Sweigart was a pioneer in studies of low-mass stellar evolution, globular clusters, deep mixing in stellar interiors, and the origin of UV-bright stars.
Allen V. Sweigart passed away on Tuesday, February 1, 2022, only four days shy of his 78th birthday, after a long and hard-fought battle against non-Hodgkin lymphoma.
Allen was a distinguished theoretical astrophysicist who dedicated most of his career to studying the structure and evolution of low-mass stars, particularly those in globular clusters. He was born in Ephrata, Pennsylvania, and attended Franklin & Marshall College in nearby Lancaster, graduating (magna cum laude) with an A.B. in mathematics. He then pursued a Ph.D. degree at Princeton University, graduating in 1970 with a thesis titled “A Method for Suppression of the Thermal Instability in Helium Shell Burning Stars.” Martin Schwarzschild was his thesis advisor. Even prior to receiving his Ph.D. degree, Allen had already co-authored, with Peter Bodenheimer, an influential study of the dynamical collapse of an isothermal gas cloud — a topic completely unrelated to his thesis.
Upon graduation, Allen moved to Yale University, to take on his first postdoctoral appointment. At Yale, he collaborated with Pierre Demarque, Peter Gross, and John Mengel on a series of classical papers on the evolution of low-mass stars. He paid particularly close attention to the progression of the nuclear burning shells in advanced stages of stellar evolution, expertly implementing the so-called “shell-shifting technique” that had been idealized by M. Schwarzschild, in collaboration with R. Härm, a few years earlier. His painstaking numerical work was crucial in making those complex model computations more affordable, without compromising accuracy. Allen’s efforts would soon pay off, as they allowed him to present some of the first extensive grids of evolutionary tracks for red giant and horizontal-branch stars to appear in the literature. His grids covered wide ranges in metallicity, mass, and helium abundance, and became widely used by the astronomical community. Among Allen’s many other contributions from this period, one finds studies of meridional circulation and the origin of CNO abundance anomalies in red giants, the so-called “R-method” of helium abundance determination in globular star clusters, and even the solar neutrino problem. With Italian collaborator Alvio Renzini, he also studied the effects of semiconvection on the evolution of horizontal-branch stars, along with its impact on the properties of pulsating RR Lyrae stars. His publications from this period allowed him to establish a solid reputation as one of the leading names in low-mass stellar evolution, and to gain notoriety for the accuracy of his calculations, attention to detail, and meticulous yet succinct writing style.
In 1977, Allen took on an NRC Senior Research Associateship at NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, where he became a civil servant three years later. He would remain at Goddard until his retirement in 2012. While at Goddard, Allen would consolidate his career, becoming frequently invited to present review papers in international meetings. Over the years, in addition to publishing improved model grids, he made great and sustained efforts to refine and update the numerical methods (along with, though perhaps to a lesser extent, the input physics) adopted in his stellar evolution code, and also to automate it, so that long sequences of models covering the different evolutionary phases of low-mass stars could be covered in a single run.
In the late 1980s and 1990s, Allen developed a strong desire to establish new collaborations, and revisit some of his previous ones, to help ensure that his latest models were used to interpret state-of-the-art astronomical observations. He participated in numerous observing proposals with the Hubble Space Telescope and other ground- and space-based facilities. With Roger Bell, from the University of Maryland, he mentored Marc Houdashelt after the latter received his Ph.D., and at around the same time, he recruited postdoctoral fellows Ben Dorman and Márcio Catelan. With the latter, a fruitful and long-lasting collaboration ensued, in which they studied the evolutionary properties of horizontal-branch and RR Lyrae stars. Years later, with Catelan’s then-Ph.D. student Aldo Valcarce, an extensive update of the input physics of Allen’s stellar evolution code was carried out, leading to the publication, in 2012, of extensive grids of evolutionary tracks and isochrones for a wide range of metallicities and helium abundances, covering from the zero-age main sequence to the end of the horizontal-branch phase of low-mass stars.
In the late 1990s to early 2000s, Allen joined forces with Tom Brown, Sabine Möhler, and others, to study the origin and nature of extreme horizontal-branch stars and the so-called “blue-hook” stars in globular clusters, as well as the origin of the UV flux from both resolved and unresolved stellar systems. These collaborations would result in an impressive number of publications, including several that only appeared in print up to several years after he had formally retired.
While not an extrovert, Allen had a captivating smile, a very pleasant personality, a big heart, and a great sense of humor. He was serious but friendly when talking about his or other people’s work. He went to great lengths to help his younger collaborators feel welcome and unleash their scientific careers. An amateur pianist, he enjoyed listening to, and playing, classical music, especially Bach, Beethoven, and Schubert. Woodworking and photography were also among his favorite hobbies. In later years, as his lymphoma progressed, Allen developed a habit of reading the professional medical literature, becoming so knowledgeable that he was able to seamlessly discuss the latest developments in the field with his doctors.
Allen will be dearly missed by all who had the privilege of knowing him. He is survived by his wife, Tamsen Sweigart; their daughter, Christine Van Norstrand, and her husband Erik; son, David Sweigart; along with two grandchildren, Aiden and Luke Van Norstrand.
See also Sweigart’s AstroGen information.