270 research outputs found
Visual and ultraviolet flux variability of the bright CP star Aur
Chemically peculiar stars of the upper part of the main sequence show
periodical variability in line intensities and continua, modulated by the
stellar rotation, which is attributed to the existence of chemical spots on the
surface of these stars. The flux variability is caused by the changing
redistribution rate of the radiative flux predominantly from the
short-wavelength part of the spectra to the long-wavelength part, which is a
result of abundance anomalies. We study the nature of the multi-spectral
variability of one of the brightest chemically peculiar stars, Aur. We
predict the flux variability of Aur from the emerging intensities
calculated for individual surface elements of the star taking into account
horizontal variation of chemical composition derived from Doppler abundance
maps. The simulated optical variability in the Str\"omgren photometric system
and the ultraviolet flux variability agree well with observations. The IUE flux
distribution is reproduced in great detail by our models. The resonance lines
of magnesium and possibly also some lines of silicon are relatively weak in the
ultraviolet domain, which indicates non-negligible vertical abundance gradients
in the atmosphere. We also derive a new period of the star,
d, from all available photometric and magnetic measurements and show that the
observed rotational period is constant over decades. The ultraviolet and visual
variability of Aur is mostly caused by silicon bound-free absorption
and chromium and iron line absorption. These elements redistribute the flux
mainly from the far-ultraviolet region to the near-ultraviolet and optical
regions in the surface abundance spots. The light variability is modulated by
the stellar rotation. The ultraviolet domain is key for understanding the
properties of chemically peculiar stars. (abridged)Comment: 12 pages, accepted for publication in Astronomy & Astrophysic
X-ray emission from hydrodynamical simulations in non-LTE wind models
Hot stars are sources of X-ray emission originating in their winds. Although
hydrodynamical simulations that are able to predict this X-ray emission are
available, the inclusion of X-rays in stationary wind models is usually based
on simplifying approximations. To improve this, we use results from
time-dependent hydrodynamical simulations of the line-driven wind instability
(seeded by the base perturbation) to derive the analytical approximation of
X-ray emission in the stellar wind. We use this approximation in our non-LTE
wind models and find that an improved inclusion of X-rays leads to a better
agreement between model ionization fractions and those derived from servations.
Furthermore, the slope of the L_x-L relation is in better agreement with
observations, however the X-ray luminosity is underestimated by a factor of
three. We propose a possible solution for this discrepancy.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic
Variations of the high-level Balmer line spectrum of the helium-strong star Sigma Orionis E
Using the high-level Balmer lines and continuum, we trace the density
structure of two magnetospheric disk segments of the prototypical Bp star sigma
Ori E (B2p) as these segments occult portions of the star during the rotational
cycle. High-resolution spectra of the Balmer lines >H9 and Balmer edge were
obtained on seven nights in January-February 2007 at an average sampling of
0.01 cycles. We measured equivalent width variations due to the star
occultations by two disk segments 0.4 cycles apart and constructed differential
spectra of the migrations of the corresponding absorptions across the Balmer
line profiles. We first estimated the rotational and magnetic obliquity angles.
We then simulated the observed Balmer jump variation using the model atmosphere
codes synspec/circus and evaluated the disk geometry and gas thermodynamics. We
find that the two occultations are caused by two disk segments. The first of
these transits quickly, indicating that the segment resides in a range of
distances, perhaps 2.5-6R_star, from the star. The second consists of a more
slowly moving segment situated closer to the surface and causing two
semi-resolved absorbing maxima. During its transit this segment brushes across
the star's "lower" limb. Judging from the line visibility up to H23-H24 during
the occultations, both disk segments have mean densities near 10^{12} cm^{-3}
and are opaque in the lines and continuum. They have semiheights less than 1/2
of a stellar radius, and their temperatures are near 10500K and 12000K,
respectively. In all, the disks of Bp stars have a much more complicated
geometry than has been anticipated, as evidenced by their (sometimes)
non-coplanarity, de-centerness, and from star to star, differences in disk
height.Comment: Accepted by Astron. Astrophys, 13 pages, 4 embedded figure
Revisiting the Rigidly Rotating Magnetosphere model for Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability
The initial success of the Rigidly Rotating Magnetosphere (RRM) model
application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005)
triggered a renewed era of observational monitoring of this archetypal object.
We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging
(MDI) technique to simultaneously determine the magnetic configuration, which
is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller
non-axisymmetric quadrupolar contribution, as well as the surface distribution
of abundance of He, Fe, C, and Si. We describe a revised RRM model that now
accepts an arbitrary surface magnetic field configuration, with the field
topology from the MDI models used as input. The resulting synthetic Ha emission
and broadband photometric observations generally agree with observations,
however, several features are poorly fit. To explore the possibility of a
photospheric contribution to the observed photometric variability, the MDI
abundance maps were used to compute a synthetic photospheric light curve to
determine the effect of the surface inhomogeneities. Including the computed
photospheric brightness modulation fails to improve the agreement between the
observed and computed photometry. We conclude that the discrepancies cannot be
explained as an effect of inhomogeneous surface abundance. Analysis of the UV
light variability shows good agreement between observed variability and
computed light curves, supporting the accuracy of the photospheric light
variation calculation. We thus conclude that significant additional physics is
necessary for the RRM model to acceptably reproduce observations of not only
sigma Ori E, but also other similar stars with significant stellar
wind-magnetic field interactions.Comment: 16 pages, 17 figures, accepted for publication in MNRA
A structural evaluation of the tungsten isotopes via thermal neutron capture
Total radiative thermal neutron-capture -ray cross sections for the
W isotopes were measured using guided neutron beams from
the Budapest Research Reactor to induce prompt and delayed rays from
elemental and isotopically-enriched tungsten targets. These cross sections were
determined from the sum of measured -ray cross sections feeding the
ground state from low-lying levels below a cutoff energy, E, where
the level scheme is completely known, and continuum rays from levels
above E, calculated using the Monte Carlo statistical-decay code
DICEBOX. The new cross sections determined in this work for the tungsten
nuclides are: b and
b;
b and b; b and
b; and,
b and b. These results are consistent with
earlier measurements in the literature. The W cross section was also
independently confirmed from an activation measurement, following the decay of
W, yielding values for that are consistent
with our prompt -ray measurement. The cross-section measurements were
found to be insensitive to choice of level density or photon strength model,
and only weakly dependent on E. Total radiative-capture widths
calculated with DICEBOX showed much greater model dependence, however, the
recommended values could be reproduced with selected model choices. The decay
schemes for all tungsten isotopes were improved in these analyses.Comment: 25 pages, 15 figures, 15 table
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