Feedback and Yields from Very Massive Stars

``How did the Universe begin and evolve'' is a key science challenge. We address this question by modelling physical processes from the micro (nuclear scales) to the macro scales (stars & galaxies), studying the ionising and chemical feedback from stars. BRIDGCE is a multidisciplinary collaboration between nuclear, stellar and extra-galactic astrophysicists, which aims to achieve a comprehensive understanding of the evolution of the Universe from the era of reionisation up to now, using chemical elements as fingerprints of the physical processes that occur in stars and galaxies.
Elements heavier than helium are produced in stars and supernovae on different timescales, and the stellar populations and interstellar medium within galaxies keep a record of star formation and chemical enrichment histories of galaxies.

Very massive stars (VMS) are important objects in astronomy for a number of reasons. Because of their brightness (up to a million times that of the Sun), they can be seen out to large distances, offering a glimpse of the Universe in our distant past. VMS possess strong outflows and produce black holes at the end. Due to these outflows and explosions, massive stars are thought to trigger star formation in galaxies throughout the Universe. They are also responsible for the chemical enrichment of the Universe. As the big bang only produced hydrogen and helium, the heavy elements, such as carbon, nitrogen, oxygen - critical for forming life - were synthesised in the cores of massive stars, and are returned to the surrounding medium through mass loss and explosions.

The primary aim of the proposed project is to predict the amount of mass loss throughout the Universe, as it is this property that determines the life and death of a massive star. Although the life of solar-type stars is driven by the physical process of hydrogen burning in their cores, the life path of a massive star is largely dominated by mass loss. For instance, an object as massive as 60 times the mass of the Sun, will only end up with 6 times the mass of the Sun before it explodes. The amount of outflow however depends critically on the chemical environment.

Our models have been shown to be successful in predicting the mass-loss properties in the Milky Way, but we now wish to extend our models to the early Universe. We will develop, test, and apply the new massive star models to map the strength of the outflow, their path to death, and the way they die. We will determine the final stellar masses and mass boundaries for the formation of neutron stars and black holes.

The predictions will be extended to stars representative for conditions in the early Universe, when the cosmos contained no elements heavier than hydrogen and helium, and we will determine whether these objects make neutron stars or black holes and how they explode as supernovae or gamma-ray bursts. Current state-of-the-art models do not take VMS mass-loss into account in a realistic way. Our models use the actual presence of the individual chemical elements - recently been shown to be of paramount importance for objects in the early Universe - and will bring the state-of-the-art to a new level.

During the course of the programme, we will predict both the direct chemical enrichment by stellar outflows, and set constraints on their fate. Using both ingredients allows us - for the very first time to provide meaningful predictions for the chemical build-up of the early Universe.