A Study of Pulsation in RR Lyrae Models

Non-linear calculations of intermediate amplitude to investigate stability have been carried out for one hundred RR Lyrae models of varying composition, mass, luminosity, and effective temperature. Of these, the calculations have been extended to the maximum amplitude for fifty models. The dependence of the location of the instability strip on composition, mass, and luminosity has been explored. In particular, the high-temperature boundary of the strip increases by 500° K for each 15 per cent increase in the mass fraction of helium. The dependence on effective temperature of the phase shift, the ratio of radius and luminosity amplitudes, and the shape of the light-curve has been explored, and the correlations of these three measures have been found. These measures may then be applied to observed variables to determine their location in the instability strip and the helium content of their envelopes. The examples studied give best agreement at about 30 per cent helium by mass. The calculations reveal many details of the shapes of the light- and velocity-curves which may be compared to observation. A number of features of these curves of known variables are discussed, and the underlying explanation is investigated. The sources of the driving energy of these models have been explored. In the models of the most favored composition, 30 per cent helium, the hydrogen ionization region is almost as important to the driving as is the He II ionization. The effects that cause the amplitude to cease growing and level off at the observed maximum amplitude are explored: they appear to involve several non-linear effects, particularly a steep velocity front, in the He II ionization zone. A new kind of period-luminosity relation has been discovered. The shortest period at which the fundamental mode persists, before it changes to the first harmonic, has been found to be independent of mass and composition and to depend only on the luminosity. This relation was found to be Ptr = 0 057 (L/L☉)0.6 days. This has been used to interpret the varying period distributions of RR Lyrae stars in clusters and in the field with the result that the luminosities are found to vary from Mbol = 0.57 for very metal-weak clusters to Mbol = 0.96 for clusters only slightly weak in metals. Metal-rich variables are slightly less luminous still. These results lead to the conclusion that the masses of RR Lyrae stars are near 0.5 M☉, which must imply a significant mass loss. These conclusions appear to be consistent with the possible interior models.</p

Oppenheimer

by Robert Serber, Victor F. Weisskopf, Abraham Pais, and Glenn T. Seaborg. Introduction by I. I. Rab

Men and Atoms

by William L. Laurenc

Interview with Robert F. Christy

Robert F. Christy was born in Vancouver in 1916, received his undergraduate education at the University of British Columbia, and took his Ph.D. degree with J. Robert Oppenheimer at Berkeley in 1941. He was an early participant on the Manhattan Project, working with Enrico Fermi at the Metallurgical Laboratory of the University of Chicago on the first atomic pile. In 1943 he went to Los Alamos as a member of the Theoretical Division under Hans Bethe, where he devised what came to be known as the Christy bomb, or the Christy gadget--the plutonium implosion device tested at Alamogordo on July 16, 1945. After the war he returned briefly to the University of Chicago, where he and his wife shared living quarters for a time with Edward Teller and his wife. Caltech was then seeking to build up its theoretical physics faculty, and Oppenheimer, who was teaching there part time, recommended that the institute hire Christy. In 1946 Christy accepted Caltech's offer of an associate professorship. He worked chiefly on the application of theory to cosmic-ray experiments in particle physics, later moving into nuclear physics and astrophysics, including important work in the 1960s on the pulsations of RR Lyrae stars, which are similar to but smaller than the Cepheid variables used as cosmic yardsticks. In 1967 this work earned Christy the Eddington Medal of the Royal Astronomical Society. In 1970, Christy became Caltech's provost, a post he held for the next ten years. After Caltech president Harold Brown left to join the Carter Administration as Secretary of Defense in 1977, Christy was also acting president of the institute, until the advent of Marvin L. (Murph) Goldberger a year later. In the mid-1980s he became a member of the National Research Council's Committee on Dosimetry, which investigated the radiation effects of the Hiroshima and Nagasaki bombs. In the interview Christy recalls his childhood in British Columbia; his undergraduate years at the University of British Columbia; his graduate work with J. Robert Oppenheimer at Berkeley; and his work on the Manhattan Project, first with Enrico Fermi at the Metallurgical Laboratory of the University of Chicago and then as a member of the Theoretical Division at Los Alamos. He recounts his wartime work on the critical assembly for the plutonium bomb ("the Christy bomb"); the Alamogordo test, July 16, 1945; the postwar concerns of ALAS (Association of Los Alamos Scientists); his brief return to the University of Chicago and move to Caltech; friendship with and later alienation from Edward Teller; work with Charles and Tommy Lauritsen and William A. Fowler in Caltech's Kellogg Radiation Laboratory; Freeman Dyson's Orion Project; work on the meson and RR Lyrae stars; fellowship at Cambridge University; 1950s Vista Project at Caltech; his opposition to the Strategic Defense Initiative; and his post-retirement work for the National Research Council's Committee on Dosimetry and on inertial-confinement fusion

Classical Cepheid Pulsation Models: IX. New Input Physics

We constructed several sequences of classical Cepheid envelope models at solar chemical composition ($Y=0.28, Z=0.02$) to investigate the dependence of the pulsation properties predicted by linear and nonlinear hydrodynamical models on input physics. To study the dependence on the equation of state (EOS) we performed several numerical experiments by using the simplified analytical EOS originally developed by Stellingwerf and the recent analytical EOS developed by Irwin. Current findings suggest that the pulsation amplitudes as well as the topology of the instability strip marginally depend on the adopted EOS. We also investigated the dependence of observables predicted by theoretical models on the mass-luminosity (ML) relation and on the spatial resolution across the Hydrogen and the Helium partial ionization regions. We found that nonlinear models are marginally affected by these physical and numerical assumptions. In particular, the difference between new and old models in the location as well as in the temperature width of the instability strip is on average smaller than 200 K. However, the spatial resolution somehow affects the pulsation properties. The new fine models predict a period at the center of the Hertzsprung Progression ($P_{HP}=9.65$$-$9.84 days) that reasonably agree with empirical data based on light curves ($P_{HP}=10.0\pm 0.5$ days; \citealt{mbm92}) and on radial velocity curves ($P_{HP}=9.95\pm 0.05$ days; \citealt{mall00}), and improve previous predictions by Bono, Castellani, and Marconi (2000, hereinafter BCM00).Comment: 35 pages, 7 figures. Accepted for publication in the Astrophysical Journa

Prospectus, October 5, 1990

https://spark.parkland.edu/prospectus_1990/1023/thumbnail.jp

Linked MRI signatures of the brain\u27s acute and persistent response to concussion in female varsity rugby players

Acute brain changes are expected after concussion, yet there is growing evidence of persistent abnormalities well beyond clinical recovery and clearance to return to play. Multiparametric MRI is a powerful approach to non-invasively study structure-function relationships in the brain, however it remains challenging to interpret the complex and heterogeneous cascade of brain changes that manifest after concussion. Emerging conjunctive, data-driven analysis approaches like linked independent component analysis can integrate structural and functional imaging data to produce linked components that describe the shared inter-subject variance across images. These linked components not only offer the potential of a more comprehensive understanding of the underlying neurobiology of concussion, but can also provide reliable information at the level of an individual athlete. In this study, we analyzed resting-state functional MRI (rs-fMRI) and diffusion tensor imaging (DTI) within a cohort of female varsity rugby players (n = 52) through the in-and off-season, including concussed athletes (n = 21) who were studied longitudinally at three days, three months and six months after a diagnosed concussion. Linked components representing co-varying white matter microstructure and functional network connectivity characterized (a) the brain\u27s acute response to concussion and (b) persistent alterations beyond clinical recovery. Furthermore, we demonstrate that these long-term brain changes related to specific aspects of a concussion history and allowed us to monitor individual athletes before and longitudinally after a diagnosed concussion