1,230 research outputs found
Evolution of zero-metallicity massive stars
We discuss the evolutionary properties of primordial massive and very massive
stars, supposed to have formed from metal-free gas. Stellar models are
presented over a large range of initial masses (8 Msun <= Mi <= 1000 Msun),
covering the hydrogen- and helium-burning phases up to the onset of carbon
burning. In most cases the evolution is followed at constant mass. To estimate
the possible effect of mass loss via stellar winds, recent analytic formalisms
for the mass-loss rates are applied to the very massive models (Mi >= 120
Msun).Comment: Invited talk at IAU Symp. 212, ``A Massive Star Odyssey, from Main
Sequence to Supernova'', K.A. van der Hucht, A. Herrero, C. Esteban (eds.), 7
pages, 5 postscript figure
Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF
We investigate the bright cut-off of the [OIII]l5007 planetary nebula
luminosity function (PNLF), that has been suggested as a powerful extragalactic
distance indicator. Theoretical PNLFs are constructed via Monte-Carlo
simulations of populations of PNe, whose individual properties are described
with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a
detailed photoionisation code (CLOUDY). The basic dependences of the cut-off
magnitude M* are then discussed. We find that: (i) In galaxies with recent or
ongoing star formation, the modelled PNLF present M* values between -4 and -5,
very close to the observationally-calibrated value for the LMC. (ii) In these
galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about
2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence
M* is expected to depend strongly on the age of the last burst of star
formation, dimming by as much as 5 mag as we go from young to 10-Gyr old
populations. (iii) Rather than on the initial metallicity of a stellar
population, M* depends on the actual [O/H] of the observed PNe, a quantity that
may differ significantly from the initial value (due to dredge-up episodes),
especially in young and intermediate-age PN populations. (iv) Also the
transition time from the end of AGB to the PN phase, and the nuclear-burning
properties (i.e. H- or He-burning) of the central stars introduce
non-negligible effects on M*. The strongest indication derived from the present
calculations is a serious difficulty to explain the age-invariance of the
cut-off brightness over an extended interval, say from 1 to 13 Gyr, that
observations of PNLFs in galaxies of late-to-early type seem to suggest.Comment: 22 pages, to appear in Astronomy & Astrophysic
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
Homogenenized interface model describing defects periodically distributed on a surface
Titre du rĂ©sumĂ© français joint : ModeÌle d'interface homogeÌneÌiseÌe deÌcrivant des deÌfauts peÌriodiquement reÌpartis sur une surfaceNational audienceWe undertake the homogenization of a three-dimensional elastic medium, which contains heterogeneities localized on a surface. These can be either reinforcements, like steel reinforcements in concrete, or defects, like microcracks periodically distributed. We propose a simple model describing the effective behaviour of these heterogeneities. This model, formulated in an energetic framework, com- bines an elastic interface behaviour with that of a membrane. Its interest is discussed through several applications.See http://hal.archives-ouvertes.fr/docs/00/59/28/84/ANNEX/r_O1H9ABAW.pd
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
Reduce, Reuse, Recycle: Planetary Nebulae as Green Galactic Citizens
We review gas-phase abundances in PNe and describe their dual utility as
archives of original progenitor metallicity via the alpha elements, as well as
sources of processed material from nucleosynthesis during the star's evolution,
i.e., C, N, and s-process elements. We describe the analysis of PN spectra to
derive abundances and discuss the discrepancies that arise from different
choices at each step. Abundance results for the Milky Way and Magellanic Clouds
from various groups of investigators are presented; the observational results
are compared with theoretical predictions of AGB stellar yields. Finally, we
suggest areas where more work is needed to improve our abilities to determine
abundances in PNe.Comment: 8 pages, 7 figures, 1 table. Invited review presented at the IAU
Symposium No. 283, Planetary Nebulae: an Eye to the Futur
Stellar Models and Yields of Asymptotic Giant Branch Stars
We present stellar yields calculated from detailed models of low and
intermediate-mass asymptotic giant branch (AGB) stars. We evolve models with a
range of mass from 1 to 6Msun, and initial metallicities from solar to 1/200th
of the solar metallicity. Each model was evolved from the zero age main
sequence to near the end of the thermally-pulsing AGB phase, and through all
intermediate phases including the core He-flash for stars initially less
massive than 2.5Msun. For each mass and metallicity, we provide tables
containing structural details of the stellar models during the TP-AGB phase,
and tables of the stellar yields for 74 species from hydrogen through to
sulphur, and for a small number of iron-group nuclei. All tables are available
for download. Our results have many applications including use in population
synthesis studies and the chemical evolution of galaxies and stellar systems,
and for comparison to the composition of AGB and post-AGB stars and planetary
nebulae.Comment: 26 pages; to appear in Publications of the Astronomical Society of
Australia (PASA); typos fixed in the text and in Tables 4 and
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