Where did the chemical elements come from? Where did the carbon, oxygen, iron and other elements essential to life originate? The question of the origin of the elements in nature is intimately tied to the intertwined stories of stellar evolution - how stars change with time - and stellar nucleosynthesis - how elements are created through thermonuclear reactions. Primordial element synthesis during the Big Bang 13.8 billion years ago created hydrogen and helium. All elements heavier than lithium came from stars. The quest for the stellar sites that produced these elements is fundamental to modern science because it is closely linked to questions concerning the origins of planetary systems, life and astrobiology, and the processes of galaxy formation and evolution. Different types of stars contribute to the build-up of metals at different times in the Universe’s past. The study of this build-up of metals is known as galactic chemical evolution.
The questions we want to address with this workshop are: How well do we know the nuclear processes that make elements? How well do we know how stars of different mass die and disperse their newly created elements into space? How and where does stellar dust form, and how are chemical elements dispersed and mixed in galaxies? How do these processes change over cosmic time? While the theory of stellar evolution has a long history starting in the mid-20th century, models of stars are still mostly 1-dimensional in space, and mostly assume that stars are single, don’t have magnetic fields, and don’t rotate. We also don’t have a good understanding of how these and possibly other processes affect how elements are mixed inside stellar interiors, how stars lose mass via outflows or winds or indeed how a companion may enhance the mass-loss process by which stars enrich galaxies, how exactly binary interactions result in thermonuclear supernovae, and how massive stars explode as core-collapse supernovae. Supernova explosions don’t just make elements but also inject considerable energy into galaxies, which drives galactic evolutionary processes such as star formation.
To make progress on these questions we need to couple state-of-the art theoretical models of stars with observations. This is where this workshop comes in. There is an increasing number of large-scale surveys and instruments dedicated to studying stars, such as Gaia, APOGEE, HERMES-GALAH, WEAVE, and 4MOST, many key science goals of SDSS-V involve stars and using stars to map the Milky Way, and the Rubin Observatory has started a 10-year survey of the southern sky (LSST), where it will discover thousands of explosive stellar events known as transients. New data from existing cutting-edge facilities such as the Atacama Large Millimetre Array (ALMA) and the James Webb Space Telescope (JWST) are making paradigm-shifting discoveries such as large nitrogen enrichments in galaxies in the early universe, and that the outflows from dying low-mass stars are shaped by companions more often than we expected. This is the right time to organise a workshop where we focus on difficult theoretical questions surrounding stellar nucleosynthesis and chemical enrichment, with observations as our key guide. There is true transformative potential here to change our field by the joint efforts of the theoretical and observational communities.
The aim of this workshop is to bring together stellar modellers and observers to identify and attack the specific problems that still hamper our ability to interpret and explain the chemical make-up of the Universe as traced by stars and galaxies. We plan for stellar modellers to compare their predictions to each other and debate the various ways in which different physics is implemented. Such discussion will be guided by current cutting-edge observations, including a careful evaluation of their uncertainties as well as their significance in comparison to the models.