It didn't take long for Jean-Pierre Lasota-Hirszowicz, a principal investigator at the Institut d'Astrophysique de Paris  , to recognise that Guillaume Dubus was destined for an outstanding career in astrophysics. "When he was doing his Ph.D., I asked him to introduce effects of irradiation in the description of the structure of accretion discs"--a common structure in astrophysics that forms when matter falls into a gravitational source--Lasota-Hirszowicz writes in an e-mail. "I told him where to find the standard formula describing the effects of irradiation. After a couple days, he came back and said, 'The formula is wrong.' I answered, 'You must be joking. Everybody has been using this formula for years. Check your calculations.' He looked at me, said nothing, and went back to his office. Next day, he was back. 'I insist. The formula is wrong.' "
"He was right. And many papers written by distinguished authors were wrong. Guillaume is different from most of his peers: They would just copy the formula without thinking." The work, which Dubus published soon after his Ph.D., is fast becoming a seminal work on how irradiated accretion works.
Dubus is driven by a desire to "really ... understand how things work," says Lasota-Hirszowicz. He doesn't give in to "easy, superficial solutions," and "in addition to being a very good physicist, he is also both a skilful observer and an excellent theorist." Dubus, who recently added particle physics to his arsenal, says it’s not always easy to do such interdisciplinary science. Still, he "wouldn't like to change anything," he says.
Feet firmly grounded ...
Dubus, who grew up in France, made up his mind to become an astrophysicist when he was 9. Dubus was "always ... interested in astrophysics as a kid," he recalls. "[The TV series] Cosmos  had a huge influence on me. I decided to do research after watching this."
Dubus finished high school in 1989 and then spent 2 years preparing for the national competition for entry to the French engineering Grandes Ecoles. Ranking near the top, Dubus was invited to study at the prestigious Ecole Polytechnique  near Paris. Polytechnique "is a little bit different from the other" Grandes Ecoles, says Dubus. One difference is that Polytechnique makes its students spend their first year in a military or civil organisation. "For 1 year, I was an officer in the navy," says Dubus. Another important difference is that over the two following years, Polytechnique "covers all the sciences, including biology. You get a very good exposure to science in general."
And a head in the stars
After graduating from Polytechnique in 1994, Dubus did an M.Sc. in astrophysics and space science at the Observatoire de Paris  in Meudon, on a French Department of Defence scholarship , modelling the atmosphere of Jupiter. Dubus stayed on to do a Ph.D. in high-energy astrophysics--the study of astronomical sources of x- and gamma rays--in Lasota-Hirszowicz's lab, working on x-ray compact binaries--"systems in which a normal star loses matter to a white dwarf, neutron star, or black hole," Dubus explains. He secured a Ph.D. grant from an exchange programme at his university that allowed him to spend a year in the astrophysics department  of the University of Oxford with Phil Charles, now director of the South African Astronomical Observatory . "What I actually wanted was to do both observational and theoretical [work]," Dubus says.
"Guillaume's Ph.D. project ... was really rather difficult, as his co-supervisor in Paris ... had him doing theoretical and numerical modelling of the structure of accretion discs, whereas with me he was working with x-ray satellite observations of variable x-ray sources in a nearby galaxy," Charles writes in an e-mail. "So he had to learn all the fundamentals of both theoretical and observational astronomy. Guillaume was superb in both, and to see this in a graduate student was really quite remarkable." Like Lasota-Hirszowicz, Charles found that Dubus stood out from his peers. "With Guillaume, I found we interacted much more as collaborating scientists than as a supervisor-student relation. ... This told me he had a great career ahead of him."
Dubus graduated in 1998, defending his thesis "on the day France played its first game [at the World Cup soccer championship]. ... So I could watch the whole event in no hurry," he says. Dubus then joined "a very good group in the observation of binaries" at the Astronomical Institute of Amsterdam , The Netherlands, for a 2-year postdoc funded by a Marie Curie Fellowship . Dubus was hoping to learn from principal investigator Jan van Paradijs, who "had a nose for this subject," says Dubus. Unfortunately, Paradijs died from cancer after just 1 year.
Keen to get more experience and to go to the United States, Dubus did another postdoc at the California Institute of Technology  in Pasadena, which he calls "a Mecca for astronomy." This time his approach to binaries was "highly theoretical." During his two postdocs, Dubus gained "good exposure" to many great minds in astronomy, which, he says, "gives you a lot confidence." Dubus also managed to publish "a few papers," but, he says, "my main achievement was to learn from people who set the standards [in] the field."
A foot in four worlds
"It's not very common to do both" observational and theoretical work, says Dubus. "It is hard to get recognitions because observers think you are a theorist, and vice versa." Still, he says, there are great advantages. "I am able to judge if observations make sense or are important in my theoretical work. Because I did theoretical work, I am able to identify things we could observe, then go out to observe them."
After his postdocs, Dubus further expanded his expertise by changing subfields. Although he still looks at the transfer of matter between stars and compact objects--black holes, neutron stars, and white dwarves--and remains in high-energy astrophysics, he switched x-rays for very high-energy gamma rays--thus entering astroparticle physics.
"Particle astrophysics," says Lasota-Hirszowicz, "does not describe scientific reality but a sociological one: fields of astrophysics on which many particle physicists are working." Particle physicists have gained interest in astrophysics in recent years as the capacity of cosmic-ray detectors has increased. Today's detectors allow scientists to observe cosmic particles with energies surpassing those obtained in particle accelerators, making of the universe a new laboratory for particle physicists, who have contributed their expertise to the study of cosmic gamma rays. "The techniques that are used in this field are techniques from particle physics," says Dubus.
Behind Dubus's move was the possibility of a tenured position from the French Centre National de la Recherche Scientifique (CNRS) , which was looking for a high-energy astrophysicist to join a particle physics lab at the Laboratoire Leprince-Ringuet  near Paris. He got the job and has been working ever since on a NASA-led project that will see the launch of a Gamma-ray Large Area Space Telescope (GLAST)  next Fall and on European High Energy Stereoscopic System (HESS)  experiments. "Most of the people involved in these experiments are particle physicists," says Dubus. "They wanted an astronomer to provide astronomy expertise to [their] observations."
"I knew nothing about ... the way they do experiments and had to learn about the physics behind the gamma-rays emissions," says Dubus. A major challenge was learning to work in teams of 100 or more when astrophysics teams traditionally include no more than four. "For me, it was a very different experience to see how it functions and how ... you can make your voice heard," he says. "Particle astrophysics brings the approach of the large consortia that build experiments for huge accelerators"--such as CERN--"to bear on an astrophysical environment. Telescopes usually operate by allocating observing time in small chunks ... to individuals in a large community of observational astronomers," says Charles. "Consequently, the ability to work with many collaborators spread over many countries, resolving many different views on how things should be done, was crucial. Guillaume clearly made the transition and did it very well."
Many new space- and ground-based astronomy research facilities have seen the light in the last decade. "Associated with this infrastructure development has come growth in job opportunities, especially for new Ph.D. graduates," says Charles. To be successful, young scientists interested in entering the field need a broad background in physics, solid grounding in maths, computing skills, and--for observational work--a good knowledge of signal processing. Then they should "look for the new observational facilities that open up new areas of astronomy, or which have dramatically enhanced capabilities," Charles advises.
"The main challenge, however, is that most of these posts are soft money, as the competition for permanent astronomy positions is intense," says Charles. As Lasota-Hirszowicz sees it, funding opportunities are expanding, but "they pay mainly for sophisticated instruments and hordes of data-processing personnel." Still, better career prospects may be emerging as the number of students entering the sciences has been dropping. "We are putting more and more efforts [in]to getting students," says Dubus, who is currently looking for a Ph.D. student.
Dubus has now returned to an astrophysics laboratory , joining the Observatoire de Grenoble  last September. He moved once more from observational to theoretical work, but his work on astroparticle physics continues. "I didn’t really know when I got that position [at the CNRS], but it turned out to be a great move. ... I have been lucky to get into that field at the right time." Astroparticle physics "opened a new window on the universe," and this translates into "lots and lots of [research] opportunities. ... I am pretty sure I will see something that intrigues me or interests me" every day, says Dubus.
Elisabeth Pain is contributing editor for South and West Europe.
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