Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure

Puzzling trends in surface stress were reported experimentally for Ni/W(110) as a function of Ni coverage. In order to explain this behavior, we have performed a density-functional-theory study of the surface stress and atomic structure of the pseudomorphic and of several different possible 1x7 configurations for this system. For the 1x7 phase, we predict a different, more regular atomic structure than previously proposed based on surface x-ray diffraction. At the same time, we reproduce the unexpected experimental change of surface stress between the pseudomorphic and 1x7 configuration along the crystallographic surface direction which does not undergo density changes. We show that the observed behavior in the surface stress is dominated by the effect of a change in Ni adsorption/coordination sites on the W(110) surface.Comment: 14 pages, 3 figures Published in J. Phys.: Condens. Matter 24 (2012) 13500

The rapid evolution of the exciting star of the Stingray Nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&

Imaging Electron Wave Functions of Quantized Energy Levels in Carbon Nanotubes

Carbon nanotubes provide a unique system to study one-dimensional quantization phenomena. Scanning tunneling microscopy is used to observe the electronic wave functions that correspond to quantized energy levels in short metallic carbon nanotubes. Discrete electron waves are apparent from periodic oscillations in the differential conductance as a function of the position along the tube axis, with a period that differs from that of the atomic lattice. Wave functions can be observed for several electron states at adjacent discrete energies. The measured wavelengths are in good agreement with the calculated Fermi wavelength for armchair nanotubes.Comment: 11 pages, 4 figures in seperate PDF fil

Diamond thin Film Detectors for Beam Monitoring Devices

Diamonds offer radiation hard sensors, which can be used directly in primary beams. Here we report on the use of a polycrystalline CVD diamond strip sensor as beam monitor of heavy ion beams with up to 2.10^9 lead ions per bunch. The strips allow for a determination of the transverse beam profile to a fraction of the pitch of the strips, while the timing information yields the longitudinal bunch length with a resolution of the order of a few mm.Comment: 6 pages, 7 figures, to appear in the Proceedings of the Hasselt Diamond Workshop (Hasselt, Belgium, Feb. 2006), v4: accidentally submitted figure, appearing at end, remove

Memory Rehabilitation Strategies in Nonsurgical Temporal Lobe Epilepsy: A Review

open8siPeople with temporal lobe epilepsy (TLE) who have not undergone epilepsy surgery often complain of memory deficits. Cognitive re- habilitation is employed as a remedial intervention in clinical settings, but research is limited and findings concerning efficacy and the criteria for choosing different approaches have been inconsistent. We aimed to appraise existing evidence on memory rehabilitation in nonsurgical individuals with temporal lobe epilepsy and to ascertain the effectiveness of specific strategies. A scoping review was preferred given the het- erogeneous nature of the interventions. A comprehensive literature search using MEDLINE, EMBASE, CINAHL, AMED, Scholars Portal/ PSYCHinfo, Proceedings First, and ProQuest Dissertations and Theses identified articles published in English before February 2016. The search retrieved 372 abstracts. Of 25 eligible studies, six were included in the final review. None included pediatric populations. Strategies included cognitive training, external memory aids, brain training, and noninvasive brain stimulation. Selection criteria tended to be general. Overall, there was insufficient evidence to make definitive conclusions regarding the efficacy of traditional memory rehabilitation strategies, brain training, and noninvasive brain stimulation. The review suggests that cognitive rehabilitation in nonsurgical TLE is underresearched and that there is a need for a systematic evaluation in this population.embargoed_20180216DEL FELICE, Alessandra; Alderighi, Marzia; Martinato, Matteo; Grisafi, Davide; Bosco, Anna; Thompson, Pamela J.; Sander, Josemir W.; Masiero, StefanoDEL FELICE, Alessandra; Alderighi, Marzia; Martinato, Matteo; Grisafi, Davide; Bosco, Anna; Thompson, Pamela J.; Sander, Josemir W.; Masiero, Stefan

Observational properties of massive black hole binary progenitors

The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (PoWR), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. We provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.Comment: 64 pages, 30 figures, accepted for publication in Astronomy & Astrophysics, v2: typos correcte

Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars

Context. Metal-poor massive stars are supposed to be progenitors of certain supernovae, gamma-ray bursts and compact object mergers, potentially contributing to the early epochs of the Universe with their strong ionizing radiation. However, they remain mainly theoretical as individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the SMC. Aims. This work aims at exploring what our state-of-the-art theories of stellar evolution combined with those of stellar atmospheres predict about a certain type of metal-poor (0.02 Z$_{\odot}$) hot massive stars, the chemically homogeneously evolving ones, called TWUIN stars. Methods. Synthetic spectra corresponding to a broad range in masses (20-130 M$_{\odot}$) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage were computed. Results. We find that TWUIN stars show almost no emission lines during most of their {core hydrogen-burning} lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission and even some metal lines (oxygen and carbon, but still almost no nitrogen) show up. Mass loss and clumping play a significant role in line-formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early O type star observed in a low-metallicity galaxy could be the outcome of chemically homogeneous evolution $-$ and therefore the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe due to their being hot without showing prominent emission lines during the majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres

First principles calculation of structural and magnetic properties for Fe monolayers and bilayers on W(110)

Structure optimizations were performed for 1 and 2 monolayers (ML) of Fe on a 5 ML W(110) substrate employing the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. The magnetic moments were also obtained for the converged and optimized structures. We find significant contractions ($\sim$ 10 %) for both the Fe-W and the neighboring Fe-Fe interlayer spacings compared to the corresponding bulk W-W and Fe-Fe interlayer spacings. Compared to the Fe bcc bulk moment of 2.2 $\mu_B$, the magnetic moment for the surface layer of Fe is enhanced (i) by 15% to 2.54 $\mu_B$ for 1 ML Fe/5 ML W(110), and (ii) by 29% to 2.84 $\mu_B$ for 2 ML Fe/5 ML W(110). The inner Fe layer for 2 ML Fe/5 ML W(110) has a bulk-like moment of 2.3 $\mu_B$. These results agree well with previous experimental data

Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars

Massive stars are the key agents of feedback. Consequently, quantitative analysis of massive stars are required to understand how the feedback of these objects shapes/ creates the large scale structures of the ISM. The giant HII region N206 in the Large Magellanic Cloud contains an OB association that powers a X-ray superbubble, serving as an ideal laboratory in this context. We obtained optical spectra with the muti-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we use the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical parameters and nitrogen abundances of our sample stars are determined by fitting synthetic spectra to the observations. The stellar and wind parameters of nine Of-type stars are used to construct wind momentum,luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant which has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age less than 4 million years, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.Comment: Accepted for the pubblication in Astronomy & Astrophysic