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Evolved stars and their shells: Laboratories for stellar physics (STARLAB)

Research project BR/143/A2/STARLAB (Research action BR)


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PROJECT DESCRIPTION

Context

Most stars of our Galaxy have rather modest masses (smaller than 8 solar masses; they are called ”low- or intermediate-mass stars” – LIMS). Despite their modest mass, they play a crucial role in the production of elements heavier than iron (the so-called “heavy elements”). Such nucleosynthesis occurs in the deep stellar layers. Convective-mixing processes bring these heavy elements to the stellar surface, where a stellar wind blows into the interstellar medium. The LIMS thus play an active role in the chemical evolution of our Galaxy, which in turn impacts the star formation process.

Objectives

The goal of this project is to boost our understanding of (some of) the physical and chemical processes at work in LIMS, like mixing processes, tidal effects (in case of stars involved in binary systems), or the mass-loss process. These processes imprint observable signatures on surface abundances, on orbital properties (for LIMS involved in binary systems), and on mass-loss properties. Data will be collected in relation with these three aspects, and their comparison to model predictions should allow us to validate the models and their ingredients (the underlying description of the physical and chemical processes involved).

Methodology

Specific studies will be devoted to the three aspects described above, putting together the complementary expertise of the teams involved, and their modelling tools (ULB: abundance determination, nucleosynthesis and evolution of single and binary stars; KULeuven: abundance determination, modelling circumstellar environments of single and binary stars, mass-loss studies; Royal Observatory: modelling dust shells around LIMS). The data necessary for this analysis come from ground-based instruments (VLT, ALMA radiotelescope in the Atacama desert, HERMES spectrograph installed on the Mercator telescope run by the KULeuven team) as well space-borne instruments (PACS/SPIRES on board the Herschel satellite).

Specifically, the three following projects will be performed:

1) to derive the surface abundances in specific LIMS and confront them with nucleosynthesis predictions. Progress is expected from the fact that we will use well-selected targets with known distances (like AGB and post-AGB stars in the Magellanic Clouds or the Galactic Bulge), allowing us to locate them in the Hertzsprung-Russell diagram, thus facilitating the confrontation with the models, or by deriving isotopic ratios (from ALMA data) rather than elemental abundances. The determination of the atmospheric composition of evolved LIMS provides strong diagnostics on the internal nuclear burning and mixing processes at play in the stellar interior. Owing to their rich nucleosynthesis, AGB and post-AGB stars are thus exceptional laboratories to test the robustness of stellar models.

2) to explore new binary evolutionary channels that may link classes of LIMS in binary systems. Progress is expected from the modelling of physical processes not dealt with so far (tidal interaction with a circumbinary disc for instance, to pump up the orbital eccentricity), and from the recent availability of a key diagnostic tool like the eccentricity – period diagram. The latter is obtained from our on-going effort to monitor the radial velocity of very diverse classes of LIM binaries to get their orbital elements. After 5 years of operation of our HERMES spectrograph, even wide orbits are now becoming available. Ultimately, this project should assess how the various classes of LIM binaries as well as their surface composition fit within a global picture.

3) to study the circumstellar shells around LIMS at different spatial scales using mid-IR interferometry, Herschel and ALMA data. We will address questions on the enigma of the dust formation and condensation process in oxygen-rich stars, and the behaviour of mass loss as a function of metallicity.


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