The search for roAp stars: null results and new candidates from Strömgren-Crawford photometry (original) (raw)
Related papers
MOST observations of the roAp stars HD 9289, HD 99563, and HD 134214
Astronomy & Astrophysics, 2011
We report on the analysis of high-precision space-based photometry of the roAp (rapidly oscillating Ap) stars HD 9289, HD 99563, and HD 134214. All three stars were observed by the MOST satellite for more than 25 days, allowing unprecedented views of their pulsation. We find previously unknown candidate frequencies in all three stars. We establish the rotation period of HD 9289 (8.5 d) for the first time and show that the star is pulsating in two modes that show different mode geometries. We present a detailed analysis of HD 99563's mode multiplet and find a new candidate frequency which appears independent of the previously known mode. Finally, we report on 11 detected pulsation frequencies in HD 134214, 9 of which were never before detected in photometry, and 3 of which are completely new detections. Thanks to the unprecedentedly small frequency uncertainties, the p-mode spectrum of HD 134214 can be seen to have a well defined large frequency spacing similar to the well-studied roAp star HD 24712 (HR 1217).
Monthly Notices of the Royal …, 2004
We present the analysis of 3 hrs of a rapid time series of precise stellar radial velocity (RV) measurements (σ = 4.5 m s −1 ) of the cool Ap star β CrB. The integrated RV measurements spanning the wavelength interval 5000-6000Å show significant variations (false alarm probability = 10 −5 ) with a period of 16.21 min (ν = 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 λ6272.0Å 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.
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.
Some recent discoveries in roAp stars
Proceedings of the International Astronomical Union, 2004
Research in roAp stars is being vigorously pursued, both theoretically and observationally by many groups. We report the discovery of a 21-min period, luminous roAp star, HD 116114. Longer periods for more luminous stars have been predicted theoretically and this is the first discovery of such a star. We discuss a model for the blue-to-red line profile variability observed in some roAp stars involving a shock wave high in the atmosphere of roAp stars, yet show that the Hα line in 33 Lib has the blue-to-red-to-blue line profile variability expected for subsonic dipolar pulsation concentrated towards the pulsation pole. Further we report for 33 Lib unprecedented observations of the amplitudes and phases of its principal mode at 2.015 mHz and its first harmonic of that at 4.030 mHz.
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.
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.
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.
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.
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.
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.