Steven Federman (Interstellar Matter)

Professor of Astronomy
Ph.D., 1979, New York University

Dr. Federman uses spectroscopy as the means to study the physical environment of interstellar gas clouds. Star formation takes place in these clouds, and by studying the physical environment of the clouds, a better understanding of the processes involved in star formation is possible. Another area of interest involves isotopic ratios, such as 7Li/6Li, 11B/10B, and 85Rb/87Rb, and seeks a better understanding of the sites where these elements are produced by nuclear reactions. He makes many of his measurements with telescopes at McDonald Observatory in West Texas, with the Hubble Space Telescope, and with the Far Ultraviolet Spectroscopic Explorer or FUSE. 

Measurements of atomic and molecular abundances in interstellar clouds allow a determination of gas density and temperature. In order to extract information on density and temperature, Dr. Federman conducts two types of analyses. In one case, he examines relative populations among energy levels in the C2 molecule and neutral atomic carbon. The other technique is based on the chemical network that connects the production of CN with the abundances of CH and C2. These analyses reveal that typical densities are 100-400 atoms (and molecules) per cubic centimeter and typical temperatures are 40-60 K.

The research on isotopic ratios is focused on determining the importance of three processes, cosmic ray spallation, neutrino spallation, and neutron capture. Spallation involves the breaking apart of nuclei resulting from a collision. Cosmic ray spallation occurs when relativistic protons and helium nuclei travel through our Galaxy and collide within interstellar carbon, nitrogen, and oxygen. This process plays a role in the production of the light elements, Li, Be, and B. Neutrino spallation takes place in supernova explosions associated with the death of stars 10 or more times more massive than the Sun. Here, the neutrinos break apart helium, carbon, and neon nuclei and in the process synthesize 7Li, 11B, and 19F (the sole stable isotope of fluorine). Neutron capture also occurs in asymptotic giant branch stars, massive stars undergoing helium and carbon fusion, and supernova explosions involving the death of a massive star; the rubidium isotopes probe this type of nuclear reaction.

To study these and other atomic/molecular species, one requires accurate laboratory absorption cross sections (or transition probabilities). The data yield oscillator strengths that are especially relevant for our astronomical studies. In many instances, the interstellar spectra provide the first accurate measurements. In a related laboratory program, Dr. Federman is collaborating with Dr. Schectman on the determination of ultraviolet oscillator strengths for atoms of interest to interstellar studies and with colleagues at Toledo (Dr. Cheng) and in France on cross sections for CO. Facilities used in this research include the Toledo Heavy Ion Accelerator , the Synchrotron Radiation Center of the University of Wisconsin-Madison, and the LURE synchrotron in Orsay, France. Future plans include measurements taken at SOLEIL, a third generation synchrotron in France.

A sampling of my publications is given below, grouped by topic -- structure and chemistry of interstellar clouds, CO photochemistry, production of the elements, and atomic oscillator strengths.

Structure and Chemistry of Interstellar Clouds:

CO Photochemistry:

Production of the Elements:

Atomic Oscillator Strengths: