1,297 research outputs found
Evolution of AGB stars at varying surface C/O ratio: The crucial effect of molecular opacities
This study calls attention to the importance of properly coupling the
molecular opacities to the actual surface abundances of TP-AGB stars that
experience the third dredge-up and/or hot-bottom burning, i.e. with surface
abundances of carbon and oxygen varying with time. New TP-AGB calculations with
variable opacities -- replacing the usually adopted solar-scaled opacity tables
-- have proven to reproduce, for the first time, basic observables of carbon
stars, like their effective temperatures, C/O ratios, and near-infrared
colours. Moreover, it turns out that the effect of envelope cooling -- due to
the increase in molecular opacities -- may cause other important effects,
namely: i) shortening of the C-star phase; ii) possible reduction or shut-down
of the third dredge-up in low-mass carbon stars; and iii) weakening or even
extinction of hot-bottom burning in intermediate-mass stars.Comment: 6 pages, 5 postscript figures, proceedings of contributed talk at the
St. Luc conference ``CNO in the Universe'', eds. C. Charbonnel, D. Schaerer,
& G. Meynet, to be published in the ASP Conference Serie
Evolution and chemical yields of AGB stars: effects of low-temperature opacities
The studies focused on the Thermally-Pulsing Asymptotic Giant Branch phase
experienced by low- and intermediate-mass stars are extremely important in many
astrophysical contexts. In particular, a detailed computation of their chemical
yields is essential for several issues, ranging from the chemical evolution of
galaxies, to the mechanisms behind the formation of globular clusters. Among
all the uncertainties affecting the theoretical modelling of this phase, and
described in the literature, it remains to be fully clarified which results are
severely affected by the use of inadequate low-temperature opacities, that are
in most cases calculated on the basis of the original chemical composition of
the stars, and do not consider the changes in the surface chemistry due to the
occurrence of the third dredge-up and hot-bottom burning. Our investigation is
aimed at investigating this point. By means of full evolutionary models
including new set of molecular opacities computed specifically with the AESOPUS
tool, we highlight which stellar models, among those present in the literature,
need a substantial revision, mainly in relation to the predicted chemical
yields. The interplay among convection, hot bottom burning and the
low-temperature opacity treatment is also discussedComment: 6 pages, 2 figure
Constraining the third dredge-up via carbon stars in the Magellanic Clouds
We use the available data for Magellanic Cloud carbon stars to constrain the
efficiency of the third dredge-up process in TP-AGB models. We show that star
counts in LMC clusters provide quite stringent limits to the lifetime of the
C-star phase, with a duration between 2 and 3 Myr for stars in the mass range
from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars,
this information allows us to re-calibrate the third dredge-up parameters log
T_b_dred and lambda in TP-AGB models that include variable molecular opacities
(Marigo 2002). Preliminary results are presented here.Comment: in proceedings of the St. Luc conference ``CNO in the Universe'',
eds. C. Charbonnel, D. Schaerer, & G. Meynet, ASP Conference Series, in pres
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
A unified formalism for the core mass-luminosity relations of shell-burning stars
The luminosity evolution of stars with highly condensed cores surrounded by
nuclear-burning shell(s) is analytically investigated with the aid of homology
relations. With respect to earlier works using a similar approach (e.g. Refsdal
& Weigert 1970; Kippenhahn 1981), the major improvement is that we derive all
the basic dependences (i.e. on core mass, core radius, and chemical
composition) in a completely generalised fashion, then accounting for a large
range of possible physical properties characterising the burning shell(s).
Parameterised formulas for the luminosity are given as a function of the (i)
relative contribution of the gas to the total pressure (gas plus radiation),
(ii) opacity source, and (iii) dominant nuclear reaction rates. In this way,
the same formalism can be applied to shell-burning stars of various
metallicities and in different evolutionary phases. In particular, we present
some applications concerning the luminosity evolution of RGB and AGB stars with
different chemical compositions, including the case of initial zero
metallicity. It turns out that homology predictions provide a good
approximation to the results of stellar model calculations. Therefore, the
proposed formalism is useful to understand the possible differences in the
luminosity evolution of shell-burning stars within a unified interpretative
framework, and can be as well adopted to improve the analytical description of
stellar properties in synthetic models.Comment: 11 pages, 7 postscript figures, accepted for publication in A&A, Main
Journa
Pre-MS depletion, accretion and primordial 7Li
We reconsider the role of pre-main sequence (pre-MS) Li depletion on the
basis of new observational and theoretical evidence: i) new observations of
Halpha emissions in young clusters show that mass accretion could be continuing
till the first stages of the MS, ii) theoretical implications from
helioseismology suggest large overshooting values below the bottom of the
convective envelopes. We argue here that a significant pre-MS 7Li destruction,
caused by efficient overshoot mixing, could be followed by a matter accretion
after 7Li depletion has ceased on MS thus restoring Li almost to the pristine
value. As a test case we show that a halo dwarf of 0.85 Msun with an extended
overshooting envelope starting with an initial abundance of A(Li) = 2.74 would
burn Li completely, but an accretion rate of the type 1e-8xe^{-t/3e6} Msun
yr would restore Li to end with an A(Li) = 2.31. A self-regulating
process is required to produce similar final values in a range of different
stellar masses to explain the PopII Spite plateau. However, this framework
could explain why open cluster stars have lower Li abundances than the
pre-solar nebula, the absence of Li in the most metal poor dwarfs and a number
of other features which lack of a satisfactory explanation.Comment: To be published in Memorie della Societ\`a Astronomica Italiana
Supplementi Vol. 22, Proceedings of Lithium in the cosmos, Iocco F.,
Bonifacio P., Vangioni E., ed
An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds
The purpose of this work is to present an extensive grid of dynamical
atmosphere and wind models for M-type AGB stars, covering a wide range of
relevant stellar parameters. We used the DARWIN code, which includes
frequency-dependent radiation-hydrodynamics and a time-dependent description of
dust condensation and evaporation, to simulate the dynamical atmosphere. The
wind-driving mechanism is photon scattering on submicron-sized MgSiO
grains. The grid consists of models, with luminosities from
to and
effective temperatures from 2200K to 3400K. For the first time different
current stellar masses are explored with M-type DARWIN models, ranging from
0.75M to 3M. The modelling results are radial atmospheric
structures, dynamical properties such as mass-loss rates and wind velocities,
and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of
condensed Si). We find that the mass-loss rates of the models correlate
strongly with luminosity. They also correlate with the ratio :
increasing by an order of magnitude increases the mass-loss rates by
about three orders of magnitude, which may naturally create a superwind regime
in evolution models. There is, however, no discernible trend of mass-loss rate
with effective temperature, in contrast to what is found for C-type AGB stars.
We also find that the mass-loss rates level off at luminosities higher than
, and consequently at pulsation periods longer
than days. The final grain radii range from 0.25 micron to 0.6
micron. The amount of condensed Si is typically between 10% and 40%, with
gas-to-dust mass ratios between 500 and 4000.Comment: Accepted to A&A, 17 pages, 15 figure
Coupling emitted light and chemical yields from stars: a basic constraint to population synthesis models of galaxies
In this paper we emphasize the close connection between the chemical and
spectrophotometric evolution of stellar systems: Chemical yields from stars
correspond to a precise fraction of their emitted light. We translate this
concept quantitatively. Starting from simple stellar populations, we derive
useful analytical relations to calculate the stellar fuel consumption (emitted
light) as a function of basic quantities predicted by stellar models, i.e. the
mass of the core and the chemical composition of the envelope. The final
formulas explicate the relation between integrated light contribution (total or
limited to particular evolutionary phases), chemical yields and stellar
remnants. We test their accuracy in the case of low- and intermediate-mass
stars, and indicate the way to extend the analysis to massive stars. This
formalism provides an easy tool to check the internal consistency between the
different stellar inputs adopted in galaxy models: The fuel computed by means
of the analytical formulas (corresponding to a given set of chemical yields)
should be compared to the exact values given by the luminosity integration
along the stellar evolutionary tracks or isochrones (corresponding to a given
set of spectrophotometric models). Only if both estimates of the fuel are
similar, the stellar inputs can be considered self-consistent in terms of their
energetics. This sets an important requirement to galaxy models, also in
consideration of the fact that different sources of input stellar data are
frequently used to model their spectro-photometric and chemical evolution.Comment: 17 pages, 7 figures, 3 tables, accepted for publication in A&
Effects of dark matter annihilation on the first stars
We study the evolution of the first stars in the universe (Population III)
from the early pre-Main Sequence until the end of helium burning in the
presence of WIMP dark matter annihilation inside the stellar structure. The two
different mechanisms that can provide this energy source are the contemporary
contraction of baryons and dark matter, and the capture of WIMPs by scattering
off the gas with subsequent accumulation inside the star. We find that the
first mechanism can generate an equilibrium phase, previously known as a "dark
star", which is transient and present in the very early stages of pre-MS
evolution. The mechanism of scattering and capture acts later, and can support
the star virtually forever, depending on environmental characteristic of the
dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star
Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and
R. Schneider ed
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
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