Towards a global Understanding of Massive Stars Evolution – TUMSE

We have defined the following strategy:

1- In a first step, we combine evolutionary and atmosphere models. Typically, evolutionary models give access to the internal structure and the global surface properties of stars. Observations provide spectroscopy and probe the surface of stars. In order to link evolutionary models to observations, it is thus necessary to known how such models would look like in terms of spectroscopy. Hence, we add a new layer on top of evolutionary models: we compute atmosphere models which give a spectroscopic view of stellar evolution. We thus produce theoretical evolutionary sequences and study the effects of initial mass, metallicity and rotation.

2- In parallel, we observe young massive clusters which host numerous massive stars. Such clusters are thought to have formed in a single burst of star formation. Their observations is thus a snapshot of the evolution of the massive stars they contain. Studying many massive clusters helps us define observational evolutionary sequences that can be directly compared to the theoretical ones. We also investigate the role of very fast rotation on the formation of potential gamma-ray bursts progenitors.

3- Finally, we study the impact of massive stars on their close environment in HII region, with a special focus on the triggering of new star formation events.

Our published results are the following:

1- We have shown that quasi-chemically homogeneous evolution, predicted on theorretical grounds, was indeed followed by some stars (the rare class of WN3-5h stars). This type evolution is due to very fast initial rotation. It was predicted but never observed before our study which showed that the surface properties of WN3-5h stars cannot be explained by any other type of evolution. Such an evolution is thought to lead to gamma-ray bursts. Paper: Martins et al. 2013, A&A, 554, A23.

2- A second study focused on the comparison of the predictions of various types of evolutionary models, in order to assess the sources of uncertainties before computing atmosphere models. We showed that beyond the main sequence, the behaviour of stellar evolution can be very different depending on which code is used to compute the tracks. Paper: Martins & Palacios, 2013, A&A, 560, A16

To be completed at the end of the project

Articles publiés:
Martins, F.; Depagne, E.; Russeil, D.; Mahy, L., 2013, A&A, 554, A23
Martins & Palacios, 2013, A&A, 560, A16

Massive stars are key objects in the Universe, with major roles in galaxy evolution, cosmology, galactic dynamics and chemical evolution, nucleosynthesis, physics of the interstellar medium, star formation. However, from their formation to their death, unknowns still pave their evolution. In the present project, we propose to answer a few key questions regarding massive star formation and evolution.

We first want to establish whether massive stars can truly trigger the formation of new generations of stars on the borders of the HII regions they create. For this, we will determine age differences between OB stars ionizing HII regions and newly formed stars on the borders of these regions.
Secondly, we want to establish clear spectroscopic evolutionary sequences for massive stars and study the effect of metallicity and rotation on these sequences. We will do this both theoretically and observationally. Evolutionary models will be computed for different metallicities and rotation rates. Atmosphere models will be subsequently computed all along the evolutionary tracks to follow the spectroscopic evolution of massive stars. In parallel, we will observe and analyze the content of young massive clusters to infer observational evolutionary sequences, as we have already done for two clusters. We will thus obtain a clear view of massive stars evolution.
Third, we want to test the existence of a metallicity threshold for a peculiar type of evolution : quasi homogeneous evolution. It is thought to be the route to long-soft gamma-ray bursts, the most energetic explosions in the Universe, observed so far only in low metalliticy environments. We will study the properties of possible gamma-ray bursts progenitors (a special type of Wolf-Rayet stars) both in the Large Magellanic Cloud and in the Galaxy.

Several by-products will come out of this project, the most important one being the production of large grids of synthetic spectra for all types of massive stars. We intend to make them available to the largest public throughout a widely open database. These data will be extremely valuable for the preparation of future studies in the ELT area.

Our project, built around a tight team of experts on massive stars, is expected to lead to the emergence of a research pole on massive stars in the Montpellier/Marseille environment. An international workshop dedicated to massive stars evolution will be held in Montpellier at the end of the project.