Miller Fellow Focus

Miller Institute News, vol. 2, issue 1 - winter 2000

Second-year Miller Fellow Dr. Michiel Hogerheijde uses molecules and dust particles in interstellar space to trace the formation of stars and planets. He is hosted by Professor Leo Blitz, head of the Radio Astronomy Laboratory at UC Berkeley.

Radio emission from molecules and dust grains is an ideal probe of the conditions in interstellar clouds. In this cold and dilute environment, the most common molecule, molecular hydrogen, does not emit any observable radiation. But up to a few hundred parts per million in these clouds consist of carbon monoxide, hydrogen cyanide, formaldehyde, silicon monoxide, carbon monosulfide, protonated nitrogen, the formyl ion, and other exotic molecules. These species do have emission lines observable at wavelengths of a few millimeter and shorter, associated with transitions between rotational energy levels.

The intensity and shape of the emission lines provide valuable information on physical conditions in interstellar clouds, such as their density, temperature, and velocity field, and on their chemical composition. At these same (sub) millimeter wavelengths, small dust particles emit thermal radiation, which gives a complementary view of the distribution of the material.

Interstellar clouds are the nurseries of stars and planets. As dense condensations inside these clouds become unstable against their own gravity, they collapse and form a star or a small group of stars. Around each star, the collapsing cloud forms an accretion disk through which material is funneled onto the star. It is thought that planets condense from the remains of such disks.

Dr. Hogerheijde's research focuses on comparatively small condensations in star-forming clouds located in the constellations Taurus, Serpens, and Ophiucus. These regions form relatively isolated stars with masses up to that of our own Sun. They are also quite close, at a few hundred light years away, so that they can be studied in large detail. The ultimate aim of Hogerheijde's research is to help understand the link between interstellar clouds and planetary systems. How does the physics of cloud-core collapse determine the formation, properties, and evolution of a circumstellar disk? How does the chemical composition of the cloud material evolve throughout this process? What was the chemical composition of the early solar system during the formation of the planets?

To find answers to these questions, Dr. Hogerheijde has been collecting data at several millimeter-wave telescopes. The molecules the Earth's atmosphere, especially water, obscure the view, so these telescopes are only found in high and dry places. The Radio Astronomy Laboratory, together with the University of Illinois and the Universoty of Maryland, operates an array of ten antennas, each with a diameter of six meters, in Hat Creek near Mount Lassen in Northern California. The California Institute of Technology operates a similar array consisting of six ten-meter antennas in Owens Valley, on the eastern side of the Sierra Nevada. By using these arrays as interferometers, a resolving power can be generated from a telescope as big as the largest separation between two antennas in the array. For the Hat Creek array, that can be as large as two kilometers. These two arrays operate at wavelengths between one and three millimeters. Observing at even shorter wavelengths, where absorption due to atmospheric water is more severe, requires a higher site, such as the summit of Mauna Kea on the Big Island of Hawaii. There the United Kingdom, Canada, and The Netherlands jointly operate the 15 meter James Clerk Maxwell Telescope.

The ultimate aim of Hogerheijde's research is to help understand the link between interstellar clouds and planetary systems

Over the past year and a half in Berkeley, Dr. Hogerheijde has collected data at all of these observatories during several observing runs, each lasting a few days to a week. After a successful run, the data are transferred to his computer in Berkeley. Then, the obtained spectra and images are calibrated and analyzed with specialized software. An essential part of the subsequent interpretation is formed by a computer code developed by Dr. Hogerheijde which solves the transfer of radiation through a model cloud together with the excitation of the molecules in that cloud. In general, this is a complex problem, because the radiation transfer depends on the excitation of the molecules through their emission and absorption of photons, while the excitation depends on the amount of radiation each molecule receives. By using his code, Hogerheijde is able to produce `synthetic' observations of model clouds which can be directly compared to the actual data.

Currently, Dr. Hogerheijde is refining a popular theoretical model that describes the collapse of cloud cores, and captures the essence of their density and velocity structure by taking a closer look at the inner regions of these cores, which appear flattened, and from which the accretion disk around the young star is presumed to originate. Extending this to later stages of protostellar evolution, where only a disk surrounds the young star but the cloud core has dispersed, he plans to investigate the chemical composition of these disks through observations and model calculations based on recent theoretical predictions of the disk's physical structure. Together with researchers from Caltech, Hogerheijde has analyzed the chemical composition of the comet Hale-Bopp. Comets are thought to preserve the chemical composition of the early solar system, and they found that at least 15 to 40 percent of the comet consists of pristine interstellar ices.

Dr. Hogerheijde's research is interdisciplinary, touching on the fields of astronomy, chemistry, and planetary science. Currently, there is much interest in this area, with the recent discovery of planetary systems around other stars and the ongoing exploration of Mars and Jupiter's moons for possible life-sustaining environments. Several new, powerful (sub) millimeter and far-infrared telescopes will become operational in the next decade. Dr. Hogerheijde hopes to use these telescopes to continue his work on the interstellar link of the origin of stars, planets, and life.