14,388 research outputs found
The spectroscopic Hertzsprung-Russell diagram
The Hertzsprung-Russell diagram is an essential diagnostic diagram for
stellar structure and evolution, which has now been in use for more than 100
years. Our spectroscopic Hertzsprung-Russell (sHR) diagram shows the inverse of
the flux-mean gravity versus the effective temperature. Observed stars whose
spectra have been quantitatively analyzed can be entered in this diagram
without the knowledge of the stellar distance or absolute brightness. Observed
stars can be as conveniently compared to stellar evolution calculations in the
sHR diagram as in the Hertzsprung-Russell diagram. However, at the same time,
our ordinate is proportional to the stellar mass-to-luminosity ratio, which can
thus be directly determined. For intermediate- and low-mass star evolution at
constant mass, we show that the shape of an evolutionary track in the sHR
diagram is identical to that in the Hertzsprung-Russell diagram. We also
demonstrate that for hot stars, their stellar Eddington factor can be directly
read off the sHR diagram. For stars near their Eddington limit, we argue that a
version of the sHR diagram may be useful where the gravity is exchanged by the
effective gravity. We discuss the advantages and limitations of the sHR
diagram, and show that it can be fruitfully applied to Galactic stars, but also
to stars with known distance, e.g., in the LMC or in galaxies beyond the Local
Group.Comment: 9 pages, 8 figures, Astronomy and Astrophysics, in pres
The Evolution of Helium Star Plus Carbon-Oxygen White Dwarf Binary Systems and Implications for Diverse Stellar Transients and Hypervelocity Stars
Helium accretion induced explosions in CO white dwarfs (WDs) are considered
promising candidates for a number of observed types of stellar transients,
including supernovae (SNe) of Type Ia and Type Iax. However, a clear favorite
outcome has not yet emerged. We explore the conditions of helium ignition in
the white dwarf and the final fates of helium star-WD binaries as function of
their initial orbital periods and component masses. We compute 274 model binary
systems with the Binary Evolution Code (BEC), where both components are fully
resolved. Stellar and orbital evolution is computed simultaneously, including
mass and angular momentum transfer, tides, and gravitational wave emission, as
well as differential rotation and internal hydrodynamic and magnetic angular
momentum transport. We find that helium detonations are expected only in
systems with the shortest initial orbital periods, and for initially massive
white dwarfs (MWD > 1.0 MSun ) and lower mass donors (Mdonor < 0.8 MSun), with
accumulated helium layers mostly exceeding 0.1 MSun. Upon detonation, these
systems would release the donor as a hypervelocity pre-WD runaway star, for
which we predict the expected range of kinematic and stellar properties.
Systems with more massive donors or initial periods exceeding 1.5 h will likely
undergo helium deflagrations after accumulating 0.1 - 0.001 MSun of helium.
Helium ignition in the white dwarf is avoided in systems with helium donor
stars below - 0.6 MSun, and lead to three distinctly different groups of double
white dwarf systems. The size of the parameter space open to helium detonation
corresponds to only about 3 % of the galactic SN Ia rate, and to 10 % of the SN
Iax rate, while the predicted large amounts of helium (>0.1 MSun) in
progenitors cannot easily be reconciled with observations of archetypical SN
Ia. ...Comment: Accepted for publication in A&A, 28 pages, 16 figures, 6 table
Frustrated Rotations in Nematic Monolayers
Tabe and Yokoyama found recently that the optical axis in a chiral monolayer
of a ferronematic rotates when water evaporates from the bath: the chiral
molecules act as propellers. When the axis is blocked at the lateral walls of
the trough, the accumulated rotation inside creates huge splays and bends. We
discuss the relaxation of these tensions, assuming that a single dust particle
nucleates disclination pairs. For the simplest geometry, we then predict a long
delay time followed by a non-periodic sequence of ``bursts''. These ideas are
checked by numerical simulations.Comment: 5 pages, 4 figures, submitted to Eur. Phys. J.
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Spectroscopically Peculiar Type Ia Supernovae and Implications for Progenitors
In a recent paper Li et al. (2000) reported that 36 percent of 45 Type Ia
supernovae (SNe Ia) discovered since 1997 in two volume-limited supernova
searches were spectroscopically peculiar, and they suggested that because this
peculiarity rate is higher than that reported for an earlier observational
sample by Branch et al. (1993), it is now more likely that SNe Ia are produced
by more than one kind of progenitor. In this paper I discuss and clarify the
differences between the results of Li et al. and Branch et al. and I suggest
that multiple progenitor systems are now less likely than they were before.Comment: 11 pages; accepted by PASP; several minor changes, 2 references
added, main conclusions unchange
Search for Stable Magnetohydrodynamic Equilibria in Barotropic Stars
It is now believed that magnetohydrodynamic equilibria can exist in stably
stratified stars due to the seminal works of Braithwaite & Spruit (2004) and
Braithwaite & Nordlund (2006). What is still not known is whether
magnetohydrodynamic equilibria can exist in a barotropic star, in which stable
stratification is not present. It has been conjectured by Reisenegger (2009)
that there will likely not exist any magnetohydrodynamical equilibria in
barotropic stars. We aim to test this claim by presenting preliminary MHD
simulations of barotropic stars using the three dimensional stagger code of
Nordlund & Galsgaard (1995).Comment: 4 pages, 2 figures, to appear in the proceedings of IAUS 302:
"Magnetic Fields Throughout Stellar Evolution
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