59 research outputs found
Testing pre-main sequence models: the power of a Bayesian approach
Pre-main sequence (PMS) models provide invaluable tools for the study of star
forming regions as they allow to assign masses and ages to young stars. Thus it
is of primary importance to test the models against observations of PMS stars
with dynamically determined mass. We developed a Bayesian method for testing
the present generation of PMS models which allows for a quantitative comparison
with observations, largely superseding the widely used isochrones and tracks
qualitative superposition.
Using the available PMS data we tested the newest PISA PMS models
establishing their good agreement with the observations. The data cover a mass
range from ~0.3 to ~3.1 Msun, temperatures from ~3x10^3 to ~1.2x10^4 K and
luminosities from ~3x10^-2 to ~60 Lsun. Masses are correctly predicted within
20% of the observed values in most of the cases and for some of them the
difference is as small as 5%. Nevertheless some discrepancies are also observed
and critically discussed.
By means of simulations, using typical observational errors, we evaluated the
spread of log \tau_sim - log \tau_rec, i.e. simulated minus recovered ages
distribution of the single objects. We also found that stars in binary systems
simulated as coeval might be recovered as non coeval, due to observational
errors. The actual fraction of fake non coevality is a complex function of the
simulated ages, masses and mass ratios. We demonstrated that it is possible to
recover the systems' ages with better precision than for single stars using the
composite age-probability distribution, i.e. the product of the components' age
distributions. Using this valuable tool we estimated the ages of the presently
observed PMS binary systems.Comment: Accepted for publication in MNRAS. Fig.2 presented in low-resolution
in this versio
Grid-based estimates of stellar ages in binary systems: SCEPtER: Stellar CharactEristics Pisa Estimation gRid
Aims. We investigate the performance of grid-based techniques in estimating the age of stars in detached eclipsing binary systems. We evaluate the precision of the estimates due to the uncertainty in the observational constraints-masses, radii, effective temperatures, and [Fe/H]-and the systematic bias caused by the uncertainty in convective core overshooting, element diffusion, mixing-length value, and initial helium content. Methods. We adopted the SCEPtER grid, which includes stars with mass in the range [0.8; 1.6] M⊙ and evolutionary stages from the zero-age main sequence to the central hydrogen depletion. Age estimates have been obtained by a generalisation of the maximum likelihood technique described in our previous work. Results. We showed that the typical 1σ random error in age estimates-due only to the uncertainty affecting the observational constraints-is about ± 7%, which is nearly independent of the masses of the two stars. However, such an error strongly depends on the evolutionary phase and becomes larger and asymmetric for stars near the zero-age main sequence where it ranges from about + 90% to-25%. The systematic bias due to the including convective core overshooting-for mild and strong overshooting scenarios-is about 50% and 120% of the error due to observational uncertainties. A variation of ± 1 in the helium-to-metal enrichment ratio ΔY/ΔZ accounts for about ± 150% of the random error. The neglect of microscopic diffusion accounts for a bias of about 60% of the error due to observational uncertainties. We also introduced a statistical test of the expected difference in the recovered age of two coeval stars in a binary system. We find that random fluctuations within the current observational uncertainties can lead genuine coeval binary components to appear to be non-coeval with a difference in age as high as 60%
Single stars in the Hyades open cluster. Fiducial sequence for testing stellar and atmospheric models
Age and mass determinations for isolated stellar objects remain
model-dependent. While stellar interior and atmospheric theoretical models are
rapidly evolving, we need a powerful tool to test them. Open clusters are good
candidates for this role. We complement previous studies on the Hyades
multiplicity by Lucky Imaging observations with the AstraLux Norte camera. This
allows us to exclude possible binary and multiple systems with companions
outside 2--7 AU separation and to create a "single-star sequence" for the
Hyades. The sequence encompasses 250 main-sequence stars ranging from A5V to
M6V. Using the "Tool for Astrophysical Data Analysis" (TA-DA), we create
various theoretical isochrones applying different combinations of interior and
atmospheric models. We compare the isochrones with the observed Hyades
single-star sequence on J vs. J - K_s, J vs. J - H and K_s vs. H - K_s
color-magnitude diagrams. As a reference we also compute absolute fluxes and
magnitudes for all stars from X-ray to mid-infrared based on photometric
measurements available in the literature(ROSAT X-ray, GALEX UV, APASS gri,
2MASS JHK_s, and WISE W1 to W).We find that combinations of both PISA and
DARTMOUTH stellar interior models with BT-Settl 2010 atmospheric models
describe the observed sequence well. The full sequence covers the mass range
0.13 to 2.3 Msun, and effective temperatures between 3060 K and 8200 K. Within
the measurement uncertainties, the current generation of models agree well with
the single-star sequence. The primary limitations are the uncertainties in the
measurement of the distance to individual Hyades members, and uncertainties in
the photometry. Additionally, a small (~0.05 mag) systematic offset can be
noted on J vs. J - K and K vs. H - K diagrams - the observed sequence is
shifted to redder colors from the theoretical predictions.Comment: 6 pages, 2 figures, 1 table. The extended version of the table will
be available online soon. Accepted for publication in Astronomy &
Astrophysic
ASTROPHYSICAL IMPACT of the UPDATED 9Be(p,)6Li and 10B(p,)7Be REACTION RATES AS DEDUCED by THM
The complete understanding of the stellar abundances of lithium, beryllium, and boron represents one of the most interesting open problems in astrophysics. These elements are largely used to probe stellar structure and mixing phenomena in different astrophysical scenarios, such as pre-main-sequence or main-sequence stars. Their different fragility against (p,) burning reactions allows one to investigate different depths of the stellar interior. Such fusion mechanisms are triggered at temperatures between T ≈ (2-5) × K, thus defining a corresponding Gamow energy between ≈ 3-10 keV, where S(E)-factor measurements need to be performed to get reliable reaction rate evaluations. The Trojan Horse Method is a well defined procedure to measure cross sections at Gamow energies overcoming the uncertainties due to low-energy S(E)-factor extrapolation as well as electron screening effects. Taking advantage of the measure of the 9Be(p,)6Li and 10B(p,)7Be cross sections, the corresponding reaction rates have been calculated and compared with the evaluations by the NACRE collaboration, widely used in the literature. The impact on surface abundances of the updated 9Be and 10B (p,) burning rates is discussed for pre-MS stars
When the tale comes true: multiple populations and wide binaries in the Orion Nebula Cluster
The high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula
Cluster (ONC) in r, and i filters by Beccari et al.(2017) revealed three
well-separated pre-main sequences in the color-magnitude diagram (CMD). The
objects belonging to the individual sequences are concentrated towards the
center of the ONC. The authors concluded that there are two competitive
scenarios: a population of unresolved binaries and triples with an exotic mass
ratio distribution, or three stellar populations with different ages. We use
Gaia DR2 in combination with the photometric OmegaCAM catalog to test and
confirm the presence of the putative three stellar populations. We also study
multiple stellar systems in the ONC for the first time using Gaia DR2. We
confirm that the second and third sequence members are more centrally
concentrated towards the center of the ONC. In addition we find an indication
that the parallax and proper motion distributions are different among the
members of the stellar sequences. The age difference among stellar populations
is estimated to be 1-2 Myr. We use Gaia measurements to identify and remove as
many unresolved multiple system candidates as possible. Nevertheless we are
still able to recover two well-separated sequences with evidence for the third
one, supporting the existence of the three stellar populations. We were able to
identify a substantial number of wide binary objects (separation between
1000-3000 au). This challenges previously inferred values that suggested no
wide binary stars exist in the ONC. Our inferred wide-binary fraction is approx
5%. We confirm the three populations correspond to three separated episodes of
star formation. Based on this result, we conclude that star formation is not
happening in a single burst in this region. (abridged)Comment: Astronomy and Astrophysics (A&A) accepted. 12 pages, 9 figures +
appendix. New version with language corrections and new ID values in Tab.A.
Lithium evolution in young open clusters from the Gaia-ESO Survey
The Gaia-ESO Survey provides the largest database of homogeneously-determined lithium abundances and stellar parameters for open star clusters of different age and metallicity. It is therefore well suited to investigate young stellar evolution and to provide independent age estimates in young clusters. We present the lithium results for a sample of young clusters of ages between 10 and 100 Myr, and compare the observed lithium depletion patterns with models of lithium depletion in pre-main sequence stars
Lithium and age of pre-main sequence stars: The case of Parenago 1802
With the aim to test the present capability of the stellar surface lithium abundance in providing an estimation for the age of PMS stars, we analyze the case of the detached, double-lined, eclipsing binary system PAR 1802. For this system, the lithium age has been compared with the theoretical one, as estimated by applying a Bayesian analysis method on a large grid of stellar evolutionary models. The models have been computed for several values of chemical composition and mixing length, by means of the code FRANEC updated with the Trojan Horse reaction rates involving lithium burning
Calibration of White Dwarf cooling sequences: theoretical uncertainty
White Dwarf luminosities are powerful age indicators, whose calibration
should be based on reliable models. We discuss the uncertainty of some chemical
and physical parameters and their influence on the age estimated by means of
white dwarf cooling sequences. Models at the beginning of the white dwarf
sequence have been obtained on the base of progenitor evolutionary tracks
computed starting from the zero age horizontal branch and for a typical halo
chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear
reaction rates, convection, mass loss and initial chemical composition are
discussed. Then, various cooling sequences for a typical white dwarf mass
(M=0.6 Mo) have been calculated under different assumptions on some input
physics, namely: conductive opacity, contribution of the ion-electron
interaction to the free energy and microscopic diffusion. Finally we present
the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much
effort has been spent to extend the equation of state down to the low
temperature and high density regime. An analysis of the latest improvement in
the physics of white dwarf interiors is presented. We conclude that at the
faint end of the cooling sequence (log L/Lo=-5.5) the present overall
uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr.
We suggest that this uncertainty could be substantially reduced by improving
our knowledge of the conductive opacity (especially in the partially degenerate
regime) and by fixing the internal stratification of C and O.Comment: 14 figures, accepted by Ap
The Araucaria Project: A study of the classical Cepheid in the eclipsing binary system OGLE LMC562.05.9009 in the Large Magellanic Cloud
We present a detailed study of the classical Cepheid in the double-lined,
highly eccentric eclipsing binary system OGLE-LMC562.05.9009. The Cepheid is a
fundamental mode pulsator with a period of 2.988 days. The orbital period of
the system is 1550 days. Using spectroscopic data from three 4-8-m telescopes
and photometry spanning 22 years, we were able to derive the dynamical masses
and radii of both stars with exquisite accuracy. Both stars in the system are
very similar in mass, radius and color, but the companion is a stable,
non-pulsating star. The Cepheid is slightly more massive and bigger (M_1 = 3.70
+/- 0.03M_sun, R_1 = 28.6 +/- 0.2R_sun) than its companion (M_2 = 3.60 +/-
0.03M_sun, R_2 = 26.6 +/- 0.2R_sun). Within the observational uncertainties
both stars have the same effective temperature of 6030 +/- 150K. Evolutionary
tracks place both stars inside the classical Cepheid instability strip, but it
is likely that future improved temperature estimates will move the stable giant
companion just beyond the red edge of the instability strip. Within current
observational and theoretical uncertainties, both stars fit on a 205 Myr
isochrone arguing for their common age. From our model, we determine a value of
the projection factor of p = 1.37 +/- 0.07 for the Cepheid in the
OGLE-LMC562.05.9009 system. This is the second Cepheid for which we could
measure its p-factor with high precision directly from the analysis of an
eclipsing binary system, which represents an important contribution towards a
better calibration of Baade-Wesselink methods of distance determination for
Cepheids.Comment: Accepted to be published in Ap
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