125 research outputs found
How reliable is Zeeman Doppler Imaging without simultaneous temperature reconstruction?
Aims: The goal of this study is to perform numerical tests of Zeeman Doppler
Imaging (ZDI) to asses whether correct reconstruction of magnetic fields is at
all possible without taking temperature into account for stars in which
magnetic and temperature inhomogeneities are spatially correlated. Methods: We
used a modern ZDI code employing a physically realistic treatment of the
polarized radiative transfer in all four Stokes parameters. We generated
artificial observations of isolated magnetic spots and of magnetic features
coinciding with cool temperature spots and then reconstructed magnetic and
temperature distributions from these data. Results: Using Stokes I and V for
simultaneous magnetic and temperature mapping for the star with a homogeneous
temperature distribution yields magnetic field strengths underestimated by
typically 10-15% relative to their true values. When temperature is kept
constant and Stokes I is not used for magnetic mapping, the underestimation is
30-60%. At the same time, the strength of magnetic field inside cool spots is
underestimated by as much as 80-95% and the spot geometry is also poorly
reconstructed when temperature variations are ignored. On the other hand, the
inversion quality is greatly improved when temperature variations are accounted
for in magnetic mapping. When using all four Stokes parameters the
reconstructed field strength inside cool spots is underestimated by 30-40% but
the spot geometry can be recovered very accurately compared to the experiments
with circular polarization alone. Conclusions: Reliable magnetic field
reconstruction for a star with high-contrast temperature spots is essentially
impossible if temperature inhomogeneities are ignored. A physically realistic
line profile modeling method, which simultaneously accounts for both types of
inhomogeneities, is required for meaningful ZDI of cool active stars
Simulations of magneto-hydrodynamic waves in atmospheres of roAp stars
We report 2D time-dependent non-linear magneto-hydrodynamical simulations of
waves in the atmospheres of roAp stars. We explore a grid of simulations in a
wide parameter space. The aim of our study is to understand the influence of
the atmosphere and the magnetic field on the propagation and reflection
properties of magneto-acoustic waves, formation of shocks and node layers.Comment: 2 pages, Proceedings of the IAU Symposium 259, "Cosmic Magnetic
Fields: From Planets, to Stars and Galaxies", November 200
Magnetic Field of the Eclipsing M Dwarf Binary YY Gem
YY Gem is a short-period eclipsing binary system containing two nearly
identical, rapidly rotating, very active early-M dwarfs. This binary represents
an important benchmark system for calibrating empirical relations between
fundamental properties of low-mass stars and for testing theories of interior
structure and evolution of these objects. Both components of YY Gem exhibit
inflated radii, which has been attributed to poorly understood magnetic
activity effects. Despite a long history of magnetic activity studies of this
system no direct magnetic field measurements have been made for it. Here we
present a comprehensive characterisation of the surface magnetic field in both
components of YY Gem. We reconstructed the global field topologies with the
help of a tomographic inversion technique applied to high-resolution
spectropolarimetric data. This analysis revealed moderately complex global
fields with a typical strength of 200-300 G and anti-aligned dipolar
components. A complementary Zeeman intensification analysis of the disentangled
intensity spectra showed that the total mean field strength reaches 3.2-3.4 kG
in both components of YY Gem. We used these results together with other recent
magnetic field measurements of M dwarfs to investigate the relation between the
global and small-scale fields in these stars. We also assessed predictions of
competing magnetoconvection interior structure models developed for YY Gem,
finding that only one of them anticipated the surface field strength compatible
with our observations. Results of our star spot mapping of YY Gem do not
support the alternative family of theoretical stellar models which attempts to
explain the radii inflation by postulating a large spot filling factor.Comment: 17 pages, 8 figures; accepted for publication in Ap
The effect of isotopic splitting on the bisector and inversions of the solar Ca II 854.2 nm line
The Ca II 854.2 nm spectral line is a common diagnostic of the solar
chromosphere. The average line profile shows an asymmetric core, and its
bisector shows a characteristic inverse-C shape. The line actually consists of
six components with slightly different wavelengths depending on the isotope of
calcium. This isotopic splitting of the line has been taken into account in
studies of non-solar stars, but never for the Sun. We performed non-LTE
radiative transfer computations from three models of the solar atmosphere and
show that the asymmetric line-core and inverse C-shape of the bisector of the
854.2 nm line can be explained by isotopic splitting. We confirm this finding
by analysing observations and showing that the line asymmetry is present
irrespective of conditions in the solar atmosphere. Finally, we show that
inversions based on the Ca II 854.2 nm line should take the isotopic splitting
into account, otherwise the inferred atmospheres will contain erroneous
velocity gradients and temperatures.Comment: Accepted for ApJ
Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy
We aim to characterise the surface magnetic fields of a sample of 8 T Tauri
stars from high-resolution near-IR spectroscopy. Some stars in our sample are
known to be magnetic from previous spectroscopic or spectropolarimetric
studies. Our goals are 1) to apply Zeeman broadening modelling to T Tauri stars
with high-resolution data, 2) to expand the sample of stars with measured
surface magnetic field strengths, 3) to investigate possible rotational or
long-term magnetic variability by comparing spectral time series of given
targets, and 4) to compare the magnetic field modulus tracing small-scale
magnetic fields to those of large-scale magnetic fields derived by Stokes V
Zeeman Doppler Imaging. We modelled the Zeeman broadening of magnetically
sensitive spectral lines in the near-IR K-band from high-resolution spectra by
using magnetic spectrum synthesis based on realistic model atmospheres and by
using different descriptions of the surface magnetic field. We developped a
Bayesian framework that selects the complexity of the magnetic field
prescription based on the information contained in the data. We obtain
individual magnetic field measurements for each star in our sample using four
different models. We find that the Bayesian Model 4 performs best in the range
of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not
detect a strong rotational variation of with a mean peak-to-peak variation
of 0.3 kG. Our confidence intervals are of the same order of magnitude, which
suggests that the Zeeman broadening is produced by a small-scale magnetic field
homogeneously distributed over stellar surfaces. A comparison of our results
with mean large-scale magnetic field measurements from Stokes V ZDI show
different fractions of mean field strength being recovered, from 25-42% for
relatively simple poloidal axisymmetric field topologies to 2-11% for more
complex fields.Comment: 14 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
- …