27,082 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
Spectrophotovoltaic orbital power generation
A subscale model of a photovoltaic power system employing spectral splitting and 1000:1 concentration was fabricated and tested. The 10-in. aperture model demonstrated 15.5% efficiency with 86% of the energy produced by a GaAs solar cell and 14% of the energy produced by an Si cell. The calculated efficiency of the system using the same solar cells, but having perfect optics, would be approximately 20%. The model design, component measurements, test results, and mathematical model are presented
New continua for absorption spectroscopy from 40 to 2000 Å
The spectra of plasmas produced by focusing the output of a Q-switched ruby laser (output 1 J) on the rare-earth metals have been studied. From samarium (Z = 70), strong quasi-uniform continua are emitted in the wavelength range 40–2000 Å. Line emission from the target elements is absent over most of this wavelength region, particularly below about 600 Å. The use of these continua as simple, reliable background sources for absorption spectroscopy in the vacuum-ultraviolet and soft x-ray region down to 40 Å is demonstrated
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
Standard Transistor Array (STAR). Volume 1, addendum 1: CAPSTAR user's guide
The cell placement techniques developed for use with the standard transistor array were incorporated in the cell arrangement program for STAR (CAPSTAR). Instructions for use of this program are given
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
Advanced fiber placement of composite fuselage structures
The Hercules/NASA Advanced Composite Technology (ACT) program will demonstrate the low cost potential of the automated fiber placement process. The Hercules fiber placement machine was developed for cost effective production of composite aircraft structures. The process uses a low cost prepreg tow material form and achieves equivalent laminate properties to structures fabricated with prepreg tape layup. Fiber placement demonstrations planned for the Hercules/NASA program include fabrication of stiffened test panels which represent crown, keel, and window belt segments of a typical transport aircraft fuselage
The evolution of organizational niches : U.S. automobile manufacturers, 1885-1981.
Although the niche figures prominently in contemporary theories of organization, analysts often fail to tie micro processes within the niche to long-term changes in the broader environment. In this paper, we advance arguments about the relationship between an organization's niche and evolution in the structure of its organizational population over time. We focus on the technological niche and processes of positioning and crowding among firms in the niche space, relating them to the level of concentration among all firms in the market. Building on previous empirical studies in organizational ecology, we study the evolution of concentration in the American automobile industry from 1885 to 1981 and estimate models of the hazard of exit of individual producers from the market. The findings show that niche and concentration interact in complex ways, yielding a more unified depiction of organizational evolution than typically described or reported
Tripropellant combustion process
The addition of small amounts of hydrogen to the combustion of LOX/hydrocarbon propellants in large rocket booster engines has the potential to enhance the system stability. Programs being conducted to evaluate the effects of hydrogen on the combustion of LOX/hydrocarbon propellants at supercritical pressures are described. Combustion instability has been a problem during the development of large hydrocarbon fueled rocket engines. At the higher combustion chamber pressures expected for the next generation of booster engines, the effect of unstable combustion could be even more destructive. The tripropellant engine cycle takes advantage of the superior cooling characteristics of hydrogen to cool the combustion chamber and a small amount of the hydrogen coolant can be used in the combustion process to enhance the system stability. Three aspects of work that will be accomplished to evaluate tripropellant combustion are described. The first is laboratory demonstration of the benefits through the evaluation of drop size, ignition delay and burning rate. The second is analytical modeling of the combustion process using the empirical relationship determined in the laboratory. The third is a subscale demonstration in which the system stability will be evaluated. The approach for each aspect is described and the analytical models that will be used are presented
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&
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