Interuniversity Attraction Pole
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Modern Aspects of Theoretical and Observational
(ground-based and space-borne) Astrophysics
Coordinator : J.-P. SWINGS (ULg)
Partners : H. HENSBERGE (Royal Observatory, Brussels)
C. STERKEN (VUB)
C. WAELKENS (KU Leuven)
The present report describes the scientific activities performed in 2004 in the frame of Inter University Attraction Pole P 5/36, "Modern aspects of theoretical and observational (ground-based and space-borne) astrophysics": they are given in the five chapters dealing with the five work-packages of our network. New extensions are also indicated, for example:
the Mercator telescope on La Palma (see IAP 4/05 reports) is now being used jointly by KU Leuven and ULg not only in the frame of asteroseismological studies, but also in order to monitor light curves of gravitational lenses;
more and more work is devoted to interferometry both in Liège and in Leuven;
studies on the evolution of galaxies have begun (ROB and KU Leuven);
research on quasars and active galaxies has been extended to the high energy domain (XMM-Newton satellite; see details in chapter IV of the report)
thanks to a substantial investement in hardware and software development, Belgium is represented at the co-PI level in the consortium for the PACS instrument for ESA’s Herschel cornerstone mission, to be launched in 2007. This involvement implies the access to 20% of the guaranteed time with the PACS instrument for Belgian astronomers. This IAP has served as the nucleus of a science team which aims at preparing the scientific use of this guaranteed time, so that all research units which could benefit from Herschel could be involved, and so that optimal coordination of the available observing time is achieved (some details are given later in the report).
The personnel involved in the IAP research topics is given in the tables requested by the Belgian Office of Science Policy: these tables are appended for information to the present report (Appendix 1).
A steering committee meeting was organized at OSP1 on January 4, 2005 : on that occasion a presentation of 2004 activities and highlights was delivered by the four partners and a discussion took place on some recent developments : for instance C. Aerts described the strong involvement of the Belgian Asteroseismology Group in the European excellence network on the subject; J. Surdej gave some information on the evolution of the ILMT project; J.P. Swings insisted on the importance of the Joint European and National Astronomy Meeting (July 4-7, 2005, Liège) whose preparation started in 2004 and which will be organized around the five themes of IAP P5/36. A meeting with the members of the follow-up committee (who actually serve on the Scientific Organizing Committee of JENAM 2005) is scheduled on that occasion.
As far as the web-pages2 are concerned, I take pleasure in indicating that they are regularly extended and updated since early 2004.
The present report consists in an update of the text of the 2003 report that was made available on the web in 2004.
During the past several years, asteroseismologists of the Instituut voor Sterrenkunde of the K.U.Leuven, of the Institut d'Astrophysique et Géophysique of the ULg and of the Royal Observatory have very succesfully collaborated on many different topics in asteroseismology (we refer to the publication list in the annual reports of the previous IUAP). The collaboration is now extended to the Observational Astronomy Group of the V.U.B. This implies the participation of all the members of the Belgian Asteroseismology Group (BAG). The BAG was founded in 2000 with the specific aim to coordinate all the Belgian initiatives and expertises in asteroseismology, in order to orient the relevant research towards the upcoming data of pulsating stars from space. It mostly concerns COROT, a French-led European mission, which is in full preparation at present, with launch foreseen in 2005 and lifetime of about two years. Several members of the BAG are involved in the development of these missions.
1) Hare and Hound exercises for the COROT mission
Solar type stars
In the context of the preparation of the COROT space mission, "hare-hound" exercises were independently performed by several teams of the COROT Seismology Working Group. The aim of such exercises, which are commonly used in helioseismology, is to prepare the data analysis and scientific interpretation of the asteroseismic COROT observations. This will help in the development and testing of the tools which are needed for the interpretation.
The exercise was divided in four consecutive steps, each performed independently by a different group:
Produce a set of theoretical oscillation frequencies (with degrees = 0,1,2,3) and their rotational splittings for a stellar model satisfying given constraints on luminosity L, effective temperature Teff and chemical composition Z/X. A solar type star was chosen.
Construct a COROT simulated time series including the set of theoretical frequencies, COROT noise, stellar noise, inclination angle of the rotation axis, amplitude and life time of the modes.....
Extract from the synthetic spectra the frequencies and splittings with their formal errors leading to an "observed" set of frequencies.
Derive the structure and rotation of the stellar model from the set of frequencies.
For the COROT Week 3, which was organized in Liège, we joined the "hare and hound" exercice at step 4, from a different angle than the other teams. We compared our numerical stellar evolution code and our oscillation code to those used by other (French) teams. By comparing the results of different "theoretical" codes, it is possible to get an estimate of the "theoretical uncertainty" on the results. This is of course essential for the interpretation of the future seismic data.
We found that for the solar-type star chosen for the exercise, we had an almost perfect agreement with the results of the French Team, at least for the evolutionary track of that star in the HR-diagram, and for the oscillations of the star. We had to use slightly different parameters for the star. This will need more in-depth analysis.
2) A Cephei stars
A seismic modelling of the massive star 16 Lac has been made. We have compared a new spectroscopic mode identification with a photometric identification based upon a non-adiabatic description of the eigenfunctions in the star atmosphere. Numerous theoretical models have been computed in order to select the best model fitting the observed frequency values as well as the mode identifications. The derived mass is 9.62 M with an age of 15.7 million years.
We have obtained photometric data assembled by the NASA space mission WIRE of the Cephei star beta Crucis (Mimosa). A total of 5 million observations covering 17 days was analysed and the three main periods found in this way are in perfect agreement with the results derived earlier from line profile variations. The photometric amplitudes are small (3, 2.7 and 0.6 millimag for the dominant modes), but this is not surprising in view of the mode identifications derived earlier from the line profiles. Additional periods of low-amplitude modes (between 0.2-0.3 mmag) are also derived, including one suggested earlier by the radial velocity data.
We have performed an observational study of the orbital motion and the intrinsic variability of the double-lined spectroscopic binary Cen. Using 463 high signal-to-noise, high-resolution spectra obtained over a timespan of 12 years it is shown that the radial velocity of Cen varies with an orbital period of 357.0 days. We derive for the first time the orbital parameters of the system and find a very eccentric orbit (e=0.81) and similar component masses with a mass ratio M1/M2=1.02. Cen forms a challenge for current evolution scenarios in close binaries and it is also a puzzle how a massive binary with such a large eccentricity could have formed in the first place. Both the primary and the secondary exhibit line-profile variations. A period analysis performed on the radial velocity variations of the primary after prewhitening the orbital motion leads to the detection of at least 3 pulsation frequencies while the star does not show any periodic photometric variability.
We try to model Eri by fitting the four most significant, independent frequencies that are present in both the photometric and spectroscopic data. So far we encounter a puzzle : a non adiabatic analysis of the solutions best fitting the observed frequencies shows that these modes are not excited. We now study the influence of a different mixture of heavy elements.