1,275 research outputs found
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
Presupernova Evolution of Rotating Massive Stars and the Rotation Rate of Pulsars
Rotation in massive stars has been studied on the main sequence and during
helium burning for decades, but only recently have realistic numerical
simulations followed the transport of angular momentum that occurs during more
advanced stages of evolution. The results affect such interesting issues as
whether rotation is important to the explosion mechanism, whether supernovae
are strong sources of gravitational radiation, the star's nucleosynthesis, and
the initial rotation rate of neutron stars and black holes. We find that when
only hydrodynamic instabilities (shear, Eddington-Sweet, etc.) are included in
the calculation, one obtains neutron stars spinning at close to critical
rotation at their surface -- or even formally in excess of critical. When
recent estimates of magnetic torques (Spruit 2002) are added, however, the
evolved cores spin about an order of magnitude slower. This is still more
angular momentum than observed in young pulsars, but too slow for the collapsar
model for gamma-ray bursts.Comment: 10 pages, 2 figures, to appear in Proc. IAU 215 "Stellar Rotation
Mixing of CNO-cycled matter in massive stars
Aims: We test predictions of evolution models on mixing of CNO-cycled
products in massive stars from a fundamental perspective. Relative changes
within the theoretical C:N:O abundance ratios and the buildup of helium are
compared with observational results. Methods: A sample of well-studied Galactic
massive stars is presented. High-quality optical spectra are carefully analysed
using improved NLTE line-formation and comprehensive analysis strategies. The
results are put in the context of the existing literature data. Results: A
tight trend in the observed N/C vs. N/O ratios and the buildup of helium is
found from the self-consistent analysis of main-sequence to supergiant stars
for the first time. The catalytic nature of the CNO-cycles is confirmed
quantitatively, though further investigations are required to derive a fully
consistent picture. Our observational results support the case of strong
mixing, as predicted e.g. by evolution models that consider magnetic fields or
by models that have gone through the first dredge-up in the case of many
supergiants.Comment: 6 pages, 6 figures. A&A, in pres
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Evaluation Study of Australian Telehealth Projects
This paper appeared at the 8th Australasian Workshop on Health Informatics and Knowledge Management (HIKM 2015), Sydney, Australia, January 2015. Conferences in Research and Practice in Information Technology (CRPIT), Vol. 164, Anthony Maeder and Jim Warren, Ed. Reproduction for academic, not-for profit purposes permitted provided this text is included.This paper provides an overview of selected telehealth projects involving pilot or trial implementations, undertaken in Australia in the period since 2000, which have undergone substantial formal evaluations reported in the peer reviewed scientific literature. Barriers and enablers reported for these telehealth projects are identified and the evaluation aspects are presented using a recently proposed generalised evaluation framework
Single star progenitors of long gamma-ray bursts I: Model grids and redshift dependent GRB rate
We present grids of massive star evolution models at four different
metallicities (Z=0.004, 0.002, 0.001, 0.00001). The effects of rotation on the
stellar structure and the transport of angular momentum and chemical elements
through the Spruit-Tayler dynamo and rotationally induced instabilities are
considered. After discussing uncertainties involved with the adopted physics,
we elaborate the final fate of massive stars as a function of initial mass and
spin rate, at each considered metallicity. In particular, we investigate for
which initial conditions long gamma-ray bursts (GRBs) are expected to be
produced in the frame of the collapsar model. Then, using an empirical spin
distribution of young massive metal-poor stars and a specified
metallicity-dependent history of star-formation, we compute the expected GRB
rate as function of metallicity and redshift based on our stellar evolution
models. The GRB production in our models is limited to metallicities of Z \lsim
0.004, with the consequence that about 50 % of all GRBs are predicted to be
found at redshifts above z = 4, with most supernovae occurring at redshifts
below z\simeq 2.2. The average GRB/SN ratio predicted by our model is about
1/200 globally, and 1/1250 at low redshift. Future strategies for testing the
considered GRB progenitor scenario are briefly discussed.Comment: 17 pages, 10 figures, 6 tables, accpeted by A&A, corrected, reference
adde
Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, with the MHD equation of state
We present stellar evolutionary models covering the mass range from 0.4 to 1
Msun calculated for metallicities Z=0.020 and 0.001 with the MHD equation of
state (Hummer & Mihalas, 1988; Mihalas et al. 1988; D\"appen et al. 1988). A
parallel calculation using the OPAL (Rogers et al. 1996) equation of state has
been made to demonstrate the adequacy of the MHD equation of state in the range
of 1.0 to 0.8 Msun (the lower end of the OPAL tables). Below, down to 0.4 Msun,
we have justified the use of the MHD equation of state by theoretical arguments
and the findings of Chabrier & Baraffe (1997).
We use the radiative opacities by Iglesias & Rogers (1996), completed with
the atomic and molecular opacities by Alexander & Fergusson (1994). We follow
the evolution from the Hayashi fully convective configuration up to the red
giant tip for the most massive stars, and up to an age of 20 Gyr for the less
massive ones. We compare our solar-metallicity models with recent models
computed by other groups and with observations.
The present stellar models complete the set of grids computed with the same
up-to-date input physics by the Geneva group [Z=0.020 and 0.001, Schaller et
al. (1992), Bernasconi (1996), and Charbonnel et al. (1996); Z=0.008, Schaerer
et al. (1992); Z=0.004, Charbonnel et al. (1993); Z=0.040, Schaerer et al.
(1993); Z=0.10, Mowlavi et al. (1998); enhanced mass loss rate evolutionary
tracks, Meynet et al. (1994)].Comment: Accepted for publication in A&A Supplement Serie
Millimeter-Wave Aperture Synthesis Imaging of Vega: Evidence for a Ring Arc at 95 AU
We present the first millimeter-wave aperture synthesis map of dust around a
main sequence star. A 3'' resolution image of 1.3 mm continuum emission from
Vega reveals a clump of emission 12'' from the star at PA 45 deg, consistent
with the location of maximum 850 micron emission in a lower resolution
JCMT/SCUBA map. The flux density is 4.0+/-0.9 mJy. Adjacent 1.3 mm peaks with
flux densities 3.4+/-1.0 mJy and 2.8+/-0.9 mJy are located 14'' and 13'' from
the star at PA 67 deg and 18 deg, respectively. An arc-like bridge connects the
two strongest peaks. There is an additional 2.4 +/-0.8 mJy peak to the SW 11''
from the star at PA 215 deg and a marginal detection, 1.4+/-0.5 mJy, at the
stellar position, consistent with photospheric emission. An extrapolation from
the 850 micron flux, assuming F_{1.3mm-0.85mm} proportional to lambda^{-2.8},
agrees well with the total detected flux for Vega at 1.3 mm, and implies a dust
emissivity index, beta, of 0.8. We conclude that we have detected all but a
very small fraction of the dust imaged by SCUBA in our aperture synthesis map
and that these grains are largely confined to segments of a ring of radius 95
AU.Comment: 4 pages, 1 figure, 1 table, accepted for publication in Astrophysical
Journal Letter
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