A frequency analysis of the p-mode pulsations of the Ap SrEu(Cr) star HD 119027

We present new high-speed, multisite photometric observations of the rapidly oscillating Ap star HD 119027 acquired over seven nights during 1996. A frequency analysis of these observations reveals the presence of oscillations at 1835, 1875, 1888, 1913, 1940, 1942 and (possibly) 1953 μHz. These frequencies are consistent with a spacing of either 13 or 26 μHz, depending on the reality of the oscillations at 1875 and 1953 μHz. The data in hand do not permit us to discriminate between the two possible spacings. If the smaller value of the spacing is correct, it suggests that HD 119027 is outside the main-sequence band. Two of the frequencies listed above are separated by only 1.95 μHz, suggesting that they are modes of ( n ,ℓ) and ( n  − 1, ℓ + 2), which in roAp stars is a quantity governed by the internal magnetic field.     

The Dushak-Erekdag survey of roAp stars

The search of roAp stars at Mt. Dushak-Erekdag Observatory was started in 1992 using the 0.8 m Odessa telescope equipped with a two-star high-speed photometer. We have observed more than a dozen stars so far and discovered HD 99563 as roAp star while BD+8087 is suspected to have rapid oscillations. Negative results of our observations for the search of rapid oscillations in four stars in NGC 752 are also discussed.     

Time-series spectroscopy of the rapidly oscillating Ap star HR 3831

We present time-series spectroscopy of the rapidly oscillating Ap (roAp) star HR 3831. This star has a dominant pulsation period of 11.7 min and a rotation period of 2.85 d. We have analysed 1400 intermediate-resolution spectra of the wavelength region 6100–7100 Å obtained over one week, using techniques similar to those we applied to another roAp star, α  Cir.
We confirm that the H α 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 α and other lines reveals rotational modulations that cannot easily be explained using the oblique pulsator model. In particular, the phase of the pulsation as measured by the width of the H α line varies with height in the line.
The variation of the H α bisector shows a very similar pattern to that observed in α Cir, which we have previously attributed to a radial node in the stellar atmosphere. However, the striking similarities between the two stars, despite the much shorter period of α Cir (6.8 min), argues against this interpretation unless the structure of the atmosphere is somewhat different between the two stars. Alternatively, the bisector variation is a signature of the degree ℓ of the mode and not the overtone value n .
High-resolution studies of the metal lines in roAp stars are needed to understand fully the form of the pulsation in the atmosphere.     

HD 12098 and other results from Nainital-Cape Survey

Nainital-Cape Survey was started with an aim to search for new rapidly oscillating Ap stars in the northern hemisphere. We discovered one new mono-periodic roAp star HD 12098. The frequency separation of HD 12098 suggests a rotation period of 5.5 days for the star. We summarize here the observations of HD 12098 and briefly discuss the results of the multi-site observation campaign organized to resolve the ambiguity in the determination of the rotation period of HD 12098. Other interesting results like non-oscillating Ap stars discovered and two candidate stars in which roAp periodicity is seen but not confirmed are also discussed.     

How to drive roAp stars

Rapidly oscillating Ap stars constitute a unique class of pulsators with which to study non-radial 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 κ mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of the chemical peculiarity and strong magnetic field on the oscillation modes and what separates these pulsators from δ Scuti and Am-type stars.     

Observations of pulsating Ap stars in South Africa

The rapidly oscillating Ap (roAp) stars currently represent the only main sequence stars other than the Sun which exhibit non-radial acoustic pulsations of high overtone. This makes them excellent subjects for asteroseismology, an approach which promises to yield accurate knowledge of the interior structures of stars. Of the 27 known roAp stars, 24 were discovered in Sutherland despite extensive searches conducted elsewhere. This paper reviews the discovery of the roAp phenomenon and describes the factors that contribute to the high discovery rate for these stars at Sutherland. Two long-term observational projects in progress at Sutherland are discussed,viz. the Cape roAp Star Survey and long-term monitoring of frequency variations in roAp stars.     

On the excitation mechanism in roAp stars

We investigate a model for the excitation of high-order oscillations in roAp stars. In this model we assume that the strong concentration of magnetic field about the magnetic poles is enough to suppress convection. Thus the model considered is composed of two polar regions, in which convection is presumed to be suppressed totally, and an equatorial region, where the convection is unaffected. This model is generated by building pairs of locally spherically symmetrical equilibria to represent the polar and equatorial regions of the star, which are patched together below the base of the convection zone. Gravitational settling of heavy elements is taken into account by choosing appropriate chemical composition profiles for both the polar and equatorial regions. Our results indicate that the composite model is unstable against axisymmetric non-radial high-order modes of pulsation that are aligned with the magnetic poles. The oscillations are excited by the κ mechanism acting principally in the hydrogen ionization zones of the polar regions. The effect of the lateral inhomogeneity on the second frequency differences is also investigated; we find that the perturbation to them by the inhomogeneity is of the same order as the second differences themselves, thereby hindering potential attempts to use such differences to identify the degrees of the modes in a straightforward way.     

Are Pulsation and Magnetic Axes Aligned in roAp Stars?

It is commonly assumed that in the rapidly oscillating Ap (roAp) stars the mode axis is aligned or nearly aligned with the magnetic field axis. This would be possible if the field is the only important effect causing departure from spherical symmetry. We show that even though these stars are slow rotators, the centrifugal force cannot be neglected. The consequence is that the modes cannot be in general symmetric about the magnetic field. We argue that such a symmetry is not implied by the observed coincidence between the field and pulsation amplitude maxima.     

Temperature behavior of elemental abundances in the atmospheres of magnetic peculiar stars

We analyze the temperature dependence of the abundances of the chemical elements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line (Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp) stars in the range 6000–15000 K. The Cr and Fe abundances in the atmospheres of Ap stars increase rapidly as the temperature rises from 6000 to 9000–10000 K. Subsequently, the Cr abundance decreases to values that exceed the solar abundance by an order of magnitude, while the Fe abundance remains enhanced by approximately +1.0 dex compared to the solar value. The temperature dependence of the abundances of these elements in the atmospheres of normal and Am stars is similar in shape, but its maximum is several orders of magnitude lower than that observed for Ap stars. In the range 6000–9500 K, the observed temperature dependences for Ap stars are satisfactorily described in terms of element diffusion under the combined action of gravitational settling and radiative acceleration. It may well be that diffusion also takes place in the atmospheres of normal stars, but its efficiency is very low due to the presence of microturbulence. We show that the magnetic field has virtually no effect on the Cr and Fe diffusion in Ap stars in the range of effective temperatures 6000–9500 K. The Ca abundance and its variation in the atmospheres of Ap stars can also be explained in terms of the diffusion model if we assume the existence of a stellar wind with a variable moderate rate of ～(2–4) × 10^{? 15}M_⊙ yr^?1.     

The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode

We undertook a time-series photometric multisite campaign for the rapidly oscillating Ap (roAp) star HD 99563 and also acquired mean light observations over four seasons. The pulsations of the star, which show flatter light maxima than minima, can be described with a frequency quintuplet centred on 1557.653 μHz and some first harmonics of it. 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 up the distorted oscillation mode into its pure spherical harmonic components and find it is dominated by the ℓ= 1 pulsation, and also has a notable ℓ= 3 contribution, with weak ℓ= 0 and 2 components. The geometrical configuration of the star allows us to see both pulsation poles for about the same amount of time; HD 99563 is only the fourth roAp star for which both pulsation poles are seen and only the third where the distortion of the pulsation modes has been modelled. We point out that HD 99563 is very similar to the well-studied roAp star HR 3831. Finally, we note that the visual companion of HD 99563 is located in the δ Scuti instability strip and may thus show pulsation. We show that if the companion was physical, the roAp star would be a 2.03-M_⊙, object, seen at a rotational inclination of 44°, which then predicts a magnetic obliquity     .     

Magnetic braking of Ap/Bp stars: application to compact black-hole X-ray binaries

We examine the proposal that the subset of neutron-star and black-hole X-ray binaries that form with Ap or Bp star companions will experience systemic angular-momentum losses due to magnetic braking, not otherwise operative with intermediate-mass companion stars. We suggest that for donor stars possessing the anomalously high magnetic fields associated with Ap and Bp stars, a magnetically coupled, irradiation-driven stellar wind can lead to substantial systemic loss of angular momentum. Hence, these systems, which would otherwise not be expected to experience 'magnetic braking', evolve to shorter orbital periods during mass transfer. In this paper, we detail how such a magnetic braking scenario operates. We apply it to a specific astrophysics problem involving the formation of compact black-hole binaries with low-mass donor stars. At present, it is not understood how these systems form, given that low-mass companion stars are not likely to provide sufficient gravitational potential to unbind the envelope of the massive progenitor of the black hole during a prior 'common-envelope' phase. On the other hand, intermediate-mass companions, such as Ap and Bp stars, could more readily eject the common envelope. However, in the absence of magnetic braking, such systems tend to evolve to long orbital periods. We show that, with the proposed magnetic braking properties afforded by Ap and Bp companions, such a scenario can lead to the formation of compact black-hole binaries with orbital periods, donor masses, lifetimes and production rates that are in accord with the observations. In spite of these successes, our models reveal a significant discrepancy between the calculated effective temperatures and the observed spectral types of the donor stars. Finally, we show that this temperature discrepancy would still exist for other scenarios invoking initially intermediate-mass donor stars, and this presents a substantial unresolved mystery.     

High-precision magnetic field measurements of Ap and Bp stars

In this paper we describe a new approach for measuring the mean longitudinal magnetic field and net linear polarization of Ap and Bp stars. As was demonstrated by Wade et al., least-squares deconvolution (LSD; Donati et al.) provides a powerful technique for detecting weak Stokes V , Q and U Zeeman signatures in stellar spectral lines. These signatures have the potential to apply strong new constraints to models of stellar magnetic field structure. Here we point out two important uses of LSD Stokes profiles. First, they can provide very precise determinations of the mean longitudinal magnetic field. In particular, this method allows one frequently to obtain 1 σ error bars better than 50 G, and smaller than 20 G in some cases. This method is applicable to both broad- and sharp-lined stars, with both weak and strong magnetic fields, and effectively redefines the quality standard of longitudinal field determinations. Secondly, LSD profiles can in some cases provide a measure of the net linear polarization, a quantity analogous to the broad-band linear polarization recently used to derive detailed magnetic field models for a few stars (e.g. Leroy et al.). In this paper we report new high-precision measurements of the longitudinal fields of 14 magnetic Ap/Bp stars, as well as net linear polarization measurements for four of these stars, derived from LSD profiles.     

Magnetic fields of Ap stars as a result of the Tayler instability

Ap star magnetism is often attributed to fossil magnetic fields which have not changed much since the pre‐main‐sequence epoch of the stars. Stable magnetic field configurations are known which could persist probably for the entire mainsequence life of the star, but they may not show the complexity and diversity exhibited by the Ap stars observed. We suggest that the Ap star magnetism is not a result of stable configurations, but is the result of an instability based on strong toroidal magnetic fields buried in the stars. The highly nonaxisymmetric remainders of the instability are reminiscent of the diversity of fields seen on Ap stars. The strengths of these remnant magnetic fields are actually between a few per cent up to considerable fractions of the internal toroidal field; this means field strengths of the order of kGauss being compatible with what is observed. The magnetic fields emerge at the surface rather quickly; rough estimates deliver time‐scales of the order of a few years. Since rotation stabilizes the instability, normal A stars may still host considerable, invisible toroidal magnetic fields (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Asteroseismology: Past, present and future

Asteroseismology studies stars with a wide variety of interior and surface conditions. For two decades asteroseismic techniques have been applied to many pulsating stars across the HR diagram. Asteroseismology is now a booming field of research with stunning new discoveries; I highlight a personal selection of these in this review, many of which are discussed in more detail elsewhere in these proceedings. For many years the Nainital-Cape Survey for northern roAp stars has been running at ARIES, so I emphasise new spectroscopic results for roAp stars and point out the outstanding prospects for the planned ARIES 3-m telescope at Devastai. High precision spectroscopy has revolutionised the asteroseismic study of some types of stars — particularly solar-like oscillators and roAp stars — while photometry is still the best way to study the frequency spectra that are the basic data of asteroseismology. New telescopes, new photometers and space missions are revolutionising asteroseismic photometry. In addition to the ground-based potential of asteroseismic spectroscopy, India has the knowledge and capability for space-based asteroseismic photometry. The future for asteroseismology is bright indeed, especially for Indian astronomers.     

Study of pulsations of chemically peculiar a stars

Rapidly oscillating chemically peculiar A stars (roAp) pulsate in high-overtone, low degree p-modes and form a sub-group of chemically peculiar magnetic A stars (Ap). Until recently, the classical asteroseismic research, i.e., frequency analysis, of these stars was based on photometric observations both ground-based and space-based. Significant progress has been achieved by obtaining uninterrupted, ultra-high precision data from the MOST, COROT, and Kepler satellites. Over the last ten years, a real breakthrough was achieved in the study of roAp stars due to the time-resolved, high spectral resolution spectroscopic observations. Unusual pulsational characteristics of these stars, caused by the interaction between propagating pulsationwaves and strong stratification of chemical elements, provide an opportunity to study the upper roAp star atmosphere in more detail than is possible for any star but the Sun, using spectroscopic data. In this paper the results of recent pulsation studies of these stars are reviewed.     

The pulsational behaviour of the rapidly oscillating Ap star HD 122970 during two photometric multisite campaigns

We undertook two time-series photometric multisite campaigns for the rapidly oscillating Ap star HD 122970. The first one, conducted in 1998, resulted in 119 h of data and in the detection of three pulsation frequencies. The presence of possible further modes which held the promise of deriving a mode identification motivated a second worldwide campaign in the year 2001. This second campaign resulted in 203 h of measurement, but did not reveal further modes. Rather, one of the previously detected signals disappeared. The two modes common to both data sets have different spherical degree. They also showed slight frequency modulation, and one of them varied in amplitude as well. Possible causes of the latter behaviour include intrinsic instability of the pulsation spectrum or precession of the pulsational axis and orbital motion in a binary system. Frequency analysis of the Hipparcos observations of the star did not allow us to determine the stellar rotation period. The amplitude and phase behaviour of the two modes of HD 122970 in the Strömgren uvby bands is quite similar to that observed for other roAp stars.     

Spectroscopy of the roAp star α Cir — I. Velocities of Hα and metal lines

We present radial velocity measurements of the rapidly oscillating Ap (roAp) star α Cir, obtained from dual-site observations with medium-dispersion spectrographs. The amplitude and phase of the principal pulsation mode vary significantly, depending on which line is being measured. The amplitude is observed to be as high as 1000 m s⁻¹ in some wavelength bands, despite a previous upper limit of 36 m s⁻¹. Furthermore, some lines are apparently pulsating in anti-phase with others. We suggest this indicates a high-overtone standing wave with a velocity node in the atmosphere of the star.     

Dynamo action in the presence of an imposed magnetic field

Dynamo action within the cores of Ap stars may offer intriguing possibilities for understanding the persistent magnetic fields observed on the surfaces of these stars. Deep within the cores of Ap stars, the coupling of convection with rotation likely yields magnetic dynamo action, generating strong magnetic fields. However, the surface fields of the magnetic Ap stars are generally thought to be of primordial origin. Recent numerical models suggest that a primordial field in the radiative envelope may possess a highly twisted toroidal shape. We have used detailed 3-D simulations to study the interaction of such a twisted magnetic field in the radiative envelope with the core-dynamo operating in the interior of a 2 solar mass A-type star. The resulting dynamo action is much more vigorous than in the absence of such a fossil field, yielding magnetic field strengths (of order 100 kG) much higher than their equipartition values relative to the convective velocities. We examine the generation of these fields, as well as the growth of large-scale magnetic structure that results from imposing a fossil magnetic field. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Do the physical properties of Ap binaries depend on their orbital elements?

We reveal sufficient evidence that the physical characteristics of Ap stars are related to binarity. The Ap star peculiarity [represented by the  Δ( V 1- G )  value and magnetic field strength] diminishes with eccentricity, and it may also increase with orbital period ( P _orb). This pattern, however, does not hold for large orbital periods. A striking gap that occurs in the orbital period distribution of Ap binaries at 160–600 d might well mark a discontinuity in the above-mentioned behaviour. There is also an interesting indication that the Ap star eccentricities are relatively lower than those of corresponding B9–A2 normal binaries for   P _orb>10 d  . All this gives serious support to the pioneering idea of Abt & Snowden concerning a possible interplay between the magnetism of Ap stars and their binarity. Nevertheless, we argue instead in favour of another mechanism, namely that it is binarity that affects magnetism and not the opposite, and suggest the presence of a new magnetohydrodynamical mechanism induced by the stellar companion and stretching to surprisingly large P _orb.     

Seismic signatures of strange stars with crust

We study acoustic oscillations (eigenfrequencies, velocity distributions, damping times) of normal crusts of strange stars. These oscillations are very specific because of huge density jump at the interface between the normal crust and the strange matter core. The oscillation problem is shown to be self-similar. For a low (but non-zero) multipolarity l , the fundamental mode (without radial nodes) has a frequency of ∼300 Hz and mostly horizontal oscillation velocity; other pressure modes have frequencies ≳20 kHz and almost radial oscillation velocities. The latter modes are similar to radial oscillations (having approximately the same frequencies and radial velocity profiles). The oscillation spectrum of strange stars with crust differs from the spectrum of neutron stars. If detected, acoustic oscillations would allow one to discriminate between strange stars with crust and neutron stars and constrain the mass and radius of the star.