304 research outputs found
Carbon burning in intermediate mass primordial stars
The evolution of a zero metallicity 9 M_s star is computed, analyzed and
compared with that of a solar metallicity star of identical ZAMS mass. Our
computations range from the main sequence until the formation of a massive
oxygen-neon white dwarf. Special attention has been payed to carbon burning in
conditions of partial degeneracy as well as to the subsequent thermally pulsing
Super-AGB phase. The latter develops in a fashion very similar to that of a
solar metallicity 9 M_s star, as a consequence of the significant enrichment in
metals of the stellar envelope that ensues due to the so-called third dredge-up
episode. The abundances in mass of the main isotopes in the final ONe core
resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28
and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly
composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4}
M_sComment: 11 pages, 11 figures, accepted for publication in A&
The late stages of the evolution of intermediate-mass primordial stars: the effects of overshooting
We compute and analyze the evolution of primordial stars of masses at the ZAMS between 5 M_sun and 10 M_sun, with and without overshooting. Our main goals are to determine the nature of the remnants of massive intermediate-mass primordial stars and to check the influence of overshooting in their evolution. Our calculations cover stellar evolution from the main sequence phase until the formation of the degenerate cores and the thermally pulsing phase. We have obtained the values for the limiting masses of Population III progenitor stars leading to carbon-oxygen and oxygen-neon compact cores. Moreover, we have also obtained the limiting mass for which isolated primordial stars would lead to core-collapse supernovae after the end of the main central burning phases. Considering a moderate amount of overshooting the mass thresholds at the ZAMS for the formation of carbon-oxygen and oxygen-neon degenerate cores shifts to smaller values by about 2 M_sun. As a by-product of our calculations, we have also obtained the structure and composition profiles of the resulting compact remnants. Opposite to what happens with solar metallicity objects, the final fate of primordial stars is not straightforward determined from the mass of the compact cores at the end of carbon burning. Instead, the small mass-loss rates typically associated to stellar winds of low metallicity stars might allow the growth of the resulting degenerate cores up to the Chandrasekhar mass, on time scales one or two orders of magnitude shorter than the time required to loose the envelope. This would lead to the formation of supernovae for initial masses as small as about 5 M_sun
Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production
Our main goals are to get a deeper insight into the evolution and final fates
of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to
investigate their C, N, and O yields. Using the Monash University Stellar
Evolution code we computed and analysed the evolution of stars of metallicity Z
= 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the
late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our
model stars experience a strong C, N, and O envelope enrichment either due to
the second dredge-up, the dredge-out phenomenon, or the third dredge-up early
during the TP-(S)AGB phase. Their late evolution is therefore similar to that
of higher metallicity objects. When using a standard prescription for the mass
loss rates during the TP-(S)AGB phase, the computed stars lose most of their
envelopes before their cores reach the Chandrasekhar mass, so our standard
models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we
find that the reduction of only one order of magnitude in the mass-loss rates,
which are particularly uncertain at this metallicity, would prevent the
complete ejection of the envelope, allowing the stars to either explode as an
SNI1/2 or become an electron-capture SN. Our calculations stop due to an
instability near the base of the convective envelope that hampers further
convergence and leaves remnant envelope masses between 0.25 M_sun for our 4
M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N,
and O yields derived from our full calculations and computed under two
different assumptions, namely, that the instability causes a practically
instant loss of the remnant envelope or that the stars recover and proceed with
further thermal pulses. Our results have implications for the early chemical
evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&
Nucleosynthetic yields of Z= intermediate-mass stars
Abridged: Observed abundances of extremely metal-poor (EMP) stars in the Halo
hold clues for the understanding of the ancient universe. Interpreting these
clues requires theoretical stellar models at the low-Z regime. We provide the
nucleosynthetic yields of intermediate-mass Z= stars between 3 and 7.5
, and quantify the effects of the uncertain wind rates. We expect
these yields can be eventually used to assess the contribution to the chemical
inventory of the early universe, and to help interpret abundances of selected
C-enhanced EMP stars. By comparing our models and other existing in the
literature, we explore evolutionary and nucleosynthetic trends with wind
prescriptions and with initial metallicity. We compare our results to
observations of CEMP-s stars belonging to the Halo. The yields of
intermediate-mass EMP stars reflect the effects of very deep second dredge-up
(for the most massive models), superimposed with the combined signatures of
hot-bottom burning and third dredge-up. We confirm the reported trend that
models with initial metallicity Z <= 0.001 give positive yields of
, and . The , and
yields, which were reported to be negative at Z = 0.0001, become
positive for Z=. The results using two different prescriptions for
mass-loss rates differ widely in terms of the duration of the thermally-pulsing
(Super) AGB phase, overall efficiency of the third dredge-up episode, and
nucleosynthetic yields. The most efficient of the standard wind rates
frequently used in the literature seems to favour agreement between our yield
results and observational data. Regardless of the wind prescription, all our
models become N-enhanced EMP stars.Comment: 18 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The formation of DA white dwarfs with thin hydrogen envelopes
We study the formation and evolution of DA white dwarfs, the progenitors of which have experienced a late thermal pulse (LTP) shortly after the departure from the thermally pulsing AGB. To this end, we compute the complete evolution of an initially 2.7 Mâ star all the way from the zero-age main sequence to the white dwarf stage. We find that most of the original H-rich material of the post-AGB remnant is burnt during the post-LTP evolution, with the result that, at entering its white dwarf cooling track, the remaining H envelope becomes 10-6 Mâ in agreement with asteroseismological inferences for some ZZ Ceti stars.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto de AstrofĂsica de La Plat
The best fit for the observed galaxy Counts-in-Cell distribution function
The Sloan Digital Sky Survey (SDSS) is the first dense redshift survey
encompassing a volume large enough to find the best analytic probability
density function that fits the galaxy Counts-in-Cells distribution ,
the frequency distribution of galaxy counts in a volume . Different analytic
functions have been previously proposed that can account for some of the
observed features of the observed frequency counts, but fail to provide an
overall good fit to this important statistical descriptor of the galaxy
large-scale distribution. Our goal is to find the probability density function
that better fits the observed Counts-in-Cells distribution . We have
made a systematic study of this function applied to several samples drawn from
the SDSS. We show the effective ways to deal with incompleteness of the sample
(masked data) in the calculation of . We use LasDamas simulations to
estimate the errors in the calculation. We test four different distribution
functions to find the best fit: the Gravitational Quasi-Equilibrium
distribution, the Negative Binomial Distribution, the Log Normal distribution
and the Log Normal Distribution including a bias parameter. In the two latter
cases, we apply a shot-noise correction to the distributions assuming the local
Poisson model. We show that the best fit for the Counts-in-Cells distribution
function is provided by the Negative Binomial distribution. In addition, at
large scales the Log Normal distribution modified with the inclusion of the
bias term also performs a satisfactory fit of the empirical values of .
Our results demonstrate that the inclusion of a bias term in the Log Normal
distribution is necessary to fit the observed galaxy Count-in-Cells
distribution function.Comment: 12 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
The frequency of occurrence of novae hosting an ONe white dwarf
In this paper, we revisit the problem of the determination of the frequency
of occurrence of galactic nova outbursts which involve an oxygen-neon (ONe)
white dwarf. The improvement with respect to previous work on the subject
derives from the fact that we use the results that our evolutionary
calculations provide for the final mass and for the chemical profiles of
intermediate-to-massive primary components of close binary systems. In
particular, the final evolutionary stages, such as the carbon burning phase,
have been carefully followed for the whole range of masses of interest. The
chemical profiles obtained with our evolutionary code are of interest in
determining the chemical composition of the ejecta after being processed
through the thermonuclear runaway, although such other factors as the
efficiency of the mixing between the accreted material and that of the
underlying white dwarf must also be considered. In our calculations of the
frequency of occurrence of nova outbursts involving an ONe white dwarf, we also
take into account the observational selection effects introduced by the
different recurrence times of the outbursts and by the spatial distribution of
novae. In spite of the very different evolutionary sequences, we find that
approximately 1/3 of the novae observed in outburst should involve an
oxygen-neon white dwarf, in agreement with previous theoretical estimates.Comment: 9 pages, 6 figures, accepted for publication in A&
Nucleosynthetic yields of intermediate-mass primordial to extremely metal-poor stars
Context. Stellar models and nucleosynthetic yields of primordial to extremely metal-poor (EMP) stars are crucial to interpret the surface abundances of the most metal-poor stars observed and, ultimately, to better understand the earliest stellar populations. In addition, they are key ingredients of Galactic chemical evolution models. Aims: We aim to better characterise the evolution and fates, and determine updated nucleosynthetic yields of intermediate-mass stars between primordial and EMP metallicity (Z = 10â10, 10â8, 10â7, 10â6, and 10â5). We also probed uncertainties in the nucleosynthesis of the oldest intermediate-mass stars, namely those related to the treatment of convection and convective boundaries and those related to wind prescriptions during the asymptotic giant branch (AGB) phase. Methods: We analyse the evolution of models from their main sequence, through the thermally pulsing AGB (TP-AGB), to the latest stages of their evolution, using the Monash-Mount Stromlo stellar evolution code MONSTAR. The results were post-processed with the code MONSOON, which allowed for the determination of the nucleosynthetic yields of 77 species up to 62Ni. By comparing them to similar calculations existing in the literature, we inspected the effects of input physics on the nucleosynthesis of EMP models. Results: From the evolutionary point of view, as reported in former works, we identified proton ingestion episodes (PIEs) in our lowest-mass lowest-metallicity models. Models of Z = 10â10 and Z = 10â8 in a narrow initial mass range around 5 Mâ experience the cessation of thermal pulses, and their final fates as type-I1/2 supernovae cannot be discarded. However, the initial mass range of models eventually leading to the formation of type-I1/2 and electron-capture supernovae is considerably reduced compared to former works. All the models of initial mass âł6-7 Mâ experience a corrosive second dredge-up and, analogously to those experiencing PIEs, undergo significant metal enrichment in their envelopes. The associated increase in their opacities allows them to develop a solar-like TP-AGB or TP-super-AGB, ultimately becoming white dwarfs. Except for those undergoing the cessation of thermal pulses, all of our models show the nucleosynthetic signatures of both efficient third dredge-up and hot-bottom burning, with the activation of the NeNa cycle and the MgAlSi chains. This leads to the creation of vast amounts of CNO, with typical [N/Fe] > 4), and the characteristic abundance signature [N/Fe] > [C/Fe] > [O/Fe]. Our nucleosynthetic yields present dramatic differences with respect to recent results existing in the literature for intermediate-mass models of similar metallicities. The reason for these discrepancies lay in the poorly known input physics related to stellar winds and, above all, the treatment of convection and convective boundaries. Nucleosynthetic yield data are only available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/668/A10
- âŠ