Bulletin American Astronomical Society, 2000
CORNELIS ZWAAN, 1928{1999
Cornelis (\Kees") Zwaan was a well-known solar
physicist at Utrecht University in The Netherlands. To-
gether with his graduate students he also ventured out to
other cool stars, concentrating on their magnetic activity.
He has been a member of the AAS and the Solar Physics
Division and had close ties with many American astro-
physicists. He died from cancer in his house at Doorn,
The Netherlands on June 16, 1999. He had had major
surgery in 1998, but recovered so fast that we and his
other friends expected to have him with us much longer.
The end came far too soon.
Kees' research focused initially on sunspots and
NLTE spectroscopy, but quickly expanded to the MHD
theory of ux tubes, and to stars in 1977 when he initi-
ated studies of solar and stellar magnetic activity utiliz-
ing solar{stellar complementarity. His own careful work
and the large amount of research which he guided and
inspired have greatly advanced our understanding of the
sun and other cool stars. He was directly involved in
the development of the ux tube paradigm, stimulated
boundary-layer dynamo theory, pioneered in diagnosing
sunspot patterns, and led a sequence of coworkers in stud-
ies of stellar activity. His PhD students that continued
careers in astronomy include ourselves, Henk Spruit, Al-
bert Greve, Aad van Ballegooijen, Rene Rutten, Karen
Harvey, Louis Strous, and Ken Tapping; in addition, he
was PhD co-advisor to Jan van Paradijs, Han Uitenbroek,
Jo Bruls, Nick Hoekzema and others. The production of
this \school", the largest astronomical one to come out of
Utrecht, was what he was most proud of. He also inspired
many colleagues, both in The Netherlands and abroad,
in a wide range of topics including Stokes prole mod-
eling, the theory of molecular Zeeman splitting, stellar
polarimetry, binary merging through magnetic braking,
cluster evolution, and many other endeavours. He once
remarked that he should score higher in acknowledge-
ments than in citations | being much surprised when we
showed him earlier this year that his citation record ranks
in the top solar physics bracket. That fact re ects the
wide range of his research interests and accomplishments.
He was also instrumental in founding the Solar Physics
Section of the European Physical Society (subsequently
also of the European Astronomical Society) and chaired
the Utrecht astronomy department and the Dutch astro-
nomical society (\Nederlandse Astronomenclub").
Kees lived most of his life in the quiet residential vil-
lage of Doorn where he was born as elder son into a civil-
servant family. A schoolmaster-uncle advocated the no-
tion of an academic career; Kees displayed his didactic
inclination early by being a rather demanding teacher of
bird and plant life to his younger brother Joost. He stud-
ied physics and astronomy at Utrecht University during
1946{1954 and then became a high school physics teacher
during ve years, interrupted by two years of mandatory
military service (which he fullled as Russian language
interpreter). He often remarked that high-school teach-
ing makes for good habits, for example acquiring legi-
ble handwriting. Afterwards, as university professor, he
spent much eort on introducing astrophysics into the
Dutch high schoolcurriculum.
Kees started a PhD thesis on sunspot spectroscopy
while being a teacher, but then obtained a grant from the
Dutch national science foundation (ZWO) and after that
a temporary position at Sterrewacht Sonnenborgh (now
Sterrekundig Instituut Utrecht) to complete it. He de-
fended his thesis \Sunspot Models |A Study of Sunspot
Spectra" in 1965, with M.G.J. Minnaert as supervisor. It
made him an authority on sunspots and the treatment of
scattered light.
After the thesis, the Zwaans (Kees married Prisca van
Diggelen in 1954) spent 1966{1967 at the Sacramento
Peak Observatory, at the time when Jack Evans turned
that Airforce facility into the premier solar physics insti-
tution in the world. It gave Kees opportunity to study
active region morphology on the large lm collection at
the Big Dome (now the Evans Facility), making him an
expert on the structural and evolutionary patterns of so-
lar magnetism. The stay was a rich experience that made
Kees and Prisca lovers of South-West scenery and Indian
culture, and turned Kees into a life-long bolo-tie wearer
much to the dismay of his Dutch relatives unfamiliar with
the ornament. Also, the American frankness suited Kees
and Prisca well | many of their American colleagues be-
came friends and have enjoyed their hospitality at Doorn.
Kees became university lecturer (associate professor)
upon his return to Utrecht in 1968, full professor in 1980.
The early seventies saw large involvement in JOSO, an
assembly of European solar physicists setting out to nd
the best site for optical solar observing in Europe and
eventually selecting sites on Tenerife and La Palma. At
Utrecht, Kees inspired engineer Rob Hammerschlag to
start on his open tower and telescope, a long develop-
ment that nally bears fruit now as the \Dutch Open
Telescope" on La Palma. Its concept relies on wind ush-
ing rather then vacuum, turns out highly successful, and
now inspires designs for future solar telescopes. Kees and
Prisca were present at the First Light Ceremony; Kees'
last activity was to set up and bequest a fund to support
its operation.
During his last winter, Kees completed a major
project: co-authoring \Solar and Stellar Magnetic Ac-
tivity" which appears this winter. Most parts written by
Kees convey his uncanny ability to recognize basic phys-
ical principles in varied, incomplete and highly confusing
data. His awe for the inspiring questions posed by stellar
magnetism lives on in these pages.
Rob Rutten
Sterrekundig Instituut Utrecht
Karel Schrijver
Lockheed-Martin Solar and Astrophysics Lab., Palo Alto
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