An Essay on the Prize in Astronomy 2009
In a career that spans forty-five years, Professor Frank H Shu has spurred fundamental changes in the existing paradigms of three different fields of astrophysics: galactic dynamics, stellar evolution, and the astrophysics of small bodies in the solar system.
Shu’s research career began auspiciously in 1964 with the publication, together with his adviser C C Lin, on the density-wave theory of spiral structure in disk galaxies. Although highly controversial at its inception, this theory has since gathered widespread observational support and computational elaboration. The philosophy that useful theories must be testable has informed all of his subsequent research. Shu has been at the forefront of theoretical developments, particularly with his group’s semi-analytical investigations of the nonlinear, non-ideal theory as applied to galactic shocks in the 1970s and resonantly forced analogues in Saturn’s rings in the 1980s.
Shu has also applied his talents to the problem of interacting binary stars. His study using matched asymptotic analysis in 1975 with his student Lubow on the gas dynamics of semi-detached binaries is considered the definitive theoretical work on this subject. Their work is widely cited by observers and constitutes a benchmark for numerical accuracy in computer simulations.
In 1977, Shu published his ideas on the inside-out gravitational collapse of the singular isothermal sphere as a model for star formation from molecular cloud cores. This paper allowed the definitive study in 1980, with his student Stahler and collaborator Taam, on the physical properties of accreting protostars, which settled a controversy then raging amongst the supporters of Richard Larson and Chushiro Hayashi on this subject.
In 1984, together with his student Terebey and his co-worker Cassen, he published a seminal paper showing how circumstellar disks form around protostars as a natural consequence of the collapse of rotating molecular cloud cores. With his student Adams and colleague Lada, he showed in 1987 how radiative reprocessing of starlight through the dusty envelope and in the surrounding disk provides a spectral classification of young stellar objects, a scheme that, with one modification, provides the basis for modern empirical identifications of the so-called class 0, I, II, and III objects in low-mass star formation.
In 1987, Shu and his student Lizano published an influential paper on bimodal star formation. They proposed that ambipolar diffusion in magnetized molecular cloud cores underlies the formation of isolated low-mass stars, whereas gravitational collapse through dynamical flows leads to supercritical states that yield cluster formation. Shu, Adams, and Lizano summarized the theoretical and observational state of affairs in a review article in 1987, one of the most referenced articles in the Annual Reviews of Astronomy and Astrophysics.
In 1988, Shu and his colleagues discovered observationally that bipolar outflows are driven by neutral stellar winds. This discovery led his group to propose what came to be called the “X-wind” theory of stellar jets and bipolar outflows. Their model is widely regarded as the most likely magnetohydrodynamic explanation for this fascinating and important phenomenon. For the past decade, Shu and his co-workers have proposed several observational tests of the model, each of which has been successfully passed.
In 1996, with his student Shang and his collaborator Lee, Shu applied the X-wind model to explain a long-standing puzzle concerning primitive meteorites: namely, chondrules and calcium-aluminium-rich inclusions (CAIs) thermally processed at temperatures above 2000 K are intimately mixed with a hydrated carbonaceous matrix that has never been above 600 K. The same theory also allows a particle-irradiation origin near the protosun for the generation of short-lived radioactivities that are inferred for the CAIs and chondrules of such meteorites. These suggestions also generated controversy when first proposed, but the prediction that CAIs and chondrules would be found in comets, which were previously thought to be formed from pristine material far removed from the Sun, was spectacularly confirmed in a 2006 dust sample returned from Comet Wild.
In 2007, Shu and co-workers published an analytic theory of the mechanisms of turbulent viscosity and resistivity in the magnetized accretion disks that ubiquitously surround young stellar objects. Their prediction that such disks become sub-Keplerian in their rotation when they age awaits confrontation with observations.
Astronomy Selection Committee
The Shaw Prize
7 October 2009, Hong Kong