2,986 research outputs found
Small, low power analog-to-digital converter
A small, low-power, high-speed, 8-bit analog-to-digital converter using silicon chip integrated circuits is suitable for use in airborne test data systems. The successive approximation method of analog-to-digital conversion is used to generate the digital output
Wind variability of B supergiants
We present the most suitable data sets available in the International Ultraviolet Explorer (IUE) archive for the study of time-dependent stellar winds in early B supergiants. The UV line profile variability in 11 B0 to B3 stars is analysed, compared and discussed, based on 16 separate data sets comprising over 600 homogeneously reduced high-resolution spectrograms. The targets include ``normal'' stars with moderate rotation rates and examples of rapid rotators. A gallery of grey-scale images (dynamic spectra) is presented, which demonstrates the richness and range of wind variability and highlights different structures in the winds of these stars. This work emphasises the suitability of B supergiants for wind studies, under-pinned by the fact that they exhibit unsaturated wind lines for a wide range of ionization. The wind activity of B supergiants is substantial and has highly varied characteristics. The variability evident in individual stars is classified and described in terms of discrete absorption components, spontaneous absorption, bowed structures, recurrence, and ionization variability and stratification. Similar structures can occur in stars of different fundamental parameters, but also different structures may occur in the same star at a given epoch. We discuss the physical phenomena that may be associated with the spectral signatures. The diversity of wind patterns evident likely reflects the role of stellar rotation and viewing angle in determining the observational characteristics of azimuthally extended structure rooted at the stellar surface. In addition, SEI line-synthesis modelling of the UV wind lines is used to provide further information about the state of the winds in our program stars. Typically the range, implied by the line profile variability, in the product of mass-loss rate and ion fraction (mdot q_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ; it can however be several times larger over localised velocity regions. At a given effective temperature the mean relative ion ratios can differ by a factor of 5. The general excess in predicted (forward-scattered) emission in the low velocity regime is discussed in terms of structured outflows. Mean ion fractions are estimated over the B0 to B1 spectral classes, and trends in the ionic ratios as a function of wind velocity are described. The low values obtained for the ion fractions of UV resonance lines may reflect the role of clumping in the wind
The effects of clumping on wind line variability
We review the effects of clumping on the profiles of resonance doublets. By
allowing the ratio of the doublet oscillator strenghts to be a free parameter,
we demonstrate that doublet profiles contain more information than is normally
utilized. In clumped (or porous) winds, this ratio can lies between unity and
the ratio of the f-values, and can change as a function of velocity and time,
depending on the fraction of the stellar disk that is covered by material
moving at a particular velocity at a given moment. Using these insights, we
present the results of SEI modeling of a sample of B supergiants, zeta Pup and
a time series for a star whose terminal velocity is low enough to make the
components of its Si IV 1400 doublet independent. These results are interpreted
within the framework of the Oskinova et al. (2007) model, and demonstrate how
the doublet profiles can be used to extract infromation about wind structure.Comment: 3 pages, to appear in Clumping in Hot Star Winds, W.-R. Hamann, A.
Feldmeier & L. Oskinova, eds., Potsdam: Univ.-Verl., 2007, URN:
http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-1398
Mass loss rates from mid-IR excesses in LMC and SMC O stars
We use a combination of BVJHK and Spitzer [3.6], [5.8] and [8.0] photometry
to determine IR excesses for a sample of 58 LMC and 46 SMC O stars. This sample
is ideal for determining IR excesses because the very small line of sight
reddening minimizes uncertainties due to extinction corrections. We use the
core-halo model developed by Lamers & Waters (1984a) to translate the excesses
into mass loss rates and demonstrate that the results of this simple model
agree with the more sophisticated CMFGEN models to within a factor of 2. Taken
at face value, the derived mass loss rates are larger than those predicted by
Vink et al. (2001), and the magnitude of the disagreement increases with
decreasing luminosity. However, the IR excesses need not imply large mass loss
rates. Instead, we argue that they probably indicate that the outer atmospheres
of O stars contain complex structures and that their winds are launched with
much smaller velocity gradients than normally assumed. If this is the case, it
could affect the theoretical and observational interpretations of the "weak
wind" problem, where classical mass loss indicators suggest that the mass loss
rates of lower luminosity O stars are far less than expected.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
Temperature dependent nucleation and propagation of domain walls in a sub-100 nm perpendicularly magnetized Co/Ni multilayer
We present a study of the temperature dependence of the switching fields in
Co/Ni-based perpendicularly magnetized spin-valves. While magnetization
reversal of all-perpendicular Co/Ni spin valves at ambient temperatures is
typically marked by a single sharp step change in resistance, low temperature
measurements can reveal a series of resistance steps, consistent with
non-uniform magnetization configurations. We propose a model that consists of
domain nucleation, propagation and annihilation to explain the temperature
dependence of the switching fields. Interestingly, low temperature (<30 K) step
changes in resistance that we associate with domain nucleation, have a bimodal
switching field and resistance step distribution, attributable to two competing
nucleation pathways.Comment: 5 pages, 4 figure
Temperature dependence of the switching field distributions in all-perpendicular spin-valve nanopillars
We present temperature dependent switching measurements of the Co/Ni
multilayered free element of 75 nm diameter spin-valve nanopillars. Angular
dependent hysteresis measurements as well as switching field measurements taken
at low temperature are in agreement with a model of thermal activation over a
perpendicular anisotropy barrier. However, the statistics of switching (mean
switching field and switching variance) from 20 K up to 400 K are in
disagreement with a N\'{e}el-Brown model that assumes a temperature independent
barrier height and anisotropy field. We introduce a modified N\'{e}el-Brown
model thats fit the experimental data in which we take a dependence
to the barrier height and the anisotropy field due to the temperature dependent
magnetization and anisotropy energy.Comment: 5 pages, 4 figure
CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?
We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star xi
Per, together with contemporaneous H alpha observations. The X-ray spectrum of
this star is similar to other single O stars, and not pathological in any way.
Its UV wind lines are known to display cyclical time variability, with a period
of 2.086 days, which is thought to be associated with co-rotating interaction
regions (CIRs). We examine the Chandra and H alpha data for variability on this
time scale. We find that the X-rays vary by about 15% over the course of the
observations and that this variability is out of phase with variable absorption
on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV
absorption associated with CIRs). While not conclusive, both sets of data are
consistent with models where the CIRs are either a source of X-rays or modulate
them.Comment: Accepted by MNRAS. 9 pages, 9 figure
Time-Resolved Magnetic Relaxation of a Nanomagnet on Subnanosecond Time Scales
We present a two-current-pulse temporal correlation experiment to study the
intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during
and following a pulse excitation. This method is applied to a model
spin-transfer system, a spin valve nanopillar with perpendicular magnetic
anisotropy. Two-pulses separated by a short delay (< 500 ps) are shown to lead
to the same switching probability as a single pulse with a duration that
depends on the delay. This demonstrates a remarkable symmetry between magnetic
excitation and relaxation and provides a direct measurement of the magnetic
relaxation time. The results are consistent with a simple finite temperature
Fokker-Planck macrospin model of the dynamics, suggesting more coherent
magnetization dynamics in this short time nonequilibrium limit than near
equilibrium
A Revised Geometry for the Magnetic Wind of theta^1 Orionis C
Theta^1 Ori is thought to be a hot analog of Bp variables because its optical
and UV line and X-ray continuum fluxes modulate regularly over the
magnetic/rotational period. A flattened magnetosphere surrounding co-rotates
with these stars, producing a periodic modulation of emission and absorption
components of the UV resonance lines, as well as of optical H and He lines. In
this paper we examine these modulations in detail and point out that the
far-blue and near-red wings of C IV and N V resonance lines exhibit
anticorrelated modulations, causing mild flux elevations at moderate redshifts
at edge-on phase (phi=0.5). However, the lines do not exhibit rest-frame
absorption features, the usual signatures of cool static disks surrounding Bp
stars. We suggest that this behavior can be explained by the existence of two
geometrically distinct wind regions separated by the local magnetic Alfven
radius. Wind streams emerging outside this point are forced outward by
radiative forces and eventually expand outward radially to infinity - this
matter produces the far-blue wing absorptions at phi=0.5. Interior streams
follow closed loops and collide at the magnetic equator with counterstreams.
There they coalesce and fall back to the star along their original field lines
- these are responsible for mild emissions at this same phase. The rapid
circulation of the interior wind component back to the star is responsible for
the absence of static disk features.Comment: 7 figure
- …