A. Jacques Sauval

Belgium

1937-2023

Obituary:

Jacques Sauval obtained his PhD at the Université de Liège. He started his career at the Royal Observatory of Belgium in 1963, and there he became head of the Section "Structures of stellar atmospheres".

His whole career was devoted to the Sun, a subject he pursued with passion and dedication, well into his retirement. For the analysis of the Sun's spectral lines, and for the derivation of the abundances, he needed a lot of atomic and molecular data. These he collected with much effort, and also contributed to the partition functions and equilibrium constants of many diatomic molecules. During 45 years he had a long and fruitful collaboration in this work with his friend and colleague Nicolas Grevesse of the Université de Liège. This collaboration resulted in key papers on the solar chemical composition.

For the observed spectrum of the Sun, he could rely on a number of high-resolution and high signal-to-noise atlases, one of which was based on observations from Jungfraujoch in Switzerland. Jacques was also a frequent observer at this “Hochalpine Forschungsstation Jungfraujoch”, where he collaborated with the Liège team in taking spectra of the Sun. This collaboration was of mutual benefit: the Liège team used the Sun to study the absorption by molecules in the Earth atmosphere, and Jacques got his solar spectra.

A similar win-win collaboration that Jacques was involved in, is the ATMOS experiment, where ATMOS stands for Atmospheric Trace Molecular Spectroscopy. This experiment was flown a number of times onboard the space shuttle, most notably during the Atlantis flight where Dirk Frimout served as payload specialist. By looking to the sunrise and sunset from space, infrared spectra of both the Earth atmosphere and of the Sun are obtained at the same time. Jacques eagerly took the opportunity to analyse the solar spectra, which cover a wavelength domain that contains a large number of molecular lines.

An important advance in Jacques’ research occurred when three-dimensional hydrodynamical models of the solar photosphere became available. This led to a collaboration with the Asplund group and to a revision of the solar abundances. Even to this day, these revised abundances present challenges to modellers of the solar interior.

Jacques also played an important part in outreach. For 12 years he was editor-in-chief of the "Ciel et Terre" journal. His own contributions frequently addressed two other subjects he was passionate about, namely comets and meteor showers. For many years he also contributed to these subjects for the Yearbook of the Royal Observatory of Belgium.

During a number of years, Jacques was the secretary for the Belgian National Committee on Astronomy. In that role, he was instrumental in reviving the astronomical "Contact Days". These 1-day meetings allow young astronomers to present their work to a wider audience of their peers. For the organization of these contact days, he took up the role of secretary of the "Groupe de contact du FNRS Astronomie et astrophysique".

Finally, we should mention Jacques' rescue of a nearly-forgotten Belgian astronomer from obscurity: Charles Fievez. Fievez had seen that spectral lines broaden in a magnetic field, but failed to deduce any conclusions from that, contrary to Zeeman, who did so 12 years later. In his efforts, Jacques did not limit himself to writing a booklet about Fievez, but he also prepared an exhibition about Fievez, organized an international meeting of spectroscopists, had a minor planet named for Fievez, and gave a public talk about Fievez’ life and works. It is this dedication to the subjects of his interest that very much shows the character of Jacques Sauval.

(Written by former colleagues at the Royal Observatory of Belgium)