Two Dimensional Stellar Evolution

* Context

Stars are, for the most part, spherical objects. For this reason, theoretical and computational models of stars are usually assumed to be just that: they are spheres. Such models work amazingly well and the those working in the field of stellar evolution have, for a century or so, exploited them very successfully. We now understand how stars burn hydrogen to helium to make them shine for billions of years, for example.

However, in the last decade, it has become increasingly clear that many of the most exciting stars are not the simple, spherical objects we are used to. Rotation is key. Stars that rotate expand around their equator with important consequences for their internal structure. Some stars, known as the Be stars, rotate is so fast that they are nearly flying apart. These stars are more egg-shaped than spherical!

Existing models of these rotating stars make some assumptions about their internal structure to construct distorted sphere models which are based on spherical models with small, non-spherical perturbations. These assume, for example, that stars rotate as nearly-spherical shells internally. Compare this to the internal structure of our Sun. At first glance it is spherical but we know from observing sunspots that it rotates once every 24 days at the equator. This is slow rotation because the distortion at the solar surface is very small. Even so the internal rotation is not on shells - latitudinal differential rotation is visible at the surface and helioseismolgy confirms that this differential rotation extends through the convective zone. In the interior it appears to rotate more solidly.

Many stars rotate much faster than the Sun and show far more severe physical symptoms. For example, rotation causes mixing of material from the centre of the star to its surface, leading to chemical pollution which can be observed with our telescopes. If, as in the Sun, the rotation rate changes with depth, we expect a dynamo to generate magnetic fields in the star. In binary-star systems material can move from one star to the other. This can spin up a star to its maximum, break up, rotation rate and induce significant chemical mixing and magnetic field formation. In turn magnetic fields transport angular momentum.

* Aims, objectives and applications

These processes can be modelled with simple spheres. A multi-dimensional model of a star is much more apt and that is the ultimate aim of this project. While three-dimensional stellar evolution is not currently possible because computers are simply not fast or powerful enough, two-dimensional stellar evolution is within our grasp. In two dimensions the fundamental processes of rotation and magnetic fields can be modelled properly without the many, often poorly justified, assumptions currently employed in stellar evolution models. The leap from one to two dimensions is a big. It needs to be made carefully.

The first achievable goal is to model two-dimensional stellar evolution by writing a new code to self-consistently solve for the structure of stars in two dimensions. This will include, as standard features, stellar rotation, magnetic field generation by dynamos, nuclear burning in the core of the star, mixing throughout the star and basic binary star interactions. The models will cover evolutionary phases from hydrogen burning, like our Sun, through red giants and supergiants, the biggest and brightest stars known, to stellar death.

The models constructed in this project will be made available to collaborators. This include the BRIDGCE project in the UK, to study the impact of rotation in stars on the chemical evolution of galaxies. Internationally the VLT-FLAMES collaboration needs stellar evolutionary models to interpret and understand the rapidly rotating stars they observe. The results of this project will be ideal for this task and for comparison with other surveys such as that made by NASA's Kepler satellite.

Who will benefit from this research and how?

* Astronomers and astrophsicists.

The prime benefits will naturally be felt in the field of stellar
structure and evolution which will take a leap forward. This is
fully explained in the "Academic Beneficiaries" section of this
proposal.

* Science communication.

Efficient visualisation of the results of the project is key to
its success. The material will be disseminated to the public
primarily through the project website. The material made in this
process will be designed as educational material. It will
interest science outreach providers, those providing online
teaching, etc. Online teaching is a booming industry and this
research can help teach students about the basic physics of
stars, rotation and magnetic fields.

* Software algorithm development and big data handling

The algorithms that will be developed in this project, mainly
involving extremely large-sparse-matrix inversion techniques, are
likely to provide excellent test cases for both third-party
commercial providers and other research groups around the UK who
are working on this, such as the STFC Computational Science
and Engineering Department through its Numerical Analysis Group
and its continuing development of HSL software library
(http://www.hsl.rl.ac.uk/). The modelled data will be large, so
previous STFC-funded research into big-data handling will be of
great relevance.

* Training in computational and astrophysical skills

The postdoc employed will, by the end, be fully trained in
parallel programming techniques, numerical analysis of large,
sparse matrices and software engineering. The plan is to hire a
software engineer but, because this is an astrophysical modelling project, the
postdoc will continue to develop his or her existing skills in stellar
evolution and hydrodynamics. The PI will also develop skills in
these fields which are highly sought after in the private sector.

There are many future students and researchers who will benefit from
the results of this project (see "Academic Beneficiaries"). Masters,
PhD and postdoc projects will definitely be spun-off as a direct
result, adding to the national knowledge and skills base.

The project will potentially use UK computing facilities, such
the STFC's DIRAC-3 facility which will be online in late 2015,
and will benefit directly from our collaboration.


* Science policy.

This project will open the door to the next generation of stellar
modelling which is relevant to many aspects of astronomy research
in the UK and worldwide. The results will greatly improve our
ability to understand observations made with upcoming large
telescopes (such as the ground-based E-ELT and forthcoming space
based missions such as JWST) and the latest large-scale satellite
surveys (such as Gaia) to which the UK contributes significant
resources in both money and manpower.