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Astrochemistry - April 2008

van Dishoeck Prof. Dr. Ewine van Dishoeck
From the Special Topic of Astrochemistry

In our recent analysis of astrochemistry research over the past decade, the work of Prof. Dr. Ewine van Dishoeck ranks at #1, with 43 papers cited a total of 687 times. Three of these papers are included in the list of the 20 most-cited papers in this field for the past 10 years, and another three are on the two-year paper list.


According to Essential Science IndicatorsSM from Thomson Scientific, Prof. Dr. van Dishoeck's record includes 179 papers, the majority of which are classified in the field of Space Science, cited a total of 3,663 times between January 1, 1997 and December 31, 2007. She is also a Highly Cited Researcher in Space Sciences.

Prof. Dr. van Dishoeck is on the Faculty of the Leiden Observatory in the Netherlands, where she holds the title of Professor of Molecular Astrophysics.

In the interview below, she talks with about her highly cited research.

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

I studied chemistry at Leiden University and was originally determined to continue with my Ph.D. in theoretical quantum chemistry. However, the professor had died a few years earlier and it became clear that there was not going to be a successor, so I was forced to look elsewhere. My then-boyfriend (now husband), Tim de Zeeuw, was studying astronomy and had just had a lecture about interstellar molecules. "Isn't that something for you?" he asked. And indeed it was!

Supervised by the world's expert at Harvard, Prof. Dalgarno, and Prof. Habing in Leiden, I made the switch to astrochemistry and did my thesis on this interdisciplinary topic. I then moved to the US for six years, spending time at Harvard, Princeton, and Caltech, where I continued to broaden my astronomy background with observations at a variety of wavelengths.

van Dishoeck

In front of the Very Large Telescope at Cerro Paranal in Chile.

I returned to Holland in 1990 to set up an interdisciplinary group within the astronomy department, but with close collaboration with chemistry. At any given time, my group consists of a mix of astronomers and chemists. The Raymond & Beverly Sackler Laboratory for Astrophysics, which simulates chemical processes in interstellar space, is also closely connected with my research. While most of our publications are in astronomy journals, we do also regularly publish in chemical physics journals.

  What drew your interest to this particular area of study?

My interests are both in basic molecular processes (e.g., how does a molecule fall apart under the influence of UV radiation?) and the use of these molecules as diagnostics of the regions in which they are found (e.g., can they tell us how hot or cold it is?). Astrochemistry is the perfect combination of these two interests.

Lately, I have also become fascinated in the question of how new stars like our Sun and planets like Earth formed, and especially which chemical building blocks are available that could potentially lead to life on new planets. But, as discussed above, my career happened at least partly by accident!

  I understand that your research group focuses on basic chemistry in space as well as the formation of stars and planets. Would you talk a little about each aspect of your research and the methods you use?

Our research can be divided into several related parts:

1. Basic chemistry:

Our aim is to provide insight into basic chemical processes under the unusual conditions in space (extremely low densities and temperatures, harsh UV) and provide actual numbers (rates) for astronomers to use in their models (e.g., how rapidly does a molecule fall apart in a typical interstellar cloud?). Thus, it has a basic chemistry aspect (mechanisms) and an application (rate) aspect.

We use ab initio quantum chemistry and molecular dynamics techniques, in collaboration with the chemistry department. Also, together with Harold Linnartz, we simulate interstellar ices and gases in the Sackler Laboratory for Astrophysics.

2. Formation of stars and planets

Our aim is to use molecules to understand the physics of how new stars and planets are formed from the tenuous clouds, as well as to probe the chemistry during star and planet formation. The different aspects are:

  • Molecules as physical diagnostics: use molecules as remote thermometers and pressure meters to determine how hot or cold a cloud is (note: in astronomy we cannot go to a cloud and put in a pressure meter; we have to infer all this information indirectly and remotely from the radiation that the molecules emit).

  • Molecules as chemical diagnostics: as a new star is formed and heats up its surrounding envelope, the chemistry changes. Thus, some molecules are very good tracers of the coldest collapsing clouds, others of the hotter gas when the star is forming.

  • Astrochemical evolution: how does the chemistry change during star formation, and which molecules are available as the raw material from which new planets are formed?

  Your most-cited paper in our astrochemical Special Topic is the 1998 Annual Review of Astronomy and Astrophysics article, "Chemical evolution of star-forming regions," (36: 317-68, 1998). Would you sum up the main points of this review for our readers?

In this review, Geoff Blake and I made a synthesis of the available literature up to 1997 on observations and models of regions in which stars have only just been born and are still deeply embedded in their natal clouds. It was very timely, because systematic studies of these very young stars, ranging from those as massive as the stars in Orion to low-mass stars comparable to our "baby" Sun, had only just become possible due to the increased sensitivity of ground-based and space-based telescopes.

"Complex organic molecules, which could form the building blocks for the origin of life, are widespread in regions where new stars and planets are formed."

Prof. Dr. Ewine van Dishoeck is featured in

In particular, the launch of the Infrared Space Observatory (ISO) had opened up the infrared window from space (from the ground, much of it is not accessible due to H2O and CO2 in our own atmosphere). ISO was particularly powerful for studying the dust grains in space, i.e., the tiny (submicron-sized) solid particles present in every cloud, often surrounded by ice mantles. These dust grains play an important role in the chemistry. Thus, complete chemical inventories—both of the gas and of the ice—were now possible for star-forming regions.

We outlined a general scenario applicable to both high- and low-mass stars in which gases freeze out onto the cold dust grains during the collapse phase, but then start to evaporate back into the gas when the young star heats up its surroundings, where they drive a so-called hot core chemistry leading to complex organic molecules. A key point was that we were able to identify clear chemical and physical diagnostics for each evolutionary phase.

  One of your most-cited original papers in our database is the 2000 Astrophysical Journal paper, "An inventory of interstellar ices toward the embedded protostar W33A," (Gibb EL, et al., 536[1]: 347-56, 10 June 2000). Please tell us a little bit about the goals and findings of this paper.

This paper reported one of the first complete mid-infrared spectra of a high-mass protostar, obtained with the ISO. The dust grains are so cold (10 K) that they act as a deep-freeze: molecules from the gas collide with the grains perhaps once every 10,000 years and then have a 100% chance to stick (except for H2 and He). This is just as what happens on a cold night with your car window: the H2O molecules from the atmosphere freeze-out and form an icy layer. Once on the grains, a different chemistry can occur than in the gas.

Although ices had been detected in the early 1970s from the ground, much was still unknown because of obscuration by our own atmosphere. ISO allowed the first complete inventory of interstellar ices and the source W 33A happens to be one of the most ice-rich sources. Thus, it rapidly became the standard reference to compare with for all other ice studies. The detected ices nicely confirmed a theory, postulated in the early 1980s by Xander Tielens, that hydrogenation is the primary grain surface process. Thus, C, N, and O are turned into CH4, NH3, and H2O ice, and CO into methanol ice (CH3OH).

As a side note, stimulated by these observations, we are now actually studying this hydrogenation directly in the Leiden laboratory.

Another paper that is one of my own favorites is our 2006 letter, "Hot organic molecules toward a young low-mass star: a look at inner disk chemistry," by Lahuis et al. (Astrophysical Journal 636[2]: L145-8, 10 January 2006), where we detected abundant acetylene and hydrogen cyanide in the planet-forming zones of disks around young stars. View NASA press release.

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

First, we will focus on water in star- and planet-forming regions, a key ingredient in the chemistry and for the origin of life. As noted before, water needs to be studied from space. ISO observed water and other ices; the Herschel Space Observatory, which will be launched within a year, will study water gas. Our group is leading an international team to study water from the earliest to the later phases of star formation and from low- to high-mass stars. Visit for more detail.

Second, the Atacama Large Millimeter Array (ALMA) will become operational in the next five years and will be particularly powerful to zoom in on the planet-forming disks around young stars and image the chemistry directly in zones where new planets are forming.

Also, the successor of the Hubble Space Telescope, the James Webb Telescope, will be launched around 2014 and will have a new powerful mid-infrared instrument.

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

Take-away lesson 1: Complex organic molecules, which could form the building blocks for the origin of life, are widespread in regions where new stars and planets are formed.

Take-away lesson 2: One always finds something new and unexpected with a new observational facility (opening up a new wavelength range or jump in sensitivity, spectral and/or spatial resolution). Sometimes one has to dig deep, but if one digs deep enough, there is always something exciting to find.

Take-away lesson 3: Interdisciplinary science is fun and exciting!

Prof. Dr. Ewine F. van Dishoeck
Leiden Observatory
Leiden, the Netherlands

Keywords: Prof. Dr. Ewine van Dishoeck, astrochemistry, planet formation, star formation, interstellar cloud, inner disk chemistry, interstellar ices, origin of life.

Special Topics : Astrochemistry : Prof. Dr. Ewine van Dishoeck - Special Topic of Astrochemistry