Grant: Discovery Grants Program - Individual

The outermost translucent layers of a star comprise the star's atmosphere, and is the part of the star that we can observe most directly. The primary observational data that we have for studying the atmosphere is the spectrum of the light emitted by the star. The physics of how the spectrum is determined by the structure of the atmosphere is sufficiently complex that we can only extract information about the star by computational modelling of the atmosphere and the spectrum. It follows that the accuracy with which the atmospheric structure is inferred depends on the physical realism of the computational modelling. Due to limitations in computational power and numerical methodology, models to date have been somewhat unrealistic approximations to actual stellar atmospheres. I propose to make improvements to a number of areas of realism in stellar atmosphere and spectrum modelling, including the geometry and horizontal structure of the atmosphere, the treatment of the thermodynamic state of the gas and radiation field, the inclusion of outflows (winds), and the realism with which atoms absorb, then re-emit, photons. This is an ambitious program, but the approach will emphasize making approximate improvements in some of these areas while making more realistic improvements in others of them to study the net combined effect of improvement in several areas at once. These improvements are especially relevant to the study of relatively cool stars such as the Sun and red giants. There are two broad scientific problems that will be addressed: 1) Increasing the accuracy with which the chemical composition is spectroscopically determined for very old red giant stars that trace remote global structures in our Galaxy, the understanding of which will help elucidate the history of structure formation and chemical element synthesis in the Universe, and 2) The structure of the mysterious dramatic temperature inversion that is found in the outer atmosphere of the Sun and most cool stars, the understanding of which will reveal physical processes that occur in such stellar atmospheres that are currently unknown.