Grant: Discovery Grants Program - Individual

The outermost translucent layers of a star comprise the star's atmosphere, and is the part of the star thatwe can observe most directly.  The primary observational data that we have for studying the atmosphere isthe spectrum of the light emitted by the star.  The physics of how the spectrum is determined by thestructure of the atmosphere is sufficiently complex that we can only extract information about the star bycomputational modelling of the atmosphere and the spectrum.  It follows that the accuracy with which theatmospheric structure is inferred depends on the physical realism of the computational modelling.  Due tolimitations in computational power and numerical methodology, models to date have been somewhatunrealistic approximations to actual stellar atmospheres.  I propose to make improvements to a number ofareas of realism in stellar atmosphere and spectrum modelling, including the geometry and horizontalstructure of the atmosphere, the treatment of the thermodynamic state of the gas and radiation field, theinclusion of outflows (winds), and the realism with which atoms absorb, then re-emit, photons.  This is anambitious program, but the approach will emphasize making approximate improvements in some of theseareas while making more realistic improvements in others of them to study the net combined effect ofimprovement in several areas at once.  These improvements are especially relevant to the study ofrelatively cool stars such as the Sun and red giants.  There are two broad scientific problems that will beaddressed: 1) Increasing the accuracy with which the chemical composition is spectroscopically determinedfor very old red giant stars that trace remote global structures in our Galaxy, the understanding of whichwill help elucidate the history of structure formation and chemical element synthesis in the Universe, and2) The structure of the mysterious dramatic temperature inversion that is found in the outer atmosphere ofthe Sun and most cool stars, the understanding of which will reveal physical processes that occur in suchstellar atmospheres that are currently unknown.