2,916 research outputs found
Cumulative physical uncertainty in modern stellar models. II. The dependence on the chemical composition
We extend our work on the effects of the uncertainties on the main input
physics for the evolution of low-mass stars. We analyse the dependence of the
cumulative physical uncertainty affecting stellar tracks on the chemical
composition. We calculated more than 6000 stellar tracks and isochrones, with
metallicity ranging from Z = 0.0001 to 0.02, 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. The
analysis was performed using a latin hypercube sampling design. We examine in a
statistical way the dependence on the variation of the physical inputs of the
turn-off (TO) luminosity, the central hydrogen exhaustion time (t_H), the
luminosity and the helium core mass at the red-giant branch (RGB) tip, and the
zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. For the
stellar tracks, an increase from Z = 0.0001 to Z = 0.02 produces a cumulative
physical uncertainty in TO luminosity from 0.028 dex to 0.017 dex, while the
global uncertainty on t_H increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip,
the cumulative uncertainty on the luminosity is almost constant at 0.03 dex,
whereas the one the helium core mass decreases from 0.0055 M_sun to 0.0035
M_sun. The dependence of the ZAHB luminosity error is not monotonic with Z, and
it varies from a minimum of 0.036 dex at Z = 0.0005 to a maximum of 0.047 dex
at Z = 0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical
uncertainty on the predicted TO luminosity and mass increases respectively from
0.012 dex to 0.014 dex and from 0.0136 M_sun to 0.0186 M_sun. Consequently, for
ages typical of galactic globular clusters, the uncertainty on the age inferred
from the TO luminosity increases from 325 Myr to 415 Myr.Comment: Accepted for publication in A&
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
The Pisa Stellar Evolution Data Base for low-mass stars
The last decade showed an impressive observational effort from the
photometric and spectroscopic point of view for ancient stellar clusters in our
Galaxy and beyond. The theoretical interpretation of these new observational
results requires updated evolutionary models and isochrones spanning a wide
range of chemical composition. With this aim we built the new "Pisa Stellar
Evolution Database" of stellar models and isochrones by adopting a well-tested
evolutionary code (FRANEC) implemented with updated physical and chemical
inputs. In particular, our code adopts realistic atmosphere models and an
updated equation of state, nuclear reaction rates and opacities calculated with
recent solar elements mixture. A total of 32646 models have been computed in
the range of initial masses 0.30 - 1.10 Msun for a grid of 216 chemical
compositions with the fractional metal abundance in mass, Z, ranging from
0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models
were computed for both solar-scaled and alpha-enhanced abundances with
different external convection efficiencies. Correspondingly, 9720 isochrones
were computed in the age range 8 - 15 Gyr, in time steps of 0.5 Gyr. The whole
database is available to the scientific community on the web. Models and
isochrones were compared with recent calculations available in the literature
and with the color-magnitude diagram of selected Galactic globular clusters.
The dependence of relevant evolutionary quantities on the chemical composition
and convection efficiency were analyzed in a quantitative statistical way and
analytical formulations were made available for reader's convenience.Comment: Accepted for publication in A&
Uncertainties in grid-based estimates of stellar mass and radius. SCEPtER: Stellar CharactEristics Pisa Estimation gRid
Some aspects of the systematic and statistical errors affecting grid-based
estimation of stellar masses and radii have still not been investigated well.
We study the impact on mass and radius determination of the uncertainty in the
input physics, in the mixing-length value, in the initial helium abundance, and
in the microscopic diffusion efficiency adopted in stellar model computations.
We consider stars with mass in the range [0.8 - 1.1] Msun and evolutionary
stages from the zero-age main sequence to the central hydrogen exhaustion.
Stellar parameters were recovered by a maximum-likelihood technique, comparing
the observations constraints to a grid of stellar models. Synthetic grids with
perturbed input were adopted to estimate the systematic errors arising from the
current uncertainty in model computations. We found that the statistical error
components, owing to the current typical uncertainty in the observations, are
nearly constant in all cases at about 4.5% and 2.2% on mass and radius
determination, respectively. The systematic bias on mass and radius
determination due to a variation of 1 in Delta Y/Delta Z is 2.3%
and 1.1%; the one due to a change of 0.24 in the value of the
mixing-length is 2.1% and 1.0%; the one due to a variation of
5% in the radiative opacity is 1.0% and 0.45%. An important bias
source is to neglect microscopic diffusion, which accounts for errors of about
3.7% and 1.5% on mass and radius. The cumulative effects of the considered
uncertainty sources can produce biased estimates of stellar characteristics.
Comparison of the results of our technique with other grid techniques shows
that the systematic biases induced by the differences in the estimation grids
are generally greater than the statistical errors involved.Comment: Accepted for publication in A&A. Abstract shortene
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