This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Dr. Duncan Lorimer and collaborators will undertake a detailed statistical study of the neutron star population in the globular clusters of the Milky Way Galaxy. The dense stellar environments provided by globular clusters act as astrophysical laboratories to probe stellar evolution and the dynamics of binary and multiple star systems. Neutron stars in globular clusters are long-lived tracers of the integrated star formation history in clusters and have a wealth of physical applications. Radio pulsars - rapidly spinning highly magnetized neutron stars - provide a very powerful window into this population. Timing observations of radio pulsars, for example, have demonstrated for the first time the existence of ionized gas in globular clusters and provide recent evidence for the existence of high-mass neutron stars which challenge many equations of state for super-dense matter. Future discoveries expected from globular clusters include binary pairs of millisecond pulsars and pulsar-black hole binary systems.
Currently, 140 radio pulsars are known in 26 globular clusters. Around half of these discoveries have been made in the past five years by groups using the NSF-funded Green Bank Telescope in West Virginia. Despite these significant advances, the last major census of the radio pulsar population in globular clusters was carried out over 15 years ago when less than two-dozen objects were known in a handful of clusters. At that time, it was not clear whether the progenitor population of millisecond pulsars (the low-mass X-ray binaries) was sufficient to account for the estimated number of 10,000 pulsars suspected to exist in the Galactic globular cluster population. Dr. Lorimer plans a comprehensive new analysis of the population to take account of selection effects due to binary motion, interstellar propagation and radio frequency interference. His team has a broad range of experience in pulsar observations and population modeling necessary to develop detailed Monte Carlo models of the underlying population of radio pulsars in globular clusters.
In addition to characterizing the overall size and birth-rate of radio pulsars in globular clusters, they will apply both frequentist and Bayesian statistical analyses to determine the underlying distributions of spin period, luminosity and (for binary systems) orbital parameters. They will also address the smaller subset of younger pulsars known to exist in globular clusters, and those binaries which will merge on sub-Hubble timescales due to the emission of gravitational radiation. These results are expected to provide vital input to complementary studies of the neutron star population which are based on models of stellar evolution. They will also help determine the likely size of the population of compact object binaries that contribute as gravitational-wave sources.
The proposed research will impact upon a number of other areas which will benefit the state of West Virginia, teaching astronomy courses and astronomical research in general. Specifically, the PI and his team will (a) develop a freely-available laboratory component for undergraduate astronomy students; (b) develop an inquiry-based learning tool for high-school science teachers and their students; (c) raise the international research profile of West Virginia University and help to increase the number of students in Science, Technology, Engineering and Mathematics disciplines; (d) enhance the science case for future national and international radio facilities; (e) make available an open-source library of tools for Monte Carlo simulations of astronomical populations.
