17,029 research outputs found
Detecting and quantifying stellar magnetic fields -- Sparse Stokes profile approximation using orthogonal matching pursuit
In the recent years, we have seen a rapidly growing number of stellar
magnetic field detections for various types of stars. Many of these magnetic
fields are estimated from spectropolarimetric observations (Stokes V) by using
the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of
this method rapidly deteriorates with increasing noise and thus calls for a
more robust procedure that combines signal detection and field estimation. We
introduce an estimation method that provides not only the effective or mean
longitudinal magnetic field from an observed Stokes V profile but also uses the
net absolute polarization of the profile to obtain an estimate of the apparent
(i.e., velocity resolved) absolute longitudinal magnetic field. By combining
the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able
to decompose observed Stokes profiles with an overcomplete dictionary of
wavelet-basis functions to reliably reconstruct the observed Stokes profiles in
the presence of noise. The elementary wave functions of the sparse
reconstruction process were utilized to estimate the effective longitudinal
magnetic field and the apparent absolute longitudinal magnetic field. A
multiresolution analysis complements the OMP algorithm to provide a robust
detection and estimation method. An extensive Monte-Carlo simulation confirms
the reliability and accuracy of the magnetic OMP approach.Comment: A&A, in press, 15 pages, 14 figure
Spot evolution on the red giant star XX Triangulum. A starspot-decay analysis based on time-series Doppler imaging
Solar spots appear to decay linearly proportional to their size. The decay
rate of solar spots is directly related to magnetic diffusivity, which itself
is a key quantity for the length of a magnetic-activity cycle. Is a linear spot
decay also seen on other stars, and is this in agreement with the large range
of solar and stellar activity cycle lengths? We investigate the evolution of
starspots on the rapidly-rotating ( 24 d) K0 giant XX
Tri, using consecutive time-series Doppler images. Our aim is to obtain a
well-sampled movie of the stellar surface over many years, and thereby detect
and quantify a starspot decay law for further comparison with the Sun. We
obtained continuous high-resolution and phase-resolved spectroscopy with the
1.2-m robotic STELLA telescope on Tenerife over six years. For each observing
season, we obtained between 5 to 7 independent Doppler images, one per stellar
rotation, making up a total of 36 maps. To quantify starspot area decay and
growth, we match the observed images with simplified spot models based on a
Monte Carlo approach. It is shown that the surface of XX Tri is covered with
large high-latitude and even polar spots and with occasional small equatorial
spots. Just over the course of six years, we see a systematically changing spot
distribution with various timescales and morphology, such as spot fragmentation
and spot merging as well as spot decay and formation. An average linear decay
of = 0.022 0.002 SH/day is inferred. We found evidence of an
active longitude in phase toward the (unseen) companion star. Furthermore, we
detect a weak solar-like differential rotation with a surface shear of
= 0.016 0.003. From the decay rate, we determine a turbulent diffusivity
of = (6.3 0.5) 10 cm/s and predict a
magnetic activity cycle of 26 6 years
A fast method for Stokes profile synthesis -- Radiative transfer modeling for ZDI and Stokes profile inversion
The major challenges for a fully polarized radiative transfer driven approach
to Zeeman-Doppler imaging are still the enormous computational requirements. In
every cycle of the iterative interplay between the forward process (spectral
synthesis) and the inverse process (derivative based optimization) the Stokes
profile synthesis requires several thousand evaluations of the polarized
radiative transfer equation for a given stellar surface model. To cope with
these computational demands and to allow for the incorporation of a full Stokes
profile synthesis into Doppler- and Zeeman-Doppler imaging applications as well
as into large scale solar Stokes profile inversions, we present a novel fast
and accurate synthesis method for calculating local Stokes profiles. Our
approach is based on artificial neural network models, which we use to
approximate the complex non-linear mapping between the most important
atmospheric parameters and the corresponding Stokes profiles. A number of
specialized artificial neural networks, are used to model the functional
relation between the model atmosphere, magnetic field strength, field
inclination, and field azimuth, on one hand and the individual components
(I,Q,U,V) of the Stokes profiles, on the other hand. We performed an extensive
statistical evaluation and show that our new approach yields accurate local as
well as disk-integrated Stokes profiles over a wide range of atmospheric
conditions. The mean rms errors for the Stokes I and V profiles are well below
0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in
the range of 1%. Our approach does not only offer an accurate approximation to
the LTE polarized radiative transfer it, moreover, accelerates the synthesis by
a factor of more than 1000.Comment: A&A, in pres
Anti-solar differential rotation on the active sub-giant HU Virginis
Measuring surface differential rotation (DR) on different types of stars is
important when characterizing the underlying stellar dynamo. It has been
suggested that anti-solar DR laws can occur when strong meridional flows exist.
We aim to investigate the differential surface rotation on the primary star of
the RS CVn binary HU Vir by tracking its starspot distribution as a function of
time. We also aim to recompute and update the values for several system
parameters of the triple system HU Vir (close and wide orbits). Time-series
high-resolution spectroscopy for four continuous months was obtained with the
1.2-m robotic STELLA telescope. Nine consecutive Doppler images were
reconstructed from these data, using our line-profile inversion code iMap. An
image cross-correlation method was applied to derive the surface
differential-rotation law for HU Vir. New orbital elements for the close and
the wide orbits were computed using our new STELLA radial velocities (RVs)
combined with the RV data available in the literature. Photometric observations
were performed with the Amadeus Automatic Photoelectric Telescope (APT),
providing contemporaneous Johnson-Cousins and data for approximately 20
years. This data was used to determine the stellar rotation period and the
active longitudes. We confirm anti-solar DR with a surface shear parameter
of -0.029 0.005 and -0.026 0.009, using single-term and
double-term differential rotation laws, respectively. The best fit is achieved
assuming a solar-like double-term law with a lap time of 400 d. Our
orbital solutions result in a period of 10.387678 0.000003 days for the
close orbit and 2726 7 d ( 7.5 yr) for the wide orbit. A
Lomb-Scarge (L-S) periodogram of the pre-whitened -band data reveals a
strong single peak providing a rotation period of 10.391 0.008 d.Comment: Accepted for publication in A&
The Magnetic Topology of the Weak-Lined T Tauri Star V410 - A Simultaneous Temperature and Magnetic Field Inversion
We present a detailed temperature and magnetic investigation of the T Tauri
star V410 Tau by means of a simultaneous Doppler- and Zeeman-Doppler Imaging.
Moreover we introduce a new line profile reconstruction method based on a
singular value decomposition (SVD) to extract the weak polarized line profiles.
One of the key features of the line profile reconstruction is that the SVD line
profiles are amenable to radiative transfer modeling within our Zeeman-Doppler
Imaging code iMap. The code also utilizes a new iterative regularization scheme
which is independent of any additional surface constraints. To provide more
stability a vital part of our inversion strategy is the inversion of both
Stokes I and Stokes V profiles to simultaneously reconstruct the temperature
and magnetic field surface distribution of V410 Tau. A new image-shear analysis
is also implemented to allow the search for image and line profile distortions
induced by a differential rotation of the star. The magnetic field structure we
obtain for V410 Tau shows a good spatial correlation with the surface
temperature and is dominated by a strong field within the cool polar spot. The
Zeeman-Doppler maps exhibit a large-scale organization of both polarities
around the polar cap in the form of a twisted bipolar structure. The magnetic
field reaches a value of almost 2 kG within the polar region but smaller fields
are also present down to lower latitudes. The pronounced non-axisymmetric field
structure and the non-detection of a differential rotation for V410 Tau
supports the idea of an underlying -type dynamo, which is predicted
for weak-lined T Tauri stars.Comment: Accepted for A&A, 18 pages, 10 figure
Graviton localization and Newton's law for brane models with a non-minimally coupled bulk scalar field
Brane world models with a non-minimally coupled bulk scalar field have been
studied recently. In this paper we consider metric fluctuations around an
arbitrary gravity-scalar background solution, and we show that the
corresponding spectrum includes a localized zero mode which strongly depends on
the profile of the background scalar field. For a special class of solutions,
with a warp factor of the RS form, we solve the linearized Einstein equations,
for a point-like mass source on the brane, by using the brane bending
formalism. We see that general relativity on the brane is recovered only if we
impose restrictions on the parameter space of the models under consideration.Comment: 17 pages, revised versio
Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae
We present the first temperature surface map of EK Dra from
very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and
Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in
spectral line profiles are inverted to a stellar surface temperature map using
our Map code. The long-term photometric record is employed to compare our
map with previously published maps. Four cool spots were reconstructed, but no
polar spot was seen. The temperature difference to the photosphere of the spots
is between 990 and 280K. Two spots are reconstructed with a typical solar
morphology with an umbra and a penumbra. For the one isolated and relatively
round spot (A), we determine an umbral temperature of 990K and a penumbral
temperature of 180K below photospheric temperature. The umbra to photosphere
intensity ratio of EK Dra is approximately only half of that of a comparison
sunspot. A test inversion from degraded line profiles showed that the higher
spectral resolution of PEPSI reconstructs the surface with a temperature
difference that is on average 10% higher than before and with smaller surface
areas by 10-20%. PEPSI is therefore better suited to detecting and
characterising temperature inhomogeneities. With ten more years of photometry,
we also refine the spot cycle period of EK Dra to 8.90.2 years with a
continuing long-term fading trend. The temperature morphology of spot A so far
appears to show the best evidence for the existence of a solar-like penumbra
for a starspot. We emphasise that it is more the non-capture of the true umbral
contrast rather than the detection of the weak penumbra that is the limiting
factor. The relatively small line broadening of EK Dra, together with the only
moderately high spectral resolutions previously available, appear to be the
main contributors to the lower-than-expected spot contrasts when comparing to
the Sun.Comment: Accepted for A&
Time-series Doppler images and surface differential rotation of the effectively-single rapidly-rotating K-giant KU Pegasi
According to most stellar dynamo theories, differential rotation (DR) plays a
crucial role for the generation of toroidal magnetic fields. Numerical models
predict surface differential rotation to be anti-solar for rapidly-rotating
giant stars, i.e., their surface angular velocity could increase with stellar
latitude. However, surface differential rotation has been derived only for a
handful of individual giant stars to date.
The spotted surface of the K-giant KU Pegasi is investigated in order to
detect its time evolution and quantify surface differential rotation.
We present altogether 11 Doppler images from spectroscopic data collected
with the robotic telescope STELLA between 2006--2011. All maps are obtained
with the surface reconstruction code iMap. Differential rotation is extracted
from these images by detecting systematic (latitude-dependent) spot
displacements. We apply a cross-correlation technique to find the best
differential rotation law.
The surface of KU Peg shows cool spots at all latitudes and one persistent
warm spot at high latitude. A small cool polar spot exists for most but not all
of the epochs. Re-identification of spots in at least two consecutive maps is
mostly possible only at mid and high latitudes and thus restricts the
differential-rotation determination mainly to these latitudes. Our
cross-correlation analysis reveals solar-like differential rotation with a
surface shear of , i.e., approximately five times weaker
than on the Sun. We also derive a more accurate and consistent set of stellar
parameters for KU Peg including a small Li abundance of ten times less than
solar.Comment: 13 pages, 12 figures, accepted for publication in A&
- …