819 research outputs found
Utilization of Forming Tool with Variable Blankholder Force for Drawing of Al Alloys
AbstractDrawing of stampings technology is nowadays the most using technology for sheets processing. Because of stamping's weight reduction is these days actual sheet processing from aluminium alloys which by their properties differ a lot from commonly using deep-drawing sheets. To achieve maximal plastic deformation with material fracture is necessary to use sheet with required mechanical properties and properly chosen forming technological properties. Common tools for deep-drawing don’t enable to lively reply of necessity to change blankholder force during sheet drawing. On the experimental tool for deep-drawing with variable blankholder force are in the paper evaluated two variants of drawing methods for irregular shape of stamping. In the first case was stamping from AA6016 alloy designed with constant blankholder force. In the second case was blankholder force optimized and during drawing was changed in dependence on drawing depth. Calculation of blankholder force in dependence on drawing depth was carried out by means of numerical simulation in software PAM-STAMP 2G. Result of optimalization was strain distribution change in critical places of stamping. Strain distribution on stamping is measured by means of optical system ARGUS
Atmosphere above a large solar pore
A large solar pore with a granular light bridge was observed on October 15,
2008 with the IBIS spectrometer at the Dunn Solar Telescope and a 69-min long
time series of spectral scans in the lines Ca II 854.2 nm and Fe I 617.3 nm was
obtained. The intensity and Doppler signals in the Ca II line were separated.
This line samples the middle chromosphere in the core and the middle
photosphere in the wings. Although no indication of a penumbra is seen in the
photosphere, an extended filamentary structure, both in intensity and Doppler
signals, is observed in the Ca II line core. An analysis of morphological and
dynamical properties of the structure shows a close similarity to a
superpenumbra of a sunspot with developed penumbra. A special attention is paid
to the light bridge, which is the brightest feature in the pore seen in the Ca
II line centre and shows an enhanced power of chromospheric oscillations at 3-5
mHz. Although the acoustic power flux in the light bridge is five times higher
than in the "quiet" chromosphere, it cannot explain the observed brightness.Comment: 6 pages, 3 figures, accepted in Journal of Physics: Conference Serie
Localised plumes in three-dimensional compressible magnetoconvection
Within the umbrae of sunspots, convection is generally inhibited by the
presence of strong vertical magnetic fields. However, convection is not
completely suppressed in these regions: bright features, known as umbral dots,
are probably associated with weak, isolated convective plumes. Motivated by
observations of umbral dots, we carry out numerical simulations of
three-dimensional, compressible magnetoconvection. By following solution
branches into the subcritical parameter regime (a region of parameter space in
which the static solution is linearly stable to convective perturbations), we
find that it is possible to generate a solution which is characterised by a
single, isolated convective plume. This solution is analogous to the steady
magnetohydrodynamic convectons that have previously been found in
two-dimensional calculations. These results can be related, in a qualitative
sense, to observations of umbral dots.Comment: submitted to MNRA
Chromospheric heating by acoustic waves compared to radiative cooling
Acoustic and magnetoacoustic waves are among the possible candidate
mechanisms that heat the upper layers of solar atmosphere. A weak chromospheric
plage near a large solar pore NOAA 11005 was observed on October 15, 2008 in
the lines Fe I 617.3 nm and Ca II 853.2 nm with the Interferometric
Bidimemsional Spectrometer (IBIS) attached to the Dunn Solar Telescope.
Analyzing the Ca II observations with spatial and temporal resolutions of 0.4"
and 52 s, the energy deposited by acoustic waves is compared with that released
by radiative losses. The deposited acoustic flux is estimated from power
spectra of Doppler oscillations measured in the Ca II line core. The radiative
losses are calculated using a grid of seven 1D hydrostatic semi-empirical model
atmospheres. The comparison shows that the spatial correlation of maps of
radiative losses and acoustic flux is 72 %. In quiet chromosphere, the
contribution of acoustic energy flux to radiative losses is small, only of
about 15 %. In active areas with photospheric magnetic field strength between
300 G and 1300 G and inclination of 20-60 degrees, the contribution increases
from 23 % (chromospheric network) to 54 % (a plage). However, these values have
to be considered as lower limits and it might be possible that the acoustic
energy flux is the main contributor to the heating of bright chromospheric
network and plages.Comment: 9 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
Dynamics of the solar atmosphere above a pore with a light bridge
Context: Solar pores are small sunspots lacking a penumbra that have a
prevailing vertical magnetic field component. They can include light bridges at
places with locally reduced magnetic field. Like sunspots, they exhibit a wide
range of oscillatory phenomena.
Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to
obtain characteristics of a chromospheric filamentary structure around the
pore, to analyse oscillations and waves in and around the pore, and to
understand the structure and brightness of the light bridge.
Methods: Spectral imaging observations in the line Ca II 854.2 nm and
complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS
spectrometer and HINODE/SOT spectropolarimeter, were used to measure
photospheric and chromospheric velocity fields, oscillations, waves, the
magnetic field in the photosphere, and acoustic energy flux and radiative
losses in the chromosphere.
Results: The chromospheric filamentary structure around the pore has all
important characteristics of a superpenumbra: it shows an inverse Evershed
effect and running waves, and has a similar morphology and oscillation
character. The granular structure of the light bridge in the upper photosphere
can be explained by radiative heating. Acoustic waves leaking up from the
photosphere along the inclined magnetic field in the light bridge transfer
enough energy flux to balance the total radiative losses of the light-bridge
chromosphere.
Conclusions: The presence of a penumbra is not a necessary condition for the
formation of a superpenumbra. The light bridge is heated by radiation in the
photosphere and by acoustic waves in the chromosphere.Comment: 14 pages, 14 figures, 3 tables, accepted for publication in
Astrononomy & Astrophysic
A model of an expanding giant that swallowed planets for the eruption of V838 Monocerotis
In early 2002 V838 Monocerotis had an extraordinary outburst whose nature is
still unclear. The optical light curve showed at least three peaks and imaging
revealed a light echo around the object - evidence for a dust shell which was
emitted several thousand years ago and now reflecting light from the eruption.
Spectral analysis suggests that the object was relatively cold throughout the
event, which was characterized by an expansion to extremely large radii. We
show that the three peaks in the light curve have a similar shape and thus it
seems likely that a certain phenomenon was three times repeated. Our suggestion
that the outburst was caused by the expansion of a red giant, followed by the
successive swallowing of three relatively massive planets in close orbits,
supplies a simple explanation to all observed peculiarities of this intriguing
object.Comment: 5 pages, 1 LaTex file, 2 .eps figures, accepted for publication in
MNRA
Magnetic Tension of Sunspot Fine Structures
The equilibrium structure of sunspots depends critically on its magnetic
topology and is dominated by magnetic forces. Tension force is one component of
the Lorentz force which balances the gradient of magnetic pressure in
force-free configurations. We employ the tension term of the Lorentz force to
clarify the structure of sunspot features like penumbral filaments, umbral
light bridges and outer penumbral fine structures. We compute vertical
component of tension term of Lorentz force over two active regions namely NOAA
AR 10933 and NOAA AR 10930 observed on 05 January 2007 and 12 December 2006
respectively. The former is a simple while latter is a complex active region
with highly sheared polarity inversion line (PIL). The vector magnetograms used
are obtained from Hinode(SOT/SP). We find an inhomogeneous distribution of
tension with both positive and negative signs in various features of the
sunspots. The existence of positive tension at locations of lower field
strength and higher inclination is compatible with the uncombed model of the
penumbral structure. Positive tension is also seen in umbral light bridges
which could be indication of uncombed structure of the light bridge. Likewise,
the upward directed tension associated with bipolar regions in the penumbra
could be a direct confirmation of the sea serpent model of penumbral
structures. Upward directed tension at the PIL of AR 10930 seems to be related
to flux emergence. The magnitude of the tension force is greater than the force
of gravity in some places, implying a nearly force-free configuration for these
sunspot features. From our study, magnetic tension emerges as a useful
diagnostic of the local equilibrium of the sunspot fine structures.Comment: 06 pages, 6 figures; Accepted for publication in the Astronomy &
Astrophysics as a "Letter to the Editor
Properties of Umbral Dots as Measured from the New Solar Telescope Data and MHD Simulations
We studied bright umbral dots (UDs) detected in a moderate size sunspot and
compared their statistical properties to recent MHD models. The study is based
on high resolution data recorded by the New Solar Telescope at the Big Bear
Solar Observatory and 3D MHD simulations of sunspots. Observed UDs, living
longer than 150 s, were detected and tracked in a 46 min long data set, using
an automatic detection code. Total 1553 (620) UDs were detected in the
photospheric (low chromospheric) data. Our main findings are: i) none of the
analyzed UDs is precisely circular, ii) the diameter-intensity relationship
only holds in bright umbral areas, and iii) UD velocities are inversely related
to their lifetime. While nearly all photospheric UDs can be identified in the
low chromospheric images, some small closely spaced UDs appear in the low
chromosphere as a single cluster. Slow moving and long living UDs seem to exist
in both the low chromosphere and photosphere, while fast moving and short
living UDs are mainly detected in the photospheric images. Comparison to the 3D
MHD simulations showed that both types of UDs display, on average, very similar
statistical characteristics. However, i) the average number of observed UDs per
unit area is smaller than that of the model UDs, and ii) on average, the
diameter of model UDs is slightly larger than that of observed ones.Comment: Accepted by the AP
Calculation of the number of partitions with constraints on the fragment size
This article introduces recursive relations allowing the calculation of the
number of partitions with constraints on the minimum and/or on the maximum
fragment size
The dissimilar chemical composition of the planet-hosting stars of the XO-2 binary system
Using high-quality spectra of the twin stars in the XO-2 binary system, we
have detected significant differences in the chemical composition of their
photospheres. The differences correlate strongly with the elements' dust
condensation temperature. In XO-2N, volatiles are enhanced by about 0.015 dex
and refractories are overabundant by up to 0.090 dex. On average, our error bar
in relative abundance is 0.012 dex. We present an early metal-depletion
scenario in which the formation of the gas giant planets known to exist around
these stars is responsible for a 0.015 dex offset in the abundances of all
elements while 20 M_Earth of non-detected rocky objects that formed around
XO-2S explain the additional refractory-element difference. An alternative
explanation involves the late accretion of at least 20 M_Earth of planet-like
material by XO-2N, allegedly as a result of the migration of the hot Jupiter
detected around that star. Dust cleansing by a nearby hot star as well as age
or Galactic birthplace effects can be ruled out as valid explanations for this
phenomenon.Comment: ApJ, in press. Complete linelist (Table 3) available in the "Other
formats -> Source" downloa
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