Surface abundance distribution and radial velocity pulsations in roAp star HD 24712 (original) (raw)
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Spectroscopy of roAp star pulsation: HD24712
2005
We present results of the radial velocity (RV) analysis of spectroscopic time-series observations of the roAp star HD24712 (HR1217) which were carried out simultaneously with the Canadian MOST mini-satellite photometry. Only lines of the rare-earth elements (REE) show substantial amplitudes of RV pulsations. Based on new Zeeman measurements we found different shapes of the magnetic curves derived by using Fe-peak and REE separately. Frequency analysis of the spectroscopic data showed that the highest amplitude frequencies are the same in photometry and spectroscopy. Photometric and spectroscopic pulsation curves are shifted in phase, and the phase shift depends on the atomic species. The observed distribution of RV pulsation amplitudes and phases with the optical depth as well as the observed phase lag between luminosity and radius variations are explained satisfactorily by the model of nonadiabatic nonradial pulsations of a magnetic star.
Pulsation in the atmosphere of the roAp star HD 24712
Astronomy & Astrophysics, 2006
Aims. We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. Methods. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. Results. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m s −1) for REE lines and the Hα core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.
Monthly Notices of the Royal Astronomical Society, 2006
HD 154708 has an extraordinarily strong magnetic field of 24.5 kG. Using 2.5 h of high time resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we have discovered this star to be an roAp star with a pulsation period of 8 min. The radial velocity amplitudes in the rare earth element lines of Nd II, Nd III and Pr III are unusually low-∼60 m s −1-for an roAp star. Some evidence suggests that roAp stars with stronger magnetic fields have lower pulsation amplitudes. Given the central role that the magnetic field plays in the oblique pulsator model of the roAp stars, an extensive study of the relation of magnetic field strength to pulsation amplitude is desirable.
Vertical and horizontal abundance structures of the roAp star HD 24712
Proceedings of the International Astronomical Union, 2004
High-resolution spectroscopic and spectropolarimetric data of the rapidly oscillating Ap star HD 24712 (HR 1217, DO Eri) has been analysed including modelling the vertical elemental abundance structures. We study the interaction and the relation of the vertical (stratification) and the horizontal (spots) abundance characteristics of Fe and the stellar magnetic field. By this synopsis and the relation of our results to the analysis of high resolution and high time resolved observations (Sachkov et al. 2005) we are likely to gain new insights about the atmospheric structure and the geometry, the origin, and the evolution of the magnetic fields of roAp stars.
Short time-scale frequency and amplitude variations in the pulsations of an roAp star: HD 217522
Monthly Notices of the Royal Astronomical Society, 2014
Photometric observations of HD 217522 in 1981 revealed only one pulsation frequency ν 1 = 1.21529 mHz. Subsequent observations in 1989 showed the presence of an additional frequency ν 2 = 2.0174 mHz. New observations in 2008 confirm the presence of the mode with ν 2 = 2.0174 mHz. Examination of the 1989 data shows amplitude modulation over a time scale of the order of a day, much shorter than what has been observed in other roAp stars. High spectral and time resolution data obtained using the VLT in 2008 confirm the presence of ν 2 and short term modulations in the radial velocity amplitudes of rare earth elements. This suggests growth and decay times shorter than a day, more typical of solar-like oscillations. The driving mechanism of roAp stars and the Sun are different, and the growth and decay seen in the Sun are due to stochastic nature of the driving mechanism. The driving mechanism in roAp stars usually leads to mode stability on a longer timescale than in the Sun. We interpret the reported change in ν 1 between the 1982 and 1989 data as part of the general frequency variability observed in this star on many time scales.
Monthly Notices of the Royal Astronomical Society, 2005
In a high-resolution spectral survey of nearly half the 34 known rapidly oscillating Ap (roAp) stars, using the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope, we have discovered remarkably large amplitude pulsations in the roAp star HD 99563 with some spectral lines showing radial velocity amplitudes up to 5 km s −1 (10 km s −1 peak-to-peak) with a pulsation period of 10.7 min. As for many other roAp stars, we find the largest pulsation amplitudes for lines of some rare earth elements and in the core of the Hα line. The highest amplitudes of 5 km s −1 are seen in rather weak lines of Eu II and Tm II. Stronger lines of Pr III and Nd III have pulsation amplitudes in the range 0.7 to 3.5 km s −1 for different lines. In the narrow Hα core, the average amplitude is 2.6 km s −1 , but, as is the case for other lines, the amplitude and phase vary strongly with line depth (atmospheric height), with the amplitude of the radial velocity variations of the line bisector reaching a maximum of 4.3 km s −1 at the bottom of the core. Some other elements show pulsation amplitudes 0.1 to 0.7 km s −1. Variations in velocity amplitude and phase for several spectral lines were studied using linebisector measurements to obtain information about the vertical structure of the pulsation modes and the stellar atmosphere.
Search for radial velocity and magnetic field pulsational variations in the roAp star γ Equulei
Astronomy and Astrophysics, 2006
Aims. A new continuous 3.5 h series of high-resolution spectroscopic observations of the roAp star γ Equ, with about 12 points per pulsation cycle, is presented to study the mean magnetic field modulus variability with the pulsation period. Methods. Radial velocity variations with amplitudes of up to approximately 400 m s −1 are clearly detected in the rare earth lines present in the covered spectral range. Two frequencies are resolved in their analysis, which are consistent with frequencies previously observed in the star. No significant radial velocity variation is detected for the line Fe ii λ 6149.2. The mean magnetic field modulus derived from measurement of the wavelength separation of the magnetically resolved components of this line does not show any variation, with a 3σ upper limit of ∼10 G. Results. While our results are consistent with the conclusion of the previous studies, difficulties to reconcile this result with those of other works may be attributed, in part at least, to the use of different diagnostic lines in different works and to the failure of all studies so far to fully resolve all the pulsation frequencies of γ Equ.
The first evidence for multiple pulsation axes: a new roAp star in the Kepler field, KIC 10195926
2011
We have discovered a new rapidly oscillating Ap star among the Kepler Mission target stars, KIC 10195926. This star shows two pulsation modes with periods that are amongst the longest known for roAp stars at 17.1 min and 18.1 min, indicating that the star is near the terminal age main sequence. The principal pulsation mode is an oblique dipole mode that shows a rotationally split frequency septuplet that provides information on the geometry of the mode. The secondary mode also appears to be a dipole mode with a rotationally split triplet, but we are able to show within the improved oblique pulsator model that these two modes cannot have the same axis of pulsation. This is the first time for any pulsating star that evidence has been found for separate pulsation axes for different modes. The two modes are separated in frequency by 55 µHz, which we model as the large separation. The star is an α 2 CVn spotted magnetic variable that shows a complex rotational light variation with a period of P rot = 5.68459 d. For the first time for any spotted magnetic star of the upper main sequence, we find clear evidence of light variation with a period of twice the rotation period; i.e. a subharmonic frequency of ν rot /2. We propose that this and other subharmonics are the first observed manifestation of torsional modes in an roAp star. From high resolution spectra we determine T eff = 7400 K, log g = 3.6 and v sin i = 21 km s −1 . We have found a magnetic pulsation model with fundamental parameters close to these values that reproduces the rotational variations of the two obliquely pulsating modes with different pulsation axes. The star shows overabundances of the rare earth elements, but these are not as extreme as most other roAp stars. The spectrum is variable with rotation, indicating surface abundance patches.
A high resolution spectroscopic survey of the roAp stars
EAS Publications Series, 2005
We have begun a general high resolution spectroscopic survey of the entire class of roAp stars to study systematics of the pulsations for the class, and to make detailed studies of individual stars. We have high spectral resolution (R ∼ 110 000), high time resolution VLT UVES data for 16 of the 35 known members of the class, two of which were discovered during this survey. We show here some examples of studies of pulsation amplitude and phase as a function of line depth in Hα and in the 6145Å line of Nd iii. The two of these lines together give a nearly continuous map of pulsation amplitude and phase as a function of optical depth from about −5 ≤ log τ5000 ≤ −2, or even higher. Our results for these lines and those of Fe support studies showing settling of Fe and levitation of Nd. The pulsation behaviour provides an independent constraint on atmospheric structure and abundance distribution in roAp stars, hence is important in tests of radiative diffusion calculations.
Pulsation models for the roAp star HD 134214
Monthly Notices of the Royal Astronomical Society, 2012
Precise time series photometry with the MOST satellite has led to identification of 10 pulsation frequencies in the rapidly oscillating Ap (roAp) star HD 134214. We have fitted the observed frequencies with theoretical frequencies of axisymmetric modes in a grid of stellar models with dipole magnetic fields. We find that, among models with a standard composition of (X, Z) = (0.70, 0.02) and with suppressed convection, eigenfrequencies of a 1.65 M model with log T eff = 3.858 and a polar magnetic field strength of 4.1 kG agree best with the observed frequencies. We identify the observed pulsation frequency with the largest amplitude as a deformed dipole (= 1) mode, and the four next largest amplitude frequencies as deformed = 2 modes. These modes have a radial quasi-node in the outermost atmospheric layers (τ ∼ 10 −3). Although the model frequencies agree roughly with observed ones, they are all above the acoustic cutoff frequency for the model atmosphere and hence are predicted to be damped. The excitation mechanism for the pulsations of HD 134214 is not clear, but further investigation of these modes may be a probe of the atmospheric structure in this magnetic chemically peculiar star.