arXiv subjects

M. Kraus

Publications and source records attributed to M. Kraus.

At least 73 records · Page 4Linked to original sources

arXiv2015

Matching the Nagy-Soper parton shower at next-to-leading order

We present an MC@NLO-like matching of next-to-leading order QCD calculations with the Nagy-Soper parton shower. An implementation of the algorithm within the HELAC-DIPOLES Monte Carlo generator is used to address the uncertainties and ambiguities of the matching scheme. First results obtained using the Nagy-Soper parton shower implementation in DEDUCTOR in conjunction with the HELAC-NLO framework are given for the pp -> top anti-top j + X process at the LHC with sqrt(s)=8 TeV. Effects of resummation are discussed for various observables.

hep-ph↗

arXiv2015

An infrared diagnostic for magnetism in hot stars

Magnetospheric observational proxies are used for indirect detection of magnetic fields in hot stars in the X-ray, UV, optical, and radio wavelength ranges. To determine the viability of infrared (IR) hydrogen recombination lines as a magnetic diagnostic for these stars, we have obtained low-resolution (R~1200), near-IR spectra of the known magnetic B2V stars HR 5907 and HR 7355, taken with the Ohio State Infrared Imager/Spectrometer (OSIRIS) attached to the 4.1m Southern Astrophysical Research (SOAR) Telescope. Both stars show definite variable emission features in IR hydrogen lines of the Brackett series, with similar properties as those found in optical spectra, including the derived location of the detected magnetospheric plasma. These features also have the added advantage of a lowered contribution of stellar flux at these wavelengths, making circumstellar material more easily detectable. IR diagnostics will be useful for the future study of magnetic hot stars, to detect and analyze lower-density environments, and to detect magnetic candidates in areas obscured from UV and optical observations, increasing the number of known magnetic stars to determine basic formation properties and investigate the origin of their magnetic fields.

astro-ph.SR↗

arXiv2014

Evidence of the Evolved Nature of the B[e] Star MWC 137

The evolutionary phase of B[e] stars is difficult to establish due to the uncertainties in their fundamental parameters. For instance, possible classifications for the Galactic B[e] star MWC 137 include pre-main-sequence and post-main-sequence phases, with a large range in luminosity. Our goal is to clarify the evolutionary stage of this peculiar object, and to study the CO molecular component of its circumstellar medium. To this purpose, we modeled the CO molecular bands using high-resolution K-band spectra. We find that MWC 137 is surrounded by a detached cool (T = 1900 +-100 K) and dense (N = (3 +- 1) x 10^21 cm^(-2)) ring of CO gas orbiting the star with a rotational velocity, projected to the line of sight, of 84 +- 2 km/s. We also find that the molecular gas is enriched in the isotope 13C, excluding the classification of the star as a Herbig Be. The observed isotopic abundance ratio (12C / 13C = 25 +- 2) derived from our modeling is compatible with a proto-PN, main-sequence or supergiant evolutionary phase. However, based on some observable characteristics of MWC 137, we propose that the supergiant scenario seems to be the most plausible. Hence, we suggest that MWC 137 could be in an extremely short-lived phase, evolving from a B[e] supergiant to a blue supergiant with a bipolar ring nebula.

astro-ph.SR↗

arXiv2014

A K-band spectral mini-survey of Galactic B[e] stars

We present a mini-survey of Galactic B[e] stars mainly undertaken with the Large Binocular Telescope (LBT). B[e] stars show morphological features with hydrogen emission lines and an infrared excess, attributed to warm circumstellar dust. In general, these features are assumed to arise from dense, non-spherical, disk-forming circumstellar material in which molecules and dust can condensate. Due to the lack of reliable luminosities, the class of Galactic B[e] stars contains stars at very different stellar evolutionary phases like Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit K-band spectra for a sample of Galactic B[e] stars with the LBT-Luci I. Prominent spectral features, such as the Brackett gamma line and CO band heads are identified in the spectra. The analysis shows that the stars can be characterized as evolved objects. Among others we find one LBV candidate (MWC314), one supergiant B[e] candidate with 13CO (MWC137) and in two cases (MWC623 and AS 381) indications for the existence of a late-type binary companion, complementary to previous studies. For MWC84, IR spectra were taken at different epochs with LBT-Luci I and the GNIRS spectrograph at the Gemini North telescope. The new data show the disappearance of the circumstellar CO emission around this star, previously detectable over decades. Also no signs of a recent prominent eruption leading to the formation of new CO disk emission are found during 2010 and 2013.

astro-ph.SR↗

arXiv2014

Self-consistent stationary MHD shear flows in the solar atmosphere as electric field generators

Magnetic fields and flows in coronal structures, for example, in gradual phases in flares, can be described by 2D and 3D magnetohydrostatic (MHS) and steady magnetohydrodynamic (MHD) equilibria. Within a physically simplified, but exact mathematical model, we study the electric currents and corresponding electric fields generated by shear flows. Starting from exact and analytically calculated magnetic potential fields, we solveid the nonlinear MHD equations self-consistently. By applying a magnetic shear flow and assuming a nonideal MHD environment, we calculated an electric field via Faraday's law. The formal solution for the electromagnetic field allowed us to compute an expression of an effective resistivity similar to the collisionless Speiser resistivity. We find that the electric field can be highly spatially structured, or in other words, filamented. The electric field component parallel to the magnetic field is the dominant component and is high where the resistivity has a maximum. The electric field is a potential field, therefore, the highest energy gain of the particles can be directly derived from the corresponding voltage. In our example of a coronal post-flare scenario we obtain electron energies of tens of keV, which are on the same order of magnitude as found observationally. This energy serves as a source for heating and acceleration of particles.

astro-ph.SR↗

arXiv2013

Probing the ejecta of evolved massive stars in transition: A VLT/SINFONI K-band survey

Massive evolved stars in transition phases, such as Luminous Blue Variables (LBVs), B[e] Supergiants (B[e]SGs), and Yellow Hypergiants (YHGs), are not well understood, and yet crucial steps in determining accurate stellar and galactic evolution models. The circumstellar environments of these stars reveal their mass-loss history, identifying clues to both their individual evolutionary status and the connection between objects of different phases. Here we present a survey of 25 such evolved massive stars (16 B[e]SGs, 6 LBVs, 2 YHGs, and 1 Peculiar Oe star), observed in the K-band with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 8 m telescope. The sample can be split into two categories based on spectral morphology: one group includes all of the B[e]SGs, the Peculiar Oe star, and two of the LBVs, while the other includes the YHGs and the rest of the LBVs. The difference in LBV spectral appearance is due to some objects being in a quiescent phase and some objects being in an active or outburst phase. CO emission features are found in 13 of our targets, with first time detections for MWC 137, LHA 120-S 35, and LHA 115-S 65. From model fits to the CO band heads, the emitting regions appear to be detached from the stellar surface. Each star with ^12CO features also shows ^13CO emission, signaling an evolved nature. Based on the level of ^13C enrichment, we conclude that many of the B[e]SGs are likely in a pre-Red Supergiant phase of their evolution. There appears to be a lower luminosity limit of log L/L_solar = 5.0 below which CO is not detected. The lack of CO features in several high luminosity B[e]SGs and variability in others suggests that they may in fact be LBV candidates, strengthening the connection between these two very similar transition phases.

astro-ph.SR↗

arXiv2012

Unveiling the structure and kinematics of B[e] stars' disks from FEROS and CRIRES spectra

We are investigating the circumstellar material for a sample of B[e] stars using high spectral resolution data taken in the optical and near-infrared regions with ESO/FEROS and ESO/CRIRES spectrographs, respectively. B[e] stars are surrounded by dense disks of still unknown origin. While optical emission lines from [O I] and [Ca II] reflect the disk conditions close to the star (few stellar radii), the near-infrared data, especially the CO band emission, mirror the characteristics in the molecular part of the disk farther away from the star (several AU). Based on our high resolution spectroscopic data, we seek to derive the density and temperature structure of the disks, as well as their kinematics. This will allow us to obtain a better understanding of their structure, formation history and evolution. Here we present our preliminary results.

astro-ph.SR↗

arXiv2012

Discovery of Raman-scattered lines in the massive luminous emission-line star LHA 115-S 18

LHA 115-S 18 is a very peculiar emission-line star exhibiting the B[e] phenomenon. Located in the Small Magellanic Cloud, its spectrum shows features of an extremely wide range of excitation and ionization stages, extending from highly ionized atomic lines (Si IV, C IV, He II) in the UV and optical regions to molecular emission bands of CO and TiO in the optical and IR regions. The most distinguishing spectral characteristic of LHA 115-S 18 is the high variability detected in the He II λ4686 emission line, which can be a very conspicuous or completely invisible feature. In this work, we report on another peculiarity of LHA 115-S 18. From high-resolution optical spectra taken between 2000 and 2008, we discovered the appearance and strengthening of two emission features at λ6825 Å, and λ7082 Å,, which we identified as Raman-scattered lines. This is the first time these lines have been detected in the spectrum of a massive luminous B[e] star. As the classification of LHA 115-S 18 is highly controversial, we discuss how the discovery of the appearance of Raman-scattered lines in this peculiar star might help us to solve this puzzle.

astro-ph.SR↗

arXiv2012

The sudden appearance of CO emission in LHA 115-S 65

Molecular emission has been detected in several Magellanic Cloud B[e] supergiants. In this Letter, we report on the detection of CO band head emission in the B[e] supergiant LHA 115-S 65, and present a K-band near-infrared spectrum obtained with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R=4500) on the ESO VLT UT4 telescope. The observed molecular band head emission in S 65 is quite surprising in light of a previous non-detection by McGregor et al. 1989, as well as a high resolution (R=50000) Gemini/Phoenix spectrum of this star taken nine months earlier showing no emission. Based on analysis of the optical spectrum by Kraus et al. 2010, we suspect that the sudden appearance of molecular emission could be due to density build up in an outflowing viscous disk, as seen for Be stars. This new discovery, combined with variability in two other similar evolved massive stars, indicates an evolutionary link between B[e] supergiants and LBVs.

astro-ph.SR↗

arXiv2012

A new observational tracer for high-density disc-like structures around B[e] supergiants

The disc formation mechanism of B[e] supergiants is one of the puzzling phenomena in massive star evolution. Rapid stellar rotation seems to play an important role for the non-spherically symmetric mass-loss leading to a high-density disc or ring-like structure of neutral material around these massive and luminous objects. The radial density and temperature structure as well as the kinematics within this high-density material are, however, not well studied. Based on high-resolution optical spectra of a sample of B[e] supergiants in the Magellanic Clouds we especially searched for tracers of the kinematics within their discs. Besides the well-known [O I] lines, we discovered the [Ca II] λλ7291, 7324 lines that can be used as a complementary set of disc tracers. We find that these lines originate from very high-density regions, located closer to the star than the [O I] λ5577 line-forming region. The line profiles of both the [O I] and the [Ca II] lines indicate that the discs or rings of high-density material are in Keplerian rotation. We estimate plausible ranges of disc inclination angles for the sample of B[e] supergiants and suggest that the star LHA 120-S 22 might have a spiral arm rather than a disc.

astro-ph.SR↗

arXiv2011

On the nature of the Be star HR 7409 (7 Vul)

HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt and is the massive component of a 69-day spectroscopic binary. The binary parameters and properties of the Be star measured using high-dispersion spectra obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young (<50-80 Myr), then the companion is a K V star, but, following another, older evolutionary scenario, the companion is a horizontal-branch star or possibly a white dwarf star. In the latter scenario, a past episode of mass transfer from an evolved star onto a less massive dwarf star would be responsible for the peculiar nature of the present-day, fast-rotating Be star.

astro-ph.SR↗

arXiv2010

The $^{13}$Carbon footprint of B[e] supergiants

We report on the first detection of $^{13}$C enhancement in two B[e] supergiants in the Large Magellanic Cloud. Stellar evolution models predict the surface abundance in $^{13}$C to strongly increase during main-sequence and post-main sequence evolution of massive stars. However, direct identification of chemically processed material on the surface of B[e] supergiants is hampered by their dense, disk-forming winds, hiding the stars. Recent theoretical computations predict the detectability of enhanced $^{13}$C via the molecular emission in $^{13}$CO arising in the circumstellar disks of B[e] supergiants. To test this potential method and to unambiguously identify a post-main sequence B[e]SG by its $^{13}$CO emission, we have obtained high-quality $K$-band spectra of two known B[e] supergiants in the Large Magellanic Cloud, using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI). Both stars clearly show the $^{13}$CO band emission, whose strength implies a strong enhancement of $^{13}$C, in agreement with theoretical predictions. This first ever direct confirmation of the evolved nature of B[e] supergiants thus paves the way to the first identification of a Galactic B[e] supergiant.

astro-ph.SR↗

arXiv2010

Two new hot subdwarf binaries in the GALEX survey

We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844, were selected from a joint optical-ultraviolet catalogue of hot sub-luminous stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using high-dispersion spectra of the Halpha core obtained using the 2m telescope at Ondrejov Observatory we measured the radial velocities of the sdB primaries and determined orbital periods of 0.26584+/-0.00004 days and 0.46249+/-0.00007 days for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series obtained from the Northern Sky Variability Survey with an effective wavelength near the R band show that GALEX J0321+4727 is a variable star (Delta m=0.12 mag) while no significant variations are observed in GALEX J2349+3844. The period of variations in GALEX J0321+4727 coincides with the orbital period and the variability is probably caused by a reflection effect on a late-type secondary star. Lack of photometric variations in GALEX J2349+3844 probably indicates that the companion is a white dwarf star. Using all available photometry and spectroscopy, we measured the atmospheric properties of the two sdB stars and placed limits on the mass and luminosity of the companion stars.

astro-ph.SR↗

arXiv2010

Transport properties of copper phthalocyanine based organic electronic devices

Ambipolar charge carrier transport in Copper phthalocyanine (CuPc) is studied experimentally in field-effect transistors and metal-insulator-semiconductor diodes at various temperatures. The electronic structure and the transport properties of CuPc attached to leads are calculated using density functional theory and scattering theory at the non-equilibrium Green's function level. We discuss, in particular, the electronic structure of CuPc molecules attached to gold chains in different geometries to mimic the different experimental setups. The combined experimental and theoretical analysis explains the dependence of the mobilityand the transmission coefficient on the charge carrier type (electrons or holes) and on the contact geometry. We demonstrate the correspondence between our experimental results on thick films and our theoretical studies of single molecule contacts. Preliminary results for fluorinated CuPc are discussed.

cond-mat.mes-hall↗

arXiv2008

Compensation-dependent in-plane magnetization reversal processes in Ga1-xMnxP1-ySy

We report the effect of dilute alloying of the anion sublattice with S on the in-plane uniaxial magnetic anisotropy and magnetization reversal process in Ga1-xMnxP as measured by both ferromagnetic resonance (FMR) and superconducting quantum interference device (SQUID) magnetometry. At T=5K, raising the S concentration increases the uniaxial magnetic anisotropy between in-plane <011> directions while decreasing the magnitude of the (negative) cubic anisotropy field. Simulation of the SQUID magnetometry indicates that the energy required for the nucleation and growth of domain walls decreases with increasing y. These combined effects have a marked influence on the shape of the field-dependent magnetization curves; while the direction remains the easy axis in the plane of the film, the field dependence of the magnetization develops double hysteresis loops in the [011] direction as the S concentration increases similar to those observed for perpendicular magnetization reversal in lightly doped Ga1-xMnxAs. The incidence of double hysteresis loops is explained with a simple model whereby magnetization reversal occurs by a combination of coherent spin rotation and noncoherent spin switching, which is consistent with both FMR and magnetometry experiments. The evolution of magnetic properties with S concentration is attributed to compensation of Mn acceptors by S donors, which results in a lowering of the concentration of holes that mediate ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2007

Suppression of hole-mediated ferromagnetism in GaMnP by hydrogen

We report the successful passivation of the Mn acceptors in GaMnP upon exposure to a remote dc hydrogen plasma. The as-grown films are non-metallic and ferromagnetic with a Curie temperature of T_C=55K. After hydrogenation the sample resistivity increases by approximately three orders of magnitude at room temperature and six orders of magnitude at 25 K. Furthermore, the hydrogenated samples are paramagnetic, which is evidenced by a magnetization curve at 5 K that is best described by a Brillouin function with g=2 and J=5/2 expected for Mn atoms in the 3d^5 configuration. These observations unambiguously proof that the ferromagnetism is carrier-mediated also in GaMnP.

cond-mat.mtrl-sci↗

arXiv2007

Magnetocrystalline anisotropy and magnetization reversal in GaMnP synthesized by ion implantation and pulsed-laser melting

We report the observation of ferromagnetic resonance (FMR) and the determination of the magnetocrystalline anisotropy in (100)-oriented single-crystalline thin film samples of GaMnP with x=0.042. The contributions to the magnetic anisotropy were determined by measuring the angular- and the temperature-dependencies of the FMR resonance fields and by superconducting quantum interference device magnetometry. The largest contribution to the anisotropy is a uniaxial component perpendicular to the film plane; however, a negative contribution from cubic anisotropy is also found. Additional in-plane uniaxial components are observed at low temperatures, which lift the degeneracy between the in-plane [011] and [01-1] directions as well as between the in-plane [010] and [001] directions. Near T=5K, the easy magnetization axis is close to the in-plane [01-1] direction. All anisotropy parameters decrease with increasing temperature and disappear above the Curie temperature T_C. A consistent picture of the magnetic anisotropy of ferromagnetic GaMnP emerges from the FMR and magnetometry data. The latter can be successfully modeled when both coherent magnetization rotation and magnetic domain nucleation are considered.

cond-mat.mtrl-sci↗

arXiv2007

Hydrogen neutral outflowing disks of B[e] supergiants

The [O I] line emission of the LMC B[e] supergiant R126 is modeled with an outflowing disk scenario. We find that hydrogen in the disk must be ionized by less than 0.1%, meaning that the disk material is predominantly neutral. The free-free emission is calculated from the polar wind, and the minimum density contrast between disk and polar wind is found to be ~10.

astro-ph↗