Time resolved spectroscopy of the cool Ap star HD 213637* (original) (raw)
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The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
Astronomy and Astrophysics, 2008
We present the discovery of pulsational variations in the cool magnetic Ap star HD 115226-the first high-amplitude rapidly oscillating Ap star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the ESO 3.6m telescope, we detect radial velocity variations with a period of 10.86 min in Pr iii, Nd iii, Dy iii lines and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed 1 km s −1 in individual lines of Nd iii. The presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial velocity maxima of rare-earth and hydrogen lines. Our abundance analysis demonstrates that HD 115226 exhibits typical roAp spectroscopic signature, notably ionization anomaly of Pr, Nd and Dy. We discuss the discovery of pulsations in HD 115226 in the context of recent spectroscopic studies of roAp stars and point to the existence of correlation between spectroscopic pulsational amplitude and the stellar rotation rate.
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.
The discovery of two new rapidly oscillating Ap stars, HD 92499 and HD 143487
Monthly Notices of the Royal Astronomical Society: Letters, 2010
We report the discovery of short periodic radial velocity variations in the stars HD 92499 and HD 143487. Both stars show strong magnetic fields and large overabundances of rare earth elements and belong to the class of cool chemically peculiar stars of the main sequence. They are therefore new rapidly oscillating Ap stars. Pulsations were detected from analysis of high time resolution spectra obtained with the European Southern Observatory Very Large Telescope by using a cross-correlation method for large spectral bands and from combinations of lines belonging to rare earth elements. The amplitudes of the pulsations are small and do not exceed several dozens of m s −1 with periods of around 10 min. The detection of such low amplitudes is important for the determination of which magnetic Ap stars pulsate, and which are constant, a distinction important for the understanding of the pulsation driving mechanism in these stars.
High time resolution spectroscopy and magnetic variability of the cool Ap star HD 965
Monthly Notices of the Royal Astronomical Society, 2005
We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine precise radial velocities using spectra obtained with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the European Southern Observatory Very Large Telescope. Special attention is given to spectral lines of rare-earth elements which in rapidly oscillating Ap (roAp) stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of 15-20 m s −1. All properties of HD 965 are similar to the majority of roAp stars. A likely reason for the apparent lack of pulsation could be connected with the geometrical structure of the magnetic field and the aspect of the star at the time of observation. Longitudinal magnetic field measurements for HD 965 showed that our UVES spectral observations were carried out when the longitudinal field was near zero and therefore, according to the oblique rotator model, near a time when the star was viewed from the magnetic equator. For a dipole oscillation aligned with the magnetic field, as is typical of roAp stars, no variation can be detected at this aspect. We may, therefore, expect to detect rapid oscillations in HD 965 in the future, when the star will present one of the magnetic poles.
A search for rapid pulsations in the magnetic cool chemically peculiar star HD 3980 ★
Monthly Notices of the Royal Astronomical Society, 2008
The Ap star HD 3980 appears to be a promising roAp candidate based on its fundamental parameters, leading us to search for rapid pulsations with the VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcos parallax and estimated temperature of 8100 K place HD 3980 in the middle of the theoretical instability strip for rapidly oscillating Ap stars, about halfway through its main sequence evolution stage. The star has a strong, variable magnetic field, as is typical of the cool magnetic Ap stars. Dipole model parameters were determined from VLT observations using FORS1. From Doppler shift measurements for individual spectral lines of rare earth elements and the Hα line core, we find no pulsations above 20 − 30 m s −1. This result is corroborated by inspection of lines of several other chemical elements, as well as with crosscorrelation for long spectral regions with the average spectrum as a template. Abundances of chemical elements were determined and show larger than solar abundances of rare earth elements. Further, ionisation disequilibria for the first two ionised states of Nd and Pr are detected. We also find that the star has a strong overabundance of manganese, which is typical for much hotter HgMn and other Bp stars. Line profile variability with the rotation period was detected for the majority of chemical species.
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.
The discovery of rapid oscillations in the magnetic Ap stars HD 69013 and HD 96237★
Monthly Notices of the Royal Astronomical Society, 2010
We report the detection of short-period variations in the stars HD 69013 and HD 96237. These stars possess large overabundances of rare earth elements and global magnetic fields, thus belong to the class of chemically peculiar Ap stars of the main sequence. Pulsations were found from analysis of high time resolution spectra obtained with the European Southern Observatory (ESO) Very Large Telescope using a cross-correlation method for wide spectral bands, from lines belonging to rare earth elements and from the Hα core. Pulsation amplitudes reach more than 200 m s −1 for some lines in HD 69013 with a period of 11.4 min and about 100 m s −1 in HD 96237 with periods near 13.6 min. The pulsations have also been detected in photometric observations obtained at the South African Astronomical Observatory.
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, 2006
In a high-resolution spectroscopic survey of rapidly oscillating Ap (roAp) stars with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope of the European Southern Observatory, we find that almost all stars show significant variation of the radial velocity amplitudes-on a timescale of a few pulsation cycles-for lines of the rare earth ion Pr III and in the core of the Hα line. These variations in the radial velocity amplitudes are described by new frequencies in the amplitude spectra that are not seen in broad-band photometric studies of the same stars. The Pr III lines form high in the atmosphere of these stars at continuum optical depths of log τ 5000 −5 and tend to be concentrated towards the magnetic poles in many stars, and the core of the Hα line forms at continuum optical depths −5 log τ 5000 −2, whereas the photometry samples the atmosphere on average at continuum optical depths closer to log τ 5000 = 0 and averages over the visible hemisphere of the star. Therefore, there are three possible explanations for the newly discovered frequencies: (1) there are modes with nodes near to the level where the photometry samples that can be easily detected at the higher level of formation of the Pr III lines; or (2) there are higher degree, , non-radial oblique pulsation modes that are detectable in the spectroscopy because the Pr III is concentrated towards the magnetic poles where such modes have their highest amplitudes, but average out over the visible hemisphere in the photometry which samples the star's surface more uniformly; or (3) there is significant growth and decay of the principal mode amplitudes on a timescale of just a few pulsation cycles at the high level of formation of the Pr III lines and core of the Hα line. The third hypothesis implies that this level is within the magneto-acoustic boundary layer where energy is being dissipated by both outward acoustic running waves and inward magnetic slow waves. We suggest observations that can distinguish among these three possibilities. We propose that strong changes in pulsation phase seen with atmospheric height in roAp stars, in some cases more than π rad from the top to the bottom of a single spectral line, strongly affect the pulsation phases seen in photometry in various bandpasses which explains why phase differences between bandpasses for roAp stars have never been explicable with standard theories that assume single spherical harmonics within the observable atmosphere. We also discuss the photometric amplitude variations as a function of bandpass, and suggest that these are primarily caused by continuum variations, rather than by variability in the rare earth element lines. We propose further tests of this suggestion.
Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445
Astronomy and Astrophysics, 2008
We present our discovery of pulsational radial-velocity variations in the cool Ap star HD 75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star γ Equ. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in Nd ii and Nd iii lines with a period close to 9 min and amplitudes of 20−30 m s −1. Substantial variation in the pulsational amplitude during our 3.8 h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD 75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens m s −1 and several km s −1. This supports the idea that many, if not all, cool Ap stars occupying the roAp instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.