John I. Castor

John Irvin Castor, whose work helped to shape both astrophysics and high-energy-density science, died suddenly on January 19, 2021.

Castor was born in Fresno, California on January 5, 1943. As a sophomore in high school, he enrolled at Fresno State University, graduating at the age of 18 with a double major in math and physics. He earned his PhD in astronomy at CalTech in 1967. Supervised by J. Beverley Oke, Castor’s thesis presented theoretical modeling of RR Lyrae pulsation, which followed the work of Robert Christy. From 1967 to 1969, Castor worked with John Cox as a postdoc in the Joint Institute for Laboratory Astrophysics (JILA) at the University of Colorado, where he extended his work on RR Lyrae modeling.

In 1969, he joined the University of Colorado faculty in the department of physics and astrophysics and was subsequently named a Fellow of JILA and professor of astrogeophysics. Stimulated by others at JILA including David Hummer, David Van Blerkom, Peter Conti and Katy Garmany, he began modeling spectra of Wolf-Rayet stars. Castor’s theory of radiatively driven stellar winds was an extremely important contribution to astrophysics. In the late 1960s and early 1970s, astronomical observations showed that hot (O and WR type) stars were losing anomalously large amounts of mass in stellar winds. Castor explained this phenomenon in several papers. The main results demonstrated that including two additional terms in the radiative force led to an increase of one hundred times that of the existing theories, which provided a stronger driving pressure force and agreed with observations. That work, known as the CAK theory, was presented in the Astrophysical Journal in 1975, with student David Abbott and postdoc Richard Klein.

Subsequent work explained the observation of hydrogen and helium and demonstrated the presence of radiative-hydrodynamic instabilities that explained time-dependent variations and the production of x rays in the winds. Together with Richard McCray and Robert Weaver, Castor also developed a theory for the production of interstellar bubbles. Castor’s work has been highly utilized in understanding observations in stellar, interstellar, and galactic astrophysics over the past 45 years.

In 1981, Castor joined Carol Alonso’s group at Lawrence Livermore National Laboratory (LLNL). He contributed to many important aspects of weapon physics, including making key contributions to experiments at the Nevada Test Site. Klein and Castor also developed a general theory of non-local thermodynamic equilibrium (non-LTE) spectral analysis as a key component of the x-ray laser program in the 1980s. Castor developed the ALTAIR code to solve the time-dependent non-LTE problem for a multilevel atom in two-dimensional axisymmetric geometry on a Lagrangian mesh of arbitrary complexity.

In the 1990s, Castor worked with Bruce Remington, Gail Glendinning, and others to develop the nascent field of laboratory astrophysics. They conducted experiments to study supernova-relevant Raleigh-Taylor instabilities at the Nova laser, demonstrating the utility of laser-driven experiments in exploring astrophysical phenomena and establishing the basis for the field of high-energy-density science.

In the 2000s, Castor focused on improving radiation hydrodynamic modeling and simulation of those laboratory experiments. His book, Radiation Hydrodynamics provides an accessible introduction to the theory and large-scale simulation methods used in the study of the dynamics of matter interacting with high-intensity radiation. As a part of this work, Castor worked with Carlos Iglesias, Forrest Rogers, Brian Wilson, Mau Chen, and others to develop a client–server database of tabular opacity models, a revolutionary way of providing physical input for modeling laboratory-scale experiments. Castor was also part of the Cimarron Collaboration to study plasma microphysics, led by Frank Graziani.

In 2012, Castor was named a Distinguished Member of the Technical Staff. That honor, the highest at the lab, is bestowed for extraordinary, internationally recognized scientific and technical contributions.¹ Castor worked at LLNL until his retirement in 2017. A symposium in his honor featured talks about the context and impact of Castor’s work by several collaborators and folks he mentored over the course of his career.²

Castor had a love of music and the outdoors and enjoyed trips with his family to the mountains of Colorado, California, and Europe. He is survived by his wife of 43 years, Melissa Reading, daughter Emily Warren and son-in-law Frankie Warren, sister Judy Butts and brother-in-law Alan Butts, and many loving nieces, nephews, cousins, and close family friends. He was preceded in death by brother Paul Castor, in 1967, and son Ethan Castor, in 2017.

Notes

1. J. I. Castor authored over 100 research papers, which have garnered over 8200 citations, and counting.
2. The Castor Symposium was archived online at https://youtube.com/playlist?list=PLvGO_dWo8VfdmV7pSam62WZc7DBWgHIzL