On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars

The Fourier Transform method is a popular tool to derive the rotational velocities of stars from their spectral line profiles. However, its domain of validity does not include line-profile variables with time-dependent profiles. We investigate the performance of the method for such cases, by interpreting the line-profile variations of spotted B stars, and of pulsating B tars, as if their spectral lines were caused by uniform surface rotation along with macroturbulence. We perform time-series analysis and harmonic least-squares fitting of various line diagnostics and of the outcome of several implementations of the Fourier Transform method. We find that the projected rotational velocities derived from the Fourier Transform vary appreciably during the pulsation cycle whenever the pulsational and rotational velocity fields are of similar magnitude. The macroturbulent velocities derived while ignoring the pulsations can vary with tens of km/s during the pulsation cycle. The temporal behaviour of the deduced rotational and macroturbulent velocities are in antiphase with each other. The rotational velocity is in phase with the second moment of the line profiles. The application of the Fourier method to stars with considerable pulsational line broadening may lead to an appreciable spread in the values of the rotation velocity, and, by implication, of the deduced value of the macroturbulence. These two quantities should therefore not be derived from single snapshot spectra if the aim is to use them as a solid diagnostic for the evaluation of stellar evolution models of slow to moderate rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

The population of AM CVn stars from the Sloan Digital Sky Survey

The AM Canum Venaticorum stars are rare interacting white dwarf binaries, whose formation and evolution are still poorly known. The Sloan Digital Sky Survey provides, for the first time, a sample of 6 AM CVn stars (out of a total population of 18) that is sufficiently homogeneous that we can start to study the population in some detail. We use the Sloan sample to `calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density of 1-3x10^{-6} pc^{-3}, which is lower than expected. We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like LISA should be lowered from current estimates, to about 1,000 for a mission duration of one year.Comment: Accepted to MNRA

Impact and extinction signatures in complete Cretaceous-Tertiary (K-T) boundary sections

The Zumaya, Caravaca and Agost sections in Spain, the El Kef section in Tunisia and the Negev (Nahal Avdat) sections in Israel are among the most continuous, expanded and complete K-T boundary sections. The distribution patterns of the planktic faunas were quantitatively analyzed in closely spaced samples across the K-T boundary in these sections, in conjuction with the geochemistry, stable isotopes, mineralogy and magnetostratigraphy. Three hundred foraminiferal specimens were randomly selected and determined. Reliable estimates for the foraminiferal productivity changes across the K-T boundary and for the 1 to 2 Ma interval preceding the K-T boundary were made from the numbers of individuals/gram of sediment corrected for the sedimentation rates (calculated from magnetic reversals and lithology). No gradual or stepwise extinction is seen below the K-T boundary nor any productivity decrease. Stable isotope analyses show a warming just after deposition of the ejecta layer, not cooling as predicted by nuclear winter scenarios, although the duration of such cooling may be too short to be observed even in these complete sections. Low REE values and cpx spherules with quench textures idential to quench-textures in diagenetically altered spherules, strongly indicate an oceanic site of (one of) the impactor(s)

Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091

In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et al. (2004). We confirm the presence of C III and O II emission features at ~ 4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hot spot around the center of mass of the binary. We note a weak periodic signal in the red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/- 0.006 min in the GMOS spectra when the source was in the low/hard state. Due to aliasing effects the period in the GMOS and X-Shooter data could well be the same. We deem it likely that the orbital period is thus close to 30 min, however, as several photometric periods have been reported for this source in the literature already, further confirmation of the 30 min period is warranted. We compare the surface area of the donor star and the disc of 4U 0614+091 with the surface area of the donor star and the disc in typical hydrogen-rich low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue that the optical emission in 4U 0614+091 is likely dominated by the disc emission. Additionally, we search for periodic signals in all the publicly available RXTE PCA light curves of 4U 0614+091 which could be associated with the orbital period of this source. A modulation at the orbital period with an amplitude of ~ 10% such as those that have been found in other ultra-compact X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure

Photometric Variability in the Faint Sky Variability Survey

The Faint Sky Variability Survey (FSVS) is aimed at finding photometric and/or astrometric variable objects between 16th and 24th mag on time-scales between tens of minutes and years with photometric precisions ranging from 3 millimag to 0.2 mag. An area of 23 deg$^2$, located at mid and high Galactic latitudes, was covered using the Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) on La Palma. Here we present some preliminary results on the variability of sources in the FSVS.Comment: 4 pages, 3 figures, to appear in 14th European Workshop on White Dwarfs, ASP Conference Series, eds. D. Koester, S. Moehle

Partial-measurement back-action and non-classical weak values in a superconducting circuit

We realize indirect partial measurement of a transmon qubit in circuit quantum electrodynamics by interaction with an ancilla qubit and projective ancilla measurement with a dedicated readout resonator. Accurate control of the interaction and ancilla measurement basis allows tailoring the measurement strength and operator. The tradeoff between measurement strength and qubit back-action is characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. Combining partial and projective qubit measurements, we provide the solid-state demonstration of the correspondence between a non-classical weak value and the violation of a Leggett-Garg inequality.Comment: 5 pages, 4 figures, and Supplementary Information (8 figures

Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates

We present a theoretical analysis of the selective darkening method for implementing quantum controlled-NOT (CNOT) gates. This method, which we recently proposed and demonstrated, consists of driving two transversely-coupled quantum bits (qubits) with a driving field that is resonant with one of the two qubits. For specific relative amplitudes and phases of the driving field felt by the two qubits, one of the two transitions in the degenerate pair is darkened, or in other words, becomes forbidden by effective selection rules. At these driving conditions, the evolution of the two-qubit state realizes a CNOT gate. The gate speed is found to be limited only by the coupling energy J, which is the fundamental speed limit for any entangling gate. Numerical simulations show that at gate speeds corresponding to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and increases further for lower gate speeds. In addition, the effect of higher-lying energy levels and weak anharmonicity is studied, as well as the scalability of the method to systems of multiple qubits. We conclude that in all these respects this method is competitive with existing schemes for creating entanglement, with the added advantages of being applicable for qubits operating at fixed frequencies (either by design or for exploitation of coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure

Kinematics of the ultracompact helium accretor AM canum venaticorum

We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma. We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA). In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated