231 research outputs found
New models for the evolution of Post-Asymptotic Giant Branch stars and Central Stars of Planetary Nebulae
The Post Asymptotic Giant Branch (AGB) phase is arguably one of the least
understood phases of the evolution of low- and intermediate- mass stars. The
two grids of models presently available are based on outdated micro- and
macro-physics and do not agree with each other. We study the timescales of
post-AGB and CSPNe in the context of our present understanding of the micro-
and macro-physics of stars. We want to assess whether new post-AGB models,
based on the latter improvements in TP-AGB modeling, can help to understand the
discrepancies between observation and theory and within theory itself. We
compute a grid of post-AGB full evolutionary sequences that include all
previous evolutionary stages from the Zero Age Main Sequence to the White Dwarf
phase. Models are computed for initial masses between 0.8 and 4 and
for a wide range of initial metallicities (0.02, 0.01, 0.001, 0.0001),
this allow us to provide post-AGB timescales and properties for H-burning
post-AGB objects with masses in the relevant range for the formation of
planetary nebulae ( 0.5 - 0.8, ).
We find post-AGB timescales that are at least to times
shorter than those of old post-AGB stellar evolution models. This is true for
the whole mass and metallicity range. The new models are also 0.1 - 0.3
dex brighter than the previous models with similar remnant masses. Post-AGB
timescales show only a mild dependence on metallicity. The shorter post-AGB
timescales derived in the present work are in agreement with recent
semiempirical determinations of the post-AGB timescales from the CSPNe in the
Galactic Bulge. Due to the very different post-AGB crossing times,
initial-final mass relation and luminosities of the present models, they will
have a significant impact in the predictions for the formation of planetary
nebulae and the planetary nebulae luminosity function.Comment: Main Article: 16 pages, 12 figures and 3 tables. Main Article +
Appendices: 22 Pages, 16 figures and 6 tables. Accepted for publication in
A&A. (Revised to match the final version accepted for publication in A&A
The formation of giant planets in wide orbits by photoevaporation-synchronised migration
The discovery of giant planets in wide orbits represents a major challenge
for planet formation theory. In the standard core accretion paradigm planets
are expected to form at radial distances au in order to form
massive cores (with masses ) able to trigger
the gaseous runaway growth before the dissipation of the disc. This has
encouraged authors to find modifications of the standard scenario as well as
alternative theories like the formation of planets by gravitational
instabilities in the disc to explain the existence of giant planets in wide
orbits. However, there is not yet consensus on how these systems are formed.
In this letter, we present a new natural mechanism for the formation of giant
planets in wide orbits within the core accretion paradigm. If photoevaporation
is considered, after a few Myr of viscous evolution a gap in the gaseous disc
is opened. We found that, under particular circumstances planet migration
becomes synchronised with the evolution of the gap, which results in an
efficient outward planet migration. This mechanism is found to allow the
formation of giant planets with masses in wide
stable orbits as large as 130 au from the central star.Comment: Accepted for publication in MNRAS Letters. Comments are welcom
A Red Giants' Toy Story
In spite of the spectacular progress accomplished by stellar evolution theory
some simple questions remain unanswered. One of these questions is ``Why do
stars become Red Giants?''. Here we present a relatively simple analytical
answer to this question. We validate our analysis by constructing a
quantitative toy-model of a red giant and comparing its predictions to full
stellar evolutionar models.
We find that the envelope forces the value of at,
and above, the burning shell into a very narrow range of possible values.
Together with the fact that the stellar material at the burning shell both
provides and transports most of the stellar luminosity, this leads to tight
relations between the thermodynamic variables at the burning shell and the mass
and radius of the core -- , and .
When complemented by typical mass-radius relations of the helium cores, this
implies that for all stellar masses the evolution of the core dictates the
values of , and . We show that for all stellar masses
evolution leads to an increase in the pressure and density contrasts between
the shell and the core, forcing a huge expansion of the layers on top of the
burning shell.
Besides explaining why stars become red giants our analysis also offers a
mathematical demonstration of the so-called shell homology relations, and
provides simple quantitative answers to some properties of low-mass red giants.Comment: Accepted for Publication in The Astrophysical Journal. Updated with
minor corrections to match the accepted version after proofs. 27 Pages. 15
Figures. 5 appendixe
Asteroseismological constraints on the coolest GW Vir variable star (PG 1159-type)PG 0122+200
We present an asteroseismological study on PG 0122+200, the coolest known
pulsating PG1159 (GW Vir) star. Our results are based on an augmented set of
the full PG1159 evolutionary models recently presented by Miller Bertolami &
Althaus (2006). We perform extensive computations of adiabatic g-mode pulsation
periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to
0.741 Msun. We derive a stellar mass of 0.626 Msun from a comparison between
the observed period spacing and the computed asymptotic period spacing, and a
stellar mass of 0.567 Msun by comparing the observed period spacing with the
average of the computed period spacing. We also find, on the basis of a
period-fit procedure, an asteroseismological model representative of PG
0122+200 which is able to reproduce the observed period pattern with an average
of the period differences of 0.88 s. The model has an effective temperature of
81500 K, a stellar mass of 0.556 Msun, a surface gravity log g= 7.65, a stellar
luminosity and radius of log(L/Lsun)= 1.14 and log(R/Rsun)= -1.73,
respectively, and a He-rich envelope thickness of Menv= 0.019 Msun. We derive a
seismic distance of about 614 pc and a parallax of about 1.6 mas. The results
of the period-fit analysis carried out in this work suggest that the
asteroseismological mass of PG 0122+200 could be 6-20 % lower than thought
hitherto and in closer agreement (to within 5 %) with the spectroscopic mass.
This result suggests that a reasonable consistency between the stellar mass
values obtained from spectroscopy and asteroseismology can be expected when
detailed PG1159 evolutionary models are considered.Comment: 10 pages, 6 figures. To be published in Astronomy & Astrophysic
On the systematics of asteroseismological mass determinations of PG1159 stars
We analyze systematics in the asteroseismological mass determination methods
in pulsating PG 1159 stars. We compare the seismic masses resulting from the
comparison of the observed mean period spacings with the usually adopted
asymptotic period spacings, and the average of the computed period spacings.
Computations are based on full PG1159 evolutionary models with stellar masses
ranging from 0.530 to 0.741 Mo that take into account the complete evolution of
progenitor stars. We conclude that asteroseismology is a precise and powerful
technique that determines the masses to a high internal accuracy, but it
depends on the adopted mass determination method. In particular, we find that
in the case of pulsating PG 1159 stars characterized by short pulsation
periods, like PG 2131+066 and PG 0122+200, the employment of the asymptotic
period spacings overestimates the stellar mass by about 0.06 Mo as compared
with inferences from the average of the period spacings. In this case, the
discrepancy between asteroseismological and spectroscopical masses is markedly
reduced when use is made of the mean period spacing instead of the asymptotic
period spacing.Comment: 7 pages, 4 figures, 1 table. To be published in Astronomy and
Astrophysic
New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects
We provide a fine and homogeneous grid of evolutionary sequences for He-core
white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white
dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and
to study their pulsational properties. White dwarf sequences have been computed
by performing full evolutionary calculations that consider the main energy
sources and processes of chemical abundance changes during white dwarf
evolution. Initial models for the evolving white dwarfs have been obtained by
computing the non-conservative evolution of a binary system consisting of a
Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods.
To derive cooling ages and masses for He-core white dwarf we perform a least
square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our
sequences by using a scheme that takes into account the time spent by models in
different regions of the Teff-g plane. This is useful when multiple solutions
for cooling age and mass determinations are possible in the case of
CNO-flashing sequences. We also explore the adiabatic pulsational properties of
models near the critical mass for the development of CNO flashes (~0.2 Msun).
This is motivated by the discovery of pulsating white dwarfs with stellar
masses near this threshold value. We obtain reliable and homogeneous mass and
cooling age determinations for 58 very low-mass white dwarfs, including 3
pulsating stars. Also, we find substantial differences in the period spacing
distributions of g-modes for models with stellar masses ~ 0.2 Msun, which could
be used as a seismic tool to distinguish stars that have undergone CNO flashes
in their early cooling phase from those that have not. Finally, for an easy
application of our results, we provide a reduced grid of values useful to
obtain masses and ages of He-core white dwarf.Comment: 12 pages, 9 figures, to be published in Astronomy and Astrophysic
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