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PROFILE OF A KEY PROGRAMME: Investigation of the Galactic Distribution and Physical Properties of T. LE BERTRE*, ESO N. EPCHTEIN and P. FOUQUE, Paris Observatory, France J. HRON and F. KERSCHBAUM, Vienna Observatory, Austria J, L EPINE, Sa"o Paulo University, Brazil

After leaving the , stars in carbon of the stellar surface. The car- mass loss during this phase. Although of intermediate mass (1 < M/Mo < 8) bon to oxygen (C/O) abundance ratio we understand in broad lines the evolu- evolve through various stagesu con- being normally inferior to 1, the surface tion of the stars through the AGB, our trolled by helium and hydrogen com- composition is in general -rich knowledge is often only qualitative and bustion. In one of these stages, called and the stars are said to be of M-type. many relevant points stay unclear. For the (AGB), hy- However, at each thermal pulse, the sur- instance, the formation and evolution of drogen and helium burn alternately in face C/O ratio increases and, in some carbon stars are still the subject of dif- shells around an electron degenerate cases, can reach a value larger than 1, ferent interpretations. Also, the mass carbon-oxygen core [I]. This core re- meaning that the surface composition loss phenomenon is not well under- sults from the complete combustion of becomes carbon-rich (C-type). This stood quantitatively; numerical models helium during previous phases. Due to mechanism is thought to be responsible show that the through the high electronic thermal conductivity, for most of the carbon stars (at least the AGB and later is strongly dependent its temperature is insufficient for carbon those on the AGB) in our . on the mass loss history. Parameters ignition. At the beginning of the evolu- The hydrogen envelope is unstable such as helium abundance and metal- tion through the AGB, helium burns and more or less regular pulsations de- licity (and in general individual element through triple-alpha reaction in a shell velop in it. Stars on the AGB are abundances) might have a strong influ- around the core which thus increases classified, sometimes arbitrarily , as irre- ence on stellar evolution and, in particu- its mass (early AGB). This shell dis- gular, semi-regular or mira variables. It lar, on carbon formation; however, appears progressively leading to the ig- is thought that the regular pulsations of there is a lack of observational data in nition of hydrogen in the surrounding miras appear during the quiet phase of that field. shell. This second source of energy be- hydrogen burning; in general, their Finally, these objects during their comes progressively dominant. periods range from 100 to 500 days, but phases of mass loss appear to be major As a consequence, the mass of the in some extreme cases may be larger. contributors to the replenishment of helium shell increases until helium is Following a thermal pulse, the star is the interstellar medium. The knowledge reignited violently in it (thermal pulse), thought to become an irregular pulsator of the composition and of the total re- then burns quietly while hydrogen in the of lower . turn rate of the matter that they are upper shell stops burning. The star may Also, through a mechanism which is injecting into the interstellar medium at experience 10 to 20 such cycles of still not well understood but which different galacto-centric distances is of - lo5 years duration. During 10% of appears related to the pulsations, the basic importance for describing the each cycle, the dominant source of is extended well chemical evolution of the Galaxy. This energy is helium combustion, and, dur- above the photospheric radius (Rphot- requires an accurate census of the AGB ing the remaining 90%, hydrogen com- 1 A.U.). In some cases, it can reach stars as a function of their location, type bustion. When helium is burning, the such a large dimension (- 10 Rphot)that, and mass loss rate. output luminosity of the star should be in its upper layers, refractory elements In our approach of these problems, lower by 0.5-1 than when condense into dust. Dust formation has we concentrate on galactic carbon stars hydrogen is burning. This phase of the drastic consequences. Radiation which are presently undergoing mass AGB - when helium and hydrogen are pressure on grains accelerate them loss. Optical surveys are known to be burning alternately - is referred to as the away from the central star and, in their biased against mass losing carbon thermally pulsing AGB (TP-AGB) and of- motion, they drag the gas outwards. In stars; also, due to the interstellar ab- ten, by ellipsis, the AGB. After the AGB, these conditions, the star becomes sorption, they are limited to the solar the star evolves quickly (in -lO3- surrounded by an expanding envelope neighbourhood. However, we have de- lo4 years) toward the of gas and dust. This mass loss veloped a selection method of such ob- stage at approximately constant lumi- phenomenon occurs preferentially dur- jects which is based on near- nosity and then dies out as a white ing the Mira phases; however, some ir- (1 to 5 pm) and IRAS (12 and 25 pm) dwarf whose temperature and luminosi- regular and semi-regular pulsators are photometric data and which has been ty steadily decrease. known to be also losing mass, whereas shown to be efficient and safe [2]. Start- The shells in which nuclear combus- some miras, especially among those of ing from the IRAS LRS data base, we tion occurs are surrounded by a large short period (<350 days), do not appear have then studied the carbon stars in convective hydrogen envelope. Follow- to be undergoing significant mass loss. the solar neighbourhood [3]. We now ing a thermal pulse, material processed The TP-AGB corresponds to a rela- want to extend our study to several lo- by the triple-alpha reaction can be con- tively short stage of the stellar evolution cations in the Galaxy, namely the disk at vected upward leading to an enrichment (lo6-1 O7 years). However, its study is different galacto-centric distances and very important, because stars are height above the galactic plane, by sur-

* Present address: DEMIRM, Observatoire de Paris, reaching their maximum luminosity veying, in an unbiased way, the mass 92195 Meudon Principal Cedex, France. (- 1O4 Lg) and experience most of their losing AGB population in the galactic centre and anti-centre directions, and at depends on the galacto-cen- References different galactic latitudes. tric distance, its effect on the formation [l] Iben, 1.: 1981, in Physical Processes in For every sub-sample, the distribu- and evolution of carbon stars will be Red Giants, ed. by I. lben and A. Renzini, tions of objects in function of their sorted out. Also, the mass-return rate p. 3. bolometric and mass loss from carbon stars will be determined as [2] Epchtein, N., Le Bertre, T., Lepine, rates will be evaluated. As the samples a function of the distance to the galactic J. R. D., Marques dos Santos, P., Matsuu- contain stars of different pulsational be- centre. ra, O.T., Picazzio, E.: 1987, Astron. Astro- haviours, the relationships between type This investigation will thus cast new phys. Suppl. Ser. 71, 39. [3] Epchtein, N., Le Bertre, T., Lepine, of variability, luminosity and mass loss light on the physical properties of car- J. R. D.: 1990, Astron. Astrophys. 227, 82. rate will be investigated. Initial masses bon stars in different physical and of carbon star progenitors will be de- chemical environments, and on the rived through their distribution perpen- chemical evolution of the interstellar dicularly to the galactic plane. Because medium throughout the Galaxy.

PROFILE OF A KEY PROGRAMME: High Precision Determinations for the Study of the Internal Kinematical and Dynamical Structure and Evolution of Young Stellar Groups H. HENSBERGE, E. L. VAN DESSEL, M. BURGER, Royal Observatoryof Belgium P. T. DEZEEUW, J. LUB, R. S. LEPOOLE, University of Leiden, the Netherlands W. VERSCHUEREN, M. DAVID, T. THEUNS, C. DE LOORE, University ofAntwerp, Belgium E. J. DE GEUS, University of Maryland, USA R. D. MATHIEU, University of Wisconsin, USA A. BLAAUW, Universities of Leiden and Groningen, the Netherlands

Purpose is at the point that makes such observa- mation on the dynamical state of young Presently observable sites of star for- tional efforts feasible. CCD echelle stellar systems is essential as input in mation corroborate the viewpoint that on ground-based tele- such numerical experiments and in the birth of stars occurs in groups. Such scopes offers the opportunity to record theoretical models. groups remain gravitationally bound for good quality (SIN >50) spectra over the a shorter or a longer time interval and extended wavelength range required in Interpretation of the Data are catalogued as open star clusters or order to acquire sufficient spectral infor- associations depending on their com- mation in early-type stars. CASPEC at The determination of RVs in early- pactness. As far as the dynamical state the 3.6-m ESO telescope provides the type stars with the required precision is, of these groups is governed by the con- stability and efficiency to obtain a for several reasons, a challenge. The ditions in the initial phases of cloud frag- reasonably large set of radial velocities application of CORAVEL in RV work has mentation, and nebular (RVs) with an accuracy that is limited by opened new frontiers in the study of gas expulsion, they contain valuable in- stellar atmosphere physics or standard late-type stars, based on a high quality formation about the star birth processes system calibration rather than by the RV standard system. The use of the and the early dynamical evolution of a instrument. We concentrate on the 3 CORAVEL technique is unfortunately star forming region (1). Evidently, the subgroups in the Sco-Cen 06associa- not extendable to earlier types. The es- younger the groups, the more the pres- tion and on the more distant young clus- tablishment of a better standard system ent state reflects the initial characteris- ter NGC 2244 in the Rosette nebula, at early spectral types is still essential tics. aiming at a detailed survey of about and our observations are expected to An important key to the progress in 500 0-F type respectively 50 0-A0 type contribute to this task (9). RVs will be such studies is the availability of infor- stars. The radial velocity study is part of measured by the cross-correlation tech- mation on the internal kinematics in stel- a continuing observing programme on nique (1 O), although attention will have lar groups: the internal velocity disper- nearby clusters and associations, in- to be paid to discern systematic effects sion, and its spatial and cluding Walraven photometry (4, 5, 6), possibly originating from subsystems of dependence (2). That purpose imposes astrometry from HIPPARCOS, mapping spectral lines; systematic atmospheric an accuracy of the order of a few kmls of molecular gas (7), and spectroscopy motions in OB stars might bias the ap- on empirically derived kinematical quan- for classification (6). The theoretical side parent RVs and could be detectable tities for a considerable number of stars of our study consists of N-body hydro- from lines formed over different atmo- covering a large range in stellar mass dynamical simulations of stars embed- spherical depths. As a by-product, this (3). Nowadays, instrumental technology ded in gas (8). Reliable empirical infor- should almost certainly lead to addition-