2,129 research outputs found

### Combining observational techniques to constrain convection in evolved massive star models

Recent stellar evolution computations indicate that massive stars in the
range ~ 20 - 30 Msun are located in the blue supergiant (BSG) region of the
Hertzsprung-Russell diagram at two different stages of their life: immediately
after the main sequence (MS, group 1) and during a blueward evolution after the
red supergiant phase (group 2). From the observation of the pulsationnal
properties of a subgroup of variable BSGs (alpha Cyg variables), one can deduce
that these stars belongs to group 2. It is however difficult to simultaneously
fit the observed surface abundances and gravity for these stars, and this
allows to constrain the physical processes of chemical species transport in
massive stars. We will show here that the surface abundances are extremely
sensitive to the physics of convection, particularly the location of the
intermediate convective shell that appears at the ignition of the hydrogen
shell burning after the MS. Our results show that the use of the Ledoux
criterion to determine the convective regions in the stellar models leads to a
better fit of the surface abundances for alpha Cyg variables than the
Schwarzschild one.Comment: 5 pages, 2 figures, to appear in IAUS 307 proceeding

### Boron depletion in 9 to 15 M(circle dot) stars with rotation

The treatment of mixing is still one of the major uncertainties in stellar evolution models. One open question is how well the prescriptions for rotational mixing describe the real effects. We tested the mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars, such as boron and nitrogen. We followed 9, 12 and 15 M(O) models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of boton for faster rotating stars and more massive stars. The mixing at the surface is more efficient, than predicted by prescriptions used in other codes and reproduces the majority of observations very well However two observed stars with strong boron depletion but, no nitrogen enhancement still can not be explained and let the question open whether additional mixing processes are acting in these massive star

### SPINSTARS at low metallicities

The main effect of axial rotation on the evolution of massive PopIII stars is
to trigger internal mixing processes which allow stars to produce significant
amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars
produce much more primary nitrogen than PopIII stars for a given initial mass
and rotation velocity. The very metal poor stars undergo strong mass loss
induced by rotation. One can distinguish two types of rotationnaly enhanced
stellar winds: 1) Rotationally mechanical winds occurs when the surface
velocity reaches the critical velocity at the equator, {\it i.e.} the velocity
at which the centrifugal acceleration is equal to the gravity; 2) Rotationally
radiatively line driven winds are a consequence of strong internal mixing which
brings large amounts of CNO elements at the surface. This enhances the opacity
and may trigger strong line driven winds. These effects are important for an
initial value of $\upsilon/\upsilon_{\rm crit}$ of 0.54 for a 60 M$_\odot$ at
$Z=10^{-8}$, {\it i.e.} for initial values of $\upsilon/\upsilon_{\rm crit}$
higher than the one ($\sim$0.4) corresponding to observations at solar $Z$.
These two effects, strong internal mixing leading to the synthesis of large
amounts of primary nitrogen and important mass losses induced by rotation,
occur for $Z$ between about 10$^{-8}$ and 0.001. For metallicities above 0.001
and for reasonable choice of the rotation velocities, internal mixing is no
longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of
First Stars III, Santa Fe, 200

### Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: Evolution to the end of core helium burning

Massive stars are key sources of radiative, kinetic and chemical feedback in the Universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes â genec, kepler and mesa â to compute non-rotating stellar models of 15, 20 and 25âMâ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The genec and kepler models hold physics assumptions used in large grids of published models. The mesa code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by genec. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool mppnp. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than 30âperâcent â smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g. helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models

### Convective envelopes in rotating OB stars

We study the effects of rotation on the outer convective zones of massive
stars. We examine the effects of rotation on the thermal gradient and on the
Solberg--Hoiland term by analytical developments and by numerical models.
Writing the criterion for convection in rotating envelopes, we show that the
effects of rotation on the thermal gradient are much larger and of opposite
sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation
favors convection in stellar envelopes at the equator and to a smaller extent
at the poles. In a rotating 20 Msun star at 94% of the critical angular
velocity, there are two convective envelopes, with the bigger one having a
thickness of 13.2% of the equatorial radius. In the non-rotating model, the
corresponding convective zone has a thickness of only 4.6% of the radius. The
occurrence of outer convection in massive stars has many consequences.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic

### Homology and Cohomology of Topological Quandles

A homology and cohomology theory for topological quandles are introduced. The
relation between these (co)homology groups and quandle (co)homology groups are
studied. The 1 - topological quandle cocycles are used to compute state sum
invariants corresponding to knot diagrams

### Populations of rotating stars II. Rapid rotators and their link to Be-type stars

Even though it is broadly accepted that single Be stars are rapidly rotating
stars surrounded by a flat rotating circumstellar disk, there is still a debate
about how fast these stars rotate and also about the mechanisms involved in the
angular-momentum and mass input in the disk. We study the properties of stars
that rotate near their critical-rotation rate and investigate the properties of
the disks formed by equatorial mass ejections. We used the most recent Geneva
stellar evolutionary tracks for rapidly rotating stars that reach the critical
limit and used a simple model for the disk structure. We obtain that for a 9
Msun star at solar metallicity, the minimum average velocity during the Main
Sequence phase to reach the critical velocity is around 330 km/s, whereas it
would be 390 km/s at the metallicity of the Small Magellanic Cloud (SMC). Red
giants or supergiants originating from very rapid rotators rotate six times
faster and show N/C ratios three times higher than those originating from
slowly rotating stars. This difference becomes stronger at lower metallicity.
It might therefore be very interesting to study the red giants in clusters that
show a large number of Be stars on the MS band. On the basis of our single-star
models, we show that the observed Be-star fraction with cluster age is
compatible with the existence of a temperature-dependent lower limit in the
velocity rate required for a star to become a Be star. The mass, extension, and
diffusion time of the disks produced when the star is losing mass at the
critical velocity, obtained from simple parametrized expressions, are not too
far from those estimated for disks around Be-type stars. At a given
metallicity, the mass and the extension of the disk increase with the initial
mass and with age on the MS phase. Denser disks are expected in low-metallicity
regions.Comment: Accepted for publication in A&A, language edite

### Entrainment and mixed layer dynamics of a surface-stress-driven stratiified fluid

Author Posting. Â© The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 765 (2015): 653-667, doi:10.1017/jfm.2015.5.We consider experimentally an initially quiescent and linearly stratified fluid with buoyancy frequency NQ in a cylinder subject to surface-stress forcing from a disc of radius R spinning at a constant angular velocity Î©. We observe the growth of the disc-adjacent turbulent mixed layer bounded by a sharp primary interface with a constant characteristic thickness lI. To a good approximation the depth of the forced mixed layer scales as hF/RâŒ(NQ/Î©)â2/3(Î©t)2/9. Generalising the previous arguments and observations of Shravat, Cenedese & Caulfield. (2012), we show that such a deepening rate is consistent with three central assumptions that allow us to develop a phenomenological energy balance model for the entrainment dynamics. First, the total kinetic energy of the deepening mixed layer EKFâhFu2F, where uF is a characteristic velocity scale of the turbulent motions within the forced layer, is essentially independent of time and the buoyancy frequency NQ. Second, the scaled entrainment parameter E=hËF/uF depends only on the local interfacial Richardson number RiI=(N2QhFlI)/(2u2F). Third, the potential energy increase (due to entrainment, mixing and homogenisation throughout the deepening mixed layer) is driven by the local energy input at the interface, and hence is proportional to the third power of the characteristic velocity uF. We establish that internal consistency between these assumptions implies that the rate of increase of the potential energy (and hence the local mass flux across the primary interface) decreases with RiI. This observation suggests, as originally argued by Phillips (1972), that the mixing in the vicinity of the primary interface leads to the spontaneous appearance of secondary partially mixed layers, and we observe experimentally such secondary layers below the primary interface.Financial support from the National Science Foundation, the Office of Naval Research and Woods Hole Oceanographic Institution
is gratefully acknowledged. The
research activity of C.P.C. is supported by EPSRC Programme Grant EP/K034529/1
entitled `Mathematical Underpinnings of Stratified Turbulence.'2015-07-2

### The effect of suitcase concealment on the insect colonization: A pilot study in Western Australia

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