MOST photometry and modeling of the rapidly oscillating (roAp) star gamma Equ
M. Gruberbauer, H. Saio, D. Huber, T. Kallinger, W. W. Weiss, D. B. Guenther, R. Kuschnig, J. M. Matthews, A. F. J. Moffat, S. Rucinski, D. Sasselov, G. A. H. Walker
备注：
10 pages, 12 figures, published in Astronomy & Astrophysics typos corrected, and minor changes in appearance and wording
Aims. Despite photometry and spectroscopy of its oscillations obtained over the past 25 years, the pulsation frequency spectrum of the rapidly oscillating Ap (roAp) star gamma Equ has remained poorly understood. Better time-series photometry, combined with recent advances to incorporate interior magnetic field geometry into pulsational models, enable us to perform improved asteroseismology of this roAp star.
Methods. We obtained 19 days of continuous high-precision photometry of gamma Equ with the MOST (Microvariability & Oscillations of STars) satellite. The data were reduced with two different reduction techniques and significant frequencies were identified. Those frequencies were fitted by interpolating a grid of pulsation models that include dipole magnetic fields of various polar strengths.
Results. We identify 7 frequencies in gamma Equ that we associate with 5 high-overtone p-modes and 1st and 2nd harmonics of the dominant p-mode. One of the modes and both harmonics are new discoveries for this star. Our best model solution (1.8 M_sun, log T_eff ~ 3.882; polar field strength ~8.1 kG) leads to unique mode identifications for these frequencies (ell = 0, 1, 2 and 4). This is the first purely asteroseismic fit to a grid of magnetic models. We measure amplitude and phase modulation of the primary frequency due to beating with a closely spaced frequency which had never been resolved. This casts doubts on theories that such modulation - unrelated to the rotation of the star - is due to a stochastic excitation mechanism.
We have carried out the first survey of the pulsational line profile variability in rapidly oscillating Ap (roAp) stars. We analysed high signal-to-noise time-series observations of ten sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the VLT and CFHT telescopes. We investigated in detail the variations of Pr III, Nd II, Nd III and Tb III lines and discovered a prominent change of the profile variability pattern with height in the atmospheres of all studied roAp stars. In every investigated star profile variability of at least one rare-earth ion is characteriz…
Magnetic fields play a key role in the pulsations of rapidly oscillating Ap (roAp) stars since they are a necessary ingredient of all pulsation excitation mechanisms proposed so far. This implies that the proper understanding of the seismological behaviour of the roAp stars requires knowledge of their magnetic fields. However, the magnetic fields of the roAp stars are not well studied. Here we present new results of measurements of the mean longitudinal field of 14 roAp stars obtained from low resolution spectropolarimetry with FORS1 at the VLT.
NainiTal-Cape Survey is a survey started with the aim of searching for new rapidly oscillating Ap stars in the northern hemisphere and has discovered one mono-periodic roAp star HD 12098. The frequency separation of HD 12098 suggests a rotation period of 5.5 day for the star. The discovery of roAp oscillations in HD 12098 and the results of the multi-site observation campaign organized to resolve the ambiguity in the determination of the rotation period of HD 12098 is presented. The results of non oscillating Ap stars discovered in the survey and two promising roAp candidates HD 17431 and HD 2…
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. 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 Halpha core, Nd, and Pr lines are formed. There RV…
The rapidly oscillating Ap (roAp) star 10 Aql shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behavior compared to other roAp stars. More than 1000 spectra were taken during 7 nights over a time span of 21 days with high-resolution spectrographs at the 8-m ESO VLT and 3.6-m TNG telescopes giving access to radial velocity variations of about 150 lines from different chemical species. A comparison of pulsation signatures in lines formed at different atmospheric heights allowed us to resolve the vertical structure of individual…
We present 10.27$ \rm{hrs}$ of photometry of the roAp star HD 134214 obtained by the MOST satellite. The star is shown to be monoperiodic and oscillating at a frequency of 2948.97$ \pm 0.55 μ\rm{Hz}$. This is consistent with earlier ground based photometric campaigns (e.g. Kreidl et al. 1994). We do not detect any of the additional frequencies identified in the recent spectroscopic study by Kurtz et al. (2006) down to an amplitude limit of 0.36$ \rm{mmag}$ ($2σ$ significance limit).
The mechanism for excitation of p-modes in rapidly oscillating, peculiar A (roAp, or cool chemically peculiar, CP) stars is unknown. Observations strongly suggest that acoustic modes in roAp stars are causally linked to the stars' magnetic field. We propose that small fluctuations in the shape of the mean magnetic field drive magnetosonic waves, which are observed as p-modes in these stars. The dynamic edge region of roAp stars is a force-free spherical shell. When strongly coupled to the magnetic field, a force-free plasma can oscillate about a minimum in its mean magnetic energy. We describe…
Rapidly oscillating Ap stars consitute a unique class of pulsators to study nonradial oscillations under some - even for stars - unusual physical conditions. These stars are chemically peculiar, they have strong magnetic fields, and they often pulsate in several high-order acoustic modes simultaneously. We discuss here an excitation mechanism for short-period oscillation modes based on the classical kappa mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of chemical peculiarity and strong magnetic field on the osc…
We present recent results from our radial velocity (RV) studies of rapidly oscillating Ap (roAp) stars. Our measurements reveal that the pulsational amplitude of these stars depends on the spectral region that is examined. For one star, gamma Equ, the pulsational RV amplitude depends on both line strength and atomic species. The elemental difference is most likely related to the abundance patches on the surface of these stars. The line strength difference is interpreted as arising form a height effect in the atmosphere. Ultimately, these measurements may provide valuable diagnostics of the atm…
We report the first comprehensive investigation of the line profile variation caused by non-radial pulsation in a magnetic oscillating chemically peculiar star. Spectrum variation of the well-known roAp star HR 3831 is detected using very high-resolution high signal-to-noise spectroscopic time-series observations and are followed through the whole rotation cycle of the star. We confirm outstanding diversity of pulsational behaviour of different lines in the HR 3831 spectrum and attribute this phenomenon to an interplay between extreme vertical chemical inhomogeneity of the HR 3831 atmosphere a…
We present bisector measurements of the H-alpha line of the rapidly oscillating Ap (roAp) star, Alpha-Cir, obtained from dual-site observations with medium-dispersion spectrographs. The velocity amplitude and phase of the principal pulsation mode vary significantly, depending on the height in the H-alpha line, including a phase reversal between the core and the wings of the line. This supports the theory, suggested in Paper I, of a radial pulsational node in the atmosphere of the star. Blending with metal lines partially affects the H-alpha bisector results but probably not enough to explain t…
Many types of stars show periodic variations of radius and brightness, which are commonly referred to as `stellar pulsations'. Observed pulsational characteristics are determined by fundamental stellar parameters. Consequently, investigations of stellar pulsations provide a unique opportunity to verify and refine our understanding of the evolution and internal structure of stars. However, a key boundary condition for this analysis -- precise information about the geometry of pulsations in the outer stellar envelopes -- has been notoriously difficult to secure. Here we demonstrate that it is po…
We present the analysis of 65 hours of high speed photometric observations of HD 12098 taken from State Observatory, Naini Tal and Gurushikhar Observatory, Mt.Abu on sixteen nights spanning from November 1999 to November 2000. HD 12098 is the first rapidly oscillating Ap star discovered from the `Naini Tal-Cape survey for northern hemisphere roAp stars'. It is the 32nd in the complete list. HD 12098 exhibits one predominant mode of oscillation at nu_1 = 2.1738 mHz. The second-most significant frequency in our data is at nu_2 = 2.1641 mHz with a 1 cycle/day alias ambiguity. We argue that nu_2 i…
We present new constraints on the rapidly oscillating Ap star alpha Cir, derived from a combination of interferometric and photometric data obtained with the Sydney University Stellar Interferometer (SUSI) and the WIRE satellite. The highlights of our study are: 1. The first determination of the angular diameter of an roAp star. 2. A nearly model-independent determination of the effective temperature of alpha Cir, which is found to be lower than previously estimated values. 3. Detection of two new oscillation frequencies allowing a determination of the large separation of alpha Cir. Based on t…
Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the theoretical acoustic cut-off frequency. We have computed models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based on Kurucz model atmospheres and on the Hopf's purely radiative relation. We compare the values of the cut-off frequency derived from expressions for the potential from Vorontsov & Zarkhov (1989), from Gough (1986), and from the approximation of an isothermal atmosphere. Kurucz model atmospheres increase the theoretical acoustic cut-off frequency for roAp stars by about 200…
We have obtained a time series of 81 high-cadence circular polarization observations of the rapidly oscillating Ap star HD 24712 with the new ESPaDOnS spectropolarimeter at CFHT. We used the high-S/N, high-resolution Stokes I and V spectra to investigate possible variation of the mean longitudinal field over the pulsation cycle in this roAp star. Our multiline magnetic field and radial velocity measurements utilized 143 spectral lines of rare-earth elements, attaining precision better than 13 G and 19 m/s, respectively. A multiperiodic radial velocity variation with an amplitude of 40-136 m/s …
We undertook a time-series photometric multi-site campaign for the rapidly oscillating Ap star HD 99563 and also acquired mean light observations over two seasons. The pulsations of the star, that show flatter light maxima than minima, can be described with a frequency quintuplet centred on 1557.653 microHertz and some first harmonics of these. The amplitude of the pulsation is modulated with the rotation period of the star that we determine with 2.91179 +/- 0.00007 d from the analysis of the stellar pulsation spectrum and of the mean light data. We break the distorted oscillation mode up into…
Three topics are discussed: 1) Photometric observations of the rapidly oscillating Ap stars have shown that the pulsation amplitude drops dramatically as a function of wavelength from the blue to the red. A theoretical derivation, plus modelling, indicates that this is because the vertical wavelength of the pulsation mode is short compared to the scale height of the atmosphere; in fact, it indicates that we are seeing a pulsation node in the observable atmosphere. Radial velocity observations, and theoretical calculations now support this. The implication for other research on CP stars is that…
We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of alpha Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, alpha Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967+-0.066 Rs. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420+-170 K. This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates,…
The rapidly oscillating Ap stars are of importance for studying the atmospheric structure of stars where the process of chemical element diffusion is significant. We have performed a survey for rapid oscillations in a sample of 9 luminous Ap stars, selected from their location in the colour-magnitude diagram as more evolved main-sequence Ap stars that are inside the instability strip for rapidly oscillating Ap (roAp) stars. Until recently this region was devoid of stars with observed rapid pulsations. We used the VLT UV-Visual Echelle Spectrograph (UVES) to obtain high time resolution spectros…
High-resolution spectroscopic time series of rapidly oscillating Ap stars show evidence for a co-existence of standing and running waves in their atmospheric layers. With the purpose of understanding these observations we have carried out a theoretical analysis of the pulsations in the outermost layers of these stars, starting from the simplest possible model that still retains all important physical ingredients. In our analysis we considered an isothermal atmosphere in a plane-parallel approximation. Moreover we assumed that in the region considered the magnetic pressure is much larger than t…
We present time-series spectroscopy of the rapidly oscillating Ap star HR 3831. This star has a dominant pulsation period of 11.7 minutes and a rotation period of 2.85 days. We have analysed 1400 intermediate-resolution spectra of the wavelength region 6100--7100 AA obtained over one week, using techniques similar to those we applied to another roAp star, Alpha Cir. We confirm that the H-alpha velocity amplitude of HR 3831 is modulated with rotation phase. Such a modulation was predicted by the oblique pulsator model, and rules out the spotted pulsator model. However, further analysis of H-alp…
Some of the rapidly oscillating (CP2) stars, have frequencies which are larger than the theoretical acoustic cut-off frequency. As the cut-off frequency depends on the T(tau) relation in the atmosphere, we have computed models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based on Kurucz model atmospheres and on Hopf's purely radiative relation. The frequency-dependent treatment of radiative transfer as well as an improved calculation of the radiative pressure in Kurucz model atmospheres increase the theoretical acoustic cut-off frequency by about 200 microHz, which is clos…
We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. 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. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsa…
We study the influence of Stark broadening and stratification effects on Siılines in the rapidly oscillating (roAp) star 10 Aql, where the Siı6142.48 Åand 6155.13 Ålines are asymmetrical and shifted. First we have calculated Stark broadening parameters using the semiclassical perturbation method for three Siılines: 5950.2 Å, 6142.48 Åand 6155.13 Å. We revised the synthetic sp$ calculation code taking into account both Stark width and shift for these lines. From the comparison of our calculations with the observations we found that Stark broadening + the stratification effect can explain asymme…
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 gamma 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. 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 detec…
We describe an analysis of the time-resolved measurements of the surface magnetic field in the roAp star gamma Equ. We have obtained a high-resolution and high S/N spectroscopic time-series, and the magnetic field was determined using Zeeman resolved profiles of the Fe II 6149.25 A and Fe I 6173.34 A lines. Contrary to recent reports we do not find any evidence of magnetic variability with pulsation phase, and derive an upper limit of 5-10 G for pulsational modulation of the surface magnetic field in gamma Equ.
We present an analysis of 210 high-resolution time-resolved spectropolarimetric observations of the roAp star gamma Equ obtained over three nights in August and September 2003. Radial velocity variations due to p-mode non-radial pulsations are clearly detected in the lines of rare-earth elements, in particular Pr III, Nd II and Nd III. In contrast, we find absolutely no evidence for the variation of the mean longitudinal magnetic field over the pulsation period in gamma Equ at the level of 110-240 G which was recently reported by Leone & Kurtz (2003). Our investigation of the variability of ci…
Recently, Tiwari, Chaubey, & Pandey (2007) detected the bright component of the visual binary HD151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude roAp star known. As a new roAp star, HD151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star - the other being HD 116114. We used the FIES spectrograph at the Nordic Optical Telescope to ob…
Since 1992, observations of roAp stars have been carried out using the dual-channel photometer attached to the 0.8m telescope, which is situated in Central Asia, at the Mt. Dushak-Erekdag station of Odessa Astronomical Observatory. Some results of observations of gamma Equ and of HD 134214 are presented. 5 stars were investigated as roAp candidates. The Fourier spectra of 4 stars did not show any variability in the high-frequency region. The Fourier spectrum of HD 99563 revealed a peak at a frequency f=128.9 c/d and with a semi-amplitude of 3.98 mmag.
Pulsational variability is observed in several types of main sequence stars with anomalous chemical abundances. In this contribution I summarize the relationship between pulsations and chemical peculiarities, giving special emphasis to rapid oscillations in magnetic Ap stars. These magneto-acoustic pulsators provide unique opportunities to study the interaction of pulsations, chemical inhomogeneities, and strong magnetic fields. Time-series monitoring of rapidly oscillating Ap stars using high-resolution spectrometers at large telescopes and ultra-precise space photometry has led to a number o…
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 3.6-m 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 in individual lines of Nd III. The presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial veloci…
The Ap star HD3980 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 8100K place HD3980 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 mea…
We present an analysis of the secular variability of the longitudinal magnetic field B_e in the roAp star gamma Equ (HD 201601). Measurements of the stellar magnetic field B_e were mostly compiled from the literature, and append also our 33 new B_e measurements which were obtained with the 1-m optical telescope of Special Astrophysical Observatory (Russia). All the available data cover the time period of 58 years, and include both phases of the maximum and minimum B_e. We determined that the period of the long-term magnetic B_e variations equals 91.1 +/- 3.6 years, with B_e (max) = + 577 +/- 3…
We derive expressions and develop a numerical technique for the analysis of the line profile moment variations for stars pulsating in oblique non-radial modes. This represents an extension of the widely used spectroscopic moment mode identification method to the oblique distorted pulsations observed in rapidly oscillating Ap stars. We demonstrate that a non-axisymmetric superposition of the pulsation and rotation velocity fields results in a qualitatively new behaviour of some of the line profile characteristics. It is found that for the majority of roAp stars the second moment varies with the…
