David E. Mkrtichian

Asteroseismology of the β Cephei star 12 (DD) Lacertae: photometric observations, pulsational frequency analysis and mode identification

We report a multisite photometric campaign for the β Cephei star 12 Lacertae. 750 hours of high-quality differential photoelectric Strömgren, Johnson and Geneva timeseries photometry were obtained with 9 telescopes during 190 nights. Our frequency analysis results in the detection of 23 sinusoidal signals in the light curves. Eleven of those correspond to independent pulsation modes, and the remainder are combination frequencies. We find some slow aperiodic variability such as that seemingly present in several β Cephei stars. We perform mode identification from our colour photometry, derive the spherical degree l for the five strongest modes unambiguously and provide constraints on l for the weaker modes. We find a mixture of modes of 0 6 l 6 4. In particular, we prove that the previously suspected rotationally split triplet within the modes of 12 Lac consists of modes of different l; their equal frequency splitting must thus be accidental. One of the periodic signals we detected in the light curves is argued to be a linearly stable mode excited to visible amplitude by nonlinear mode coupling via a 2:1 resonance. We also find a low-frequency signal in the light variations whose physical nature is unclear; it could be a parent or daughter mode resonantly coupled. The remaining combination frequencies are consistent with simple light-curve distortions. The range of excited pulsation frequencies of 12 Lac may be sufficiently large that it cannot be reproduced by standard models. We suspect that the star has a larger metal abundance in the pulsational driving zone, a hypothesis also capable of explaining the presence of Cephei stars in the LMC.

Frequency spectrum of the rapidly-oscillating mass-accreting component of the Algol-type system AS Eri

The first multisite photometric campaign devoted to the rapidly oscillating mass-accreting (primary) component of the Algol-type eclipsing binary system AS Eri has confirmed the presence of rapid pulsations with frequency 59.03116 d −1 ,a nd revealed the second and third oscillation modes with frequencies 62.5631 d −1 and 61.6743 d −1 , respectively. These modes are related to the 5−6 overtone oscillations and are among the shortest periods excited in non-magnetic MS A-F stars. The nearly equator-on visibility of eclipsing binaries help to narrow the range of possible mode identifications for the detectable modes as radial or (l,m) = (1, ±1), (l,m) = (2, ±2) and (l,m) = (2, ±0). We checked the high-order pulsation-to-orbital synchronization (POS) using the trial mode identification and the Doppler effect correction for frequencies of non-radial pulsation. We found that (l,m,n) = (1, 1, 5) or (2, 2, 5) and (l,m,n) = (2, −2, 6) identifications for f1 and f2 modes respectively satisfied the high- order POS. These mode identifications are in agreement with the range of modes visible in disk integrated light of an equator-on visible pulsating component. The wavelength distribution of pulsation amplitudes in AS Eri is largest in the Stromgren u filter and decreases toward longer wavelengths. We place AS Eri and other known mass-accreting pulsating components of Algols on HR-diagram. They are located inside the instability strip on the Main Sequence. We also discuss the peculiar evolutionary status of primary components in Algols and stress that they are not normal δ Scuti stars, but form a separate group of pulsators. Finally, we discuss proximity and eclipse effects, and have simulated the effect of primary minimum data gaps that may produce the 1/Porb alias sidelobes in DFT analysis of eclipsing binary data. Aliases from gaps in primary minimum observations seem to be the principal limitation on spectral window functions in asteroseismic studies of eclipsing binaries.

Radial Velocity Variations in Pulsating Ap Stars. II. 33 Librae

We present precise relative radial velocity (RV) measurements for the rapidly oscillating Ap (roAp) star 33 Librae measured from high resolution data spanning the wavelength interval 5000–6200 u We find that pulsational radial velocity amplitude determined over a broad wavelength range (≈ 100 u depends on the spectral region that is examined and can be as high as 60 ms −1 at 5600 u and as low as 7 ms −1 in the 5900 u region. RV measurements of individual spectral lines can show higher amplitudes than results obtained using a “broad-band” measurement that includes many spectral lines. The acoustic cross-sections of the atmosphere, i.e. the phase and amplitude of the pulsations, as a function of optical depth is found for spectral lines of Ca, Cr, Fe, La, Ce, Gd, Er and Nd. This analysis shows that pulsation phase is variable through the atmosphere and that Nd III lines pulsate almost 180 ◦ out-of-phase with those of Nd II features and are formed significantly higher in the stellar atmosphere. This conclusively establishes the presence of at least one radial node to the pulsations in the upper stellar atmosphere. We have estimated that this acoustic node is located above an optical depth log � < −4.5 and below the level where the Nd III lines are formed. We also suspect that there may be a second atmospheric node in the lower atmosphere below, or at log � ≃ −0.9 and close to continuum formation level. The histogram of pulsational phases for all individual spectral feature shows a bi-modal Gaussian distribution with 17% of the lines having a pulsational phase ≈ 165 ◦ outof-phase with most other spectral lines. This is also consistent with the presence of a radial node in the stellar atmosphere. The accumulation of phase due to a running wave component can explain the 165 ◦ phase difference as well as the broader width (by a factor of two) of one of the Gaussian components of the phase distribution. We also found evidence for phase variations as a function of effective Lande g-factors. This may be the influence of magnetic field and magnetic intensification effect on depths of spectral lines formation and that magnetic field is controlling the pulsations. Our RV measurements for 33 Lib suggest that we are seeing evidence of vertical structure to the oscillations as well as the influence of the distribution of elements on the stellar surface. We suggest and briefly discuss a new semi-empirical tomographic procedure for monoand multi-mode roAp stars that will use acoustic cross-sections obtained on different chemical elements and different pulsation modes for restoring the abundance and acoustic profiles throughout the stellar atmosphere and across the stellar surface.

Radial velocity variations in pulsating Ap stars – III. The discovery of 16.21‐min oscillations in β CrB

We present the analysis of 3 h of a rapid time series of precise stellar radial velocity (RV) measurements (a = 4.5 m s -1 ) of the cool Ap star β CrB. The integrated RV measurements spanning the wavelength interval 5000-6000 A show significant variations (false alarm probability = 10 -5 ) with a period of 16.21 min (v = 1028.17 μHz) and an amplitude of 3.54 ± 0.56 m s -1 The RV measured over a much narrower wavelength interval reveals one spectral feature at λ6272.0 A pulsating with the same 16.21-min period and an amplitude of 138 ± 23 m s -1 . These observations establish β CrB to be a low-amplitude rapidly oscillating Ap star.

An asteroseismic study of the β Cephei star 12 Lacertae: multisite spectroscopic observations, mode identification and seismic modelling

We present the results of a spectroscopic multisite campaign for the β Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with eight different telescopes in a time span of 11 months. In addition, we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the slowly pulsating B-stars (SPB)-like g-mode with frequency 0.3428 d −1 reported before is detected in our spectroscopy. We identify the four main modes as (� 1, m1) = (1, 1), (� 2, m2) = (0, 0), (� 3, m3) = (1, 0) and (� 4, m4) = (2, 1) for f 1 = 5.178 964 d −1 , f 2 = 5.334 224 d −1 , f 3 = 5.066 316 d −1 and f 4 = 5.490 133 d −1 , respectively. Our seismic modelling shows that f 2 is likely the radial first overtone and that the core overshooting parameter αov is lower than 0.4 local pressure scale heights.

Rotational Velocities of the Components of 23 Binaries

By using high dispersion spectra obtained by the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory (South Korea), multiline least-square deconvolution of line profiles, and Fourier analysis techniques, we obtained the rotational velocities of the components of 23 binary systems. The rotational velocities for nine primary, eight secondary, and one tertiary components of these systems were determined for the first time. The rotational velocities for primary components of seven systems appeared to be significantly different than the corresponding synchronous values (more than twice as fast for five systems, and less than half as fast for two systems). Our velocities for AU Mon, RY Gem, and RZ Eri are significantly lower than the previously published values obtained by using one or very few lines. We show that these discrepancies can be explained by strong blending of the lines in the spectra of the primaries with strong lines of the secondaries, by influence of gaseous streams, and maybe by nonsolar chemical compositions.

Search for A-F spectral type pulsating components in Algol-type eclipsing binary systems

We present the results of a systematic search for pulsating components in Algol-type eclipsing binary systems. A total number of 14 eclipsing binaries with A-F spectral type primary components were observed for 22 nights. We confirmed small-amplitude oscillating features of a recently detected pulsator TW Dra, which has a pulsating period of 0.053 day and a semi-amplitude of about 5 mmag in B-passband. We discovered new pulsating components in two eclipsing binaries of RX Hya and AB Per. The primary component of RX Hya is pulsating with a dominant period of 0.052 day and a semi-amplitude of about 7 mmag. AB Per has also a pulsating component with a period of 0.196 day and a semi-amplitude of about 10 mmag in B-passband. We suggest that these two new pulsators are members of the newly introduced group of mass-accreting pulsating stars in semi-detached Algol-type eclipsing binary systems.

2006 WHOLE EARTH TELESCOPE OBSERVATIONS OF GD358: A NEW LOOK AT THE PROTOTYPE DBV

We report on the analysis of 436.1 hr of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 and 4000 μHz, with the dominant peak at 1234 μHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l = 1 modes in the limit of slow rotation. The high-k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high-k multiplets is highly suspect. The k = 9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high-k modes. Analysis of the k = 9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

The eclipsing binary star RZ Cas. I. First spectroscopic detection of rapid pulsations in an Algol system

In a first report on the results of a multi-site campaign in 2001 of photometric and spectroscopic observations of the active semi-detached Algol-type system RZ Cas, we concentrate on the radial velocity (RV) variations. Using weak absorption lines we obtain an improved orbital solution for both components. In the orbital RV curve we observe a strong, asymmetric rotation effect. For the first time we detect rapid spectroscopic multi-mode pulsations in an Algol system. Whereas the photometrically observed oscillations were dominated by monoperiodic pulsations at frequency 64.19 c d - 1 until the year 2000, we find in 2001 a multiperiodic behaviour with two dominant frequencies of f 1 = 56.600 c d - 1 and f 2 = 64.189 c d - 1 . Both modes show amplitude variations over the orbital phase with a a minimum at orbital phases Φ = 0.6-0.8 and a maximum just after the primary minimum (f 2 ) and at Φ 0.25 (f 1 ). The different shape of amplitude modulation of the f 1 and f 2 modes points to different (l, m) mode structures. The modulation itself can be explained by assuming a variable extinction due to gas streams and an inhomogeneous accretion annulus that weakens the light from different regions of the primary depending on its orbital position. This assumption is well supported by the gas density distribution obtained in preliminary hydrodynamic simulations. We found strong variations and cycle-to-cycle variable shapes of the orbital RV-curves of Balmer lines that have maximum magnitude in the Ha line indicating a strong variability of mass-transfer rates and a non-stationary circumbinary envelope.

The detection of the rich p-mode spectrum and asteroseismology of Przybylski's star

Aims. We investigate the oscillation spectrum of the most chemically-peculiar star in the sky, the rapidly-oscillating magnetic (roAp) star HD 101065. Methods. High-precision radial velocity (RV) measurements, spanning four consecutive nights on 3–6 March, 2004, were obtained with the HARPS echelle-spectrometer at the ESO 3.6-m telescope, and photometric data were acquired during 6 nights of observations at the 24-inch telescope of Siding Spring Observatory. The RVs were measured using all spectral lines in the wavelength range 4400–5100 A. Results. We detected a rich spectrum of oscillation modes with semi-amplitudes ranging between 217 m s −1 and 1.6 m s −1 .T hese belong to the complete spectrum of high-order, low � = 0–2 modes, having a large spacing of Δν = 64.07 ± 0.9 μHz. We calculated the nonadiabatic frequencies of axisymmetric high-order p-modes for main-sequence models with dipole magnetic fields of two chemical compositions (X,Z) = (0.7, 0.02) and (X,Z) = (0.695, 0.025). The former composition provided the best fit model and yielded stellar parameters of mass, M = 1.525 ± 0.025 M� ,a ge= (1.5 ± 0.1) × 10 9 yr, effective temperature log Te = 3.821 ± 0.006, luminosity logL/L� = 0.797 ± 0.026, surface gravity log g = 4.06 ± 0.04, and polar magnetic field strength, Bp = 8.7 ± 0. 3k G. We are able to identify the modes of all detected oscillation frequencies. The photometric U and B light curves have maxima that precede the RV maximum by 0.16–0.19 in phase.