Asteroseismology: from dream to reality

With the resounding success of helioseismology in determining the interior structure and rotation of the Sun, and in providing unprecedented studies of the interaction of pulsation and magnetic fields in the solar atmosphere, astronomers have been delighted, after decades of disappointing attempts, with the recent discovery of solar-like oscillations in ξ Hya, β Hyi, α Cen A and B, η Boo, ν Ind, ζ Her, δ Eri, HD 20794, HD 160191, and others as this list is growing rapidly. There is now true seismology of some solar-like stars. Asteroseismology also 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. This review describes some recent developments for some selected classes of pulsating stars other than the solar-like oscillators.     

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.     

The Eddington Mission

The Eddington mission was given full approval by the European Space Agency on the 23rd May 2002, with launch scheduled for 2007/8. Its science objectives are stellar evolution and asteroseismology, and planet finding. In its current design it consists of 4 × 60 cm folded Schmidt telescopes, each with 6^o × 6^o field of view and its own CCD array camera. Eddington will spend 2 years primarily devoted to asteroseismology with 1–3 months on different target fields monitoring up to 50,000 stars per field, and 3 years continuously on a single field monitoring upwards of 100,000 stars for planet searching. The asteroseismic goal is to be able to detect oscillations frequencies of stars with a precision 0.1–0.3 μHz, to probe their interior structure and the study the physical processes that govern their evolution. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Asteroseismic Determination of sdB Stars Fundamental Parameters

Recent observational efforts and theoretical breakthroughs have encouraged the development of detailed asteroseismic analyses of rapidly oscillating sdB stars (the so-called EC14026 stars). This led to the first seismic determinations of the fundamental parameters that define the structure of EHB stars. We briefly review the current status of these analyses, discussing some of the properties of acoustic modes in EHB models that affect the asteroseismology of these stars. We then recall the basic ideas behind the method we developed in an attempt to objectively extract, from models, asteroseismic solutions suitable to any given sdB pulsator. A preliminary application of this method to the pulsating sdB star Feige 48 is also presented.     

Identification of absorption lines of rare-earth elements in the region of the spectrum λλ 610.25–610.57 nm roAp stars,HD 101065, HD 134214, HD 137949, and HD 24712

In the spectral region λλ = 610.25−610.57 nm of the roAp star HD 101065, we performed the identification of the rare-earth element absorption lines absent from the atomic spectral line databases VALD and DREAM. The identified lines were used for calculating the synthetic spectra of the roAp stars HD 137949, HD 134214, and HD 24712. The upper limit of the identified line estimates was determined. The rotation/pulsation parameter νsini and magnetic field modulus were determined using the lines Nd III 669.083 nm and Ca I 616.217 nm. The element abundance in the roAp stars HD 134214 and HD 24712 was determined for the first time using Nd III lines. Calculations were carried out with the help of the code SynthM developed by S.A. Khan.     

Problems, Connections and Expectations of Asteroseismology: a Summary of the Workshop

The workshop took place at the beginning of what promises tobe a golden age of asteroseismology.Ground-based instrumentation is finally reaching a level of stabilitywhich allows detailed investigations of solar-like oscillations in atleast bright, slowly rotating main-sequence stars.Very extensive results are expected from the coming space missions,including data on a broad range of stars from the Eddington mission.The observational situation is therefore extremely promising.To make full use of these promises, major efforts are requiredtowards the efficient utilization of the data, through the developmentof techniques for the analysis and interpretation of the data.A broad range of topics related to these issues is discussed in the presentproceedings. Here I review some of the relevant problems,relate the asteroseismic investigations to broader areas of astrophysics and consider briefly the basis for our great expectations for the developmentof the field.     

Asteroseismology and interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.     

The Eddington Mission

The Eddington mission was given full approval by the European Space Agencyon the 23rd May 2002, as part of the new `Cosmic Vision' Science programme, with launch scheduled for 2007/8. Its twin scienceobjectives are asteroseismology and planet finding.In its current design it consists of 4 × 60 cm folded Schmidt telescopes, eachwith 6° × 6° field of view and its own CCD array camera.The current observing plan is to spend 2 years primarily devoted to asteroseismologywith 1–3 months on different target fields monitoring up to 50,000 stars per field,and 3 years continuously on asingle target field monitoring upwards of 100,000 stars as required for planetsearching. The asteroseismic goal is to be able to detect oscillationsfrequencies with a precision 0.1–0.3 Hz.     

Using the seismology of non-magnetic chemically peculiar stars as a probe of dynamical processes in stellar interiors

Chemical composition is a good tracer of the hydrodynamical processes that occur in stars as they often lead to mixing and particle transport. By comparing abundances predicted by models and those observed in stars we can infer some constraints on those mixing processes. As pulsations in the stars are often very sensitive to chemical composition, we can use asteroseismology to probe the internal chemical composition of stars where no direct observations are possible. In this paper I focus on main sequence stars Am, λ Bootis, and HgMn stars and discuss what we can learn of mixing processes in these stars from seismology.     

Photometric variability of five candidates for protoplanetary nebulae

We analyze long-term UBV observations and ASAS-3 photometry for five candidates for protoplanetary nebulae—F and G supergiants with infrared excesses at highG alactic latitudes—V340 Ser, IRAS 05113+1347, V552 Pup, V448 Lac, and RV Col. These stars exhibit quasi-periodic multifrequency light variations caused by pulsations with characteristic time scales from 83 to 139 days, depending on the stellar temperature. Cooler stars undergo variations with larger amplitudes and periods. The variations at close frequencies with a period ratio of 1.03–1.09 are responsible for the amplitude modulation revealed for most program stars.     

Overshooting and semiconvection: structural changes and asteroseismic signatures

Overshooting and semiconvection are two poorly known mechanisms which affect the extent and the efficiency of chemical mixing outside classical convection zones in stars. We discuss the uncertainties and the inferences of those processes in main sequence stars burning hydrogen in a convective core. We then focus on the asteroseismic signatures of partially or fully mixed zones surrounding the convective core, through the detailed shape of the induced chemical composition profile. We emphasize the potential power of asteroseismology to determine the internal structure of stars and thus to help us understand the physical processes at work inside the stars.     

Sounding stellar cycles with Kepler – I. Strategy for selecting targets

The long-term monitoring and high photometric precision of the Kepler satellite will provide a unique opportunity to sound the stellar cycles of many solar-type stars using asteroseismology. This can be achieved by studying periodic changes in the amplitudes and frequencies of the oscillation modes observed in these stars. By comparing these measurements with conventional ground-based chromospheric activity indices, we can improve our understanding of the relationship between chromospheric changes and those taking place deep in the interior throughout the stellar activity cycle. In addition, asteroseismic measurements of the convection zone depth and differential rotation may help us determine whether stellar cycles are driven at the top or at the base of the convection zone. In this paper, we analyse the precision that will be possible using Kepler to measure stellar cycles, convection zone depths and differential rotation. Based on this analysis, we describe a strategy for selecting specific targets to be observed by the Kepler Asteroseismic Investigation for the full length of the mission, to optimize their suitability for probing stellar cycles in a wide variety of solar-type stars.     

HD 101065, the most peculiar star: First results from precise radial velocity study

In this paper we discuss the prospects for asteroseismology with spatial resolution and motivate studies of the most chemically peculiar roAp star HD 101065. We present the first results from a high-precision radial velocity (RV) study of HD 101065 based on data spanning four nights that were acquired using the HARPS echelle-spectrometer at the ESO 3.6 m telescope. The analysis of individual nights showed the amplitude and phase modulation of the dominant mode. The analysis of the whole data set showed the presence of multi-periodic oscillations with two groups of equally-spaced modes. We find Δυ = 65.2μHz and δυ = 7.3μHz for the large and the small spacing, respectively. HD 101065 is the only roAp star to show the existence of two groups ofl = 0, 2 andl = 1, 3 excited modes.     

Anomalous abundance of lithium in the roAp star HD 12098 recently discovered in the northern hemisphere of the sky

Spectral observations of Ap-CP stars with the BTA (Special Astronomical Observatory, Russian Academy of Sciences) using the NES echelle spectrometer have revealed several stars with an anomalous lithium abundance. The oscillating star HD 12098, which is the first roAp star in the northern hemisphere of the sky, merits special attention. Strong, variable LiI 6708 Å line was observed in the spectrum of this star. There are not enough observations for a reliable analysis by Doppler mapping, but there are enough to indicate the presence of lithium spots on the surface of this star similar to the roAp stars HD 83368 and HD 60435, on whose surfaces spots with a high lithium abundance have been reliably detected. Parameters for a model of its atmosphere have been chosen using the method of synthetic spectra based on atmospheric models including lines from the VALD list and several additional blended REE lines calculated by the authors. The profile of the lithium LiI 6708 Å blend has been calculated taking the magnetic field into account using the SYNTHM code. A lithium abundance has been determined for two phases of the rotation of HD 12098 that is anomalously high compared to the solar and meteoritic abundances. The large difference in the lithium abundance in the two phases (in two different regions on the star's surface) exceeds 0.5 dex and is very close to that which we have found by analyzing the spectra of the roAp stars HD 83368 and HD 60435. Thus, we have discovered yet another roAp star, HD 12098, with lithium spots on its surface. Translated from Astrofizika, Vol. 51, No. 4, pp. 607–616 (November 2008).     

<Emphasis Type="Italic">BVRI</Emphasis> CCD-photometry of comparison stars in the neighborhoods of galaxies with active nuclei. III

Observations with a matrix photometer are reported. The stellar magnitudes in the BVRcIc bands are estimated for 80 comparison stars in the neighborhoods of 10 galaxies with active nuclei: 2 Seyfert galaxies, 4 quasars, and 4 BL Lac objects. The stellar magnitudes of the observed stars range from 11 to 19^m.5. For stars with magnitudes of 14–15^m the typical photometric errors are 0^m.011, 0^m.008, 0^m.006, and 0^m.007 in the B, V, R, and I bands, respectively. The BVRI magnitudes for most of these stars were not known previously. 14′ × 14′ finding charts for these stars are included. These results can be used for differential BVRI photometry of active galactic nuclei. __________ Translated from Astrofizika, Vol. 50, No. 1, pp. 57–72 (February 2007).     

A revised DDO abundance calibration for population I red giants

Several arguments that justify establishing a revised abundance calibration for DDO photometry of population I red giants are presented. The components of the blanketing vector in the DDOC(45–48) vsC(42–45) diagram are determined for late-type dwarfs and giants. We have redefined the DDO cyanogen anomaly and calibrated it against metallicity. The sample of field giants now available with abundances derived from high dispersion spectroscopy is substantially larger than previously available, leading to a more accurate abundance calibration. Iso-abundance lines in theC(41–42) vsC(42–45) diagram have been determined for population IG and K giants and an iterative method for deriving abundances of these stars is described. We show that the new DDO abundances are in very good agreement with those derived from high dispersion spectroscopy. The new method improves by about 0.1 dex the DDO abundances derived for early G and/or late K giants, with respect to the δCN method of Janes (1975).     

MSST observations of the pulsating sdB star PG 1605+072

We present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (also known as EC 14026 stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in May/June 2002 immediately following the Whole Earth Telescope Xcov22 run, which observed PG 1605+072 as an alternate target. We will first give an overview of the project and its feasibility, after which we will present the massive data set, made up of 399 h of photometry and 151 h of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.     

PLATO: PLAnetary Transits and Oscillations of stars

The PLAnetary Transits and Oscillations of stars Mission (PLATO), presented to ESA in the framework of its “Cosmic Vision” programme, will detect and characterize exoplanets by means of their transit signature in front of a very large sample of bright stars, and measure the seismic oscillations of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems. PLATO is the next-generation planet finder, building on the accomplishments of CoRoT and Kepler: i) it will observe significantly more stars, ii) its targets will be 2 to 3 magnitudes brighter (hence the precision of the measurements will be correspondingly greater as will be those of post-detection investigations, e.g. spectroscopy, asteroseismology, and eventually imaging), iii) it will be capable of observing significantly smaller exoplanets. The space-based observations will be complemented by ground- and space-based follow-up observations. These goals will be achieved by a long-term (4 years), high-precision, high-time-resolution, high-duty-cycle monitoring in visible photometry of a sample of more than 100,000 relatively bright (m _V  ≤ 12) stars and another 400,000 down to m _V  = 14. Two different mission concepts are proposed for PLATO: i) a “staring” concept with 100 small, very wide-field telescopes, assembled on a single platform and all looking at the same 26° diameter field, and ii) a “spinning” concept with three moderate-size telescopes covering more than 1400 degree². See for The PLATO Consortium.     

BVRI CCD-photometry of comparison stars in the fields of galaxies with active nuclei. IV

We report observations of 24 stars in the fields of the three Seyfert galaxies MCG +08-23-067, Mrk 817, and Mrk 290. The observations were made with a CCD array photometer in the BVRcIc bands. The V magnitudes of the observed stars ranged from 13.5 and 17.2. For stars of approximate magnitude 15, the typical photometric errors are 0.010, 0.011, 0.008 and 0.011 magnitude in the BVRI bands, respectively. The BVRI magnitudes of all these stars were not known previously. In the field of the galaxy Mrk 290 a star has been found that probably has a periodic brightness variation with P=1.518 days and the mean value V=14.80 and which may belong to the class of “spotted” stars. 14′x14′ charts are supplied for identifying the stars. These results can be used for differential photometry of active galactic nuclei in the BVRI bands. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 41–50 (February 2008).