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Meeting report T he RAS discussion meeting on “Stellar seismology and extrasolar planet-finding and the Eddington mission”, organized by Alan Penny (Rutherford Appleton Laboratory) and Ian Roxburgh (Queen Mary, University of London), was held at the Scientific Societies Lecture Theatre on 11 January 2002. The morning session was chaired by Alan Penny and opened with a general review of the Eddington mission and then covered the science of asteroseismology, with particular respect to that possible with Eddington. The first talk was by Fabio Favata (ESTEC), the Eddington Study Scientist, who presented a general review of Eddington. Eddington is a high-precision, long-duration space photometry mission in the ESA science programme. Two key science goals are to provide the data needed for an empirically based theory of stellar structure and evolution, through asteroseismology, and to detect a significant sample of extrasolar planets through transits. Eddington will find planets with a wide range of characteristics, including Earth-like, habitable worlds. The instrumental configuration is being studied in detail, the favourite one being a multiple (three or four) set of 600 mm Schmidt cameras with mosaic CCD cameras operating in frame transfer mode, resulting in a highperformance mission. Asteroseismology The first of the talks devoted to asteroseismology was by Ian Roxburgh (Queen Mary, University of London) who spoke on the use of asteroseismology with Eddington. He first described the principles of asteroseismology – namely that as the frequencies of oscillation of a star are determined by its internal structure, so measurements of a set of frequencies for a star encode information on that internal structure, primarily on the variation of density with radius inside the star. He went on to describe some of the methods available to extract this information ranging from the use of the small separations between frequencies of degree 0 and 2 to place a global constraint on the interior structure, to detailed inversion techniques based on the phase shift of the oscillation modes, that is the departure of a particular oscillation wave from a pure sine wave. In these inversion techniques the unknown structure of the surface layers of a star is represented by a frequency-dependent surface phase shift which is independent of degree, while the central regions of the star produce a phase shift that depends on degree 1. Matching the internal and external phase shifts beneath the surface layers gives algorithms for 6.30 Looking inside stars and looking for planets Asteroseismology and extrasolar planets are the main science goals of the Eddington mission, now approved by ESA for a 2007 launch. Alan Penny presents a summary of the January 2002 RAS meeting that discussed the sciences of this wide-field high-precision photometric space telescope. Since the date of this meeting, ESA has decided to implement the mission in the framework of a 2007–08 launch. 1: Sir Arthur Eddington, the astronomer and physicist known for his research on the motion, internal constitution and luminosity of stars, and in whose memory ESA’s space photometry mission is named. 2: Artist’s impression of the Eddington craft/satellite due for launch in 2007–08 (ESA). determining the variation of density with radius in the bulk of the stellar interior. Three examples of such inversions were described, one for the Sun using data obtained by the Birmingham Oscillation Network, the others using artificial data for stars of 0.8 and 1.45 M. He stressed that with the data from the Eddington mission we should be in a position to test the predictions of the theory of stellar evolution and to improve that theory on the basis of asteroseismic measurements. Photometric studies Next Don Kurtz (University of Central Lancashire) discussed some current results in asteroseismology, mostly from photometric studies. PG 1159, a DOV star, is the current asteroseismic record holder with 101 identified modes; its mass is known to a precision of ±0.003 M and atmospheric stratification has been detected. Studies of the EC 14026 stars are important for understanding extreme horizontal branch evolution. One EC 14026 star, PG 1336-018, is in an eclipsing binary where tomography during eclipse is allowing a unique method of mode identification, and possibly has a tidally governed pulsation axis. Study of the DAV star BPM 37093 has the potential to test theories of C–O crystallization in cool white-dwarf interiors. New theoretical understanding of the interaction of pulsation, rotation and global stellar magnetic fields are being tested in the rapidly oscillating Ap stars. All photometric studies of pulsating stars would be revolutionized by the one-to-two orders of magnitude improvement in sensitivity over ground-based observations that Eddington could achieve. Model callibration Douglas Gough (Institute of Astronomy, Cambridge) addressed how accurately one can calibrate a main-sequence model of one solar mass, assuming radius and luminosity to be known, using the Sun as an example. Perhaps the most frequently used method for inferring the internal properties of stars from Eddington data will be to use seismic oscillation frequencies to calibrate stellar models. Such calibrations will best be carried out in conjunction with other astronomical data, such as metallicity, position on the HR diagram – which December 2002 Vol 43 Meeting report determines a star’s radius – and, in the case of binary systems, the mass. Given the uncertainty in the structure of the outer boundary layer of the convection zone, whose influence on the oscillations must be eliminated by considering only suitably chosen combinations of the data, and given the fact that changes in the age and the presumed initial helium abundance have a rather similar influence on those data combinations, the precision with which one can ascertain the age is less than might naively be suspected. Nevertheless, it will be far better than any current method. Indeed, it will raise our knowledge of the structure and evolution of stars to a completely new level. Michael Thompson (Imperial College of Science Technology and Medicine) spoke on asteroseismic inversion for solar-type stars. These are, broadly speaking, F, G and K stars on the main sequence, or subgiants, which are expected to display turbulently excited modes: a defining feature is the expected low amplitudes of pulsation. He described how experiments (including blind tests) to invert frequencies of stellar models, with the anticipated mode coverage and noise properties from planned spaceborne observations, have been carried out to determine what could be achieved with the expected data. The sound speed profile throughout most of the core of a solar-type star was estimated to within a few percent with a SOLA-type inversion method, with three months of data and assuming that l = 0–3, mode frequencies were determined. The recently reported observed spectra of oscillations in a Cen and b Hyd, which are qualitatively like the solar low-degree p-mode spectrum, look very encouraging for the prospects of such inference. reflected light studies for transiting and nontransiting planets would give planet albedoes. There would be Announcements of Opportunity for the wider community to be involved. The consequences of the selection of the NASA Kepler planet transit search mission were described. Keith Horne (University of St Andrews) discussed Eddington planet catch simulations. Eddington aims to detect transits of Earthanalogue planets 8 ×10–5 mag deep, lasting ~13 hours, with one year period. The baseline design – a space telescope with area A ~ 0.6 m2 and CCD cameras observing a W ~ 10 deg2 starfield for t > 3 years – does the job. The planet catch can be boosted at fixed cost by using N co-aligned telescopes with smaller A but wider W. For the nearby main-sequence stars surveyed, the planet catch scales as W (N A t)3/2, M–3/8 T3 r6, with M the star mass, T and r the planet temperature and radius, strongly favouring large, hot planets. For a power-law planet mass function ∂n/∂m µm–1.5, Monte Carlo simulations predict that baseline and thereby provides a greater number of stars at a given magnitude than does the optical design used in Eddington. Because Kepler stares at a single FOV throughout the mission, it does much less asteroseismology than Eddington. Other comparisons were also discussed. Planets around variable stars Suzanne Aigrain (Institute of Astronomy, Cambridge) spoke about the impact of stellar variability on planetary transit detection. The variability of the parent star is a major source of non-Gaussian noise. Characterizing it will help to refine instrument design, develop data preprocessing methods and improve target selection for missions such as COROT, Eddington and Kepler. By studying the power spectrum of the solar irradiance data from the VIRGO instrument on board SOHO, a low-frequency component was identified, well correlated with chromospheric activity indicators, which can be filtered out using a high-pass filter, but higher frequency granulation-related components remain a problem. Filters based on knowledge of the shape of the transit promise to curb this problem but must be tested to the limit. Future work will therefore concentrate on producing realistic simulated light curves for stars with a range of ages, masses and luminosities, and on the potential use of colour information. The final talk of the day was by Richard Nelson (Queen Mary, University of London) who spoke about recent theoretical work on the formation of terrestrial and gas-giant planets, and summarized the likely impact of missions such as Eddington and Kepler on planet formation theories. Computer simulations of terrestrial planet formation performed by George Wetherill and co-workers provide modeldependent predications for the likely distribution of planetary masses, semi-major axes, eccentricities, and orbital inclinations. The detection of terrestrial planets via transit searches could be compared with these model predictions to constrain the initial conditions of planet formation. In addition, Eddington and Kepler will detect many giant planets and provide information about their orbital period distribution. This distribution may be used to constrain models of planetary migration via disc–planet interaction. Eddington and Kepler will be able to detect planets in binary systems, an area that is only just beginning to receive significant theoretical attention. ● “While Eddington and Kepler search for ‘hot and habitable Earths’, ground-based hourly monitoring of ~1000 galactic microlensing events can find ‘cool Earths’.” Extrasolar planet hunting The afternoon session was chaired by Ian Roxburgh and covered the science of extrasolar planets, as examined by Eddington. The first talk was by Alan Penny (Rutherford Appleton Laboratory), who spoke on the prospects for planet finding. Some 3% of nearby stars have Jupiter-mass objects orbiting within 3 AU, and theory points to the formation of terrestrialmass planets. Eddington would be able to detect planets of 0.8 Earth mass with orbits of less than a year, and determine their frequency of occurrence and distribution of masses, orbital radii and inclinations and the presence of massive moons. In the main target field the stars would be at some 500–1000 pc, and Eddington would find many thousands of Jupiters, some 2000 of terrestrial mass, including a few hundred in the habitable zone, and a few tens like the Earth. Additional planet data would come from asteroseismology. Secondary eclipses and December 2002 Vol 43 Eddington will find ~104 “hot Jupiters”, ~102 “hot Earths”, and ~10 “habitable Earths”. The 1% abundance of “hot Jupiters” is known from Doppler wobble studies, but the abundance of Earths is currently unknown. While Eddington and Kepler search for “hot and habitable Earths”, ground-based hourly monitoring of ~1000 galactic microlensing events can find “cool Earths”. If both experiments are done, we will be able to interpolate safely to estimate the abundance and hence distances to the nearest habitable Earths. William Borucki (NASA Ames Research Center), Kepler Principal Investigator, compared the Kepler and Eddington missions. These are spaceborne photometric missions with similar apertures, which are both capable of finding Earth-size extrasolar planets and both can detect p-mode oscillations in stars. The Kepler mission is optimized to find Earth-size planets in the habitability zone of Sun-like stars and does asteroseismology only as incidental science. The Eddington mission appears to be optimized for asteroseismology. The Kepler design provides a very large field of view, a low measurement cadence, a heliocentric orbit, and a long mission duration. The demand for a large field-of-view results in a Schmidt design with a massive corrector. However, the use of the corrector allows a 105 square degree field of view Alan Penny, Rutherford Appleton Laboratory, [email protected] (with thanks to all the speakers who wrote the reports on their talks). The Eddington website is sci.esa.int/home/ eddington/index.cfm. 6.31