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PURPOSE: Cerebral small vessel disease (cSVD) involves several pathologies affecting the small vessels, including blood-brain barrier (BBB) impairment. Dynamic susceptibility contrast (DSC) MRI is sensitive to both blood perfusion and BBB leakage, and correction methods may be crucial for obtaining reliable perfusion measures. These methods might also be applicable to detect BBB leakage itself. This study investigated to what extent DSC-MRI can measure subtle BBB leakage in a clinical feasibility setting.METHODS: In vivo DCE and DSC data were collected from fifteen cSVD patients (71 (±10) years, 6F/9M) and twelve elderly controls (71 (±10) years, 4F/8M). DSC-derived leakage fractions were obtained using the Boxerman-Schmainda-Weisskoff method (K2). K2 was compared with the DCE-derived leakage rate Ki, obtained from Patlak analysis. Subsequently, differences were assessed between white matter hyperintensities (WMH), cortical gray matter (CGM), and normal-appearing white matter (NAWM). Additionally, computer simulations were performed to assess the sensitivity of DSC-MRI to BBB leakage.RESULTS: K2 showed significant differences between tissue regions (P < 0.001 for CGM-NAWM and CGM-WMH, and P = 0.001 for NAWM-WMH). Conversely, according to the computer simulations the DSC sensitivity was insufficient to measure subtle BBB leakage, as the K2 values were below the derived limit of quantification (4∙10-3 min-1). As expected, Ki was elevated in the WMH compared to CGM and NAWM (P < 0.001).CONCLUSIONS: Although clinical DSC-MRI seems capable to detect subtle BBB leakage differences between WMH and normal-appearing brain tissue it is not recommended. K2 as a direct measure for subtle BBB leakage remains ambiguous as its signal effects are due to mixed T1- and T2∗-weighting. Further research is warranted to better disentangle perfusion from leakage effects.

When modeling thermoacoustic (TA) devices at high amplitude, nonlinear effects such as time-average mass flows, and the generation of higher harmonics can no longer be neglected. Thus far, modeling these effects in TA devices required a generally computationally costly time integration of the nonlinear governing equations. In this paper, a fast one-dimensional nonlinear model for TA devices is presented, which omits this costly time integration by directly solving the periodic steady state. The model is defined in the frequency domain, which eases the implementation of phase delays due to viscous resistance and thermoacoustic heat exchange. As a demonstration, the model is used to solve an experimental standing wave thermoacoustic engine. The obtained results agree with experimental results, as well as with results from a nonlinear time domain model from the literature. The low computational cost of this model opens the possibility to do optimization studies using a nonlinear TA model.

Contains fulltext : 181970.pdf (Publisher’s version ) (Open Access) Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm 0.5 Pr 0.5 )FeO 3 and (Sm 0.55 Tb 0.45 )FeO 3 , which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium. 10 p.

A one-dimensional (1D) laminar oscillating flow heat transfer model is derived and applied to parallel-plate thermoacoustic heat exchangers. The model can be used to estimate the heat transfer from the solid wall to the acoustic medium, which is required for the heat input/output of thermoacoustic systems. The model is implementable in existing (quasi-)1D thermoacoustic codes, such as DeltaEC. Examples of generated results show good agreement with literature results. The model allows for arbitrary wave phasing; however, it is shown that the wave phasing does not significantly influence the heat transfer.

The information content of data on rotationally periodic recurrent discrete absorption components (DACs) in hot star wind emission lines is discussed. The data comprise optical depths tau(w,phi) as a function of dimensionless Doppler velocity w=(Deltalambda/lambda(0))(c/v(infinity)) and of time expressed in terms of stellar rotation angle phi. This is used to study the spatial distributions of density, radial and rotational velocities, and ionisation structures of the corotating wind streams to which recurrent DACs are conventionally attributed. The simplifying assumptions made to reduce the degrees of freedom in such structure distribution functions to match those in the DAC data are discussed and the problem then posed in terms of a bivariate relationship between tau(w, phi) and the radial velocity v(r)(r), transverse rotation rate Omega(r) and density rho(r, phi) structures of the streams. The discussion applies to cases where: the streams are equatorial; the system is seen edge on; the ionisation structure is approximated as uniform; the radial and transverse velocities are taken to be functions only of radial distance but the stream density is allowed to vary with azimuth. The last kinematic assumption essentially ignores the dynamical feedback of density on velocity and the relationship of this to fully dynamical models is discussed. The case of narrow streams is first considered, noting the result of Hamann et al. (2001) that the apparent acceleration of a narrow stream DAC is higher than the acceleration of the matter itself, so that the apparent slow acceleration of DACs cannot be attributed to the slowness of stellar rotation. Thus DACs either involve matter which accelerates slower than the general wind flow, or they are formed by structures which are not advected with the matter flow but propagate upstream (such as Abbott waves). It is then shown how, in the kinematic model approximation, the radial speed of the absorbing matter can be found by inversion of the apparent acceleration of the narrow DAC, for a given rotation law. The case of broad streams is more complex but also more informative. The observed tau(w,phi) is governed not only by v(r)(r) and Omega(r) of the absorbing stream matter but also by the density profile across the stream, determined by the azimuthal (phi(0)) distribution function F- 0(phi(0)) of mass loss rate around the stellar equator. When F-0(phi(0)) is fairly wide in phi(0), the acceleration of the DAC peak tau(w, phi) in w is generally slow compared with that of a narrow stream DAC and the information on v(r)(r), Omega(r) and F-0(phi(0)) is convoluted in the data tau(w, phi). We show that it is possible, in this kinematic model, to recover by inversion, complete information on all three distribution functions v(r)(r), Omega(r) and F- 0(phi(0)) from data on tau(w, phi) of sufficiently high precision and resolution since v(r)(r) and Omega(r) occur in combination rather than independently in the equations. This is demonstrated for simulated data, including noise effects, and is discussed in relation to real data and to fully hydrodynamic models

In this paper, we use the very recent spectropolarimetric observations of β Cep collected by Henrichs et al. and propose for this star a consistent model of the large-scale magnetic field and of the associated magnetically confined wind and circumstellar environment. A re-examination of the fundamental parameters of β Cep in the light of the Hipparcos parallax indicates that this star is most likely a 12-M⊙ star with a radius of 7 R⊙, effective temperature of 26 000 K and age of 12 Myr, viewed with an inclination of the rotation axis of about 60°. Using two different modelling strategies, we obtain that the magnetic field of β Cep can be approximately described as a dipole with a polar strength of , the axis of symmetry of which is tilted with respect to the rotation axis by about . Although one of the weakest detected to date, this magnetic field is strong enough to magnetically confine the wind of β Cep up to a distance of about 8 to 9 R∗. We find that both the X-ray luminosity and variability of β Cep can be explained within the framework of the magnetically confined wind-shock model of Babel & Montmerle, in which the stellar-wind streams from both magnetic hemispheres collide with each other in the magnetic equatorial plane, producing a strong shock, an extended post-shock region and a high-density cooling disc. By studying the stability of the cooling disc, we obtain that field lines can support the increasing disc weight for no more than a month before they become significantly elongated in an effort to equilibrate the gravitational plus centrifugal force, thereby generating strong field gradients across the disc. The associated current sheet eventually tears, forcing the field to reconnect through resistive diffusion and the disc plasma to collapse towards the star. We propose that this collapse is the cause for the recurrent Be episodes of β Cep, and finally discuss the applicability of this model to He peculiar, classical Be and normal non-supergiant B stars.

We present the results of an extensive observing campaign on the O7.5 III star Persei. The UV obser- vations were obtained with the International Ultraviolet Explorer. Per was monitored continuously in October 1994 during 10 days at ultraviolet and visual wavelengths. The ground-based optical observations include mag- netic eld measurements, H and He i 6678 spectra, and were partially covered by photometry and polarimetry. We describe a method to automatically remove the variable contamination of telluric lines in the groundbased spectra. The aim of this campaign was to search for the origin of the cyclical wind variability in this star. We determined a very accurate period of 2.086(2) d in the resonance lines of Si iv and in the subordinate Niv and H line proles. The epochs of maximum absorption in the UV resonance lines due to discrete absorption com- ponents (DACs) coincide in phase with the maxima in blue-shifted H absorption. This implies that the periodic variability originates close to the stellar surface. The phase velocity relation shows a maximum at 1400 km s 1 . The general trend of these observations can be well explained by the corotating interaction region (CIR) model. In this model the wind is perturbed by one or more xed patches on the stellar surface, which are most probably due to small magnetic eld structures. Our magnetic eld measurements gave, however, only a null-detection with a1 errorbar of 70 G in the longitudinal component. Some observations are more dicult to t into this picture. The 2-day period is not detected in the photospheric/transition region line He i 6678. The dynamic spectrum of this line shows a pattern indicating the presence of non-radial pulsation, consistent with the previously reported period of 3.5 h. The edge variability around 2300 km s 1 in the saturated wind lines of C iv and N v is nearly identical to the edge variability in the unsaturated Si iv line, supporting the view that this type of variability is also due to the moving DACs. A detailed analysis using Fourier reconstructions reveals that each DAC actually consists of 2 dierent components: a \fast" and a \slow" one which merge at higher velocities.

New circular spectropolarimetric observations of the B1 IIIe star β Cep (υsini = 25 km s−1) show a sinusoidally varying weak longitudinal magnetic field (~ 200 G peak-to-peak). The period corresponds to the 12 day period in the stellar wind variations observed in ultraviolet spectral lines. Maximum field occurs at maximum emission in the UV wind lines. This gives compelling evidence for a magnetic-rotator model for this star, with an unambiguous rotation period of 12 days.The similarity between the Hα emission phases in β Cep and in Be stars suggests that the origin of the Be phenomenon does not have to be rapid rotation: we propose that in β Cep the velocity to bring material in (Keplerian) orbit is provided by the high corotation velocity at the Alfvén radius (~10 R*), whereas in Be stars this is done by the rapid rotation of the surface. In both cases the cause of the emission phases has still to be found. Weak temporary magnetic fields remain the strongest candidate.A full paper, with results including additional measurements in June and July 1999, will appear in A & A.

We present results from the analysis of spectra of the fast rotating active giant star FK Comae Berenices. The extended emission is interpreted as arising in giant active loops and prominences. The absorption is due to a shell of cold and dense gas (like solar filaments).

Although the star itself is not He enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star beta Cep are similar to what is observed in magnetic He-peculiar B stars, suggesting that beta Cep is magnetic. We searched for a magnetic field using spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data. Using 130 time-resolved circular polarisation spectra, obtained with the MuSiCoS spectropolarimeter at the 2m TBL from 1998 until 2005, we applied the least-squares deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on EW measurements of UV wind lines obtained over 17 years. We also analysed the short- and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion. beta Cep hosts a sinusoidally varying magnetic field with an amplitude 97(4) G and an average value -6(3) G. From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability. This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with the ~90-y orbit around its Be-star binary companion., 14 pages, 10 figures, 5 tables

On-the-job training can be defined as the enhancement of job competence acquisition, involving one or more of the following elements: (i) the actual work processes, (ii) the physical work environment, and (iii) the social work environment. Research into on-the-job is in its infancy, although it comprises a substantial portion of employee training. There is an important distinction between structured and unstructured on the-job training, but structure is just one of the dimensions needed to describe the variety of forms on-the-job training can take. Recent studies offer insights into conditions, processes, and effects of on-the-job training. These insights should be integrated in a theory on on-the-job training.

Time-resolved magneto-optical imaging of laser-excited rare-earth orthoferrite $(\mathrm{SmPr}){\mathrm{FeO}}_{3}$ demonstrates that a single 60 fs circularly polarized laser pulse is capable of creating a magnetic domain on a picosecond time scale with a magnetization direction determined by the helicity of light. Depending on the light intensity and sample temperature, pulses of the same helicity can create domains with opposite magnetizations. We argue that this phenomenon relies on a twofold effect of light which (i) instantaneously excites coherent low-amplitude spin precession and (ii) triggers a spin reorientation phase transition. The former dynamically breaks the equivalence between two otherwise degenerate states with opposite magnetizations in the high-temperature phase and thus controls the route of the phase transition.

Using 100-fs optical laser pulses, we have been able to excite and probe spin dynamics in the rare-earth orthoferrite ${\mathrm{ErFeO}}_{3}$. The investigation was performed in a broad temperature range with the focus on the vicinities of the compensation point ${T}_{\mathrm{comp}}\ensuremath{\approx}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the spin reorientation transition region in the interval $86\phantom{\rule{0.16em}{0ex}}\mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}99\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Spin precession excited by the laser pulse was present in a large part of the investigated temperature range, but was especially strong near the spin reorientation region. In this region the laser pulse also caused an ultrafast spin reorientation. By changing the laser pulse fluence, we could vary both the reorientation amplitude and the reorientation speed. We show that the laser-induced spin dynamics in ${\mathrm{ErFeO}}_{3}$ is caused in part by heating and in part by the inverse Faraday effect. Comparing to the results of similar experiments in other rare-earth orthoferrites, we found the speed of the laser-induced spin reorientation to be significantly lower. We attribute this finding to the weaker electron-phonon coupling of the ${\mathrm{Er}}^{3+}$ 4f electrons with the lattice.

Cyclical wind variability is an ubiquitous but as yet unexplained feature among OB stars. The O7.5 III(n)((f)) star xi Persei is the brightest representative of this class on the Northern hemisphere. As its prominent cyclical wind properties vary on a rotational time scale (2 or 4 days) the star has been already for a long time a serious magnetic candidate. As the cause of this enigmatic behavior non-radial pulsations and/or a surface magnetic field are suggested. We present a preliminary report on our attempts to detect a magnetic field in this star with high-resolution measurements obtained with the spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in 2006 and 5 nights in 2007. Only upper limits could be obtained, even with the longest possible exposure times. If the star hosts a magnetic field, its surface strength should be less than about 300 G. This would still be enough to disturb the stellar wind significantly. From our new data it seems that the amplitude of the known non-radial pulsations has changed within less than a year, which needs further investigation., Comment: 2 pages, 6 figures, contributed poster at IAU Symposium 259 "Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife, Spain, November 3-7, 2008

We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy) campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in the Balmer lines and in Na D. Our observations support the suggestion that the wind and infall signatures are out of phase on SU Aur. We present Doppler images of SU Aur that have been obtained from least-squares deconvolved profiles. Images taken about one rotation apart show only limited overlap, in particular at low latitudes. This is in part due to limitations in signal-to-noise, and in part due to line profile deformations that arise from short-lived and/or non-surface features. The agreement at high latitudes is better and suggests that at least some longer-lived features are present. The analysis of Stokes V profiles yields a marginal magnetic field detection during one of the phases., Comment: 22 pages, 20 figures, accepted for publication in MNRAS

Although thermally sprayed aluminum (TSA) has come to be recognized as an effective corrosion protection treatment, its use has been limited to off-shore structures. The fact that TSA coatings were unsuccessful as an alternative to traditional organic coatings has more to do with economics than technology. This paper presents life cycle cost models that justify the much higher level of investment required with TSA spraying while accounting for its ecological advantages in terms of energy, waste, and volatile emissions. Paper includes a German-language abstract.

We present preliminary results of the spectroscopic monitoring of AB Aur obtained during the MUSICOS 96 campaign. The analysis is mainly focussed on the He I D3 line, on the Hα line, and on a set of photospheric lines. The star was monitored irregularly for more than 200 hours.

We have measured the brightness of the optical counterpart of the black hole X-ray binary Nova Sco 1994 (GRO J1655-40) during 40 nights between 1995 May 3 and July 27. From our observations and data from the literature we refine the orbital period to be 2.620 40 ± 0.000 98 d. We model the R-band light curve as primarily arising from an X-ray heated secondary, using a model that includes a Roche lobe filling secondary star, the effects of X-ray heating on both the concave accretion disc and the X-ray illuminated surface of the secondary, shadowing of the secondary and disc, and mutual eclipses of the disc and the secondary star. From the shape of the light curve we constrain the inclination of the system to lie in the range 65°-76° and constrain the mass ratio q = M1/M2 to lie in the range 3.8-5.5. This implies a mass range for the secondary star and compact object of 0.95-1.8 M⊙ and 4.9-6.8 M⊙, respectively. The (V-R) and (R-i) colour curves do not show evidence of significant variability with orbital phase during 1995 June-July, their average values being 1.06(3) and 0.86(2) mag, respectively. © 1997 RAS.