R. New

Influence of high power densities on the composition of pulsed magnetron plasmas

The application of high power pulses with peak voltage of -2 kV and peak power density of 3 kWcm-2 to magnetron plasma sources is a new development in sputtering technology. The high power is appli ...

Ensemble asteroseismology of solar-type stars with the NASA Kepler mission.

Measurements of 500 Sun-like stars show that their properties differ from those predicted by stellar population models. In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy.

Radius Determination of Solar-type Stars Using Asteroseismology: What to Expect from the Kepler Mission

For distant stars, as observed by the NASA Kepler satellite, parallax information is currently of fairly low quality and is not complete. This limits the precision with which the absolute sizes of the stars and their potential transiting planets can be determined by traditional methods. Asteroseismology will be used to aid the radius determination of stars observed during NASA’s Kepler mission. We report on the recent asteroFLAG hare-and-hounds Exercise#2, where a group of “hares” simulated data of F–K main-sequence stars that a group of “hounds” sought to analyze, aimed at determining the stellar radii. We investigated stars in the range 9 <V <15, both with and without parallaxes. We further test different uncertainties in Teff, and compare results with and without using asteroseismic constraints. Based on the asteroseismic large frequency spacing, obtained from simulations of 4 yr time series data from the Kepler mission, we demonstrate that the stellar radii can be correctly and precisely determined, when combined with traditional stellar parameters from the Kepler Input Catalogue. The radii found by the various methods used by each independent hound generally agree with the true values of the artificial stars to within 3%, when the large frequency spacing is used. This is 5–10 times better than the results where seismology is not applied. These results give strong confidence that radius estimation can be performed to better than 3% for solar-like stars using automatic pipeline reduction. Even when the stellar distance and luminosity are unknown we can obtain the same level of agreement. Given the uncertainties used for this exercise we find that the input log g and parallax do not help to constrain the radius, and that Teff and metallicity are the only parameters we need in addition to the large frequency spacing. It is the uncertainty in the metallicity that dominates the uncertainty in the radius.

A PRECISE ASTEROSEISMIC AGE AND RADIUS FOR THE EVOLVED SUN-LIKE STAR KIC 11026764

The primary science goal of the Kepler Mission is to provide a census of exoplanets in the solar neighborhood, including the identification and characterization of habitable Earth-like planets. The asteroseismic capabilities of the mission are being used to determine precise radii and ages for the target stars from their solar-like oscillations. Chaplin et al. published observations of three bright G-type stars, which were monitored during the first 33.5 days of science operations. One of these stars, the subgiant KIC 11026764, exhibits a characteristic pattern of oscillation frequencies suggesting that it has evolved significantly. We have derived asteroseismic estimates of the properties of KIC 11026764 from Kepler photometry combined with ground-based spectroscopic data. We present the results of detailed modeling for this star, employing a variety of independent codes and analyses that attempt to match the asteroseismic and spectroscopic constraints simultaneously. We determine both the radius and the age of KIC 11026764 with a precision near 1%, and an accuracy near 2% for the radius and 15% for the age. Continued observations of this star promise to reveal additional oscillation frequencies that will further improve the determination of its fundamental properties.

The Octave (Birmingham-Sheffield Hallam) automated pipeline for extracting oscillation parameters of solar-like main-sequence stars

The number of main-sequence stars for which we can observe solar-like oscillations is expected to increase considerably with the short-cadence high-precision photometric observations from the NASA Kepler satellite. Because of this increase in the number of stars, automated tools are needed to analyse these data in a reasonable amount of time. In the framework of the asteroFLAG consortium, we present an automated pipeline which extracts frequencies and other parameters of solar-like oscillations in main-sequence and subgiant stars. The pipeline uses only the time series data as input and does not require any other input information. Tests on 353 artificial stars reveal that we can obtain accurate frequencies and oscillation parameters for about three quarters of the stars. We conclude that our methods are well suited for the analysis of main-sequence stars, which show mainly p-mode oscillations.

Solar p-mode frequencies over three solar cycles

We analyze thirty years of solar oscillations data collected by the Birmingham Solar Oscillations Network (BiSON). Estimates of the mean frequency shifts of low-degree p-modes have been extracted over a period spanning solar cycles 21-23. Two methods of analysis are used to extract the frequency shifts: one method uses results on fitted frequencies of individual modes, which are then averaged to give mean frequency shifts; the other method uses cross-correlations of power frequency spectra made from subsets of the data shifted in time. The frequency shifts are correlated against six proxies of solar activity, which are sensitive to magnetic and irradiance variability at a range of locations from the photosphere to the corona. We find proxies that have good sensitivity to the effects of weak-component magnetic flux—which is more widely distributed in latitude than the strong flux in the active regions—are those that follow the frequency shifts most consistently over the three cycles. This list includes the Mg II H and K core-to-wing data, the 10.7 cm radio flux, and the He I equivalent width data. While the two methods of analysis give consistent results, use of the cross-correlation function to measure mean frequency shifts returns less precise values in cases in which the duty cycle is greater than 30%. Estimation of uncertainties from the cross-correlation method also requires that proper allowance be made for strong correlations in the data.

FRESH INSIGHTS ON THE STRUCTURE OF THE SOLAR CORE

We present new results on the structure of the solar core, obtained with new sets of frequencies of solar low-degree p modes obtained from the BiSON network. We find that different methods used in extracting the different sets of frequencies cause shifts in frequencies, but the shifts are not large enough to affect solar structure results. We find that the BiSON frequencies show that the solar sound speed in the core is slightly larger than that inferred from data from Michelson Doppler Imager low-degree modes, and the uncertainties on the inversion results are smaller. Density results also change by a larger amount, and we find that solar models now tend to show smaller differences in density compared to the Sun. The result is seen at all radii, a result of the fact that conservation of mass implies that density differences in one region have to cancel out density differences in others, since our models are constructed to have the same mass as the Sun. The uncertainties on the density results are much smaller too. We attribute the change in results to having more, and lower frequency, low-degree mode frequencies available. These modes provide greater sensitivity to conditions in the core.

Solar internal sound speed as inferred from combined BiSON and LOWL oscillation frequencies

Observations of the Sun with the LOWL instrument provide a homogeneous set of solar p-mode frequencies from low to intermediate degree that allow one to determine the structure of much of the solar interior avoiding systematic errors that are introduced when different data sets are combined, i.e., principally the effects of solar cycle changes on the frequencies, Unfortunately, the LOWL data set contains very few of the lowest-degree modes, which are essential for determining reliably the structure of the solar core - in addition, these lowest-degree data have fairly large associated uncertainties, However, observations made by the Birmingham Solar-Oscillations Network (BiSON) in integrated sunlight provide high-accuracy measurements of a large number of low-degree modes. In this paper we demonstrate that the low-degree mode set of the LOWL data can be successfully combined with the more accurate BiSON data, provided the observations are contemporaneous for those frequencies where the solar cycle induced effects are important, We show that this leads to a factor of 2 decrease in the error on the inferred sound speed in the solar core, We find that the solar sound speed is higher than in solar models for r < 0.2 R.. The density of the solar core is, however, lower than that in solar models.

Solar p-mode frequencies and their dependence on solar-activity - recent results from the BISON network

We present here high-accuracy determinations of the frequencies of low-l solar p-modes and their solar-cycle dependence. The data were obtained using the Birmingham network of solar spectrometers (BISON). The precision of the measurements is discussed. Our previously published results of a significant frequency shift between solar minimum and solar maximum, apparently independent of l and similar to that found by other workers for intermediate-l modes, is confirmed and extended. This suggests that at most only a small fraction of the variation is due to the solar core. Sets of frequencies at high and low solar activity, and an average corrected for solar-activity effects, are presented. There is now evidence that the solar-activity dependence of the frequencies varies across the 5 minute spectrum.

High power impulse magnetron sputtering discharges: Instabilities and plasma self-organization

We report on instabilities in high power impulse magnetron sputtering plasmas which are likely to be of the generalized drift wave type. They are characterized by well defined regions of high and low plasma emissivity along the racetrack of the magnetron and cause periodic shifts in floating potential. The azimuthal mode number m depends on plasma current, plasma density, and gas pressure. The structures rotate in E→×B→ direction at velocities of ∼10 km s−1 and frequencies up to 200 kHz. Collisions with residual gas atoms slow down the rotating wave, whereas increasing ionization degree of the gas and plasma conductivity speeds it up.