arXiv subjects

M. Wolf

Publications and source records attributed to M. Wolf.

At least 91 records · Page 5Linked to original sources

arXiv2014

Properties and nature of Be stars 30. Reliable physical properties of a semi-detached B9.5e+G8III binary BR CMi = HD 61273 compared to those of other well studied semi-detached emission-line binaries

Reliable determination of the basic physical properties of hot emission-line binaries with Roche-lobe filling secondaries is important for developing the theory of mass exchange in binaries. It is a very hard task, however, which is complicated by the presence of circumstellar matter in these systems. So far, only a small number of systems with accurate values of component masses, radii, and other properties are known. Here, we report the first detailed study of a new representative of this class of binaries, BR CMi, based on the analysis of radial velocities and multichannel photometry from several observatories, and compare its physical properties with those for other well-studied systems. BR CMi is an ellipsoidal variable seen under an intermediate orbital inclination of ~51 degrees, and it has an orbital period of 12.919059(15) d and a circular orbit. We used the disentangled component spectra to estimate the effective temperatures 9500(200) K and 4655(50) K by comparing them with model spectra. They correspond to spectral types B9.5e and G8III. We also used the disentangled spectra of both binary components as templates for the 2-D cross-correlation to obtain accurate RVs and a reliable orbital solution. Some evidence of a secular period increase at a rate of 1.1+/-0.5 s per year was found. This, together with a very low mass ratio of 0.06 and a normal mass and radius of the mass gaining component, indicates that BR CMi is in a slow phase of the mass exchange after the mass-ratio reversal. It thus belongs to a still poorly populated subgroup of Be stars for which the origin of Balmer emission lines is safely explained as a consequence of mass transfer between the binary components.

astro-ph.SR↗

arXiv2014

Ultrafast exciton formation at the ZnO(10${\overline{\textbf{1}}}$0) surface

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level $E_\text{F}$ followed by ultrafast (200 fs) formation of a surface exciton (SX). Transient screening of the Coulomb interaction reduces the SX formation probability at high excitation densities near the Mott limit. Located just below the surface, the SX are stable with regard to hydrogen-induced work function modifications and thus the ideal prerequisite for resonant energy transfer applications.

cond-mat.mtrl-sci↗

arXiv2014

Homogeneous and inhomogeneous sources of optical transition broadening in room temperature CdSe/ZnS nanocrystal quantum dots

We perform photoluminescence excitation measurements on individual CdSe/ZnS nanocrystal quantum dots (NCQDs) at room temperature to study optical transition energies and broadening. The observed features in the spectra are identified and compared to calculated transition energies using an effective mass model. The observed broadening is attributed to phonon broadening, spectral diffusion and size and shape inhomogeneity. The former two contribute the broadening transitions in individual QDs while the latter contributes to the QD-to-QD variation. We find that phonon broadening is often not the dominant contribution to transition line widths, even at room temperature, and that broadening does not necessarily increase with transition energy. This may be explained by differing magnitude of spectral diffusion for different quantum-confined states.

cond-mat.mes-hall↗

arXiv2014

The period analysis of V418 Aql, SU Boo, RV CVn, CR Cas, GV Cyg, V432 Per, and BD+42 2782

The minimum timings of eclipsing binaries V418 Aql, SU Boo, RV CVn, CR Cas, GV Cyg, V432 Per, and BD+42 2782 were collected and analyzed. Their long-term behavior was studied via period analysis, revealing a periodic term in eclipse times. We derived 576 new times of minimum. Hence, to describe the periodic variation, a thirdbody hypothesis was proposed and the resulting orbital periods are as follows: 70, 7.4, 53, 37, 27, 53, and 18 yr, respectively. For the system V432 Per an additional 9.5 yr variation was also found. The predicted minimum masses of these distant bodies were calculated and their detectability discussed. The light curves of SU Boo and RV CVn were analyzed using the PHOEBE program, resulting in physical parameters of the components. New variable stars in the field of V418 Aql were discovered.

astro-ph.SR↗

arXiv2014

Revised physical elements of the astrophysically important O9.5+O9.5V eclipsing binary system Y Cyg

Thanks to its long and rich observational history and rapid apsidal motion, the massive eclipsing binary Y Cyg represents one of the cornestones to critical tests of stellar evolution theory for massive stars. Yet, the determination of the basic physical properties is less accurate than it could be given the existing number of spectral and photometric observations. Our goal is to analyze all these data simultaneously with the new dedicated series of our own spectral and photometric observations from observatories widely separated in longitude. We obtained new series of UBV observations at three observatories separated in local time to obtain complete light curves of Y Cyg for its orbital period close to 3 days. This new photometry was reduced and carefully transformed to the standard UBV system using the HEC22 program. We also obtained new series of red spectra secured at two observatories and re-analyzed earlier obtained blue electronic spectra. Our analyses provide the most accurate so far published value of the apsidal period of 47.805 +/- 0.030 yrs and the following physical elements: M1=17.72+/-0.35$ Msun, M2=17.73+/-0.30 Msun, R1=5.785+/-0.091 Rsun, and R2=5.816+/-0.063 Rsun. The disentangling thus resulted in the masses, which are somewhat higher than all previous determinations and virtually the same for both stars, while the light curve implies a slighly higher radius and luminosity for star 2. The above empirical values imply the logarithm of the internal structure constant log k2 = -1.937. A comparison with Claret's stellar interior models implies an age close to 2 millions yrs for both stars. The claimed accuracy of modern element determination of 1-2 per cent seems still a bit too optimistic and obtaining new high-dispersion and high-resolution spectra is desirable.

astro-ph.SR↗

arXiv2014

Spectroscopy Apparatus for the Measurement of The Hyperfine Structure of Antihydrogen

The ASACUSA CUSP collaboration at the Antiproton Decelerator (AD) of CERN is planning to measure the ground-state hyperfine splitting of antihydrogen using an atomic spectroscopy beamline. We describe here the latest developments on the spectroscopy apparatus developed to be coupled to the antihydrogen production setup (CUSP).

physics.ins-det↗

arXiv2013

The first analysis of extragalactic binary-orbit precession

The main aim of the present paper is the very first analysis of the binary-orbit precession out of our Galaxy. The light curves of an eclipsing binary MACHO 82.8043.171 in the Large Magellanic cloud (LMC) were studied in order to analyse the long-term evolution of its orbit. It is a detached system that is undergoing rapid orbit precession. The inclination of the orbit towards the observer has been changing, which has caused the eclipse depth to become lower over the past decade, and this is ongoing. The period of this effect was derived as only about 77 years, so it is the second fastest nodal motion known amongst such systems nowadays. This is the first analysis of an extragalactic binary with nodal precession. This effect is probably caused by a distant third body orbiting the pair, which could potentially be detected via spectroscopy. Some preliminary estimates of this body are presented. However, even such a result can tell us something about the multiplicity fraction in other galaxies.

astro-ph.SR↗

arXiv2013

Apsidal motion and absolute parameters for five LMC eccentric eclipsing binaries

Aims: As part of our observational projects at the La Silla Danish 1.54-meter telescope, we aim to measure the precise times of minimum light for eccentric eclipsing binaries in the Large Magellanic Cloud, needed for accurate determination of apsidal motion. Many new times of minima were derived from the photometric databases OGLE and MACHO. Several new minima were also observed. Five early-type and eccentric-orbit eclipsing binaries: HV 982 (P = 5.34 d, e = 0.15), HV 2274 (5.73 d, 0.17), MACHO 78.6097.13 (3.11 d, 0.05), MACHO 81.8881.47 (3.88 d, 0.22), and MACHO 79.5377.76 (2.64 d, 0.06) were studied. Methods: The O-C diagrams of the systems were analysed using all reliable timings found in the literature, and new or improved elements of apsidal motion were obtained. Light and radial velocity curves of MACHO 81.8881.47 and MACHO 79.5377.76 were analysed using the program PHOEBE. Results: We derived for the first time or significantly improved the relatively short periods of apsidal motion of 211 (12), 127 (8), 48 (13), 103 (20), and 42 (19) years, respectively. The internal structure constants, log k2, were found to be -2.37, -2.47, -2.17, -2.02, and -1.86 respectively, under the assumption that the component stars rotate pseudosynchronously. The relativistic effects are weak, up to 6% of the total apsidal motion rate. The masses for MACHO 81.8881.47 resulted in 5.51 (0.21) and 5.40 (0.19) M0, while for MACHO 79.5377.76 the masses are 11.26 (0.35) and 11.27 (0.35) M0, respectively.

astro-ph.SR↗

arXiv2013

A new look into the spectral and light variations of epsilon Aur

Investigating long series of spectral and photometric observations, we found that the orbital elements of epsilon Aur are subject to much larger uncertainties than usually believed. The H alpha emission is found to move basically with the F primary but its exact location should still be investigated. We also find strong additional absorption and large reddening of the object near the third contact during the eclipse. Episodic atmospheric mass transfer from the F primary towards its companion is tentatively suggested.

astro-ph.SR↗

arXiv2013

Earth Inner Core Periodic Motion due to Pressure Difference Induced by Tidal Acceleration

The inner structure of the earth is still a topic of discussion. Seismic measurements showed a structure of solid, liquid, solid which describes the mantle, outer core and inner core with the inner core in the center. The analysis of waveform doublets suggests now that the inner core is out of center and even of faster rotation than the mantel and crust. From the sum of Buoyancy and Gravity on the earth inner core, the position energy is plotted and together with the tangential tidal acceleration, it is derived that Earth Inner Core cannot be in a center position without additional force. The Earth Core System is explained as Hydrodynamic Bearing. The tidal acceleration is identified as the reason for the periodic motion of the inner core and certain frequencies of nutation. The Eccentricities out of nutation due to the effects from the sun and moon are calculated as an approximation.

astro-ph.EP↗

arXiv2013

Multi-Wavelength Observations of Supernova 2011ei: Time-Dependent Classification of Type IIb and Ib Supernovae and Implications for their Progenitors

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (> 12,000 km/s) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v = 0.13c and a progenitor star mass-loss rate of 1.4 x 10^{-5} Msun yr^{-1} (assuming wind velocity v_w=10^3 km/s). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lower-mass (3-4 Msun), compact (R* <= 1x10^{11} cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN 2011ei's rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star's evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations.

astro-ph.HE↗

arXiv2013

Abell 48 - a rare WN-type central star of a planetary nebula

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. Almost all of these H-deficient central stars (CSs) display spectra with strong carbon and helium lines. Most of them exhibit emission line spectra resembling those of massive WC stars. Therefore these stars are classed as CSPNe of spectral type [WC]. Recently, quantitative spectral analysis of two emission-line CSs, PB 8 and IC 4663, revealed that these stars do not belong to the [WC] class. Instead PB 8 has been classified as [WN/WC] type and IC 4663 as [WN] type. In this work we report the spectroscopic identification of another rare [WN] star, the CS of Abell 48. We performed a spectral analysis of Abell 48 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. We find that the expanding atmosphere of Abell 48 is mainly composed of helium (85 per cent by mass), hydrogen (10 per cent), and nitrogen (5 per cent). The residual hydrogen and the enhanced nitrogen abundance make this object different from the other [WN] star IC 4663. We discuss the possible origin of this atmospheric composition.

astro-ph.SR↗

arXiv2012

Engineering ultrafast spin currents and terahertz transients by magnetic heterostructures

In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz (THz) regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin-current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is employed to drive spins from a ferromagnetic Fe thin film into a nonmagnetic cap layer that has either low (Ru) or high (Au) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect that converts the spin flow into a THz electromagnetic pulse. We find that the Ru cap layer yields a considerably longer spin-current pulse because electrons are injected in Ru d states that have a much smaller mobility than Au sp states. Thus, spin current pulses and the resulting THz transients can be shaped by tailoring magnetic heterostructures, which opens the door for engineering high-speed spintronic devices as well as broadband THz emitters in particular covering the elusive range from 5 to 10THz.

cond-mat.mes-hall↗

arXiv2012

Unique sextuple system: 65 Ursae Majoris

Context. 65 UMa belongs to rather small group of stellar systems of higher multiplicity, whose inner and outer orbits are well-known. Methods: Photometric, spectroscopic, and interferometric data were analyzed, revealing the basic physical properties of the system 65 UMa. A disentangling technique is used to perform the spectra decomposition. This combined approach allows us to study the long-term period changes in the system, identifying the period variation due to the motion on the visual orbit, in addition to a short-term modulation. Results: We find that the system contains one more component, hence 65 UMa is a sextuple hierarchical system. The most inner pair of components consists of an eclipsing binary orbiting around a barycenter on a circular orbit, both components being almost identical of spectral type about A7. This pair orbits on an eccentric orbit around a barycenter, and the third component orbits with a period of about 640 days. This motion is reflected in the period variation in the minima times of the eclipsing pair, as well as in the radial velocities of the primary, secondary, and tertiary components. Moreover, this system orbits around a barycenter with the distant component resolved interferometrically, whose period is of about 118 years. Two more distant components (4" and 63") are also probably gravitationally bound to the system. The nodal period of the eclipsing-pair orbit is on the order of only a few centuries, which makes this system even more interesting for a future prospective detection of changing the depths of minima. Conclusions: We identify a unique solution of the system 65 UMa, decomposing the individual components and even shifting the system to higher multiplicity. The study of this kind of multiple can help us to understand the origin of stellar systems. Besides 65 UMa, only another 11 sextuple systems have been studied.

astro-ph.SR↗

arXiv2012

Recent photometry of symbiotic stars - XIII

We present new multicolour (UBVRcIc) photometric observations of classical symbiotic stars, EG And, Z And, BF Cyg, CH Cyg, CI Cyg, V1329 Cyg, TX CVn, AG Dra, Draco C1, AG Peg and AX Per, carried out between 2007.1 and 2011.9. The aim of this paper is to present new data of our monitoring programme, to describe the main features of their light curves and to point problems for their future investigation. The data were obtained by the method of the classical photoelectric and CCD photometry.

astro-ph.SR↗

arXiv2012

The quest for companions to post-common envelope binaries. II. NSVS14256825 and HS0705+6700

We report new mid-eclipse times of the two close binaries NSVS14256825 and HS0705+6700, harboring an sdB primary and a low-mass main-sequence secondary. Both objects display clear variations in the measured orbital period, which can be explained by the action of a third object orbiting the binary. If this interpretation is correct, the third object in NSVS14256825 is a giant planet with a mass of roughly 12 M_Jup. For HS0705+6700, we provide evidence that strengthens the case for the suggested periodic nature of the eclipse time variation and reduces the uncertainties in the parameters of the brown dwarf implied by that model. The derived period is 8.4 yr and the mass is 31 M_Jup, if the orbit is coplanar with the binary. This research is part of the PlanetFinders project, an ongoing collaboration between professional astronomers and student groups at high schools.

astro-ph.SR↗

arXiv2012

Relaxation dynamics of a protein solution investigated by dielectric spectroscopy

In the present work, we provide a dielectric study on two differently concentrated aqueous lysozyme solutions in the frequency range from 1 MHz to 40 GHz and for temperatures from 275 to 330 K. We analyze the three dispersion regions, commonly found in protein solutions, usually termed beta-, gamma-, and delta-relaxation. The beta-relaxation, occurring in the frequency range around 10 MHz and the gamma-relaxation around 20 GHz (at room temperature) can be attributed to the rotation of the polar protein molecules in their aqueous medium and the reorientational motion of the free water molecules, respectively. The nature of the delta-relaxation, which often is ascribed to the motion of bound water molecules, is not yet fully understood. Here we provide data on the temperature dependence of the relaxation times and relaxation strengths of all three detected processes and on the dc conductivity arising from ionic charge transport. The temperature dependences of the beta- and gamma-relaxations are closely correlated. We found a significant temperature dependence of the dipole moment of the protein, indicating conformational changes. Moreover we find a breakdown of the Debye-Stokes-Einstein relation in this protein solution, i.e., the dc conductivity is not completely governed by the mobility of the solvent molecules. Instead it seems that the dc conductivity is closely connected to the hydration shell dynamics.

cond-mat.soft↗

arXiv2012

Discovery of two new Galactic candidate luminous blue variables with WISE

We report the discovery of two new Galactic candidate luminous blue variable (cLBV) stars via detection of circular shells (typical of known confirmed and cLBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 um in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered cLBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R- and I-bands by 0.68\pm0.10 mag and 0.61\pm0.04 mag, respectively, during the last 13-18 years, while the second one (known as Hen 3-1383) varies its B,V,R,I and K_s brightnesses by \simeq 0.5-0.9 mag on time-scales from 10 days to decades. We also found significant changes in the spectrum of Hen 3-1383 on a timescale of \simeq 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered cLBVs might be directly related to their possible runaway status.

astro-ph.SR↗