Stellar Convection and Spectroscopy

Significant Achievements, Anticipated Outcomes and Future Work

The progress of the project during 2003 has been severely restricted by the bushfires in January 2003 which destroyed much of Mt Stromlo Observatory. Due to this major disruption, much of the research of the PI has been on hold in order to attend to the necessary rebuilding activities at the observatory together with the other staff. As a consequence most of the planned activities during 2003 were not carried out. With the reconstruction now underway and the arrival of an ARC-funded postdoc, Regner Trampedach, the progress is expected to improve during 2004.

During 2004 we intend to extend our 3D modelling to giant stars, which has previously never been accomplished. It is expected that convection is quite different in giants compared with solar-like dwarf stars with much more vigorous convective motions and even supersonic convection. This will place much more severe demands on the numerical codes to handle properly. We also intend to continue our studies of the effects of chemical composition on stellar convection and perform detailed line formation calculations for astrophysically important elements like oxygen, magnesium and iron.

Computational Techniques Used

We perform radiative hydrodynamical simulations of stellar convection in 3D. The code adopts a Eulerian xyz-geometry with an explicit third-order leap-frog time-integration. The boundaries are transmitting in the vertical direction and periodic in the horizontal direction. Typical numerical grid resolutions are 100^3-200^3 with numerical stabilization provided by a hyper-viscosity. Since stellar convection is driven by the radiative cooling at the surface layers, we treat the energy exchange between radiation and gas in detail by solving the radiative transfer equation at each hydrodynamical time-step. Most of the computing time is in fact spent on solving the radiative transfer taking into account the effects of spectral lines rather than than the hydrodynamical part of the simulation. The radiative transfer equation is solved along typically 10 different rays in the simulation box using the Featrier long-characteristic method. The post-processing of the simulations done with the supercomputing facilities consists of detailed spectral line formation calculations in 3D and under non-local thermodynamical equilibrium conditions. This includes solving simultaneously the statistical equilibrium rate equations for the atomic level populations and the detailed radiative transfer equation for 20-50 directions for typically 100 radiative transitions each consisting of about 50-100 frequencies. Convergence is achieved through an iterative process involving 10-30 iterations depending on the element in question.

Publications, Awards and External Funding

External Funding and Awards

The project is supported by an ARC Discovery Project grant (DP0342613) entitled "The first stars and the chemical enrichment of the Universe" for the period 2003-2005. The grant is worth $375,000 in total and includes a postdoc to work on 3D stellar modelling using the APAC National Facility.

A collaborative program between RSAA, ANU, and Department of Astronomy and Space Physics, Uppsala University, Sweden, has been awarded by STINT (The Swedish Foundation for International Cooperation in Research and Higher Education). The funded project ($80,000) will run during the period 2003-2005 and includes visits by overseas colleagues for collaboration on radiative-hydrodynamics calculations using the APAC National Facility.

Publications

M. Asplund, M. Carlsson, A.V. Botnen, "Lithium in metal-poor halo stars: 3D non-LTE line formation in HD140283 and HD84937", Astronomy and Astrophysics, 399, 2003, L31-L34
P.S. Barklem, A.K. Belyaev, M. Asplund, "Inelastic H+Li and H- + Li+ collisions and non-LTE Li I line formation in stellar atmospheres", Astronomy and Astrophysics, 409, 2003, L1-4
C.J. Akerman, L. Carigi, P.E. Nissen, M. Pettini, M. Asplund, "The evolution of the C/O ratio in metal-poor halo stars", Astronomy and Astrophysics, 414, 2004, 931-942
P.E. Nissen, Y.Q. Chen, M. Asplund, M. Pettini, "Sulphur and zinc abundances in Galactic stars and damped Ly α systems", Astronomy and Astrophysics, 415, 2004, 993-1007
M. Asplund, N. Grevesse, A.J. Sauval, C. Allende Prieto, D. Kiselman, "Line formation in solar granulation: IV. [O I], O I and OH lines and the photospheric O abundance", Astronomy and Astrophysics, 417, 2004, 751-768
M. Asplund, "Line formation in solar granulation: V. Missing UV opacity and the photospheric Be abundance", Astronomy and Astrophysics, 417, 2004, 769-774