Astrophysicist Dimitri Manuel Mihalas died in his sleep at home in Santa Fe, New Mexico, on 21 November 2013. At his request, his body was donated to the University of New Mexico Medical School and his library to the New Mexico Institute of Mining and Technology.
Born on 20 March 1939 in Los Angeles, Dimitri received a BA with a triple major in physics, mathematics, and astronomy from UCLA at age 20. In 1963 he earned his PhD from Caltech. His thesis, under John Beverley Oke, was on theoretical models of hydrogen and helium line strengths and profiles in extremely hot, bright type O stars. He interpreted the observational data with the best theoretical analysis possible at the time and computed the models using the simplifying approximation of local thermodynamic equilibrium (LTE). Although primitive by today’s standards, that well-cited work had a major impact on the field.
After joining the faculty at Princeton University in 1964, Dimitri undertook new analyses of stellar abundances and devised a method for constructing a constant-flux atmosphere with convection in the full-transport regime. His computation of the first line-blanketed spectrum of a hot B star showed that continuum-only models overestimated the effective temperature by 10% and hence luminosity by 40%.
Realizing that the widely accepted approximation of LTE in stellar atmospheres was inadequate, Dimitri set out to solve the analogous non-LTE problem. Only the highly idealized case of a two-level atom with a single line and two continua had been solved previously. He and Lawrence Auer, working first at the University of Colorado in 1967–68 and later when Dimitri was at the University of Chicago, reformulated the non-LTE stellar atmosphere problem by applying the Newton–Raphson root-finding technique to the full nonlinear system and iterating to convergence. It was a breakthrough in the field and revolutionized all further work on computing stellar atmospheres.
With the new methodology, Dimitri constructed extensive non-LTE models for hot stars and used them to evaluate the effects of departures from LTE on observable stellar-line and continuum indices. Dimitri and Auer then performed several critical studies not previously possible: They achieved good agreement between the computed and observed strengths of the hydrogen and helium lines, and they found surface gravities consistent with the stars’ observed masses and element abundances obtained from nebular analyses. That widely cited work has served as a standard for verifying present-day calculations that use much faster iterative methods.
Between 1987 and 1994, in support of the international Opacity Project, Dimitri worked with Lawrence Anderson, David Hummer, and Barbara Mihalas to develop a modern equation-of-state code for stellar envelopes. The project’s results were found to be in excellent agreement with the independent computations using the OPAL model at Lawrence Livermore National Laboratory. The new data helped resolve several previous discrepancies between calculation and observation, especially with the interpretation of Cepheid variable pulsations.
With one of us (Stone), Dimitri developed more accurate numerical methods for time-dependent radiation transport. In collaboration with Karl-Heinz Winkler and Michael Norman, Dimitri developed novel implicit adaptive grid methods to solve one-dimensional radiation hydrodynamics problems that have multiple time and space scales ranging over many orders of magnitude. He derived the “adaptive-grid transport theorem” rigorously. In 1994–97, he developed TITAN, a 1D implicit adaptive-grid code. In collaboration with another of us (Cheng) at Los Alamos National Laboratory, Dimitri used TITAN not only to compute the best-ever numerical solution of the infamous Noh stagnating shock problem in planar, cylindrical, and spherical geometry, but also to extend the numerical solution to multiple reflections; give analytical solutions for the planar, cylindrical, and geometrical cases; and show that the numerical solutions agree with the analytical solutions for large numbers of shock reflections.
Dimitri received many appointments, including at the National Center for Atmospheric Research from 1971 to 1985 and the University of Illinois at Urbana-Champaign from 1985 to 1999. He joined Los Alamos National Laboratory as a consultant in 1981, became a full-time staff member in 1998, and retired in 2012. His numerous honors include the 1974 Helen B. Warner Prize from the American Astronomical Society and a 1984 Alexander von Humboldt Foundation US Senior Scientist Award.
Among 11 books that he wrote, cowrote, or edited, 3 have been used worldwide as undergraduate and graduate textbooks. Foundations of Radiation Hydrodynamics (Oxford University Press, 1984) is considered the bible of the radiation hydrodynamics community, and Stellar Atmospheres (W. H. Freeman, 1970) remains the standard in the field even after almost 45 years. In addition to his many scientific contributions, Dimitri also published several collections of poetry and other writings, including Depression, Bipolar Disorder, and Spiritual Growth (2002). Those books have had as profound an impact as his professional textbooks and articles.
Dimitri gave generously of himself as an adviser, role model, confidant, and friend. To him, everyone was an individual, with his or her own strengths and weaknesses; he would encourage and enthusiastically praise each person. He is cherished by the many students and colleagues whose lives and careers he touched.
