191 research outputs found
The Structure and Evolution of Stars: Introductory Remarks
In this introductory chapter of the Special Issue entitled âThe Structure and Evolution of Starsâ, we highlight the recent major progress made in our understanding of the physics that governs stellar interiors. In so doing, we combine insight from observations, 1D evolutionary modelling and 2D + 3D rotating (magneto)hydrodynamical simulations. Therefore, a complete and compelling picture of the necessary ingredients in state-of-the-art stellar structure theory and areas in which improvementsstillneedtobemadearecontextualised. Additionally, the over-arching perspective linking all the themes of subsequent chapters is presented
The Mass-Loss Induced Eccentric Kozai Mechanism: A New Channel for the Production of Close Compact Object-Stellar Binaries
Over a broad range of initial inclinations and eccentricities an appreciable
fraction of hierarchical triple star systems with similar masses are
essentially unaffected by the Kozai-Lidov mechanism (KM) until the primary in
the central binary evolves into a compact object. Once it does, it may be much
less massive than the other components in the ternary, enabling the "eccentric
Kozai mechanism (EKM):" the mutual inclination between the inner and outer
binary can flip signs driving the inner binary to very high eccentricity,
leading to a close binary or collision. We demonstrate this "Mass-loss Induced
Eccentric Kozai" (MIEK) mechanism by considering an example system and defining
an ad-hoc minimal separation between the inner two members at which tidal
affects become important. For fixed initial masses and semi-major axes, but
uniform distributions of eccentricity and cosine of the mutual inclination,
~10% of systems interact tidally or collide while the primary is on the MS due
to the KM or EKM. Those affected by the EKM are not captured by earlier
quadrupole-order secular calculations. We show that fully ~30% of systems
interact tidally or collide for the first time as the primary swells to AU
scales, mostly as a result of the KM. Finally, ~2% of systems interact tidally
or collide for the first time after the primary sheds most of its mass and
becomes a WD, mostly as a result of the MIEK mechanism. These findings motivate
a more detailed study of mass-loss in triple systems and the formation of close
NS/WD-MS and NS/WD-NS/WD binaries without an initial common envelope phase.Comment: 12 pages, 6 figures, 1 table. Accepted for publication in ApJ. For a
brief video explaining this paper, see http://youtu.be/4CdTOF17q5
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
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