515 research outputs found
Lithium-7 surface abundance in pre-MS stars. Testing theory against clusters and binary systems
The disagreement between theoretical predictions and observations for surface
lithium abundance in stars is a long-standing problem, which indicates that the
adopted physical treatment is still lacking in some points. However, thanks to
the recent improvements in both models and observations, it is interesting to
analyse the situation to evaluate present uncertainties. We present a
consistent and quantitative analysis of the theoretical uncertainties affecting
surface lithium abundance in the current generation of models. By means of an
up-to-date and well tested evolutionary code, FRANEC, theoretical errors on
surface 7Li abundance predictions, during the pre-main sequence (pre-MS) and
main sequence (MS) phases, are discussed in detail. Then, the predicted surface
7Li abundance was tested against observational data for five open clusters,
namely Ic 2602, \alpha Per, Blanco1, Pleiades, and Ngc 2516, and for four
detached double-lined eclipsing binary systems. Stellar models for the
aforementioned clusters were computed by adopting suitable chemical
composition, age, and mixing length parameter for MS stars determined from the
analysis of the colour-magnitude diagram of each cluster. We restricted our
analysis to young clusters, to avoid additional uncertainty sources such as
diffusion and/or radiative levitation efficiency. We confirm the disagreement,
within present uncertainties, between theoretical predictions and 7Li
observations for standard models. However, we notice that a satisfactory
agreement with observations for 7Li abundance in both young open clusters and
binary systems can be achieved if a lower convection efficiency is adopted
during the pre-MS phase with respect to the MS one.Comment: 10 pages, 5 figures. Accepted for publication in A&
Effect of planet ingestion on low-mass stars evolution: the case of 2MASS J08095427--4721419 star in the Gamma Velorum cluster
We analysed the effects of planet ingestion on the characteristics of a
pre-MS star similar to the Gamma Velorum cluster member 2MASS
J08095427--4721419 (#52). We discussed the effects of changing the age at
which the accretion episode occurs, the mass of the ingested planet and its
chemical composition. We showed that the mass of the ingested planet required
to explain the current [Fe/H]^#52 increases by decreasing the age and/or
by decreasing the Iron content of the accreted matter.
We compared the predictions of a simplified accretion method -- where only
the variation of the surface chemical composition is considered -- with that of
a full accretion model that properly accounts for the modification of the
stellar structure. We showed that the two approaches result in different
convective envelope extension which can vary up to 10 percent. We discussed the
impact of the planet ingestion on a stellar model in the colour-magnitude
diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07
dex in magnitude are expected and that such variations persist even much later
the accretion episode. We also analysed the systematic bias in the stellar mass
and age inferred by using a grid of standard non accreting models to recover
the characteristics of an accreting star. We found that standard non accreting
models can safely be adopted for mass estimate, as the bias is <= 6 percent,
while much more caution should be used for age estimate where the differences
can reach about 60 percent.Comment: Accepted for publication in MNRAS. 13 pages, 3 tables, 9 figure
On the age of Galactic bulge microlensed dwarf and subgiant stars
Recent results by Bensby and collaborators on the ages of microlensed stars
in the Galactic bulge have challenged the picture of an exclusively old stellar
population. However, these age estimates have not been independently confirmed.
In this paper we verify these results by means of a grid-based method and
quantify the systematic biases that might be induced by some assumptions
adopted to compute stellar models. We explore the impact of increasing the
initial helium abundance, neglecting the element microscopic diffusion, and
changing the mixing-length calibration in theoretical stellar track
computations. We adopt the SCEPtER pipeline with a novel stellar model grid for
metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60;
1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We
show for the considered evolutionary phases that our technique provides
unbiased age estimates. Our age results are in good agreement with Bensby and
collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than
9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta
Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich
stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the
upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr
and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar
models computed by neglecting microscopic diffusion or by assuming a
super-solar mixing-length value leads to a mean increase in the age estimates
of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values
for the age estimates, there are four stars estimated younger than 5 Gyr is in
both cases. Thus, the assessment of a sizeable fraction of young stars among
the microlensed sample in the Galactic bulge appears robust.Comment: Accepted for publication in A&A. Abstract shortene
Cumulative physical uncertainty in modern stellar models I. The case of low-mass stars
Using our updated stellar evolutionary code, we quantitatively evaluate the
effects of the uncertainties in the main physical inputs on the evolutionary
characteristics of low mass stars from the main sequence to the zero age
horizontal branch (ZAHB). We calculated more than 3000 stellar tracks and
isochrones, with updated solar mixture, by changing the following physical
inputs within their current range of uncertainty: 1H(p,nu e+)2H,
14N(p,gamma)15O, and triple-alpha reaction rates, radiative and conductive
opacities, neutrino energy losses, and microscopic diffusion velocities. We
performed a systematic variation on a fixed grid, in a way to obtain a full
crossing of the perturbed input values. The effect of the variations of the
chosen physical inputs on relevant stellar evolutionary features, such as the
turn-off luminosity, the central hydrogen exhaustion time, the red-giant branch
(RGB) tip luminosity, the helium core mass, and the ZAHB luminosity in the RR
Lyrae region are statistically analyzed. For a 0.9 Msun model, the cumulative
uncertainty on the turn-off, the RGB tip, and the ZAHB luminosities accounts
for 0.02 dex, 0.03 dex, and 0.045 dex respectively, while the
central hydrogen exhaustion time varies of about 0.7 Gyr. The most
relevant effect is due to the radiative opacities uncertainty; for the later
evolutionary stages the second most important effect is due to the triple-alpha
reaction rate uncertainty. For an isochrone of 12 Gyr, we find that the
isochrone turn-off log luminosity varies of 0.013 dex, the mass at the
isochrone turn-off varies of 0.015 Msun, and the difference between ZAHB
and turn-off log-luminosity varies of 0.05 dex. The effect of the
physical uncertainty affecting the age inferred from turn-off luminosity and
from the vertical method are of 0.375 Gyr and 1.25 Gyr
respectively.Comment: Accepteted for pubblication in A&A. The abstract is shortened to fill
in the arxiv abstract fiel
Mixing-length estimates from binary systems. A theoretical investigation on the estimation errors
We performed a theoretical investigation on the mixing-length parameter
recovery from an eclipsing double-lined binary system. We focused on a syntetic
system composed by a primary of mass M = 0.95 Msun and a secondary of M = 0.85
Msun. Monte Carlo simulations were conducted at three metallicities, and three
evolutionary stages of the primary. For each configuration artificial data were
sampled assuming an increasing difference between the mixing-length of the two
stars. The mixing length values were reconstructed using three alternative
set-ups. A first method, which assumes full independence between the two stars,
showed a great difficulty to constrain the mixing-length values: the recovered
values were nearly unconstrained with a standard deviation of 0.40. The second
technique imposes the constraint of common age and initial chemical composition
for the two stars in the fit. We found that values match the
ones recovered under the previous configuration, but values are
peaked around unbiased estimates. This occurs because the primary star provides
a much more tight age constraint in the joint fit than the secondary. Within
this second scenario we also explored, for systems sharing a common
, the difference in the mixing-length values of the two stars only
due to random fluctuations owing to the observational errors. The posterior
distribution of these differences was peaked around zero, with a large standard
deviation of 0.3 (15\% of the solar-scaled value). The third technique also
imposes the constraint of a common mixing-length value for the two stars, and
served as a test for identification of wrong fitting assumptions. In this case
the common mixing-length is mainly dictated by the value of .
[...] For less than half of the systems can be
recovered and only 20% at .Comment: Abstract abridge
Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions
We study the temporal evolution of the habitable zone (HZ) of low-mass stars
- only due to stellar evolution - and evaluate the related uncertainties. These
uncertainties are then compared with those due to the adoption of different
climate models. We computed stellar evolutionary tracks from the pre-main
sequence phase to the helium flash at the red-giant branch tip for stars with
masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 -
0.04], and various initial helium contents. We evaluated several
characteristics of the HZ, such as the distance from the host star at which the
habitability is longest, the duration of this habitability, the width of the
zone for which the habitability lasts one half of the maximum, and the
boundaries of the continuously habitable zone (CHZ) for which the habitability
lasts at least 4 Gyr. We developed analytical models, accurate to the percent
level or lower, which allowed to obtain these characteristics in dependence on
the mass and the chemical composition of the host star. The metallicity of the
host star plays a relevant role in determining the HZ. The initial helium
content accounts for a variation of the CHZ boundaries as large as 30% and 10%
in the inner and outer border. The computed analytical models allow the first
systematic study of the variability of the CHZ boundaries that is caused by the
uncertainty in the estimated values of mass and metallicity of the host star.
An uncertainty range of about 30% in the inner boundary and 15% in the outer
one were found. We also verified that these uncertainties are larger than that
due to relying on recently revised climatic models, which leads to a CHZ
boundaries shift within 5% with respect to those of our reference scenario. We
made an on-line tool available that provides both HZ characteristics and
interpolated stellar tracks.Comment: Accepted for publication in A&A, abstract abridge
Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities
The expected distribution of Cepheids within the instability strip is
affected by several model inputs, reflecting upon the predicted
Period-Luminosity relation. On the basis of new and updated sets of
evolutionary and pulsational models, we quantitatively evaluated the effects on
the theoretical PL relation of current uncertainties on the chemical abundances
of Cepheids in the Large Magellanic Cloud and on several physical assumptions
adopted in the evolutionary models. We analysed how the different factors
influence the evolutionary and pulsational observables and the resulting PL
relation. As a result, we found that present uncertainties on the most relevant
H and He burning reaction rates do not influence in a relevant way the loop
extension in temperature. On the contrary, current uncertainties on the LMC
chemical composition significantly affect the loop extension and also reflect
in the morphology of the instability strip; however their influence on the
predicted pulsational parameters is negligible. We also discussed how
overshooting and mass loss influence the ML relation and the pulsational
parameters. In summary, the present uncertainties on the physical inputs
adopted in the evolutionary codes and in the LMC chemical composition are
negligible for the prediction of the main pulsational properties; the inclusion
of overshooting in the previous H burning phase and/or of mass loss is expected
to significantly change the resulting theoretical pulsational scenario for
Cepheids, as well as the calibration of their distance scale. These systematic
effects are expected to influence the theoretical Cepheid calibration of the
secondary distance indicators and in turn the resulting evaluation of the
Hubble constant.Comment: accepted for publication on A&
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