Dr. Vassiliki Kalogera of Northwestern University will undertake a comprehensive research project to address fundamental questions concerning the origin of neutron stars and black holes. The research will take advantage of a unique opportunity provided by recent growth in observational constraints for a sizable sample of Galactic and extragalactic X-ray binaries harboring accreting neutron stars or black holes. She will engage in theoretical computations which incorporate many elements necessary for the modeling of X-ray binary evolutionary histories: motion in the Galaxy, mass transfer and its dependence on orbital eccentricity, evolution of detached binary systems due to nuclear burning, stellar tides and angular momentum losses,and the effect of the core-collapse event on binary orbital dynamics. She will also engage in statistical analyses of known systems and binary populations to determine most likely progenitor properties and X-ray binary formation channels.
The results will include physical constraints on the immediate progenitors of compact objects and statistical statements about the likelihood of their constrained properties. These will be used as initial conditions for core-collapse hydrodynamic simulations which will test and further our understanding of the core-collapse process. The work will contribute to answering currently open, basic questions about compact object formation. What, for example, is the mass relationship between compact objects and their progenitors? Do some neutron stars acquire natal kicks with magnitudes considerably lower than was thought before? What are the implications for asymmetries in the core-collapse process and the role of electron-capture supernovae? Do most black holes require sizable natal kicks and how is their formation different from that of neutron stars, if at all? What is the true distribution of black hole masses, corrected for observational selection biases?
Dr. Kalogera will continue her outreach efforts through public lectures, talks at minority conferences and institutions, participation in school science fairs in the Chicago area and beyond. She is working in collaboration with visualization and graphics experts at Northwestern and the Adler Planetarium and Astronomy Museum in Chicago to develop visualization segments appropriate for the general public telling the story of how compact objects form and evolve in interacting binaries. The products will be presented at Northwestern, the Adler Planetarium, and will be available through web sites for broad dissemination.
Intellectual Merit: A wide range of projects were completed all targeting to uncover some of the astrophysics of the core-collapse mechanism of massive stars when they run out of nuclear fuel, how black holes form, what are they progenitors, whether they receive natal kicks, what is the origin of spin for compact objects, what is their mass distribution and how does it depend on metallicity. The most important results (all published in peer-reviewed journals like Nature and The Astrophysical Journal) include: - We calculated the progenitor masses and natal kick magnitudes that are consistent with all observational constraints available to date for three X-ray binaries harboring black holes which accrete matter from their massive companions (known as M33 X-7, Cyg X-1, IC!0 X-1). - we showed that the current black-home mass measurements imply the existence of a potential mass gap in the range of 3-5 times the mass of the Sun; we also showed that a previously unrecognized observational selection bias could be responsible for the presence of this gap, but that, even correcting for that, a paucity in the aforementioned mass regime still remains. - We investigated the implications of the existence of such a black-hole mass gap (or paucity) for the physical mechanism of core collapse in massive stars and find that relatively fast explosions (100-200ms) are favored, which are due to instabilities in the core that develop over 10-20ms. - We calculated the maximum black-hole mass in view of new results on massive-star wind mass loss and we also calculated the black-home mass function and its dependence on the progenitors' metallicity. - We also uncovered a unique implication of the double pulsar (the only binary system known where both components are pulsars): the measure spin tilts imply without much dount that angular momentum must be generated during the collapse and the spins of compact objects cannot be due to the progenitor angular momentum alone. Overall our studies led to 12 publications, 9 of which in peer-reviewed journals, and 8 presentations at research conferences (6 of them oral and 2 posters). Furthermore this grant enabled the start of two other projects (one investigating the hydrodynamics of mass fallback during core collapse and compact object formation, and one on the statistical analysis of our results for black-hole progenitor masses and natal kicks); both are ongoing and articles are being prepared for submission to The Astrophysical Journal. Broader Impacts: This grant contributed to the development of exceptional human resources and the training of 7 young researchers in computational thinking, programming, and analysis and statistical methods of data. In this way we contributed to the preparation of a competitive workforce in STEM disciplines. The PI recruited 1 postdoctoral associate, 3 graduate students (2 of them female and 2 over having completed their PhD), and 3 undergraduate students (2 of them female) all working on this research. We also development state-of-the-art visualizations (movies) for the evolution of single and binary stars. These are all publicly available (http://ciera.northwestern.edu/Research/stellar_evolution/stellar_evolution.php) and have been used in numerous presentations to the public and K-12 students and teachers. Also team members participated in an Elementary School Science Fair (Park Ridge, IL) and at a Junior Science Cafe discussing this research with the local public.
