880 research outputs found
No magnetic field variation with pulsation phase in the roAp star gamma Equulei
We present an analysis of 210 high-resolution time-resolved
spectropolarimetric observations of the roAp star gamma Equ obtained over three
nights in August and September 2003. Radial velocity variations due to p-mode
non-radial pulsations are clearly detected in the lines of rare-earth elements,
in particular Pr III, Nd II and Nd III. In contrast, we find absolutely no
evidence for the variation of the mean longitudinal magnetic field over the
pulsation period in gamma Equ at the level of 110-240 G which was recently
reported by Leone & Kurtz (2003). Our investigation of the variability of
circularly polarized profiles of 13 Nd III lines demonstrates that, at the 3
sigma confidence level, no magnetic field variation with an amplitude above
40-60 G was present in gamma Equ during our monitoring of this star.Comment: accepted for publication by Astronomy & Astrophysic
Least squares deconvolution of the stellar intensity and polarization spectra
Least squares deconvolution (LSD) is a powerful method of extracting
high-precision average line profiles from the stellar intensity and
polarization spectra. Despite its common usage, the LSD method is poorly
documented and has never been tested using realistic synthetic spectra. In this
study we revisit the key assumptions of the LSD technique, clarify its
numerical implementation, discuss possible improvements and give
recommendations how to make LSD results understandable and reproducible. We
also address the problem of interpretation of the moments and shapes of the LSD
profiles in terms of physical parameters. We have developed an improved,
multiprofile version of LSD and have extended the deconvolution procedure to
linear polarization analysis taking into account anomalous Zeeman splitting of
spectral lines. This code is applied to the theoretical Stokes parameter
spectra. We test various methods of interpreting the mean profiles,
investigating how coarse approximations of the multiline technique translate
into errors of the derived parameters. We find that, generally, the Stokes
parameter LSD profiles do not behave as a real spectral line with respect to
the variation of magnetic field and elemental abundance. This problem is
especially prominent for the Stokes I variation with abundance and Stokes Q
variation with magnetic field. At the same time, the Stokes V LSD spectra
closely resemble profile of a properly chosen synthetic line for the magnetic
field strength up to 1 kG. We conclude that the usual method of interpreting
the LSD profiles by assuming that they are equivalent to a real spectral line
gives satisfactory results only in a limited parameter range and thus should be
applied with caution. A more trustworthy approach is to abandon the single-line
approximation of the average profiles and apply LSD consistently to
observations and synthetic spectra.Comment: Accepted for publication in Astronomy & Astrophysics; 15 pages, 12
figures; second version includes minor language correction
Global magnetohydrodynamical models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids
We present global 3D MHD simulations of disks of gas and solids, aiming at
developing models that can be used to study various scenarios of planet
formation and planet-disk interaction in turbulent accretion disks. A second
goal is to show that Cartesian codes are comparable to cylindrical and
spherical ones in handling the magnetohydrodynamics of the disk simulations, as
the disk-in-a-box models presented here develop and sustain MHD turbulence. We
investigate the dependence of the magnetorotational instability on disk scale
height, finding evidence that the turbulence generated by the magnetorotational
instability grows with thermal pressure. The turbulent stresses depend on the
thermal pressure obeying a power law of 0.24+/-0.03, compatible with the value
of 0.25 found in shearing box calculations. The ratio of stresses decreased
with increasing temperature. We also study the dynamics of boulders in the
hydromagnetic turbulence. The vertical turbulent diffusion of the embedded
boulders is comparable to the turbulent viscosity of the flow. Significant
overdensities arise in the solid component as boulders concentrate in high
pressure regions.Comment: Changes after peer review proces
Inhomogeneous distribution of mercury on the surfaces of rapidly rotating HgMn stars
Starspots are usually associated with the action of magnetic fields at the
stellar surfaces. However, recently an inhomogeneous chemical distribution of
mercury was found for the mercury-manganese (HgMn) star alpha And -- a
well-established member of a non-magnetic subclass of the chemically peculiar
stars of the upper main sequence. In this study we present first results of the
high-resolution survey of the HgII 3984 resonance line in the spectra of
rapidly rotating HgMn stars with atmospheric parameters similar to those of
alpha And. We use spectrum synthesis modelling and take advantage of the
Doppler resolution of the stellar surfaces to probe horizontal structure of
mercury distribution. Clear signatures of spots are found in the HgII 3984 line
profiles of HR 1185 and HR 8723. Two observations of the latter star separated
by two days give evidence for the line profile variability. We conclude that
inhomogeneous distribution of Hg is a common phenomenon for the rapidly
rotating HgMn stars in the 13000--13800 K effective temperature range
independently of the stellar evolutionary stage. These results establish
existence of a new class of spectrum variable spotted B-type stars. It is
suggested that the observed Hg inhomogeneities arise from dynamical
instabilities in the chemical diffusion processes and are unrelated to magnetic
phenomena.Comment: 6 pages, 3 figures, accepted by Astronomy & Astrophysic
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