Research
Since about 1980 our group has been at the forefront in the study of massive star evolution in galaxies. The main research topics are the following:
1. Stellar evolution with rotation
(A. Maeder and G. Meynet)The construction of better stellar models is the core of our research group. The high resolution spectroscopic observations as well as the satellite projects COROT, EDDINGTON show that the subject is very topical.
- Our aim is to make full numerical simulations of stars accounting for the various effects of rotation, including a detailed physics for internal mixing and surface effects in particular the losses of mass and angular momentum.
- We presently make grids of models of massive stars in the range of 9 to 120 solar masses at Z = 0.02. The lifetimes, the tracks in the HR diagram, the evolution of surface compositions of CNO elements, the evolution of rotation velocities will be among the predictions given. Extensive comparisons with recent abundance determinations are envisaged.The grids already calculated are presented in the following page: database.
- We also do grids of stellar models at other metallicities, in particular at Z = 0.001-0.004 well appropriate for BCDG (Blue Compact Dwarf Galaxies) and DLA (Damped Lyman-alpha systems), which are galaxy precursors or galaxies at a very early stage of their nuclear evolution. Extensive comparisons with recent abundance determinations are envisaged, particularly with recent results for SMC (Small Magellanic Cloud) stars.
- Extension of the grids to lower masses, including the Sun. There the magnetic coupling between the outer convective zone and the stellar wind is an essential feature to be included. These works require a strong additional theoretical investment, as well as numerical developments, thus they will likely extend over the next years.
- An example of results obtained with the models is the mass loss flux of a rotating star. The figures below present a surface of equal mass loss of stars with Teff= 30'000 K on the left and 25'000 K on the right. A hot bi-polar structure (blue color) with a rapid rotation is noted as well as the possible production of an equatorial disk colder (red color) and with a slow rotation:
2. Advanced nucleosynthesis and gamma-rays
(G. Meynet, A. Maeder and R. Hirschi)- Advantage will be taken from the new models with rotation to extend the calculations to the pre-supernova stages, in order to obtain new stellar yields (i.e. the input data for the chemical evolution of galaxies at various initial metallicities). These developments are the objectives of the Thesis of R. Hirschi. For these models, the new nuclear cross-sections available from recent data in nuclear astrophysics as well as the detailed opacities from recent developments in atomic physics will be accounted for.
- We search for the origin of primary nitrogen in relation with observations of halo stars, BCDG and DLA. In that respect a close comparison will be performed with the results of VLT observations obtained by M. Dessauges (c. f. point 4). A cooperation with Prof. Y. Izotoov of Kiev is also being active.
- The predicted chemical abundances are extensively compared to recent observations obtained by various groups, in particular by the group of Prof. A. Willis at University Colloge London and by Dr. K. Venn, in Macalester College, Minnesota (USA).
- Further work are made on the study of the production of gamma-rays by radioactive isotopes by rotating stars in relation with INTEGRAL. This work will be carried on by G. Meynet in relation with N. Mowlawi, who works at the ISDC. Our first results show quite interestingly that the production of radioactive elements as well as the production of most chemical species are considerably modified by the inclusion of stellar rotation in the models.
- Models have been calculated for Wolf-Rayet stars (ex:
)
and the surface abundances obtained are presented in this figure: 
(click on icon). This figure represents the evolution of the surface abundances
of a 60 solar masses star as a function of the remaining mass for different
initial rotation velocity (vini = 0 km/s top and 300 km/s bottom).
The phases during which the star is a type O, a LBV (Luminous Blue Variable)
and a Wolf-Rayet (WR) are indicated. The WR phase is divided in three parts:
WN, WN/WC transition and WC.
3. Stellar formation
(A. Maeder and I. Elshikh)- A first grid of models with accretion rates growing with the stellar masses is now being completed. It will be followed by other grids including the results from VLA observations (Churchwell, 1998) and from IRAS (Henning et al. 2000), which support large inflows and outflows from massive protostars.
- We make 2D models of accretion disks around massive stars. This work is made by R. Behrend in his thesis. These disks models are coupled to internal models for pre-Main Sequence evolution now working at Geneva Observatory. Such models are quite new, they are extremely complex and represent a step forward in the field. We plan also to be able to provide results concerning the temperature of the disks and their evolution. The model predictions are compared with observations.
- These new models change our views on the origin of the IMF and maximum
stellar masses. The maximum mass is essentially reached when the accretion
rate (which increases with the stellar mass) becomes so large that the
luminosity generated by the shock of the accreting matter reaches the Eddington
luminosity.
4. Starbursts observations, DLA and Nucleosynthesis at ESO (M. Pindao and M. Dessauges-Zavadsky)
- Observation of WR stars in starbursts are continued in the line of the work now by M. Pindao. The objective is to extract all possible information from the spectral features in the integrated spectrum of galaxies.
- A better modelisation of the IR properties of starbursts at various metallicities is performed.
- Better criterion for the distinction in the integrated spectrum of Seyfert galaxies between the light contribution from starbursts and from central black holes will be worked out in the continuation of the work made by M. Dessauges et al. (Astron. Astrophys. 355, 89)
- The collaboration with Dr. S. D'Odorico (ESO) on the CNO abundances in DLA, which are precursors of galaxies at very high redshifts,is carried on by Miroslava Dessauges in her thesis (including observations on the VLT).
- We study Starbursts in outer galaxies but similar sites can be found
in our galaxy, for example in the following clusters: 
(click on icon). The density of stars in these clusters is about 300'000
stars over only a few light years (for more info click here).
Last update: 2002/09/22