Massive Star Birth, Life and Death: Closing the Loop in the Local Group of Galaxies

Lowell Observatory, Flagstaff AZ

Investigators

Abstract

Massive stars are the 'cosmic engines' of the Universe. These stars shine because of nuclear reactions in their cores. This process also has made most of the oxygen and carbon atoms that exist today. When these massive stars die, lighter elements are turned into heavier ones, such as iron. We are literally made up of this 'star stuff'. These stars also give off so much energy that they cause new stars to form. Although we understand most kinds of stars pretty well, there are many things about massive stars that remain a mystery. The goal of this project is to see how well our theories work by comparing predictions against observations. This project will give us a clearer picture of what happens as a massive star ages. The investigators will compare the type of stars found in different nearby galaxies, using data taken on large telescopes in Arizona and in Chile. This project will also fund two early-career research students, and provide research projects for two undergraduates. The investigators will study massive stars in several ways. They plan to observe Local Group galaxies, which are close enough to be studied in great detail. They will observe all massive stars in these galaxies. Specifically they will determine 1) how changing orbits of binary stars modify the element formation in massive stars and 2) how the most massive stars end their lives. The investigator and his collaborators have begun a survey for Wolf-Rayet (WR) stars in the Magellanic Clouds. With half of the area covered, the project has already confirmed the existence of 15 new WR stars, 10 of which appear to be a new type, never before seen. These discoveries suggest that the population of WR stars in the Magellanic Clouds may have been underestimated by as much as 40%, and yet for decades the Magellanic Clouds have served as the linchpins for evaluating massive star models at low metallicities. When complete, the Magellanic Cloud WR Survey will permit the determination of what fraction of WR stars have changed through binary interactions (Roche-lobe overflow) and fraction show changes in element abundance through single-star material losses due to stellar winds. Thirty WR binaries have been identified in M31 and M33, and repeated radial velocity measurements are being made with the 6.5-m MMT telescope, along with photometric monitoring at Lowell Observatory?s new 4.3-m telescope. This project will provide similar data on the frequency of binary interactions at higher metallicity, and provide direct measurements of the masses of WR stars for comparison with model predictions. The nature of the mysterious, oxygen-rich, WR stars (WO stars) will be explored by determining accurate surface abundances of carbon and oxygen. Comparing these observations to models will determine if the WO stars are truly the "last hurrah" of the most massive stars. Each of these experiments provides a key piece of information to improve models of massive star nuclear energy production. The investigator will bring undergraduates into his research program, providing a boost to students from small, liberal arts colleges. This experience enables their students to successfully compete for entrance to best graduate schools in the country. These students are drawn from the REU program administered through Northern Arizona University, as well as Lowell Observatory's MIT Field Camp, which now includes students from the all-women?s Wellesley College.