216 research outputs found
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&
The solar-like CoRoT target HD 170987: spectroscopic and seismic observations
The CoRoT mission is in its third year of observation and the data from the
second long run in the galactic centre direction are being analysed. The
solar-like oscillating stars that have been observed up to now have given some
interesting results, specially concerning the amplitudes that are lower than
predicted. We present here the results from the analysis of the star HD
170987.The goal of this research work is to characterise the global parameters
of HD 170987. We look for global seismic parameters such as the mean large
separation, maximum amplitude of the modes, and surface rotation because the
signal-to-noise ratio in the observations do not allow us to measure individual
modes. We also want to retrieve the stellar parameters of the star and its
chemical composition.We have studied the chemical composition of the star using
ground-based observations performed with the NARVAL spectrograph. We have used
several methods to calculate the global parameters from the acoustic
oscillations based on CoRoT data. The light curve of the star has been
interpolated using inpainting algorithms to reduce the effect of data gaps. We
find power excess related to p modes in the range [400 - 1200]muHz with a mean
large separation of 55.2+-0.8muHz with a probability above 95% that increases
to 55.9 +-0.2muHz in a higher frequency range [500 - 1250] muHz and a rejection
level of 1%. A hint of the variation of this quantity with frequency is also
found. The rotation period of the star is estimated to be around 4.3 days with
an inclination axis of i=50 deg +20/-13. We measure a bolometric amplitude per
radial mode in a range [2.4 - 2.9] ppm around 1000 muHz. Finally, using a grid
of models, we estimate the stellar mass, M=1.43+-0.05 Msun, the radius,
R=1.96+-0.046 Rsun, and the age ~2.4 Gyr.Comment: 12 pages, 15 figures, accepted for publication in A&
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
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
Fundamental Properties of Stars using Asteroseismology from Kepler & CoRoT and Interferometry from the CHARA Array
We present results of a long-baseline interferometry campaign using the PAVO
beam combiner at the CHARA Array to measure the angular sizes of five
main-sequence stars, one subgiant and four red giant stars for which solar-like
oscillations have been detected by either Kepler or CoRoT. By combining
interferometric angular diameters, Hipparcos parallaxes, asteroseismic
densities, bolometric fluxes and high-resolution spectroscopy we derive a full
set of near model-independent fundamental properties for the sample. We first
use these properties to test asteroseismic scaling relations for the frequency
of maximum power (nu_max) and the large frequency separation (Delta_nu). We
find excellent agreement within the observational uncertainties, and
empirically show that simple estimates of asteroseismic radii for main-sequence
stars are accurate to <~4%. We furthermore find good agreement of our measured
effective temperatures with spectroscopic and photometric estimates with mean
deviations for stars between T_eff = 4600-6200 K of -22+/-32 K (with a scatter
of 97K) and -58+/-31 K (with a scatter of 93 K), respectively. Finally we
present a first comparison with evolutionary models, and find differences
between observed and theoretical properties for the metal-rich main-sequence
star HD173701. We conclude that the constraints presented in this study will
have strong potential for testing stellar model physics, in particular when
combined with detailed modelling of individual oscillation frequencies.Comment: 18 pages, 12 figures, 7 tables; accepted for publication in Ap
Enhanced lithium depletion in Sun-like stars with orbiting planets
The surface abundance of lithium on the Sun is 140 times less than
protosolar, yet the temperature at the base of the surface convective zone is
not hot enough to burn Li. A large range of Li abundances in solar type stars
of the same age, mass and metallicity is observed, but theoretically difficult
to understand. An earlier suggestion that Li is more depleted in stars with
planets was weakened by the lack of a proper comparison sample of stars without
detected planets. Here we report Li abundances for an unbiased sample of
solar-analogue stars with and without detected planets. We find that the
planet-bearing stars have less than 1 per cent of the primordial Li abundance,
while about 50 per cent of the solar analogues without detected planets have on
average 10 times more Li. The presence of planets may increase the amount of
mixing and deepen the convective zone to such an extent that the Li can be
burned.Comment: 13 pages, 2 figure
Pre-main-sequence Lithium Depletion
In this review I briefly discuss the theory of pre-main-sequence (PMS) Li
depletion in low-mass (0.075<M<1.2 Msun) stars and highlight those uncertain
parameters which lead to substantial differences in model predictions. I then
summarise observations of PMS stars in very young open clusters, clusters that
have just reached the ZAMS and briefly highlight recent developments in the
observation of Li in very low-mass PMS stars.Comment: 8 pages, invited review at "Chemical abundances and mixing in stars
in the Milky Way and its satellites", eds. L. Pasquini, S. Randich. ESO
Astrophysics Symposium (Springer-Verlag
Lithium abundances of halo dwarfs based on excitation temperature. I. LTE
The discovery of the Spite plateau in the abundances of 7Li for metal-poor
stars led to the determination of an observationally deduced primordial lithium
abundance. However, after the success of the Wilkinson Microwave Anisotropy
Probe (WMAP) in determining the baryon density, OmegaBh^2, there was a
discrepancy between observationally determined and theoretically determined
abundances in the case of 7Li. One of the most important uncertain factors in
the calculation of the stellar 7Li abundance is the effective temperature,
Teff. We use sixteen metal-poor halo dwarfs to calculate new Teff values using
the excitation energy method. With this temperature scale we then calculate new
Li abundances for this group of stars in an attempt to resolve the 7Li
discrepancy. Using high signal-to-noise (S/N ~ 100) spectra of 16 metal-poor
halo dwarfs, obtained with the UCLES spectrograph on the AAT, measurements of
equivalent widths from a set of unblended FeI lines are made. These equivalent
widths are then used to calculate new Teff values with the use of the single
line radiative transfer program WIDTH6, where we have constrained the gravity
using either theoretical isochrones or the Hipparcos parallax, rather than the
ionization balance. The lithium abundances of the stars are calculated with
these temperatures. The physical parameters are derived for the 16 programme
stars, and two standards. These include Teff, log g, [Fe/H], microturbulence
and 7Li abundances. A comparison between the temperature scale of this work and
those adopted by others has been undertaken. We find good consistency with the
temperatures derived from the Halpha line by Asplund et al. (2006), but not
with the hotter scale of Melendez & Ramirez (2004).Comment: 13 pages, 9 figure
- …