Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?

In the light of the recent and unexpected discovery of a brand new type of white dwarfs, those with carbon-dominated atmospheres, we examine the asteroseismological potential of such stars. The motivation behind this is based on the observation that past models of carbon-atmosphere white dwarfs have partially ionized outer layers that bear strong resemblance with those responsible for mode excitation in models of pulsating DB (helium-atmosphere) and pulsating DA (hydrogen-atmosphere) white dwarfs. Our exciting main result is that, given the right location in parameter space, some carbon-atmosphere white dwarfs are predicted to show pulsational instability against gravity modes. We are eagerly waiting the results of observational searches for luminosity variations in these stars.Comment: 4-page letter + 4 figure

On local-hidden-variable no-go theorems

The strongest attack against quantum mechanics came in 1935 in the form of a paper by Einstein, Podolsky and Rosen. It was argued that the theory of quantum mechanics could not be called a complete theory of Nature, for every element of reality is not represented in the formalism as such. The authors then put forth a proposition: we must search for a theory where, upon knowing everything about the system, including possible hidden variables, one could make precise predictions concerning elements of reality. This project was ultimatly doomed in 1964 with the work of Bell Bell, who showed that the most general local hidden variable theory could not reproduce correlations that arise in quantum mechanics. There exist mainly three forms of no-go theorems for local hidden variable theories. Although almost every physicist knows the consequences of these no-go theorems, not every physicist is aware of the distinctions between the three or even their exact definitions. Thus we will discuss here the three principal forms of no-go theorems for local hidden variable theories of Nature. We will define Bell inequalities, Bell inequalities without inequalities and pseudo-telepathy. A discussion of the similarities and differences will follow.Comment: 7 pages, no figure, replaced "Bell inequalities" with "Bell theorems" and updated the reference

Quantum computers can search rapidly by using almost any selective transformations

The search problem is to find a state satisfying certain properties out of a given set. Grover's algorithm drives a quantum computer from a prepared initial state to the target state and solves the problem quadratically faster than a classical computer. The algorithm uses selective transformations to distinguish the initial state and target state from other states. It does not succeed unless the selective transformations are very close to phase-inversions. Here we show a way to go beyond this limitation. An important application lies in quantum error-correction, where the errors can cause the selective transformations to deviate from phase-inversions. The algorithms presented here are robust to errors as long as the errors are reproducible and reversible. This particular class of systematic errors arise often from imperfections in apparatus setup. Hence our algorithms offer a significant flexibility in the physical implementation of quantum search.Comment: 8 pages, Accepted for publication in PR

Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48

As part of a multifaceted effort to exploit better the asteroseismological potential of the pulsating sdB star Feige 48, we present an improved spectroscopic analysis of that star based on new grids of NLTE, fully line-blanketed model atmospheres. To that end, we gathered four high S/N time-averaged optical spectra of varying spectral resolution from 1.0 \AA\ to 8.7 \AA, and we made use of the results of four independent studies to fix the abundances of the most important metals in the atmosphere of Feige 48. The mean atmospheric parameters we obtained from our four spectra of Feige 48 are : Teff= 29,850 $\pm$ 60 K, log $g$ = 5.46 $\pm$ 0.01, and log N(He)/N(H) = $-$2.88 $\pm$ 0.02. We also modeled for the first time the He II line at 1640 \AA\ from the STIS archive spectrum of the star and we found with this line an effective temperature and a surface gravity that match well the values obtained with the optical data. With some fine tuning of the abundances of the metals visible in the optical domain we were able to achieve a very good agreement between our best available spectrum and our best-fitting synthetic one. Our derived atmospheric parameters for Feige 48 are in rather good agreement with previous estimates based on less sophisticated models. This underlines the relatively small effects of the NLTE approach combined with line blanketing in the atmosphere of this particular star, implying that the current estimates of the atmospheric parameters of Feige 48 are reliable and secure.Comment: Accepted for publication in ApJ, April 201

The design of a linear L-band high power amplifier for mobile communication satellites

A linear L-band solid state high power amplifier designed for the space segment of the Mobile Satellite (MSAT) mobile communication system is described. The amplifier is capable of producing 35 watts of RF power with multitone signal at an efficiency of 25 percent and with intermodulation products better than 16 dB below carrier

Quantum Computation of a Complex System : the Kicked Harper Model

The simulation of complex quantum systems on a quantum computer is studied, taking the kicked Harper model as an example. This well-studied system has a rich variety of dynamical behavior depending on parameters, displays interesting phenomena such as fractal spectra, mixed phase space, dynamical localization, anomalous diffusion, or partial delocalization, and can describe electrons in a magnetic field. Three different quantum algorithms are presented and analyzed, enabling to simulate efficiently the evolution operator of this system with different precision using different resources. Depending on the parameters chosen, the system is near-integrable, localized, or partially delocalized. In each case we identify transport or spectral quantities which can be obtained more efficiently on a quantum computer than on a classical one. In most cases, a polynomial gain compared to classical algorithms is obtained, which can be quadratic or less depending on the parameter regime. We also present the effects of static imperfections on the quantities selected, and show that depending on the regime of parameters, very different behaviors are observed. Some quantities can be obtained reliably with moderate levels of imperfection, whereas others are exponentially sensitive to imperfection strength. In particular, the imperfection threshold for delocalization becomes exponentially small in the partially delocalized regime. Our results show that interesting behavior can be observed with as little as 7-8 qubits, and can be reliably measured in presence of moderate levels of internal imperfections

The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing

We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind both in terms of the accuracy of the Teff determinations as well as the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs increases gradually from a constant value of ~0.25 between Teff = 15,000 K and 10,000 K to a value twice as large in the range 10,000 > Teff > 8000 K, suggesting that convective mixing, which occurs when the bottom of the hydrogen convection zone reaches the underlying convective helium envelope, is responsible for this gradual transition. The comparison of our results with an approximate model used to describe the outcome of this convective mixing process implies hydrogen mass layers in the range log M_H/M_tot = -10 to -8 for about 15% of the DA stars that survived the DA to DB transition near Teff ~ 30,000 K, the remainder having presumably more massive layers above log M_H/M_tot ~ -6.Comment: 30 pages, 11 figures, accepted for publication in the Astrophysical Journa

Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9

Context. SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from ~60 s to ~120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle. Aims. Our primary goal is to examine quantitatively how pulsation modes can be excited in an object such as J1600+0748. Given the failure of uniform-metallicity models as well documented in the recent Ph.D. thesis of C. Rodríguez-López, we consider the effects of radiative levitation on iron as a means to boost the efficiency of the opacity-driving mechanism in models of J1600+0748. Methods. We combine high sensitivity time-averaged optical spectroscopy and full nonadiabatic calculations to carry out our study. In the first instance, this is used to estimate the location of J1600+0748 in the log $g-T_{\rm eff}$ plane. Given this essential input, we pulsate stellar models consistent with these atmospheric parameters. We construct both uniform-metallicity models and structures in which the iron abundance is specified by the condition of diffusive equilibrium between gravitational settling and radiative levitation. Results. On the basis of NTLE H/He synthetic spectra, we find that the target star has the following atmospheric parameters: log g = 5.93 $\pm$ 0.11, $T_{\rm eff}$ = 71 070 $\pm$ 2725 K, and log N(He)/N(H) = -0.85 $\pm$ 0.08. This takes into account our deconvolution of the spectrum of J1600+0748 as it is polluted by the light of a main sequence companion. We confirm that uniform-metallicity stellar models with Z in the range from 0.02 to 0.10 cannot excite pulsation modes of the kind observed. On the other hand, we find that the inclusion of radiative levitation, as we implemented it, leads to pulsational instabilities in a period range that overlaps with, although it is narrower than, the observed range in J1600+0748. The excited modes correspond to low-order, low-degree p-modes. Conclusions. We infer that radiative levitation is a likely essential ingredient in the excitation physics at work in J1600+0748