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.
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."
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
Sciencearticle,
"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 takenyour 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