83 research outputs found
Effect of episodic accretion on the structure and the lithium depletion of low-mass stars and planet-hosting stars
Following up our recent analysis devoted to the impact of non steady
accretion on the location of young low-mass stars or brown dwarfs in the
Herzsprung-Russell diagram, we perform a detailed analysis devoted to the
effect of burst accretion on the internal structure of low-mass and solar type
stars. We find that episodic accretion can produce objects with significantly
higher central temperatures than the ones of the non accreting counterparts of
same mass and age. As a consequence, lithium depletion can be severely enhanced
in these objects. This provides a natural explanation for the unexpected level
of lithium depletion observed in young objects for the inferred age of their
parent cluster. These results confirm the limited reliability of lithium
abundance as a criterion for assessing or rejecting cluster membership. They
also show that lithium is not a reliable age indicator, because its fate
strongly depends on the past accretion history of the star. Under the
assumption that giant planets primarily form in massive disks prone to
gravitational instability and thus to accretion burst episodes, the same
analysis also explains the higher Li depletion observed in planet hosting
stars. At last, we show that, depending on the burst rate and intensity,
accretion outbursts can produce solar mass stars with lower convective envelope
masses, at ages less than a few tens of Myr, than predicted by standard (non or
slowly accreting) pre-main sequence models. This result has interesting,
although speculative, implications for the recently discovered depletion of
refractory elements in the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Secondary Stars in CVs: The Theoretical Perspective
We apply the new generation of theoretical models of low-mass stars to
secondaries in CVs, focussing on systems above the period gap. The models
confirm that the spectral type should be a good indicator of the donor mass.
The orbital period-spectral type diagram can potentially constrain the
long-term mean mass transfer rate. A transfer rate that increases with
decreasing period is most easily reconciled with the observational data.Comment: 6 pages; to appear in proceedings of Warner Symposium on Cataclysmic
Variable
Significant uncertainties from calibrating overshooting with eclipsing binary systems
The precise measurement of the masses and radii of stars in eclipsing binary
systems provides a window into uncertain processes in stellar evolution,
especially mixing at convective boundaries. Recently, these data have been used
to calibrate models of convective overshooting in the cores of main sequence
stars. In this study we have used a small representative sample of eclipsing
binary stars with to test how precisely this
method can constrain the overshooting and whether the data support a universal
stellar mass--overshooting relation. We do not recover the previously reported
stellar mass dependence for the extent of overshooting and in each case we find
there is a substantial amount of uncertainty, that is, the same binary pair can
be matched by models with different amounts of overshooting. Models with a
moderate overshooting parameter (using the
scheme from Herwig et al. 1997) are consistent with all eight systems studied.
Generally, a much larger range of is suitable for individual
systems. In the case of main sequence and early post-main sequence stars, large
changes in the amount of overshooting have little effect on the radius and
effective temperature, and therefore the method is of extremely limited
utility.Comment: Accepted for publication in A&
Comparison of different nonlinear solvers for 2D time-implicit stellar hydrodynamics
Time-implicit schemes are attractive since they allow numerical time steps
that are much larger than those permitted by the Courant-Friedrich-Lewy
criterion characterizing time-explicit methods. This advantage comes, however,
with a cost: the solution of a system of nonlinear equations is required at
each time step. In this work, the nonlinear system results from the
discretization of the hydrodynamical equations with the Crank-Nicholson scheme.
We compare the cost of different methods, based on Newton-Raphson iterations,
to solve this nonlinear system, and benchmark their performances against
time-explicit schemes. Since our general scientific objective is to model
stellar interiors, we use as test cases two realistic models for the convective
envelope of a red giant and a young Sun. Focusing on 2D simulations, we show
that the best performances are obtained with the quasi-Newton method proposed
by Broyden. Another important concern is the accuracy of implicit calculations.
Based on the study of an idealized problem, namely the advection of a single
vortex by a uniform flow, we show that there are two aspects: i) the nonlinear
solver has to be accurate enough to resolve the truncation error of the
numerical discretization, and ii) the time step has be small enough to resolve
the advection of eddies. We show that with these two conditions fulfilled, our
implicit methods exhibit similar accuracy to time-explicit schemes, which have
lower values for the time step and higher computational costs. Finally, we
discuss in the conclusion the applicability of these methods to fully implicit
3D calculations.Comment: Accepted for publication in A&
The radius anomaly in the planet/brown dwarf overlapping mass regime
The recent detection of the transit of very massive substellar companions
(CoRoT-3b, Deleuil et al. 2008; CoRoT-15b, Bouchy et al. 2010; WASP-30b,
Anderson et al. 2010; Hat-P-20b, Bakos et al. 2010) provides a strong
constraint to planet and brown dwarf formation and migration mechanisms.
Whether these objects are brown dwarfs originating from the gravitational
collapse of a dense molecular cloud that, at the same time, gave birth to the
more massive stellar companion, or whether they are planets that formed through
core accretion of solids in the protoplanetary disk can not always been
determined unambiguously and the mechanisms responsible for their short orbital
distances are not yet fully understood.
In this contribution, we examine the possibility to constrain the nature of a
massive substellar object from the various observables provided by the
combination of Radial Velocity and Photometry measurements (e.g. M_p, R_p, M_s,
Age, a, e...).
In a second part, developments in the modeling of tidal evolution at high
eccentricity and inclination - as measured for HD 80 606 with e=0.9337 (Naef et
al. 2001), XO-3 with a stellar obliquity >37.3+-3.7 deg (H\'ebrard et al. 2008;
Winn et al. 2009) and several other exoplanets - are discussed along with their
implication in the understanding of the radius anomaly problem of extrasolar
giant planets.Comment: Proceedings of the conference: "Detection and dynamics of transiting
exoplanets" held at the OHP, 23-27 August 2010. 7 pages, 3 figure
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