The Acoustic Cut-Off Frequency of roAp Stars

Audard, N.; Kupka, F.; Morel, Pierre; Provost, J.; Weiss, W. W.

doi:10.1007/978-94-011-4982-2_64

Cited by 24 publications

(10 citation statements)

References 3 publications

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“…This star has one of the shortest pulsation periods, 5.7 min, which is close to the acoustic cut-off frequency calculated for HD 134214 by Audard et al (1998). The centre-of-gravity and bisector amplitude-phase diagrams are shown for HD 134214 in …”

Section: Hd 134214supporting

confidence: 79%

Pulsation tomography of rapidly oscillating Ap stars

Ryabchikova

Sachkov²,

Kochukhov³

et al. 2007

A&A

113

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Aims. We present detailed analysis of the vertical pulsation mode cross-section in ten rapidly oscillating Ap (roAp) stars based on spectroscopic time-series observations. The aim of this analysis is to derive from observations a complete picture of how the amplitude and phase of magnetoacoustic waves depend on depth. Methods. We use the unique properties of roAp stars, in particular chemical stratification, to resolve the vertical structure of p-modes. Our approach consists of characterising pulsational behaviour of a carefully chosen, but extensive sample of spectral lines. We analyse the resulting amplitude-phase diagrams and interpret observations in terms of pulsation wave propagation. Results. We find common features in the pulsational behaviour of roAp stars. Within a sample of representative elements the lowest amplitudes are detected for Eu ii (and Fe in 33 Lib and in HD 19918), then pulsations go through the layers where Hα core, Nd, and Pr lines are formed. There RV amplitude reaches its maximum, and after that decreases in most stars. The maximum RV of the second REE ions is always delayed relative to the first ions. The largest phase shifts are detected in Tb iii and Th iii lines. Pulsational variability of the Th iii lines is detected here for the first time. The Y ii lines deviate from this picture, showing even lower amplitudes than Eu ii lines but half a period phase shift relative to other weakly pulsating lines. We measured an extra broadening, equivalent to a macroturbulent velocity from 4 to 11−12 km s −1 (where maximum values are observed for Tb iii and Th iii lines), for pulsating REE lines. The surface magnetic field strength is derived for the first time for three roAp stars: HD 9289 (2 kG), HD 12932 (1.7 kG), and HD 19918 (1.6 kG). Conclusions. The roAp stars exhibit similarity in the depth-dependence of pulsation phase and amplitude, indicating similar chemical stratification and comparable vertical mode cross-sections. In general, pulsations waves are represented by a superposition of the running and standing wave components. In the atmospheres of roAp stars with the pulsation frequency below the acoustic cut-off frequency, pulsations have a standing-wave character in the deeper layers and behave like a running wave in the outer layers. Cooler roAp stars develop a running wave higher in the atmosphere. In stars with pulsation frequency close to the acoustic cut-off one, pulsation waves have a running character starting from deep layers. The transition from standing to running wave is accompanied by an increase in the turbulent broadening of spectral lines.

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Section: Hd 134214supporting

confidence: 79%

Pulsation tomography of rapidly oscillating Ap stars

Ryabchikova

Sachkov²,

Kochukhov³

et al. 2007

A&A

113

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“…This deficiency can also influence the reflection of the waves near the surface. Some previous studies have discussed this problem (Shibahashi & Saio 1985; Audard et al 1998; Gautschy et al 1998) and stressed the fact that a possible way to increase the critical cut‐off frequency of the models, and thus help reflecting the modes, is to have a steeper temperature gradient in the atmosphere. The surface chemical inhomogeneities (Shibahashi & Saio 1985; Gautschy et al 1998), and the possible suppression of convection by the magnetic field (Balmforth et al 2001), both contribute to steepen the temperature gradient in the surface layers of the models.…”

Section: Uncertainties In the Modelsmentioning

confidence: 99%

Seismic tests of the structure of rapidly oscillating Ap stars: HR 1217

Cunha

Fernandes

Monteiro

2003

Monthly Notices of the Royal Astronomical Society

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HR 1217 is probably the best studied multiperiodic rapidly oscillating Ap star, which makes this star a test case for the seismic analysis of these complex stars. In this paper, we combine the observational data available for HR 1217 and use these data to constrain theoretical models appropriate for this star. In particular, we try to establish which unknown model parameters are the most relevant to reproduce the seismic data. We find that the interior chemical composition has more impact in the theoretical oscillation spectrum of HR 1217 than parameters related to the modelling of convection. Models with lower abundances of heavy elements and larger abundances of helium produce oscillation spectra that are closer to that observed. However, like Matthews et al., we show that it is difficult to find a plausible model that can reproduce the seismic observations of HR 1217, when the uncertainties in the observables are not taken into account, unless a complementary effect is added in order to account for a decrease in the expected large frequency separations.

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“…The frequencies in many roAp stars exceed the acoustic cut‐off frequency of models with standard stellar atmospheres, thus also indicating abnormal atmospheric structure (see e.g. Audard et al 1998; Cunha 2006). The magnetic field directly alters the eigenfrequencies and eigenfunctions, it indirectly contributes to driving the pulsations by suppression of convection, and its geometry contributes to, or defines, the mode selection.…”

Section: Introductionmentioning

confidence: 99%

On the nature of the upper atmospheric variability in the rapidly oscillating Ap star HD 134214★

Kurtz

Elkin

Mathys

et al. 2007

Monthly Notices of the Royal Astronomical Society

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Recently, we discovered an entirely new type of variability in the upper atmospheres of rapidly oscillating Ap (roAp) stars. This manifests itself in amplitude modulation of the radial velocities that has not been previously detected in photometric studies of the same stars. To study this new variability further we obtained a full night, 8.85 h, of high time resolution (70 s), high spectral resolution (R = 105 000), high signal-to-noise ratio (on average S/N ∼ 130) data with Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) for the roAp star HD 134214. We also obtained 4.2 h of new photometric data in Johnson B with the South African Astronomical Observatory (SAAO) 0.5-m telescope 2 d later. HD 134214 has been known for years to be singly periodic with a relatively stable amplitude in photometry; it has the highest pulsation frequency of any roAp star of 2.949 mHz (P = 5.65 min). Our new UVES data show this principal frequency, plus five other frequencies in amplitude spectra of rare earth elements lines and the Hα line. The new frequencies are stable over the 8.85 h of observation, and the highest of them ν 2 = 2.782 mHz is the same as found in data taken 2 yr earlier with UVES presented in the discovery paper. The amplitudes of the new frequencies drop faster with atmospheric depth than does the amplitude of the principal frequency, hence explaining why they are generally not seen in broad-band photometric measurements that sample on average more deeply in the atmosphere. Our new photometric measurements also detect ν 2 for the first time in photometric data. Our analysis suggests that the new frequencies are associated with pulsation modes, but the nature of those modes and why they increase in amplitude with atmospheric height more strongly than the principal frequencies is not yet known.

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The Acoustic Cut-Off Frequency of roAp Stars

Cited by 24 publications

References 3 publications

Pulsation tomography of rapidly oscillating Ap stars

Pulsation tomography of rapidly oscillating Ap stars

Seismic tests of the structure of rapidly oscillating Ap stars: HR 1217

On the nature of the upper atmospheric variability in the rapidly oscillating Ap star HD 134214★

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