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Astrochemistry - April 2008

Herbst Professor Eric Herbst
From the Special Topic of Astrochemistry

According to our April 2008 Special Topic on Astrochemistry, the scientist whose work ranks at #9 by total citations is Dr. Eric Herbst, with 23 papers cited a total of 273 times. He also ranks at #4 by total number of papers and at #13 by cites/paper. In Essential Science IndicatorsSM from Thomson Scientific, Dr. Herbst's citation record includes 129 papers, mostly classified in the field of Space Science, cited a total of 2,148 times between January 1, 1997 and December 31, 2007. He is also a Highly Cited Researcher in Space Sciences.


At present, Dr. Herbst is a Distinguished University Professor in Physics, Astronomy, and Chemistry at The Ohio State University in Columbus, where he oversees the Astrophysical Chemistry Group. He is also a Fellow of both the Royal Society of Chemistry and the American Physical Society.

In the interview below, he talks with ScienceWatch.com about his highly cited work.

Please tell us a little about your research and educational background.

I received a Ph.D. in physical chemistry from the Department of Chemistry at Harvard University under Professor William Klemperer in 1972, and then continued there for a year as a postdoctoral associate in the same research group. In 1973, I moved to the Joint Institute for Laboratory Astrophysics at the University of Colorado where I was once again a postdoctoral associate, this time under Professor Carl Lineberger.

After a year in Boulder, I obtained my first academic job—in the chemistry department at the College of William and Mary, where I was first an assistant and then an associate professor. In 1980, I moved to Duke University as associate professor of physics and was subsequently promoted to Professor of Physics. I spent the year 1988-89 on sabbatical at the University of Cologne in Germany under an awardeeship from the Humboldt Foundation.

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Finally, in 1991, I made my last move—to The Ohio State University, initially as a Professor of Physics. In subsequent years, I received joint appointments in the departments of astronomy and chemistry and was awarded a Distinguished University Professorship. Perhaps my most prestigious award was the Centenary Award of the Royal Society of Chemistry, given in 2004. I am a Fellow of this organization as well as of the American Physical Society. During my career, I have taught a multitude of courses in physics, chemistry, and astronomy.

My research lies in two areas. I am known more widely for my research in astrochemistry, which is the study of molecules in environments outside of the Earth, specifically in star-forming regions of interstellar clouds. The molecules are of interest both for chemical reasons, since they grow under non-terrestrial conditions, and for astronomical reasons, as probes of their environment. In astrochemistry, I am known most for my detailed computer simulations of the chemistry.

"The molecules are of interest both for chemical reasons, since they grow under non-terrestrial conditions, and for astronomical reasons, as probes of their environment."

Prof. Eric Herbst is featured in ISIHighlyCited.com.

I am also a molecular spectroscopist. In order to detect gaseous interstellar molecules in the millimeter-wave and submillimeter-wave regions of the spectrum by their rotational motions, I do laboratory experimental work in molecular spectroscopy on the spectra of molecules likely to be found in interstellar clouds. Without knowledge of their spectra in the laboratory, it is very difficult to detect molecules in space.

What do you consider the main focus of your research, and what drew your interest to this particular area?

The main focus of my research is to simulate the chemistry of molecules in star-forming regions of interstellar clouds. Such computer simulations yield the expected concentrations of molecules as functions of time and the physical conditions of the regions. If these expected concentrations are in agreement with the concentrations obtained via spectral observations, then both the history and the current physical conditions can be determined. I am also interested in how large interstellar molecules can grow and whether or not there is a relationship between interstellar molecules and the chemistry of the early Solar System, which formed from an interstellar cloud.

I was drawn to this area as a senior graduate student and subsequent postdoctoral associate in the research group of William Klemperer. At this time, polyatomic molecules were first being detected in interstellar clouds. Our first paper on the subject—"The Formation and Depletion of Molecules in Dense Interstellar Clouds," (Astrophysical Journal 185: 505, 1973)—showed how the rich chemistry could be explained by unusual reactions that occur under the low temperature conditions of interstellar clouds. In the 35 years since this paper was published, the field has gone from the exotic playground of a few into a worldwide endeavor of chemists, physicists, and astronomers. Our rudimentary knowledge of dense interstellar clouds in 1973 has undergone dramatic change, and regions of such clouds on smaller and smaller spatial scales are being studied, both observationally and theoretically. In particular, regions of star formation, and the process of star formation itself, have become much better understood, thanks in part to the role of molecules as probes.

The list of molecules detected in space continues to grow by about four molecules every year. A current list of molecules detected by high-resolution spectral techniques is attached. Prepared by myself and by Evelyne Roueff (Meudon, France), this list includes those molecules studied in the gas mainly by rotational spectroscopy. Much larger molecules, known as polycyclic aromatic hydrocarbons (PAHs), are inferred from broader features in the infrared region of the spectrum.

 Your most-cited paper in our astrochemical Special Topic is the 2002 Planetary and Space Science article, "H3++HD« H2D++H2: low-temperature laboratory measurements and interstellar implications," (50[12-13]: 1275-85, October-November 2002)." Would you describe the aims and findings of this work for our readers?

Star formation occurs in evolutionary stages. The first major stage in the formation of low-mass stars is known as a dense cold core. In this stage, which lasts around 105 yr, atoms are converted into molecules both in the gas-phase and on the surfaces of tiny dust particles. Many of the molecules are organic in nature; i.e., they contain the element carbon. One of the most interesting aspects of the gas-phase chemistry is known as deuterium fractionation, a term that refers to differences in the concentration ratio between deuterated isotopomers and their normal counterparts compared with the actual deuterium-to-hydrogen elemental ratio. For example, throughout most of the galaxy, the elemental D/H ratio is about 10-5, and this ratio pertains to the concentration ratio HD/H2 in cold cores. But, if one studies certain other concentration ratios, such as DCN/HCN, one finds a vastly increased number of 0.01-0.03, three orders of magnitude greater!

The Planetary and Space Science paper concerns a laboratory measurement of the rates of chemical reactions that help to determine concentration ratios between deuterated and normal species in our simulations. With the laboratory results, one can calculate abundance ratios such as DCN/HCN precisely, as a function of temperature. Since the calculated ratios are strong functions of the temperature, this parameter can be determined from the observations. Deuterium fractionation is even more astounding in collapsing cold cores, the next stage in stellar evolution, where multiply deuterated isotopomers are detected (e.g. ND3), and can also be understood by the chemical processes we have studied.

 What directions have you taken your work since the 2002 paper?

Since 2002, we have studied the chemistry of various evolutionary regions along the path to star formation. It is known, for example, that as the collapsing cold core heats up due to star formation at its center, the chemistry switches from one dominated by exotic molecules to one dominated by terrestrial-like species. We have found that surface chemistry on interstellar grains is as important as gas-phase chemistry in this transformation, and is probably more important in making larger organic molecules. The so-called hot core surrounding the newly forming star can develop a rotating disk, known as a protoplanetary disk. This disk is a direct precursor to solar-type planetary systems and has a most interesting chemistry, which we and others are simulating.

 Where do you see this research going in five to ten years? 

In the next decade, the field will be revolutionized by the advent of two new telescopes: Herschel, to be flown into space, from which it will scan the sky in the far-infrared region, which is totally unobservable from the ground, and ALMA, a giant collection of millimeter-wave telescopes operating from a high valley in the Andes and capable of seeing objects with spatial resolution much greater than achieved by current instruments. The net result will be a great increase in the known complexity of the interstellar medium, and a greater need for chemical simulations in which both chemistry and dynamics (changes in physical conditions) are incorporated.

 What should the "take-away lesson" about your work be for the general public?

The "take-away lesson" about my work in astrochemistry is that an interdisciplinary field can be exciting to investigators trained in different areas of science. Chemists are interested in interstellar molecules because they are synthesized under unusual conditions (by terrestrial standards); biologists are interested in these molecules because they are probably the precursors of biological species; astronomers are interested because the study of molecules in space tells us about the physical conditions and their history in sources such as those leading to star and planetary formation.

Eric Herbst, Ph.D.
Distinguished University Professor of Physics, Astronomy, & Chemistry
Department of Physics
The Ohio State University
Columbus, OH, USA

Relevant keywords for this interview: astrochemistry, interstellar clouds, astrochemical computer simulations, molecular spectroscopy, gaseous interstellar molecules, early solar system chemistry, star formation, Professor Eric Herbst, Special Topic of Astrochemistry

 



Special Topics : Astrochemistry : Professor Eric Herbst - Special Topic of Astrochemistry