4,717 research outputs found
Radiative Ablation of Disks Around Massive Stars
Hot, massive stars (spectral types O and B) have extreme luminosities () that drive strong stellar winds through UV line-scattering.
Some massive stars also have disks, formed by either decretion from the star
(as in the rapidly rotating "Classical Be stars"), or accretion during the
star's formation. This dissertation examines the role of stellar radiation in
driving (ablating) material away from these circumstellar disks.
A key result is that the observed month to year decay of Classical Be disks
can be explained by line-driven ablation without, as previously done, appealing
to anomalously strong viscous diffusion. Moreover, the higher luminosity of O
stars leads to ablation of optically thin disks on dynamical timescales of
order a day, providing a natural explanation for the lack of observed Oe stars.
In addition to the destruction of Be disks, this dissertation also introduces a
model for their formation by coupling observationally inferred non-radial
pulsation modes and rapid stellar rotation to launch material into orbiting
Keplerian disks.
In contrast to such Be decretion disks, star-forming accretion disks are much
denser and so are generally optically thick to continuum processes. To
circumvent the computational challenges associated with radiation hydrodynamics
through optically thick media, we develop an approximate method for treating
continuum absorption in the limit of geometrically thin disks. The comparison
of ablation with and without continuum absorption shows that accounting for
disk optical thickness leads to less than a 50 reduction in ablation rate,
implying that ablation rate depends mainly on stellar properties like
luminosity.Comment: PhD dissertation, Univ Delaware, Fall 201
3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars
Context: State of the art quantitative spectroscopy of OB-stars compares
synthetic spectra (calculated by means of 1D, spherically symmetric computer
codes) with observations. Certain stellar atmospheres, however, show strong
deviations from spherical symmetry, and need to be treated in 3D. Aims: We
present a newly developed 3D radiative transfer code, tailored to the solution
of the radiation field in rapidly expanding stellar atmospheres. We apply our
code to the continuum transfer in wind-ablation models, and to the UV resonance
line formation in magnetic winds. Methods: We have used a 3D finite-volume
method for the solution of the equation of radiative transfer, to study
continuum- and line-scattering problems. Convergence has been accelerated by a
non-local approximate Lambda-iteration scheme. Particular emphasis has been put
on careful (spherically symmetric) test cases. Results: Typical errors of the
source functions, when compared to 1D solutions, are of the order of 10-20 %,
and increase for optically thick continua. In circumstellar discs, the
radiation temperatures in the (optically thin) transition region from wind to
disc are quite similar to corresponding values in the wind. For MHD simulations
of dynamical magnetospheres, the line profiles, calculated with our 3D code,
agree well with previous solutions using a 3D-SEI method. When compared with
profiles resulting from the `analytic dynamical magnetosphere' (ADM) model,
significant differences become apparent. Conclusions: Due to similar radiation
temperatures in the wind and the transition region to the disc, the same
line-strength distribution can be applied within radiation hydrodynamic
calculations for circumstellar discs in `accreting high-mass stars'. To
properly describe the UV line formation in dynamical magnetospheres, the ADM
model needs to be further developed, at least in a large part of the outer
wind
Investigating the origin of cyclical wind variability in hot, massive stars - II. Hydrodynamical simulations of co-rotating interaction regions using realistic spot parameters for the O giant Persei
OB stars exhibit various types of spectral variability historically
associated with wind structures, including the apparently ubiquitous discrete
absorption components (DACs). These features have been proposed to be caused
either by magnetic fields or non-radial pulsations. In this second paper of
this series, we revisit the canonical phenomenological hydrodynamical modelling
used to explain the formation of DACs by taking into account modern
observations and more realistic theoretical predictions. Using constraints on
putative bright spots located on the surface of the O giant Persei
derived from high precision space-based broadband optical photometry obtained
with the Microvariability and Oscillations of STars (MOST) space telescope, we
generate two-dimensional hydrodynamical simulations of co-rotating interaction
regions in its wind. We then compute synthetic ultraviolet (UV) resonance line
profiles using Sobolev Exact Integration and compare them with historical
timeseries obtained by the International Ultraviolet Explorer (IUE) to evaluate
if the observed behaviour of Persei's DACs is reproduced. Testing three
different models of spot size and strength, we find that the classical pattern
of variability can be successfully reproduced for two of them: the model with
the smallest spots yields absorption features that are incompatible with
observations. Furthermore, we test the effect of the radial dependence of
ionization levels on line driving, but cannot conclusively assess the
importance of this factor. In conclusion, this study self-consistently links
optical photometry and UV spectroscopy, paving the way to a better
understanding of cyclical wind variability in massive stars in the context of
the bright spot paradigm.Comment: 16 pages, 10 figures, accepted for publication by MNRA
Theoretical wind clumping predictions of OB supergiants from line-driven instability simulations across the bi-stability jump
(Abridged) The behaviour of mass loss across bi-stability jump is a key
uncertainty in models of massive stars. While an increase in mass loss is
theoretically predicted, this has so far not been observationally confirmed.
However, radiation-driven winds of massive stars are known to exhibit clumpy
structures triggered by the line-deshadowing instability (LDI). Wind clumping
affects empirical mass-loss rates inferred from density square-dependent
spectral diagnostics. If clumping properties differ significantly for O and B
supergiants across the bi-stability jump, this may help alleviate discrepancies
between theory and observations. We investigate with analytical and numerical
tools how the onset of clumpy structures behaves in the winds of O supergiants
(OSG) and B supergiants (BSG) across the bi-stability jump. We derive a scaling
relation for the linear growth rate of the LDI for a single optically thick
line and apply it in both regimes. We run 1D time-dependent line-driven
instability simulations to study the non-linear evolution of the LDI in clumpy
OSG and BSG winds. Linear perturbation analysis for a single line shows that
the LDI linear growth rate scales strongly with stellar effective temperature
and terminal wind speed. This implies significantly lower growth rates for
(cooler, slower) BSG winds than for OSG winds. This is confirmed by the
non-linear simulations, which show significant differences in OSG and BSG wind
structure formation, with the latter characterized by significantly weaker
clumping factors and lower velocity dispersions. This suggests that lower
correction factors due to clumping should be employed when deriving empirical
mass-loss rates for BSGs on the cool side of the bi-stability jump. Moreover,
the non-linear simulations provide a theoretical background toward explaining
the general lack of observed intrinsic X-ray emission in (single) B star winds.Comment: 10 pages, 5 figures, accepted for publication in A&
Identifying spin-triplet pairing in spin-orbit coupled multi-band superconductors
We investigate the combined effect of Hund's and spin-orbit (SO) coupling on
superconductivity in multi-orbital systems. Hund's interaction leads to
orbital-singlet spin-triplet superconductivity, where the Cooper pair wave
function is antisymmetric under the exchange of two orbitals. We identify three
d-vectors describing even-parity orbital-singlet spin-triplet pairings among
t2g-orbitals, and find that the three d-vectors are mutually orthogonal to each
other. SO coupling further assists pair formation, pins the orientation of the
d-vector triad, and induces spin-singlet pairings with a relative phase
difference of \pi/2. In the band basis the pseudospin d-vectors are aligned
along the z-axis and correspond to momentum-dependent inter- and intra-band
pairings. We discuss quasiparticle dispersion, magnetic response, collective
modes, and experimental consequences in light of the superconductor Sr2RuO4.Comment: 6 pages, 5 figure
Analytic, dust-independent mass-loss rates for red supergiant winds initiated by turbulent pressure
Context. Red supergiants are observed to undergo vigorous mass-loss. However,
to date, no theoretical model has succeeded in explaining the origins of these
objects' winds. This strongly limits our understanding of red supergiant
evolution and Type II-P and II-L supernova progenitor properties.
Aims. We examine the role that vigorous atmospheric turbulence may play in
initiating and determining the mass-loss rates of red supergiant stars.
Methods. We analytically and numerically solve the equations of conservation
of mass and momentum, which we later couple to an atmospheric temperature
structure, to obtain theoretically motivated mass-loss rates. We then compare
these to state-of-the-art empirical mass-loss rate scaling formulae as well as
observationally inferred mass-loss rates of red supergiants.
Results. We find that the pressure due to the characteristic turbulent
velocities inferred for red supergiants is sufficient to explain the mass-loss
rates of these objects in the absence of the normally employed opacity from
circumstellar dust. Motivated by this initial success, we provide a first
theoretical and fully analytic mass-loss rate prescription for red supergiants.
We conclude by highlighting some intriguing possible implications of these
rates for future studies of stellar evolution, especially in light of the lack
of a direct dependence on metallicity.Comment: 14 pages, 9 figures, 2 table
Half-quantum vortex and d-soliton in SrRuO
Assuming that the superconductivity in SrRuO is described by a planar
p-wave order parameter, we consider possible topological defects in
SrRuO. In particular, it is shown that both of the -soliton
and half-quantum vortex can be created in the presence of the magnetic field
parallel to the - plane. We discuss how one can detect the -soliton and half-quantum vortex experimentally.Comment: 8 pages, 3 figure
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