The Catholic University of America

Research Program

My interests in stellar astrophysics address atmospheric phenomena and nucleosynthesis through the study of stellar spectra. In a broad sense I am interested in the chemical evolution of the galaxy, including aspects of the solar system (Sun and meteorites) and stars across the H-R diagram. As a necessary complement to my analyses of stellar spectra, I am also involved with studies in laboratory astrophysics, through conducting experiments to create the atomic data needed to analyze stellar spectra. Please feel free to contact me if you are interested in any of these projects. Student participation is always welcome.

Stellar Astrophysics

Hot stars

Stars of spectral types B and A have surface temperatures in the approximate range from 20,000 K to 8000 K. This temperature range is home to many spectrum anomalies, which gives rise to the concept of chemically normal and peculiar stars. The term chemically normal stars implies that their chemical compositions are similar to the Sun, while the term chemically peculiar stars refers to a variety of spectrum anomalies that are usually interpreted as element abundance variations on the stellar surface. My work in determining element abundances and isotope compositions for B and A type stars aims to characterize spectrum anomalies and to determine how they may be related, as well as to determine how they may change with stellar temperature and evolve over time. The elements heavier than the iron group are particularly interesting to me in this regard. My colleagues and I have used ground-based telescopes and space observatories to acquire stellar spectra for this work, and we continue to analyze these data and collect additional spectra.

An additional interest of mine for hot stars is the presence of weak emission lines in the optical and near-IR spectral regions of B-type main sequence stars. The emission lines arise from, predominantly, iron-group elements and have equivalent widths of only up to tens of milliangstroms. This interesting phenomenon is telling us something about the outer atmospheric regions of slowly-rotating B stars. Although they are observed in the spectra of both chemically normal and peculiar stars, their stronger appearance in peculiar star spectra links the weak emission lines with the processes by which elements segregate themselves under the influence of radiation pressure. I continue to analyse the spectra of hot stars for identifying and characterizing weak emission lines and in the near future I will be appling non-LTE codes to analyze the data.

Cool Stars

Giant and supergiant stars hold the key to understanding nucleosynthesis by the slow neutron capture process, also referred to as the s-process. I am studying the chemical composition of cool giants and supergiants at near-IR wavelengths, with emphasis placed on post iron-group elements. This has never been done before, and it is exciting to plot a course of discovery through the near-IR at high spectral resolution. Most recently, my summer intern worked hard to identify spectral lines and the abundances will result as accurate oscillator strengths are placed into the code. We prioritize the supergiant alpha Orionis as a massive star that may be undergoing weak s-process nucleosynthesis, with its by-products being dredged to the surface by convective motions.

Symbiotic stars are binary systems comprised of a cool giant and a hot companion. One does not directly 'see' the hot companion, but it is inferred from the spectrum, where the combination of high excitation and low excitation spectral features. Some symbiotic stars undergo tremendous outbursts of energy and material due to the transfer of material from the cool giant to the hot companion and subsequent eruption once the white dwarf can ignite nuclear reactions on its surface. It is these symbiotic novae that are thought to be the precursors to Type II supernovae after the white dwarf has reached critical mass. My colleagues and I study symbiotic stars to understand their spectra, chemical compositions, and surrounding environments. Our work with the Spitzer Infrared Space Telescope searches for evidence of prior eruptions and its chemistry through both imaging and spectroscopy. Our ground-based programs collect spectra to follow the temporal evolution of the post eruption-excitation mechanisms and chemical compositions. We have studied the ultraviolet spectrum of several symbiotics for characterizing the excitation mechanisms of emission lines.

Laboratory Astrophysics

To determine the abundances of elements in stellar atmospheres I fit observed stellar spectra taken at high spectral resolution with synthetic spectra calculated with the ATLAS and SYNTHE suites of programs of Robert Kurucz (CfA). A critical input to these codes is the atomic data for the many spectral lines. A spectral line is characterized by parameters for its position (wavelength), strength (oscillator strength) and shape (including hyperfine structure and isotope shift).

Oscillator strength, or f-value, is related to the probability that an electron will change its energy state to that of another. It is a parameter critical to the determination of an element's abundance. Any uncertainty in the f-value is directly transferred to the abundance determined from that line. The f-value can be determined experimentally or through calculation with sophisticated atomic structure codes. Modern experiments for the oscillator strength typically involve the measurement of the lifetime of an energy level and the branching fractions for transitions from that level. I work with colleagues at NIST and Lund Observatory to produce f-values for lines of interest to our study of stellar spectra.

Hyperfine structure (HFS) arises when electrons interact with the magnetic field of the nucleus of its atom. A spectral line may be divided into a number of components that can be seen as beautiful patterns in laboratory spectra. In stellar spectra, obtained at a lower resolution, these patterns may be seen as broadened features. Neglecting to account for the HFS in stellar spectrum analysis can lead to the misidentification of the feature and/or an incorrect abundance determined from the line. I have been involved in the determination of HFS for a number of heavy elements. Without such data we would not have been able to analyze the spectra of chemically peculiar stars and galactic halo stars. I am currently working on several projects to measure HFS for stellar spectrum analysis.

Isotope shift refers to the shift in the energy levels of an element or ion due to either the difference in mass or volume of the nucleus caused by the number of neutrons. Using the variation in spectral line wavelength due to isotope shift, I study the effects of diffusion in the atmospheres of chemically peculiar stars and nucleosynthesis by the rapid neutron capture process (the r-process) at early stages of the galaxy.