Robert Michael Hjellming

Robert Michael Hjellming, an astronomer at the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, and an adjunct professor of physics at New Mexico Tech (New Mexico Institute of Mining and Technology) for more than 30 years, died of natural causes on 29 July 2000 while he and his wife Carol were making scuba certification dives in Santa Rosa, New Mexico.

The concept of scuba diving in New Mexico seems surreal to me. But Bob had a proven history of making his high-risk "adventures'' pay off handsomely throughout his professional career. As one of Bob's students, I saw a mentor who moved on, always with intensity, intelligence, agility, and unflappable civility, leaving us with the illusion that his students could do so too.

Born in Gary, Indiana, on 12 December 1938, Bob earned his degrees from the University of Chicago: BS in physics in 1960, and MS and PhD in astronomy and physics in 1961 and 1965, respectively. His doctoral thesis was on physical processes in regions of ionized hydrogen. He began his post-PhD career at Case Western Reserve University in 1965.

In 1968, Bob joined NRAO, working in Charlottesville, Virginia. From 1969 to 1971, his interests shifted from numerical models of radiation transfer in ionized nebulae to analytical models of clouds in the cold interstellar medium. He also was interested in analyzing statistical populations of upper energy levels of hydrogenic atoms to successfully explain the stimulated amplification of radio recombination lines propagating through the interstellar medium.

That was just the start of a careening career. In 1970, Bob teamed up with Campbell Wade, an NRAO colleague, to try to detect radio emission from stars, an idea so conjectural at the time that they started the program during engineering time at the Green Bank Interferometer (GBI) in West Virginia. During the two years that followed, there was a veritable blizzard of new discoveries of radio stars, novae, and x-ray sources.

In 1973, Bob and his family moved to Socorro, where he headed the team that developed the initial data calibration and analysis software for the Very Large Array (VLA)--at that time an interferometer without any antennas--an irony well suited to a theoretician! His successful conversion from theorist to astute but wary observer seemed effortless, and he wound up writing the comprehensive VLA user manual, commonly referred to as the "VLA Greenbook.''

By the late 1970s, Bob jumped off the management treadmill to reinvigorate his scientific career. He moved with agility between theory and observations, concentrating first on one type of star, then another. He published 71 papers on about 20 objects in the 1980s alone, including many reviews. Simultaneously, he mentored students, having supervised a dozen more PhD students and postdocs by the time of his death.

Bob had many long-term associates and collaborators who worked in teams to observe the evolution in shape and spectrum of various types of transient galactic radio sources. The most famous of these was the campaign to monitor SS433 shortly after its discovery. This campaign led to the discovery of precessing relativistic jets that make rotating corkscrew patterns on the sky. In the 1990s, he actively pursued the x-ray transients discovered and monitored by new satellites such as Rossi X-ray Timing Explorer (RXTE) and Compton Gamma-Ray Observatory (CGRO), emphasizing the connections between the x rays from the accretion disk and the synchrotron jets seen at radio wavelengths. He also was one of the first scientific users of the new Astronomical Imaging Processing System (AIPS⁺⁺) imaging package. The last article Bob published, on the x-ray transient V4641 Sgr, appeared in the Astrophysical Journal in December 2000. That article, in which he combined GBI and VLA data with a new imaging algorithm and a substantial extension to the model he developed for SS433, showcased Bob's unique abilities as an observer, software pundit, and theoretician.

It is highly appropriate that a number of conferences held in the fall of 2000 were dedicated to Bob. His death was tragic, but he died as he likely would have wished--extending the limits of his reach and always looking ahead.