound hard materials structured at subwavelength scale. Considering the wave equation in the time domain, jump conditions are derived for the acoustic pressure and the normal velocity across an equivalent interface with non zero thickness. These jump conditions are implemented in a numerical scheme and compared to the results of the direct problem
Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations
Stellar parameter determination is a challenging task when dealing with
galactic giant stars. The combination of different investigation techniques has
proven to be a promising approach. We analyse archive spectra obtained with the
Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric
observations from the Very Large Telescope MID-infrared Interferometric
instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The
aim of this work is to determine stellar parameters using spectroscopy and
interferometry. The observations are used to constrain the model atmosphere,
and eventually the stellar evolutionary model in the region where the tracks
map the beginning of the carbon star sequence. Two different approaches are
used to determine stellar parameters: (i) the 'classic' interferometric
approach where the effective temperature is fixed by using the angular diameter
in the N-band (from interferometry) and the apparent bolometric magnitude; (ii)
parameters are obtained by fitting a grid of state-of-the-art hydrostatic
models to spectroscopic and interferometric observations. We find a good
agreement between the parameters of the two methods. The effective temperature
and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the
H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star
just recently, which means that the star is still in a 'quiet' phase compared
to the subsequent strong-wind regime. This is in agreement with the C/O ratio
being only slightly larger than 1.Comment: 11 pages, 9 figures, 5 table
Resolving the Outer Disks and Halos of Nearby Galaxies
In a hierarchical merging scenario, the outer parts of a galaxy are a fossil
record of the galaxy's early history. Observations of the outer disks and halos
of galaxies thus provide a tool to study individual galaxy histories and test
formation theories. Locally, an impressive effort has been made to understand
the halo of the Milky Way, Andromeda, and M33. However, due to the stochastic
nature of halo formation, a better understanding of this process requires a
large sample of galaxies with known halo properties. The GHOSTS project (PI: R.
de Jong) aims to characterize the halos and outer portions of 14 nearby (D=4-14
Mpc) spiral galaxies using the Hubble Space Telescope. Detection of individual
stars in the outer parts of these galaxies enables us to study both the
morphological properties of the galaxies, and determine the stars' metallicity
and age.Comment: Contributed talk; to appear in the proceedings of "Galaxies in the
Local Volume" Sydney 8-13 July 200
Parameterising the third dredge-up in asymptotic giant branch stars
We present new evolutionary sequences for low and intermediate mass stars for
three different metallicities, Z = 0.02,0.008 and 0.004. We evolve the models
from the pre-main sequence to the thermally-pulsing asymptotic giant branch
phase. We have two sequences of models for each mass, one which includes mass
loss and one without mass loss. Typically 20 or more pulses have been followed
for each model, allowing us to calculate the third dredge-up parameter for each
case. Using the results from this large and homogeneous set of models, we
present an approximate fit for the core mass at the first thermal pulse, as
well as for the third dredge-up efficiency parameter, and the core mass at the
first third dredge-up episode as a function of metallicity and total mass. We
also examine the effect of a reduced envelope mass on the value of the third
dredge-up efficiency parameter.Comment: 23 pages, 19 figures, accepted for publication in PASA (Publications
of the Astronomical Society of Australia
A new inverse method for stellar population synthesis and error analysis
The stellar population synthesis in unresolved composite objects is a very
tricky problem. Indeed, it is a degenerate problem since many parameters affect
the observables. The stellar population synthesis issue thus deserves a deep
and rigourous analysis. In this paper we present a method of inversion which
uses as observables the intensities at each pixel of a galactic spectrum and
provides the stellar contribution to luminosity of all stars considered in a
database. The main contribution of this paper to the synthesis problem is that
it provides an analytical computation of the uncertainties accompanying a
solution. This constitutes an important improvement relative to previous
methods which do not provide such infomation except in the method described by
Pelat (1997) and Moultaka & Pelat (2000). The latter uses the equivalent widths
and intensities of stellar spectra in order to reproduce the equivalent widths
of a galactic spectrum. The novelty of this work relative to the previous one
is that the dust emission present in the IR spectra can be modeled as well as
the velocity dispersion of stars that broadens the lines of a galactic
spectrum. Tests are also performed in order to estimate the reliability of the
method and the influence on the results of an additive continuum present in a
studied spectrum, for example in the case of AGNs.Comment: 12 pages, 4 figures, 7 tables. In press in A&
A more detailed look at the Opacities for Enriched Carbon and Oxygen Mixtures
We have included opacity tables in our stellar evolution code that enable us
to accurately model the structure of stars composed of mixtures with carbon and
oxygen independently enhanced relative to solar. We present tests to
demonstrate the effects of the new tables. Two of these are practical examples,
the effect on the evolution of a thermally pulsing asymptotic giant branch star
and a Wolf-Rayet Star. The changes are small but perceptible.Comment: 6 pages, 3 figures, 5 tables. Refereed version with corrections
resubmitted to MNRA
Carbon Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,
nitrogen,and oxygen yields adopted, have been computed to reproduce the
Galactic O/H values obtained from H II regions. All the models fit the oxygen
gradient, but only two models fit also the carbon gradient, those based on
carbon yields that increase with metallicity due to stellar winds in massive
stars (MS) and decrease with metallicity due to stellar winds in low and
intermediate mass stars (LIMS). The successful models also fit the C/O versus
O/H evolution history of the solar vicinity obtained from stellar observations.
We also compare the present day N/H gradient and the N/O versus O/H and the
C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity
predicted by our two best models with those derived from H II regions and from
stellar observations. While our two best models fit the C/H and O/H gradients
as well as the C/O versus O/H history, only Model 1 fits well the N/H gradient
and the N/O values for metal poor stars but fails to fit the N/H values for
metal rich stars. Therefore we conclude that our two best models solve the C
enrichment problem, but that further work needs to be done on the N enrichment
problem. By adding the C and O production since the Sun was formed predicted by
Models 1 and 2 to the observed solar values we find an excellent agreement with
the O/H and C/H values of the solar vicinity derived from H II regions O and C
recombination lines. One of the most important results of this paper is that
the fraction of carbon due to MS and LIMS in the interstellar medium is
strongly dependent on time and on the galactocentric distance; at present about
half of the carbon in the interstellar medium of the solar vicinity has been
produced by MS and half by LIMS.Comment: 34 pages, 6 tables, 7 figures. Accepted for publication in Ap
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