200 research outputs found
The effect of heavy element opacity on pre-main sequence Li depletion
Recent 3-D analysis of the solar spectrum data suggests a significant change
of the solar chemical composition. This may affect the temporal evolution of
the surface abundance of light elements since the extension of the convective
envelope is largely affected by the internal opacity value. We analyse the
influence of the adopted solar mixture on the opacity in the convective
envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion.
The surface Li abundance depends on the relative efficiency of several
processes, some of them still not known with the required precision; this paper
thus analyses one of the aspects of this ``puzzle''. Focusing on PMS evolution,
where the largest amount of Li burning occurs, we computed stellar models for
three selected masses (0.8, 1.0 and 1.2 Msun, with Z=0.013, Y=0.27, alpha=1.9)
by varying the chemical mixture, that is the internal element distribution in
Z. We analysed the contribution of the single elements to the opacity at the
temperatures and densities of interest for Li depletion. Several mixtures were
obtained by varying the abundance of the most important elements one at a time;
we then calculated the corresponding PMS Li abundance evolution. We found that
a mixture variation does change the Li abundance: at fixed total metallicity,
the Li depletion increases when increasing the fraction of elements heavier
than O.Comment: A&A accepted, 11 pages, 18 eps figure
Lithium evolution in intermediate age and old open clusters: NGC 752 revisited
We present new high resolution spectroscopic observations of the intermediate
age (~2 Gyr) open cluster NGC 752. We investigate the Li vs. Teff distribution
and we obtain a new accurate determination of the cluster metallicity. We
compare the results for NGC 752 with other intermediate age and old clusters
spanning the age range from the Hyades (~0.6 Gyr) to NGC 188 (~6-8 Gyr). We
find that NGC 752 has a solar iron content ([Fe/H]=+0.01+/-0.04), at variance
with early reports of sub-solar metallicity. We find that NGC 752 is only
slightly more Li depleted than the younger Hyades and has a Li pattern almost
identical to that observed in the ~2 Gyr old IC 4651 and NGC 3680. As for the
latter clusters, we find that NGC 752 is characterized by a tight Li vs. Teff
distribution for solar-type stars, with no evidence for a Li spread as large as
the one observed in the solar age solar metallicity M 67. We discuss these
results in the framework of mixing mechanisms and Li depletion on the main
sequence (MS). We conclude that the development of a large scatter in Li
abundances in old open clusters might be an exception rather than the rule
(additional observations of old clusters are required), and that metallicity
variations of the order of ~0.2 dex do not affect Li depletion after the age of
the Hyades.Comment: A&A accepted, 10 pages, 5 ps figure
Two distinct halo populations in the solar neighborhood. IV. Lithium abundances
We investigate if there is a difference in the lithium abundances of stars
belonging to two halo populations of F and G main-sequence stars previously
found to differ in [alpha/Fe] for the metallicity range -1.4 < [Fe/H] < -0.7.
Li abundances are derived from the LiI 6707.8 A line measured in
high-resolution spectra using MARCS model atmospheres. Furthermore, masses of
the stars are determined from the logTeff - logg diagram by interpolating
between Yonsei-Yale evolutionary tracks. There is no significant systematic
difference in the lithium abundances of high- and low-alpha halo stars. For the
large majority of stars with masses 0.7 < M/M_sun < 0.9 and heavy-element mass
fractions 0.001 < Z < 0.006, the Li abundance is well fitted by a relation
A(Li) = a0 + a1 M + a2 Z + a3 M Z, where a0, a1, a2, and a3 are constants.
Extrapolating this relation to Z = 0 leads to a Li abundance close to the
primordial value predicted from standard Big Bang nucleosynthesis calculations
and the WMAP baryon density. The relation, however, does not apply to stars
with [Fe/H] < -1.5. We suggest that metal-rich halo stars were formed with a Li
abundance close to the primordial value, and that lithium in their atmospheres
has been depleted in time with an approximately linear dependence on stellar
mass and Z. The lack of a systematic difference in the Li abundances of high-
and low-alpha stars indicates that an environmental effect is not important for
the destruction of lithium.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Evolution of lithium beyond the solar age: a Li survey of the old open cluster NGC 188
We have determined Li abundances for 11 G-type stars in the 6-8 Gyr old open
cluster NGC 188. These data significantly enlarge the number of cluster stars
with Li measurements, allowing us to extend the investigation of Li depletion
in open clusters to ages well beyond the age of the Sun. We have also inferred
the cluster metallicity which turns out to be solar. We find that solar-type
stars in NGC 188 are only slightly more Li depleted than the much younger
Hyades and no more Li depleted than stars of similar temperature in the 2-4 Gyr
younger cluster M 67. At variance with M 67, NGC 188 members show virtually no
scatter in their Li abundances. Surprisingly, no solar- type star in NGC 188
appears as Li depleted as the Sun or as the most Li depleted stars in M 67. We
discuss the implications of these results for mechanisms of internal mixing and
Li depletion in main sequence stars.Comment: to appear in A&
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
Taxonomy of Iberian Hoplia (Col., Scarabaeoidea, Hopliinae) based on mtDNA analysis
Abstract The morphology of some Hoplia species (Scarabaeoidea: Hopliinae) is so variable that parapatric populations have often been considered different species or subspecies. In this study we analyze the nucleotide sequences of a fragment of mitochondrial gene cytochrome c oxidase subunit I (COI) of six species and two subspecies of Palaearctic Hoplia to reexamine the species limits. Based on the analysis of sequences from COI and morphological and ecological observations, we consider Hoplia freyi Baraud to be a junior synonym of Hoplia chlorophana Erichson and H. philanthus ramburi Heyden to be a junior synonym of H. philanthus philanthus (Fuessly). However, complete resolution of relationships among H. philanthus subspecies requires the addition of sequences from genes evolving faster than COI. Phylogenetic relationships among the species studied are discussed
Modelling fully convective stars in eclipsing binaries: KOI-126 and CM Draconis
We present models of the components of the systems KOI-126 and CM Draconis,
the two eclipsing binary systems known to date to contain stars with masses low
enough to have fully convective interiors. We are able to model satisfactorily
the system KOI-126, finding consistent solutions for the radii and surface
temperatures of all three components, using a solar-like value of the
mixing-length parameter \alpha in the convection zone, and PHOENIX NextGen 1D
model atmospheres for the surface boundary conditions. Depending on the
chemical composition, we estimate the age of the system to be in the range 3-5
Gyr. For CM Draconis, on the other hand, we cannot reconcile our models with
the observed radii and T_eff using the current metal-poor composition estimate
based on kinematics. Higher metallicities lessen but do not remove the
discrepancy. We then explore the effect of varying the mixing length parameter
\alpha. As previously noted in the literature, a reduced \alpha can be used as
a simple measure of the lower convective efficiency due to rotation and induced
magnetic fields. Our models show a sensitivity to \alpha (for \alpha < 1.0)
sufficient to partially account for the radius discrepancies. It is, however,
impossible to reconcile the models with the observations on the basis of the
effect of the reduced \alpha alone. We therefore suggest that the combined
effects of high metallicity and \alpha reduction could explain the observations
of CM Draconis. For example, increasing the metallicity of the system towards
super-solar values (i.e. Z = 2 Z_sun) yields an agreement within 2 \sigma with
\alpha = 1.0.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Iron and Nickel spectral opacity calculations in conditions relevant for pulsating stellar envelopes and experiments
Seismology of stars is strongly developing. To address this question we have
formed an international collaboration OPAC to perform specific experimental
measurements, compare opacity calculations and improve the opacity calculations
in the stellar codes [1]. We consider the following opacity codes: SCO,
CASSANDRA, STA, OPAS, LEDCOP, OP, SCO-RCG. Their comparison has shown large
differences for Fe and Ni in equivalent conditions of envelopes of type II
supernova precursors, temperatures between 15 and 40 eV and densities of a few
mg/cm3 [2, 3, 4]. LEDCOP, OPAS, SCO-RCG structure codes and STA give similar
results and differ from OP ones for the lower temperatures and for spectral
interval values [3]. In this work we discuss the role of Configuration
Interaction (CI) and the influence of the number of used configurations. We
present and include in the opacity code comparisons new HULLAC-v9 calculations
[5, 6] that include full CI. To illustrate the importance of this effect we
compare different CI approximations (modes) available in HULLAC-v9 [7]. These
results are compared to previous predictions and to experimental data.
Differences with OP results are discussed.Comment: 4 pages, 3 figures, conference Inertial Fusion Sciences and
Applications, Bordeaux, 12th to 16th September 2011; EPJ web of Conferences
201
Seismic and dynamical solar models i-the impact of the solar rotation history on neutrinos and seismic indicators
Solar activity and helioseismology show the limitation of the standard solar
model and call for the inclusion of dynamical processes in both convective and
radiative zones. We concentrate here on the radiative zone and first show the
sensitivity of boron neutrinos to the microscopic physics included in solar
models. We confront the neutrino predictions of the seismic model to all the
detected neutrino fluxes. Then we compute new models of the Sun including a
detailed transport of angular momentum and chemicals due to internal rotation
that includes meridional circulation and shear induced turbulence. We use two
stellar evolution codes: CESAM and STAREVOL to estimate the different terms. We
follow three temporal evolutions of the internal rotation differing by their
initial conditions: very slow, moderate and fast rotation, with magnetic
braking at the arrival on the main sequence for the last two. We find that the
meridional velocity in the present solar radiative zone is extremely small in
comparison with those of the convective zone, smaller than 10^-6 cm/s instead
of m/s. All models lead to a radial differential rotation profile but with a
significantly different contrast. We compare these profiles to the presumed
solar internal rotation and show that if meridional circulation and shear
turbulence were the only mechanisms transporting angular momentum within the
Sun, a rather slow rotation in the young Sun is favored. The transport by
rotation slightly influence the sound speed profile but its potential impact on
the chemicals in the transition region between radiation and convective zones.
This work pushes us to pursue the inclusion of the other dynamical processes to
better reproduce the present observable and to describe the young active Sun.
We also need to get a better knowledge of solar gravity mode splittings to use
their constraints.Comment: 39 pages, 9 figures, accepted in Astrophysical Journa
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