17,121 research outputs found
A 3D short-characteristics method for continuum and line scattering problems in the winds of hot stars
Context: Knowledge about hot, massive stars is usually inferred from
quantitative spectroscopy. To analyse non-spherical phenomena, the existing 1D
codes must be extended to higher dimensions, and corresponding tools need to be
developed. Aims: We present a 3D radiative transfer code that is capable of
calculating continuum and line scattering problems in the winds of hot stars.
By considering spherically symmetric test models, we discuss potential error
sources, and indicate advantages and disadvantages by comparing different
solution methods. Further, we analyse the UV resonance line formation in the
winds of rapidly rotating O stars. Methods: We consider both a (simplified)
continuum model including scattering and thermal sources, and a UV resonance
line transition approximated by a two-level-atom. We applied the
short-characteristics (SC) method, using linear or monotonic B\'ezier
interpolations, to solve the equation of radiative transfer on a non-uniform
Cartesian grid. To calculate scattering dominated problems, our solution method
is supplemented by an accelerated -iteration scheme. Results: For the
spherical test models, the mean relative error of the source function is on the
level, depending on the applied interpolation technique and the
complexity of the considered model. All calculated line profiles are in
excellent agreement with corresponding 1D solutions. The predicted line
profiles from fast rotating stars show a distinct behaviour as a function of
rotational speed and inclination. This behaviour is tightly coupled to the wind
structure and the description of gravity darkening and stellar surface
distortion. Conclusions: Our SC methods are ready to be used for quantitative
analyses of UV resonance line profiles. When calculating optically thick
continua, both SC methods give reliable results, in contrast to the alternative
finite-volume method
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
Dynamo Processes in the T Tauri star V410 Tau
We present new brightness and magnetic images of the weak-line T Tauri star
V410 Tau, made using data from the NARVAL spectropolarimeter at Telescope
Bernard Lyot (TBL). The brightness image shows a large polar spot and
significant spot coverage at lower latitudes. The magnetic maps show a field
that is predominantly dipolar and non-axisymmetric with a strong azimuthal
component. The field is 50% poloidal and 50% toroidal, and there is very little
differential rotation apparent from the magnetic images.
A photometric monitoring campaign on this star has previously revealed V-band
variability of up to 0.6 magnitudes but in 2009 the lightcurve is much flatter.
The Doppler image presented here is consistent with this low variability.
Calculating the flux predicted by the mapped spot distribution gives an
peak-to-peak variability of 0.04 magnitudes. The reduction in the amplitude of
the lightcurve, compared with previous observations, appears to be related to a
change in the distribution of the spots, rather than the number or area.
This paper is the first from a Zeeman-Doppler imaging campaign being carried
out on V410 Tau between 2009-2012 at TBL. During this time it is expected that
the lightcurve will return to a high amplitude state, allowing us to ascertain
whether the photometric changes are accompanied by a change in the magnetic
field topology.Comment: 12 pages, 11 figures, accepted by MNRA
Radiative cooling implementations in simulations of primordial star formation
We study the thermal evolution of primordial star-forming gas clouds using
three-dimensional cosmological simulations. We critically examine how
assumptions and approximations made in calculating radiative cooling rates
affect the dynamics of the collapsing gas clouds. We consider two important
molecular hydrogen cooling processes that operate in a dense primordial gas;
H_2 line cooling and continuum cooling by H_2 collision-induced emission. To
calculate the optically thick cooling rates, we follow the Sobolev method for
the former, whereas we perform ray-tracing for the latter. We also run the same
set of simulations using simplified fitting functions for the net cooling
rates. We compare the simulation results in detail. We show that the time- and
direction-dependence of hydrodynamic quantities such as gas temperature and
local velocity gradients significantly affects the optically thick cooling
rates. Gravitational collapse of the cloud core is accelerated when the cooling
rates are calculated by using the fitting functions. The structure and
evolution of the central pre-stellar disk are also affected. We conclude that
physically motivated implementations of radiative transfer are necessary to
follow accurately the thermal and chemical evolution of a primordial gas to
high densities.Comment: 25 pages, 12 figures, To appear in Ap
A new method for the spectroscopic identification of stellar non-radial pulsation modes. I. The method and numerical tests
We present the Fourier parameter fit method, a new method for
spectroscopically identifying stellar radial and non-radial pulsation modes
based on the high-resolution time-series spectroscopy of absorption-line
profiles. In contrast to previous methods this one permits a quantification of
the statistical significance of the computed solutions. The application of
genetic algorithms in seeking solutions makes it possible to search through a
large parameter space. The mode identification is carried out by minimizing
chi-square, using the observed amplitude and phase across the line profile and
their modeled counterparts. Computations of the theoretical line profiles are
based on a stellar displacement field, which is described as superposition of
spherical harmonics and that includes the first order effects of the Coriolis
force. We made numerical tests of the method on a grid of different mono- and
multi-mode models for 0 <= l <= 4 in order to explore its capabilities and
limitations. Our results show that whereas the azimuthal order m can be
unambiguously identified for low-order modes, the error of l is in the range of
pm 1. The value of m can be determined with higher precision than with other
spectroscopic mode identification methods. Improved values for the inclination
can be obtained from the analysis of non-axisymmetric pulsation modes. The new
method is ideally suited to intermediatley rotating Delta Scuti and Beta Cephei
stars.Comment: 12 pages, 14 figure
The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*
The compact, nonthermal radio source Sgr A* at the Galactic Center appears to
be coincident with a 2.6 million solar mass point-like object. Its energy
source may be the release of gravitational energy as gas from the interstellar
medium descends into its potential well. Simple attempts at calculating the
spectrum and flux based on this picture have come close to the observations,
yet have had difficulty in accounting for the low efficiency in this source.
There now appear to be two reasons for this low conversion rate: (1) the plasma
separates into two temperatures, with the protons attaining a significantly
higher temperature than that of the radiating electrons, and (2) the magnetic
field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung
emissivity, and therefore the overall power of Sgr A*. We investigate the
latter with improvement over what has been attempted before: rather than
calculating B based on a presumed model, we instead infer its distribution with
radius empirically with the requirement that the resulting spectrum matches the
observations. Our ansatz for B(r) is motivated in part by earlier calculations
of the expected magnetic dissipation rate due to reconnection in a compressed
flow. We find reasonable agreement with the observed spectrum of Sgr A* as long
as its distribution consists of 3 primary components: an outer equipartition
field, a roughly constant field at intermediate radii (~1000 Schwarzschild
radii), and an inner dynamo (more or less within the last stable orbit for a
non-rotating black hole) which increases B to about 100 Gauss. The latter
component accounts for the observed sub-millimiter hump in this source.Comment: 33 pages including 2 figures; submitted to Ap
Light Curves for Rapidly-Rotating Neutron Stars
We present raytracing computations for light emitted from the surface of a
rapidly-rotating neutron star in order to construct light curves for X-ray
pulsars and bursters. These calculations are for realistic models of
rapidly-rotating neutron stars which take into account both the correct
exterior metric and the oblate shape of the star. We find that the most
important effect arising from rotation comes from the oblate shape of the
rotating star. We find that approximating a rotating neutron star as a sphere
introduces serious errors in fitted values of the star's radius and mass if the
rotation rate is very large. However, in most cases acceptable fits to the
ratio M/R can be obtained with the spherical approximation.Comment: Accepted by the Astrophysical Journal. 13 pages & 7 figure
How to break the density-anisotropy degeneracy in spherical stellar systems
We present a new non-parametric Jeans code, GravSphere, that recovers the
density and velocity anisotropy of spherical stellar
systems, assuming only that they are in a steady-state. Using a large suite of
mock data, we confirm that with only line-of-sight velocity data, GravSphere
provides a good estimate of the density at the projected stellar half mass
radius, , but is not able to measure or ,
even with 10,000 tracer stars. We then test three popular methods for breaking
this degeneracy: using multiple populations with different
; using higher order `Virial Shape Parameters' (VSPs); and including
proper motion data.
We find that two populations provide an excellent recovery of
in-between their respective . However, even with a total of tracers, we are not able to well-constrain for either
population. By contrast, using 1000 tracers with higher order VSPs we are able
to measure over the range and broadly constrain
. Including proper motion data for all stars gives an even better
performance, with and well-measured over the range .
Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios
typical of a merger remnant, . In this case, GravSphere can become
slightly biased. However, we find that when this occurs the data are poorly
fit, allowing us to detect when such departures from spherical symmetry become
problematic.Comment: 19 pages; 1 table; 11 Figures. Version accepted for publication in
MNRAS. (Minor changes from previously. Appendix B added showing decreasing
bias of VSP estimators with increasing sampling.
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