WIRE Satellite Research Articles

Asteroseismic analysis of the roAp star α Circini: 84 d of high-precision photometry from theWIREsatellite

We present a detailed study of the pulsation of alpha Circini, the brightest of the rapidly oscillating Ap stars. We have obtained 84 days of high-precision photometry from four runs with the star tracker on the WIRE satellite. Simultaneously, we collected ground-based Johnson B observations on 16 nights at the South African Astronomical Observatory. In addition to the dominant oscillation mode at 2442 microHz, we detect two new modes that lie symmetrically around the principal mode to form a triplet. The average separation between these modes is 30.173+-0.004 microHz and they are nearly equidistant with the separations differing by only 3.9 nHz. We compare the observed frequencies with theoretical pulsation models based on constraints from the recently determined interferometric radius and effective temperature, and the recently updated Hipparcos parallax. We show that the theoretical large separations for models of alpha Cir with global parameters within the 1-sigma observational uncertainties vary between 59 and 65 microHz. This is consistent with the large separation being twice the observed value, indicating that the three main modes are of alternating even and odd degrees. The frequency differences in the triplet are significantly smaller than those predicted from our models, for all possible combinations of mode degrees, and may indicate that the effects of magnetic perturbations need to be taken into account. The WIRE light curves are modulated by a double wave with a period of 4.479 days, and a peak-to-peak amplitude of 4 mmag. This variation is due to the rotation of the star and is a new discovery, made possible by the high precision of the WIRE photometry. The rotational modulation confirms an earlier indirect determination of the rotation period.

A new level of photometric precision: WIRE observations of eclipsing binary stars

The WIRE satellite was launched in March 1999 and was the first space mission to do asteroseismology from space on a large number of stars. WIRE has produced very high-precision photometry of a few hundred bright stars (V < 6) with temporal coverage of several weeks, including K giants, solar-like stars, δ Scuti stars, and β Cepheids. In the current work we will describe the status of science done on seven detached eclipsing binary systems. Our results emphasize some of the challenges and exciting results expected from coming satellite missions like COROT and Kepler. Unfortunately, on 23 October 2006, communication with WIRE failed after almost eight years in space. Because of this sad news we will give a brief history of WIRE at the end of this paper.

Oscillating K Giants with the WIRE Satellite: Determination of Their Asteroseismic Masses

Mass estimates of K giants are generally very uncertain. Traditionally, stellar masses of single field stars are determined by comparing their location in the Hertzsprung-Russell diagram with stellar evolutionary models. Applying an additional method to determine the mass is therefore of significant interest for understanding stellar evolution. We present the time series analysis of 11 K giants recently observed with the WIRE satellite. With this comprehensive sample, we report the first confirmation that the characteristic acoustic frequency, νmax, can be predicted for K giants by scaling from the solar acoustic cutoff frequency. We are further able to utilize our measurements of νmax to determine an asteroseismic mass for each star with a lower uncertainty compared to the traditional method, for most stars in our sample. This indicates good prospects for the application of our method on the vast amounts of data that will soon come from the COROT and Kepler space missions.

Eclipsing binaries observed with the WIRE satellite

We present the most precise light curve ever obtained of a detached eclipsing binary star and use it investigate the inclusion of non-linear limb darkening laws in eclipsing binary light curve models. This light curve, of the bright system beta Aurigae, was obtained using the star tracker aboard the WIRE satellite and contains 30000 datapoints with a scatter of 0.3 mmag. We analyse it using a version of the EBOP code modified to include non-linear limb darkening and to directly incorporate observed times of minimum light and spectroscopic light ratios into the solution as individual observations. We also analyse the dataset with the WD code to ensure that the two models give consistent results. EBOP provides an excellent fit to the WIRE data. Whilst the fractional radii are only defined to a precision of 5%, including an accurate published spectroscopic light ratio improves this dramatically to 0.5%. Using non-linear limb darkening improves the quality of the fit significantly and causes the measured radii to increase by 0.4%. It is possible to derive all of the limb darkening coefficients from the light curve, although they are strongly correlated with each other, and they agree with theoretical predictions. The radii and masses of the components of beta Aur are R_A = 2.762 +/- 0.017 Rsun, R_B = 2.568 +/- 0.017 Rsun, M_A = 2.376 +/- 0.027 Msun and M_B = 2.291 +/- 0.027 Msun. Theoretical stellar models can match these parameters for a solar metal abundance and an age of 450-500 Myr. The Hipparcos trigonometric parallax and an interferometrically-derived orbital parallax give distances to beta Aur which are in excellent agreement with each other and with distances derived using surface brightness relations and several sets of empirical and theoretical bolometric corrections (abridged).

Asteroseismology with the WIRE satellite

I give a summary of results from the WIRE satellite, which has been used to observe bright stars from 1999–2000 and 2003–2006. The WIRE targets are monitored for up to five weeks with a duty cycle of 30–40%. The aim has been to characterize the flux variation of stars across the Hertzsprung-Russell diagram. I present an overview of the results for solar-like stars, δ Scuti stars, giant stars, and eclipsing binaries.

Detection of p-mode oscillations in β Hydri from photometric observations with WIRE

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Asteroseismology with the WIRE satellite

Aims.We have analysed ground-based multi-colour Strömgren photometry and single-filter photometry from the star tracker on the wire satellite of the δ Scuti star ϵ Cephei.

Eclipsing binaries observed with the WIRE satellite

Context.Determinations of stellar mass and radius with realistic uncertainties at the level of 1% provide important constraints on models of stellar structure and evolution.

Evidence for Granulation and Oscillations in Procyon from Photometry with theWIRESatellite

We report evidence for the granulation signal in the star Procyon A, based on two photometric time series from the star tracker on the WIRE satellite. The power spectra show evidence of excess power around 1 mHz, consistent with the detection of p-modes reported from radial velocity measurements. We see a significant increase in the noise level below 3 mHz, which we interpret as the granulation signal. We have made a large set of numerical simulations to constrain the amplitude and timescale of the granulation signal and the amplitude of the oscillations. We find that the timescale for granulation is Γgran = 750 ± 200 s, the granulation amplitude is 1.8 ± 0.3 times solar, and the amplitude of the p-modes is 8.5 ± 2 ppm. We found the distribution of peak heights in the observed power spectra to be consistent with that expected from p-mode oscillations. However, the quality of the data is not sufficient to measure the large separation or to detect a comblike structure, as seen in the p-modes of the Sun. Comparison with the recent negative result from the MOST satellite reveals that the MOST data must have an additional noise source that prevented the detection of oscillations.

Multiperiodicity in the light variations of the β Cephei star β Crucis

High-resolution spectroscopic observations have led recently to the discovery that the beta Cephei star beta Crucis (Mimosa) is multiperiodic with at least three non-radial pulsation modes. Data obtained by the star tracker of the WIRE satellite have now allowed us to confirm this multiperiodicity in the light variations. A total of 5 million observations covering 17 days was analyzed and the three main periods we find in this work are in perfect agreement with the results derived from the line profile variations. The photometric amplitudes are small (3, 2.7 and 0.6 millimag for the dominant modes), but this is not surprising in view of the mode identifications derived earlier from the line profiles. Additional periods of low-amplitude modes (between 0.2-0.3 mmag) are also derived, including one suggested earlier by the radial velocity data.

Asteroseismology from space: The δ Scuti star θ2 Tauri monitored by the WIRE satellite

The bright variable star (theta)(sup 2) Tau was monitored with the star camera on the Wide-Field Infrared Explorer satellite. Twelve independent frequencies were detected down to the 0.5 mmag amplitude level. Their reality was investigated by searching for them using two different algorithms and by some internal checks: both procedures strengthened our confidence in the results. All the frequencies are in the range 10.8 - 14.6 cd(exp -1). The histogram of the frequency spacings shows that 81% are below 1.8 d; rotation may thus play a role in the mode excitation. The fundamental radial mode is not observed, although it is expected to occur in a region where the noise level is very low (55 (micro)mag). The rms (root mean square) residual is about two times lower than that usually obtained from successful groundbased multisite campaigns. The comparison of the results of previous campaigns with the new ones establishes the amplitude variability of some modes.

A Flower on a WIRE: Asteroseismology of a Massive Star

AbstractRecently it became clear from a line profile study that the β Cephei star β Crucis (Mimosa) is multiperiodic with at least three modes excited. Observations of this star by the star tracker of the WIRE satellite have now confirmed the multiperiodicity in the light variations. A total of 5 million observations covering 17 days was analysed and the three main periods are in perfect agreement with the line profile results. The amplitudes are small (3, 2.7 and 0.6 millimag for the dominant modes), but this is not surprising in view of the mode identifications derived from the line profiles. More periods could still be present, including the one earlier suggested by the radial velocity data.

Oscillations of UMa and other red giants

There is growing observational evidence that the variability of red giants could be caused by self-excitation of global modes of oscillation. The most recent evidence of such oscillations was reported for α UMa by Buzasi et al. who analysed space photometric data from the WIRE satellite. Little is understood concerning the oscillation properties in red giants. In this paper we address the question as to whether excited radial and non-radial modes can explain the observed variability in red giants. In particular, we present the results of numerical computations of oscillation properties of a model of α UMa and of several models of a 2-M⊙ star in the red giant phase. The red giant stars that we have studied have two cavities that can support oscillations: the inner core that supports gravity (g) waves and the outer one that supports acoustic (p) waves. Most of the modes in the p-mode frequency range are g modes confined in the core; those modes with frequencies close to a corresponding characteristic frequency of a p mode in the outer cavity are of mixed character and have substantial amplitudes in the outer cavity. We have shown that such modes of low degree, and 2, together with the radial (p) modes, can be unstable. The linear growth rates of these non-radial modes are similar to those of corresponding radial modes. In the model of α UMa and in the 2-M⊙ models in the lower regions of the giant branch, high amplitudes in the p-mode cavity arise only for modes with . We have been unable to explain the observed oscillation properties of α UMa, either in terms of mode instability or in terms of stochastic excitation by turbulent convection. The modes with the lowest frequencies, which exhibit the largest amplitudes and may correspond to the first three radial modes, are computed to be unstable if all of the effects of convection are neglected in the stability analyses. However, if the Lagrangian perturbations of the turbulent fluxes (heat and momentum) are taken into account in the pulsation calculation, only modes with higher frequencies are found to be unstable. The observed frequency dependence of amplitudes reported by Buzasi et al. does not agree with what one expects from stochastic excitation. This mechanism predicts an amplitude of the fundamental mode about two orders of magnitude smaller than the amplitudes of modes with orders , which is in stark disagreement with the observations.

Evolutionary Model and Oscillation Frequencies for alpha Ursae Majoris: A Comparison with Observations.

Inspired by the observations of low-amplitude oscillations of alpha Ursae Majoris A by Buzasi et al. using the WIRE satellite, a grid of stellar evolutionary tracks has been constructed to derive physically consistent interior models for the nearby red giant. The pulsation properties of these models were then calculated and compared with the observations. It is found that, by adopting the correct metallicity and for a normal helium abundance, only models in the mass range of 4.0-4.5 M middle dot in circle fall within the observational error box for alpha UMa A. This mass range is compatible, within the uncertainties, with the mass derived from the astrometric mass function. Analysis of the pulsation spectra of the models indicates that the observed alpha UMa oscillations can be most simply interpreted as radial (i.e., l=0) p-mode oscillations of low radial order n. The lowest frequencies observed by Buzasi et al. are compatible, within the observational errors, with model frequencies of radial orders n=0, 1, and 2 for models in the mass range of 4.0-4.5 M middle dot in circle. The higher frequencies observed can also be tentatively interpreted as higher n-valued radial p-modes, if we allow that some n-values are not presently observed. The theoretical l=1, 2, and 3 modes in the observed frequency range are g-modes with a mixed mode character, that is, with p-mode-like characteristics near the surface and g-mode-like characteristics in the interior. The calculated radial p-mode frequencies are nearly equally spaced, separated by 2-3 µHz. The nonradial modes are very densely packed throughout the observed frequency range and, even if excited to significant amplitudes at the surface, are unlikely to be resolved by the present observations.

Observation and analysis of quantum wire structures by high-resolution X-ray diffraction

We report on the double-crystal X-ray diffraction analysis of AlGaAs/GaAs quantum wire structures. We show that X-ray scattering is very sensitive to the modulation of structural parameters parallel to the crystal surface under certain diffraction conditions. The experimental diffraction patterns exhibit quantum wire satellite peaks which allow us to determine the quantum wire period and the quantum wire width. Furthermore, we found that due to the finite lateral width of the quantum wires a partial asymmetric strain relaxation of the unit cell occurs resulting in an orthorhombic lattice deformation.