1,072 research outputs found
Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star
We have derived the mean broadening profile of the star V102 in the region of
the open cluster IC4665 from high resolution spectroscopy. At a projected
equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong
deviation from classical rotation. We discuss several scenarios, the most
plausible being strong differential rotation in latitudinal direction. For this
scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8
rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a
spectral type of A9 and support photometric measurements classifying IC4665
V102 as a non-member of IC4665. At such early spectral type this is the
strongest case of differential rotation observed so far. Together with three
similar stars, IC4665 V102 seems to form a new class of objects that exhibit
extreme latitudinal shear in a very shallow convective envelope.Comment: accepted for A&A Letter
Spectral type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070
We show individual high resolution spectra of components A, B, and C of the
nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and
C are known to have masses at the threshold to brown dwarfs. From our spectra
we measure rotation velocities and the mean magnetic field for all three
components individually. We find magnetic flux on the order of several
kilo-Gauss in all components. The rotation velocities of the two late-M objects
B and C are similar (vsini = 16km/s), the earlier A component is spinning only
at about half that rate. This suggests weakening of net rotational braking at
late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs
is real. Furthermore, we found that magnetic flux in the B component is about
twice as strong as in component C at similar rotation rate. This indicates that
rotational braking is not proportional to magnetic field strength in fully
convective objects, and that a different field topology is the reason for the
weak braking in low mass objects.Comment: accepted for publication as A&A Lette
3D simulations of M star atmosphere velocities and their influence on molecular FeH lines
We present an investigation of the velocity fields in early to late M-type
star hydrodynamic models, and we simulate their influence on FeH molecular line
shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff
of 2500 K and 4000 K. Our aim is to characterize the Teff- and log g
-dependence of the velocity fields and express them in terms of micro- and
macro-turbulent velocities in the one dimensional sense. We present also a
direct comparison between 3D hydrodynamical velocity fields and 1D turbulent
velocities. The velocity fields strongly affect the line shapes of FeH, and it
is our goal to give a rough estimate for the log g and Teff parameter range in
which 3D spectral synthesis is necessary and where 1D synthesis suffices. In
order to calculate M-star structure models we employ the 3D
radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these
models is performed with the line synthesis code LINFOR3D. We describe the 3D
velocity fields in terms of a Gaussian standard deviation and project them onto
the line of sight to include geometrical and limb-darkening effects. The micro-
and macro-turbulent velocities are determined with the "Curve of Growth" method
and convolution with a Gaussian velocity profile, respectively. To characterize
the log g and Teff dependence of FeH lines, the equivalent width, line width,
and line depth are regarded. The velocity fields in M-stars strongly depend on
log g and Teff. They become stronger with decreasing log g and increasing Teff.Comment: 14 pages, 17 figures, 3 tables, accepted by Astronomy & Astrophysic
Three-dimensional simulations of near-surface convection in main-sequence stars. IV. Effect of small-scale magnetic flux concentrations on centre-to-limb variation and spectral lines
Magnetic fields affect the local structure of the photosphere of stars. They
can considerably influence the radiative properties near the optical surface,
flow velocities, and the temperature and pressure profiles. We aim at
understanding qualitatively the influence of small magnetic flux concentrations
in unipolar plage regions on the centre-to-limb variation of the intensity and
its contrast and on the shape of spectral line profiles in cool main-sequence
stars. We analyse the bolometric and continuum intensity and its angular
dependence of 24 radiative magnetohydrodynamic simulations of the near-surface
layers of main-sequence stars with six different sets of stellar parameters
(spectral types F to early M) and four different average magnetic field
strengths (including the non-magnetic case). We also calculated disc-integrated
profiles of three spectral lines. The small magnetic flux concentrations formed
in the magnetic runs of simulations have a considerable impact on the intensity
and its centre-to-limb variation. Spectral lines are not only broadened owing
to the Zeeman effect, but are also strongly affected by the modified
thermodynamical structure and flow patterns. This indirect magnetic impact on
the line profiles is often bigger than that of the Zeeman effect. The effects
of the magnetic field on the radiation leaving the star can be considerable and
is not restricted to spectral line broadening and polarisation by the Zeeman
effect. The inhomogeneous structure of the magnetic field on small length
scales and its impact on (and spatial correlation with) the local
thermodynamical structure and the flow field near the surface influence the
measurement of the global field properties and stellar parameters. These
effects need to be taken into account in the interpretation of observations.Comment: 16 pages, 13+3 figures, 1 appendix, accepted for publication in A&
Three-dimensional simulations of near-surface convection in main-sequence stars. III. The structure of small-scale magnetic flux concentrations
The convective envelopes of cool main-sequence stars harbour magnetic fields
with a complex global and local structure. These fields affect the near-surface
convection and the outer stellar atmospheres in many ways and are responsible
for the observable magnetic activity of stars. Our aim is to understand the
local structure in unipolar regions with moderate average magnetic flux
density. These correspond to plage regions covering a substantial fraction of
the surface of the Sun (and likely also the surface of other Sun-like stars)
during periods of high magnetic activity. We analyse the results of 18
local-box magnetohydrodynamics simulations covering the upper layers of the
convection zones and the photospheres of cool main-sequence stars of spectral
types F to early M. The average vertical field in these simulations ranges from
20 to 500G. We find a substantial variation of the properties of the surface
magnetoconvection between main-sequence stars of different spectral types. As a
consequence of a reduced efficiency of the convective collapse of flux tubes, M
dwarfs lack bright magnetic structures in unipolar regions of moderate field
strength. The spatial correlation between velocity and the magnetic field as
well as the lifetime of magnetic structures and their sizes relative to the
granules vary significantly along the model sequence of stellar types.Comment: 15 pages, 12 figures, accepted for publication in A&
Three-dimensional simulations of near-surface convection in main-sequence stars - II. Properties of granulation and spectral lines
The atmospheres of cool main-sequence stars are structured by convective
flows from the convective envelope that penetrate the optically thin layers and
lead to structuring of the stellar atmospheres analogous to solar granulation.
The flows have considerable influence on the 3D structure of temperature and
pressure and affect the profiles of spectral lines formed in the photosphere.
For the set of six 3D radiative (M)HD simulations of cool main-sequence stars
described in the first paper of this series, we analyse the near-surface
layers. We aim at describing the properties of granulation of different stars
and at quantifying the effects on spectral lines of the thermodynamic structure
and flows of 3D convective atmospheres. We detected and tracked granules in
brightness images from the simulations to analyse their statistical properties,
as well as their evolution and lifetime. We calculated spatially resolved
spectral line profiles using the line synthesis code SPINOR. To enable a
comparison to stellar observations, we implemented a numerical
disc-integration, which includes (differential) rotation. Although the stellar
parameters change considerably along the model sequence, the properties of the
granules are very similar. The impact of the 3D structure of the atmospheres on
line profiles is measurable in disc-integrated spectra. Line asymmetries caused
by convection are modulated by stellar rotation. The 3D structure of cool
stellar atmospheres as shaped by convective flows has to be taken into account
when using photospheric lines to determine stellar parameters.Comment: 18 pages, 22 figures, 3 tables; accepted for publication in A&
Three-dimensional simulations of near-surface convection in main-sequence stars - I. Overall structure
The near-surface layers of cool main-sequence stars are structured by
convective flows, which are overshooting into the atmosphere. The flows and the
associated spatio-temporal variations of density and temperature affect
spectral line profiles and thus have an impact on estimates of stellar
properties such as effective temperature, gravitational acceleration, and
abundances. We aim at identifying distinctive properties of the thermodynamic
structure of the atmospheres of different stars and understand their causes. We
ran comprehensive 3D radiation hydrodynamics simulations of the near-surface
layers of six simulated stars of spectral type F3V to M2V with the MURaM code.
We carry out a systematic parameter study of the mean stratifications, flow
structures, and the energy flux in these stars.\par Results: We find monotonic
trends along the lower main sequence in granule size, flow velocity, and
intensity contrast. The convection in the M-star models differs substantially
from that of the hotter stars, mainly owing to the more gradual transition from
convective to radiative energy transport. While the basic mechanisms driving
surface convection in cool stars are the same, the properties of the convection
vary along the main sequence. Apart from monotonic trends in rms velocity,
intensity contrast, granule size, etc., there is a transition between "naked"
and "hidden" granulation around spectral type K5V caused by the (highly
non-linear) temperature dependence of the opacity. These variations have to be
taken into account when stellar parameters are derived from spectra.Comment: 14 pages, 1 appendix, 15+2 figures, 2 tables; accepted for
publication in A&
Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F and later
and derived ``overall'' broadening functions in selected wavelength regions
utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of
the projected rotational velocity were derived from the first zero
of the Fourier transformed profiles and the shapes of the profiles were
analyzed for effects of differential rotation. The broadening profiles of 70
stars rotating faster than km s show no indications of multiplicity nor of spottedness. In
those profiles we used the ratio of the first two zeros of the Fourier
transform to search for deviations from rigid rotation. In the vast
majority the profiles were found to be consistent with rigid rotation. Five
stars were found to have flat profiles probably due to cool polar caps, in
three stars cuspy profiles were found. Two out of those three cases may be due
to extremely rapid rotation seen pole on, only in one case ( km
s) solar-like differential rotation is the most plausible explanation
for the observed profile. These results indicate that the strength of
differential rotation diminishes in stars rotating as rapidly as v \sin{i} \ga
50 km s.Comment: 10 pages, accepted for publication in A&
Solving mazes with memristors: a massively-parallel approach
Solving mazes is not just a fun pastime. Mazes are prototype models in graph theory, topology, robotics, traffic optimization, psychology, and in many other areas of science and technology. However, when maze complexity increases their solution becomes cumbersome and very time consuming. Here, we show that a network of memristors - resistors with memory - can solve such a non-trivial problem quite easily. In particular, maze solving by the network of memristors occurs in a massively parallel fashion since all memristors in the network participate simultaneously in the calculation. The result of the calculation is then recorded into the memristors’ states, and can be used and/or recovered at a later time. Furthermore, the network of memristors finds all possible solutions in multiple-solution mazes, and sorts out the solution paths according to their length. Our results demonstrate not only the first application of memristive networks to the field of massively-parallel computing, but also a novel algorithm to solve mazes which could find applications in different research fields
The lived user experience of virtual environments: Initial steps of a phenomenological analysis in a safety training setting
Virtual environments (VEs) are making their way into various sectors of life to enhance and support human activity, including learning. VEs have been used in various contexts for training, and in many cases they are designed to model or simulate - as accurately and authentically as possible - a specific work context. In striving for authenticity, visual and representative realism tends to receive most of the development input, despite of several studies that challenge its importance. New training avenues have raised the importance of rigorous phenomenological descriptions for a deeper understanding of user experience in the actual context of use. This paper reports the preliminary steps in a phenomenological analysis of how employees working in actual hazardous settings experience virtual safety training environments. Such open-ended research project can reveal new aspects of user experience that can advice the development and evaluation of human-computer interaction in digital technology-enhanced training contexts
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