52 research outputs found
Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars
Magnetic fields are at the heart of the observed stellar activity in
late-type stars, and they are presumably generated by a dynamo mechanism at the
interface layer between the radiative and the convective stellar regions. Since
dynamo models are based on the interaction between differential rotation and
convective motions, the introduction of rotation in the ATON 2.3 stellar code
allows for explorations regarding a physically consistent treatment of magnetic
effects in stellar structure and evolution, even though there are formidable
mathematical and numerical challenges involved. As examples, we present
theoretical estimates for both the local (tau_c) and global (tau_g) convective
turnover times for rotating pre-main sequence solar-type stars, based on
up-to-date input physics for stellar models. Our theoretical predictions are
compared with the previous ones available in the literature. In addition, we
investigate the dependence of the convective turnover time on convection
regimes, the presence of rotation and atmospheric treatment. Those estimates,
this quantities can be used to calculate the Rossby number, Ro, which is
related to the magnetic activity strength in dynamo theories and, at least for
main-sequence stars, shows an observational correlation with stellar activity.
More important, they can also contribute for testing stellar models against
observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree
with those published by Kim & Demarque (1996). By increasing the convection
efficiency, tau_g decreases for a given mass. FST models show still lower
values. The presence of rotation shifts tau_g towards slightly higher values
when compared with non-rotating models. The use of non-gray boundary conditions
in the models yields values of tau_g smaller than in the gray approximation.Comment: 10 pages, 14 figures, accepted for publication in A&
Towards a unified model of stellar rotation II: Model-dependent characteristics of stellar populations
Rotation has a number of important effects on the evolution of stars. Apart
from structural changes because of the centrifugal force, turbulent mixing and
meridional circulation caused by rotation can dramatically affect a star's
chemical evolution. This leads to changes in the surface temperature and
luminosity as well as modifying its lifetime. Observationally rotation
decreases the surface gravity, causes enhanced mass loss and leads to surface
abundance anomalies of various chemical isotopes. The replication of these
physical effects with simple stellar evolution models is very difficult and has
resulted in the use of numerous different formulations to describe the physics.
Using stellar evolution calculations based on several physical models we
discuss the features of the resulting simulated stellar populations which can
help to distinguish between the models.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars
In close binary systems, rotation and tidal forces of the component stars
deform each other and destroy their spherical symmetry. We present new models
for low-mass, pre-main sequence stars that include the combined distortion
effects of tidal and rotational forces on the equilibrium configuration of
stars. We investigate the effects of interaction between tides and rotation on
the stellar structure and evolution. The Kippenhahn & Thomas (1970)
approximation, along with the Clairaut-Legendre expansion for the gravitational
potential of a self-gravitating body, is used to take the distortion effects
into account. We obtained values of internal structure constants for low-mass,
pre-main sequence stars from stellar evolutionary models that consider the
combined effects of rotation and tidal forces due to a companion star. We also
derived a new expression for the rotational inertia of a tidally and
rotationally distorted star. Our distorted models were successfully used to
analyze the eclipsing binary system EK Cep, reproducing the stellar radii,
effective temperature ratio, lithium depletion, rotational velocities, and the
apsidal motion rate in the age interval of 15.5-16.7 Myr. In the low-mass
range, the assumption that harmonics greater than j=2 can be neglected seems
not to be fully justified, although it is widely used when analyzing the
apsidal motion of binary systems. The non-standard evolutionary tracks are
cooler than the standard ones, mainly for low-mass stars. Distorted models
predict more mass-concentrated stars at the zero-age main-sequence than
standard models
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Determination of stellar shape in microlensing event MOA 2002-BLG-33
We report a measurement of the shape of the source star in microlensing event
MOA 2002-BLG-33. The lens for this event was a close binary whose
centre-of-mass passed almost directly in front of the source star. At this
time, the source star was closely bounded on all sides by a caustic of the
lens. This allowed the oblateness of the source star to be constrained. We
found that a/b = 1.02^{+0.04}_{-0.02} where a and b are its semi-major and
semi-minor axes respectively. The angular resolution of this measurement is
approximately 0.04 microarcsec. We also report HST images of the event that
confirm a previous identification of the source star as an F8-G2 turn-off
main-sequence star.Comment: 8 pages, 5 figures, Accepted by A&
Photoperiod Influences Growth and mll (Mixed-Lineage Leukaemia) Expression in Atlantic Cod
Photoperiod is associated to phenotypic plasticity of somatic growth in several teleost species. However, the molecular mechanisms underlying this phenomenon are currently unknown but it is likely that epigenetic regulation by methyltransferases is involved. The MLL (mixed-lineage leukaemia) family comprises histone methyltransferases that play a critical role in regulating gene expression during early development in mammals. So far, these genes have received scant attention in teleost fish. In the present study, the mean weight of Atlantic cod juveniles reared under continuous illumination was found to be 13% greater than those kept under natural photoperiod conditions for 120 days. We newly determined cDNA sequences of five mll (mll1, mll2, mll3a, mll4b and mll5) and two setd1 (setd1a and setd1ba) paralogues from Atlantic cod. Phylogenetic analysis revealed that the cod genes clustered within the appropriate mll clade and comparative mapping of mll paralogues showed that these genes lie within a region of conserved synteny among teleosts. All mll and setd1 genes were highly expressed in gonads and fast muscle of adult cod, albeit at different levels, and they were differentially regulated with photoperiod in muscle of juvenile fish. Following only one day of exposure to constant light, mll1, mll4b and setd1a were up to 57% lower in these fish compared to the natural photoperiod group. In addition, mRNA expression of myogenic regulatory factors (myog and myf-5) and pax7 in fast muscle was also affected by different photoperiod conditions. Notably, myog was significantly elevated in the continuous illumination group throughout the time course of the experiment. The absence of a day/night cycle is associated with a generalised decrease in mll expression concomitant with an increase in myog transcript levels in fast muscle of Atlantic cod, which may be involved in the observed epigenetic regulation of growth by photoperiod in this species
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
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Physical exercise augmented cognitive behaviour therapy for older adults with generalised anxiety disorder (PEXACOG): a feasibility study for a randomized controlled trial.
BACKGROUND: Generalised anxiety disorder (GAD) is a frequent and severe disorder among older adults. For older adults with GAD the effect of the recommended treatment, cognitive behaviour therapy (CBT), is reduced. Physical exercise (PE) may enhance the effect of CBT by improving cognitive function and increasing levels of brain-derived neurotrophic factor (BDNF), a predictor of the effect of CBT in patients with anxiety. The aim of the study was to assess the feasibility of a randomized controlled trial (RCT) investigating treatment effect of the combination of CBT and PE for GAD in a sample of older adults, including procedures for assessment and treatment. METHODS: Four participants aged 62-70 years (M = 65.5, SD = 3.2) with a primary diagnosis of GAD were included. Participants received 15 weeks of PE in combination with 10 weeks of CBT. Participants completed self-report measures, and clinical, biological, physiological and neuropsychological tests at pre-, interim- and post-treatment. RESULTS: Procedures, protocols, and results are presented. One participant dropped out during treatment. For the three participants completing, the total adherence to PE and CBT was 80% and 100%, respectively. An independent assessor concluded that the completers no longer fulfilled the criteria for GAD after treatment. Changes in self-report measures suggest symptom reduction related to anxiety and worry. The sample is considered representative for the target population. CONCLUSIONS: The results indicate that combining CBT and PE for older adults with GAD is feasible, and that the procedures and tests are suitable and manageable for the current sample. TRIAL REGISTRATION: ClinicalTrials.gov, NCT02690441. Registered on 24 February 2016, https://clinicaltrials.gov/ct2/show/NCT02690441
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