ANALYSIS
Developing
characteristics for low metal vs. normal stars
In order to properly analyze the spectra of the observed stars, I first needed a model to which I could compare them. In order to create these, I registered with the Vienna Atomic Line Database (VALD). The VALD creates theoretical stellar models, using the sun as its default setting. I used the ‘extract stellar’ mode and then received a theoretical spectrum. In submitting a query to the VALD, the following information was required for input: wavelengths to be displayed, the microturbulence, the effective temperature, and the log of the surface gravity. Microturbulence is a way to account for the turbulent motions that the stars’ atmospheres possess. The turbulence of the atmospheres changes line breadth. The effective temperature is a measure of a star’s temperature on the basis of the total energy emitted per unit area using the Stefan-Boltzmann law.
For each stellar spectral and luminosity class, a model star was created with VALD in order to compare any peculiarities in its spectra. In order to create such models, values for the abundances were either gathered from online sources when available or determined by analysis of the spectrum. If this was not possible, a query was sent that was based off the solar model and modified, which included changing the star parameters or its abundances. This brings the model closer to the real data and creates a good spectrum from which to base the identifications of lines.
VALD serves its greatest and quickest purpose as a reference for the theoretical central depth of the spectral lines. This allows for a ratio between the data and the theoretical models to be created and that ratio is utilized in calculating abundances. I calculated the abundances of my stars by creating a logarithmic ratio between my stars’ central depths and the central depths of the VALD model stars. A sample VALD query can be found in the data book.