1,056 research outputs found
Close binary evolution I. The tidally induced shear mixing in rotating binaries
We study how tides in a binary system induce some specific internal shear
mixing, able to substantially modify the evolution of close binaries prior to
mass transfer. We construct numerical models accounting for tidal interactions,
meridional circulation, transport of angular momentum, shears and horizontal
turbulence and consider a variety of orbital periods and initial rotation
velocities. Depending on orbital periods and rotation velocities, tidal effects
may spin down (spin down Case) or spin up (spin up Case) the axial rotation. In
both cases, tides may induce a large internal differential rotation. The
resulting tidally induced shear mixing (TISM) is so efficient that the internal
distributions of angular velocity and chemical elements are greatly influenced.
The evolutionary tracks are modified, and in both cases of spin down and spin
up, large amounts of nitrogen can be transported to the stellar surfaces before
any binary mass transfer. Meridional circulation, when properly treated as an
advection, always tends to counteract the tidal interaction, tending to spin up
the surface when it is braked down and vice versa. As a consequence, the times
needed for the axial angular velocity to become equal to the orbital angular
velocity may be larger than given by typical synchronization timescales. Also,
due to meridional circulation some differential rotation remains in tidally
locked binary systems.Comment: 10 pages, 18 figures, Accepted for publication in Astronomy and
Astrophysic
Apparent Age Spreads in Clusters and the Role of Stellar Rotation
We use the Geneva Syclist isochrone models that include the effects of stellar rotation to investigate the role that rotation has on the resulting colour-magnitude diagram (CMD) of young and intermediate age clusters. We find that if a distribution of rotation velocities exists within the clusters, rotating stars will remain on the main sequence (MS) for longer, appearing to be younger than non-rotating stars within the same cluster. This results in an extended main sequence turn-off (eMSTO) that appears at young ages (~Myr) and lasts beyond 1~Gyr. If this eMSTO is interpreted as an age spread, the resulting age spread is proportional to the age of the cluster, i.e. young clusters (~Myr) appear to have small age spreads (10s of Myr) whereas older clusters (~Gyr) appear to have much larger spreads, up to a few hundred Myr. We compare the predicted spreads for a sample of rotation rates to observations of young and intermediate age clusters, and find a strong correlation between the measured 'age spread' and the age of the cluster, in good agreement with models of stellar rotation. This suggests that the 'age spreads' reported in the literature may simply be the result of a distribution of stellar rotation velocities within clusters
Apparent age spreads in clusters and the role of stellar rotation
We use the Geneva syclist isochrone models that include the effects of stellar rotation to investigate the role that rotation has on the resulting colour-magnitude diagram of young and intermediate age clusters. We find that if a distribution of rotation velocities exists within the clusters, rotating stars will remain on the main sequence for longer, appearing to be younger than non-rotating stars within the same cluster. This results in an extended main sequence turn-off (eMSTO) that appears at young ages (âŒ30Myr) and lasts beyond 1Gyr. If this eMSTO is interpreted as an age spread, the resulting age spread is proportional to the age of the cluster, i.e. young clusters (<100Myr) appear to have small age spreads (tens of Myr) whereas older clusters (âŒ1Gyr) appear to have much large spreads, up to a few hundred Myr. We compare the predicted spreads for a sample of rotation rates to observations of young and intermediate age clusters, and find a strong correlation between the measured âage spread' and the age of the cluster, in good agreement with models of stellar rotation. This suggests that the âage spreads' reported in the literature may simply be the result of a distribution of stellar rotation velocities within cluster
A nova origin of the gas cloud at the Galactic Center ?
The recent discovery by Gillessen and collaborators of a cloud of gas falling
towards the Galactic Center on a highly eccentric orbit, diving nearly straight
into the immediate neighborhood of the central supermassive black hole, raises
the important question of its origin. Several models have already been
proposed. Here we suggest that a recent nova outburst has ejected a ring-like
shell of gas. Viewed at high inclination, that could account for the mass, head
and tail structure, and the unusually high eccentricity of the observed cloud
in a natural way, even as the nova moves on an orbit quite normal for the young
stars in the close neighborhood of the Galactic Center. We illustrate this by
calculating orbits for the head and tail parts of the ejecta and the nova that
has produced it. We briefly discuss some of the questions that this model, if
true, raises about the stellar environment close to the Galactic Center.Comment: 4 pages, 1 figure, Journal reference, correction of a minor erro
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
A study of the effect of rotational mixing on massive stars evolution: Surface abundances of Galactic O7-8 giant stars
Context. Massive star evolution remains only partly constrained. In particular, the exact role of rotation has been questioned by puzzling properties of OB stars in the Magellanic Clouds. Aims. Our goal is to study the relation between surface chemical composition and rotational velocity, and to test predictions of evolutionary models including rotation. Methods. We have performed a spectroscopic analysis of a sample of fifteen Galactic O7-8 giant stars. This sample is homogeneous in terms of mass, metallicity and evolutionary state. It is made of stars with a wide range of projected rotational velocities. Results. We show that the sample stars are located on the second half of the main sequence, in a relatively narrow mass range (25-40 Mâ ). Almost all stars with projected rotational velocities above 100 km s -1 have N/C ratios about ten times the initial value. Below 100 km s -1 a wide range of N/C values is observed. The relation between N/C and surface gravity is well reproduced by various sets of models. Some evolutionary models including rotation are also able to consistently explain slowly rotating, highly enriched stars. This is due to differential rotation which efficiently transports nucleosynthesis products and allows the surface to rotate slower than the core. In addition, angular momentum removal by winds amplifies surface braking on the main sequence. Comparison of the surface composition of O7-8 giant stars with a sample of B stars with initial masses about four times smaller reveal that chemical enrichment scales with initial mass, as expected from theory. Conclusions. Although evolutionary models that include rotation face difficulties in explaining the chemical properties of O- and B-type stars at low metallicity, some of them can consistently account for the properties of main-sequence Galactic O stars in the mass range 25-40 M â.Instituto de AstrofĂsica de La Plat
Stellar Evolution in the Early Universe
Massive stars played a key role in the early evolution of the Universe. They
formed with the first halos and started the re-ionisation. It is therefore very
important to understand their evolution. In this paper, we describe the strong
impact of rotation induced mixing and mass loss at very low . The strong
mixing leads to a significant production of primary nitrogen 14, carbon 13 and
neon 22. Mass loss during the red supergiant stage allows the production of
Wolf-Rayet stars, type Ib,c supernovae and possibly gamma-ray bursts (GRBs)
down to almost Z=0 for stars more massive than 60 solar masses. Galactic
chemical evolution models calculated with models of rotating stars better
reproduce the early evolution of N/O, C/O and C12/C13. We calculated the weak
s-process production induced by the primary neon 22 and obtain overproduction
factors (relative to the initial composition, Z=1.e-6) between 100-1000 in the
mass range 60-90.Comment: 8 pages, 4 figures, proceedings of IAU Symposium 255,
"Low-Metallicity Star Formation: From the First stars to Dwarf Galaxies",
L.K. Hunt, S. Madden & R. Schneider, ed
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