Oka in his Jones Laboratory office. (Photo by Jason Smith)
Five decades, three sciences, one molecule
The life’s work of chemistry and astronomy and astrophysics professor emeritus Takeshi Oka.

This story originally appeared in the most recent issue of Inquiry, the newsletter for physical sciences alumni.

“I’m a mixture of many things,” explains Takeshi Oka, professor emeritus of chemistry and astronomy and astrophysics. Trained as a physicist, employed as a chemist, and now researching astrophysics, Oka has been well prepared for the quest that has occupied the majority of his life: to understand a molecule that could explain the gas filling the void between the stars.

Oka was born in Tokyo in 1932. His family moved to Lushun, China, in 1937 when his father received an appointment at the Lushun Institute of Technology. Oka returned to Japan in 1949 to attend the University of Tokyo, where he received his bachelor’s degree in chemistry and earned his doctorate in the laboratory of Koichi Shimoda in physics. In 1963, he moved to Ottawa to work on molecular spectroscopy in Gerhard Herzberg’s group at the Division of Pure Physics at the National Research Council of Canada.

In 1968, the scientific community was shocked to learn of the discovery by Charlie Townes of ammonia and water in the interstellar medium (ISM), the thin cold gas that fills the otherwise empty space between stars. It had been assumed that deep space was simply too cold, and the atoms too few and far between, to combine into molecules. But the presence of ammonia and water suggested that there were other, as yet unknown, forces at work. Oka’s curiosity was piqued; since stars are born from such gas, its study is essential to understand the initial stage of star formation, the most fundamental process in astrophysics. In 1975 he decided to “drop everything” he was working on and start looking for ionized triatomic hydrogen—H3+.

H3+ is, as Oka describes it, “the origin of interstellar chemistry. Almost all molecules in the interstellar medium are produced from it.” Detecting and characterizing H3+ would be a major breakthrough in understanding the physical and chemical processes in the ISM. Before finding it in space, he first had to discover its spectrum—the wavelengths of light that it absorbs and emits when stimulated—in the lab as the fingerprint with which to hunt the molecule in space. The spectrum had been the Holy Grail of many ambitious physicists, and that first step took five years of concentrated work.

Discovered by J. J. Thomson a century ago, H3+ is a highly symmetric molecule and primarily absorbs light in the infrared. (Normally, this makes a molecule difficult to detect from Earth, since the water vapor and other gases in Earth’s atmosphere also tend to absorb infrared radiation. Luckily for Oka, no component of the atmosphere interferes strongly with the wavelengths H3+ absorbs.) Oka then turned from the lab to observation, becoming, in effect, an astronomer—career change number two.

By this time it was 1981. Oka left Canada to join the University of Chicago, jointly appointed to chemistry and astronomy and astrophysics. He and Tom Geballe, his collaborator since 1982, began to search for the molecule using a telescope on Mauna Kea, hoping to identify it by examining the infrared spectra from bright stars; if light from the stars passed through a cloud containing H3+ on its way to Earth, the molecule would leave its telltale absorption lines in its spectrum. Other scientists were skeptical that the molecule existed in amounts large enough to observe. In fact, the first search Oka attempted alone was a “miserable failure”; his infrared spectrometer was not advanced enough to detect the faint signals from H3+. But Oka and Geballe persisted, and with improved equipment they finally detected the molecule in 1996, 16 years after they started. To their delight, they realized that there was H3+ almost everywhere in the Galaxy—but, to their puzzlement, there was an enormous amount of H3+—a million times denser than elsewhere—near the center of the Milky Way, in an area called the Central Molecular Zone (CMZ). Why the CMZ should contain so much H3+ was a mystery, but the challenge heralded the beginning of a new era in Oka’s research.

That discovery heralded what Oka calls his “golden age,” the period after he retired from teaching in 2003 and became a professor emeritus. Without teaching and administrative duties, he plowed his energies into research to study this center of astrophysical activities. The last eight years, he says, have been “the most fruitful period of my astronomical observations, and the most exciting time of my research life.” He has been to observatories on Mauna Kea at least 30 times, and to one on Cerro Pachon in the Chilean Andes seven times. His study has revealed a vast amount of previously unreported warm and diffuse gas and an extremely high cosmic ray ionization rate; this is radically changing the conception of the CMZ and has implications for the presence of the three super-massive clusters unique to the region.

Looking back over his career, he finds that his discoveries, first about the spectrum of H3+ in the laboratory and then the molecule in space, have fallen into a pattern: “When I was attempting it, people said it was impossible. When I found it, people were skeptical. When it was all established and the dust was settled, people said it was obvious.” (His ongoing work on the Galactic center is still in the second phase, he adds.) Rather than dampening his spirits, the skepticism spurred him on. “Such negative reaction is encouraging since the more negative people are, the more revolutionary the finding.”

Oka estimates he needs another ten years of concentrated research to fully understand the abundance of H3+ in the CMZ; if so, he will have spent 46 years trying to learn the secret of H3+. He recently received a new NSF grant for that purpose. Even after devoting his life to studying one molecule, he refuses to be defined by his line of research. “I loved all the four countries [I lived in], I love all the sciences,” he says, “I belong to all.”