Absorption

Spectra of stellar photospheres are generally described by a continuum flux level with absorption lines superimposed upon it. The absorption lines can be characterized by their wavelength, depth and shape, which reflect plasma conditions (temperature, pressure, chemical composition, magnetic field strength) and atomic parameters (isotope structure, hyperfine structure, Zeeman splitting, oscillator strengths). The FTS measurements provide information on a spectral line’s wavelength, intensity and structure and even the transition energy levels. When coupled with experimental energy level lifetime measurements the FTS data lead to transition oscillator strengths. These data provide the necessary atomic line description that serves as input for synthetic spectrum codes.

The analysis of stellar spectra aims to determine the plasma conditions using spectral lines having known atomic parameters. In particular, the chemical composition of the plasma plays an important role in revealing the dynamical processes at work in stellar atmospheres and even the past history of the galaxy. Our analysis of the solar spectrum is directed to quantifying elemental abundances for use of the Sun as an abundance standard by using improved atomic data.

For many stars hotter than the Sun, the deviation of the chemical abundance pattern from the solar distribution is marked by peculiarities in both elemental abundances and isotopic ratios that have their origins in the dynamics of the atmosphere. It is theorized that a delicate balance of forces involving radiation and gravity is responsible for creating the quiet conditions in the stellar atmosphere that allow atom/ions to diffuse to higher or lower atmospheric depths. Our observations of these chemically peculiar stars are directed to defining peculiar element abundance distributions as well as isotopic anomalies for the various types of peculiar stars.

Chemical compositions in stellar atmospheres may also be different from the solar distribution due to the original composition of the star forming clouds. The early history of the Galaxy can be studied from the spectra of halo stars. Our laboratory data are being applied to revealing the age of the Galaxy, through observation of spectral lines of radioactive elements, and the mechanisms for the creation of elements from its earliest generations of stars.