MOST observations of the roAp stars HD 9289, HD 99563, and HD 134214 (original) (raw)
Related papers
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.
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.
Monthly Notices of The Royal Astronomical Society, 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.
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.
MOST Photometry of the roAp star HD 134214
Communications in Asteroseismology, 2007
We present 10.27 hrs of photometry of the roAp star HD 134214 obtained by the MOST 1 satellite. The star is shown to be monoperiodic and oscillating at a frequency of 2948.97 ± 0.55 μHz. This is consistent with earlier ground based photometric campaigns (e.g. Kreidl et al. 1994). We do not detect any of the additional frequencies identified in the recent spectroscopic study by Kurtz et al. (2006) down to an amplitude limit of 0.36 mmag (2σ significance limit).
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, 2005
The discovery of a new rapidly oscillating Ap star, HD 116114, with a pulsation period of 21 min, using high-resolution spectra obtained with the Ultraviolet-Visual Echelle Spectrograph at the European Southern Observatory's Very Large Telescope, is presented. The highest amplitudes of the radial velocity variations are between 50 and 125 m s −1 visible in the Eu II lines. The spectral lines of La II and the core of the Hα line have amplitudes of about 30 m s −1. The frequency obtained for the oscillations is in good agreement with theoretical predictions of longer-period, evolved roAp stars. The distinction in luminosity between the roAp and noAp stars, and the suggestion that in all roAp stars the abundance of the second ions of Pr and Nd, relative to the abundance of the first ions, is anomalously high, need to be revised in the light of this discovery.
The search for roAp stars: null results and new candidates from Strömgren-Crawford photometry
Research in Astronomy and Astrophysics, 2018
The rapidly oscillating Ap (roAp) stars exhibit pulsational photometric and/or radial velocity variations on time scales of several minutes, which are essential to test current pulsation models as well as our assumptions of atmospheric structure characteristics. In addition, their chemical peculiarity makes them very interesting for probing stellar formation and evolution in the presence of a global magnetic field. To date, a limited number of only 61 roAp stars is known to show photometric variability. On the other hand, a literature survey yields 619 unique stars that have unsuccessfully been searched for variability of this kind. Strömgren-Crawford uvbyβ photometry of stars from both subgroups was used to investigate whether there is a selection bias for the investigated stars. We also present new photometric measurements (202 hours on 59 different nights) of 55 roAp candidates. We did not detect any new roAp star. Although our detection limits are comparable to other surveys, we also did not find pulsations in the known roAp star HD 12098, which may be a consequence of temporal amplitude changes. On the other hand, we do find some evidence for photometric variability of beta CrB at its spectroscopically derived pulsation period. From the uvbyβ photometry we conclude that the blue border of the roAp instability strip appears observationally well defined, whereas the red border is rather poorly known and studied. Within these boundaries, a total of 4646 candidates were identified which appear worth investigating for short-term pulsational variability.
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.