Research and scholarship
Research and scholarship
Stars play a key role in the universe through the light they shine, all the chemical elements they produce and the supernova explosions that mark their death. They can be used to probe the universe and its evolution from its infancy. The first stars formed approximately 400 million years after the Big Bang. Even if most of those first stars and subsequent generations are long dead, their chemical "fingerprints" are stored in low-mass stars that have a total lifetime longer than the age of the universe. In particular, extremely metal-poor low-mass stars enable us to study the first stars and the early universe. Using stars and nucleosynthesis as probes of the universe requires a multi-disciplinary approach developing synergy between astronomical observations, nuclear physics experiments and stellar evolution models. The need for multi-disciplinary approaches to tackle key scientific challenges is not new. For example, 100 years ago, Albert Einstein collaborated with mathematician Marcel Grossmann to develop the theory of General Relativity. Multi-disciplinary research is strongly supported in Europe via the ERC frontiers science grants. The UK and Europe play a leading role in the multi-disciplinary research area of stellar nucleosynthesis, thanks to state-of-the-art observing (ESO-VLT, ESA-Gaia) and nuclear experimental (e.g. CERN, LUNA) facilities. For example, Observers find the most metal-poor stars using the VLT, challenging theories of star formation. The LUNA nuclear facility in Italy measured the key reaction of the CNO cycle, the 14N(p,g) reaction in the Gamow window, removing extrapolation uncertainties. Furthermore, next-generation facilities (ESO E-ELT,FAIR@GSI) are being built, which represent billions of Euros in investments.
To maximise return on these huge investments, an improved theoretical framework of interpretation is crucially needed. Theoretical models are lagging far behind observational and experimental efforts. Next-generation stellar evolution models matching these next-generation facilities are therefore the primary objective of the my research.
Due to the complex nature of stars, stellar models would ideally require three-dimensional (3D) hydrodynamic models that include all the relevant physics (convection, rotation, magnetic fields, binary interactions, mass loss). 3D hydro models must use time steps that are at most days (max convective turnover time is ~200 days for red giants). The total lifetime of stars, however, is at least 2 million years. This means it will not be possible to model the full evolution of stars with 3D models in the near future. This explains why most stellar evolution models are limited to (spherically-symmetric) one dimension (1D). There is a long and successful tradition of 1D stellar evolution with many codes developed in different countries. The models produced with these codes have many applications in astrophysics. They are used as a theoretical framework of interpretation for observational surveys (e.g. VLT Flames survey of massive stars), as well as input for supernova simulations, galactic chemical evolution simulations and asteroseismology studies. They are also used to resolve as yet unexplained observations. For example, my group's stellar evolution models explain unique abundances in extremely metal-poor stars (Cescutti,., Hirschi et al 2013) and the oldest globular cluster of the galactic bulge, thus creating for the first time a link between these two populations (Chiappini,.., Hirschi et al 2011, Nature). My group's very massive star models "weighed" the most massive stars discovered to date (Crowther,.,Hirschi et al 2010). The masses determined (up to 320 solar masses at birth) drastically upset the previous upper mass limit of stars, which was around 150 solar masses.
The predictive power of stellar evolution, however, is crippled by 1D prescriptions of 3D phenomena containing free parameters, which need to be tuned to reproduce subsets of observations. A key uncertain prescription in 1D codes is that of convection, in particular convective boundary mixing (CBM). Even though 3D hydro simulations cannot follow the entire evolution of stars, computing power has finally reached the point where convective boundaries can be resolved in the largest simulations. This means that now these simulations can provide the guidance and constraints to build the next generation of stellar evolution models including 3D-hydro-based prescriptions for convective boundary mixing. Very importantly, the constraints from 3D simulations are independent of astronomical observations and thus provide a unique new insight into CBM. The goal of my research is to constrain, improve and establish new 1D prescriptions using 3D simulations and incorporate these new prescriptions into 1D stellar models. These latest 3D models require the largest computers on the planet. The development of these computers represents a multi-disciplinary effort between applied computer science, applied mathematics and (astro-)physics, the other key multi-disciplinary aspect of my research.
The goal of my theoretical research is thus to link major nuclear physics experiments to large astronomical observing programmes, hydrodynamics simulations to stellar models and theoretical stellar astrophysics to the high performance computing industry. To work towards these goals, I currently chair the ChETEC COST Action: http://www.chetec.eu/ .
See my personal web page for further information.
