Collaborative Research: The Importance of Binarity on the Origin of Wolf-Rayet Stars

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This project is designed to demonstrate how high-mass stars lose their outermost layers, exposing their cores. Such objects are known as Wolf-Rayet (WR) stars. Most of their hydrogen is gone, and they are made up of the products of nuclear burning such as helium, carbon, and oxygen. WR stars are near the ends of their lives and are destined to explode as spectacular supernovae. There are two methods by which these WRs might form. In the first of these, the outer layers are removed by "stellar winds," in which matter is pushed off the star by the pressure of their intense light. In the second method, material is pulled off the star by the gravity of an orbiting companion star. It is likely that some WR stars form one way, while others form the other. The goal of this project is to find what fraction of WR stars is formed by each method. This result will be important scientifically, because these high-mass stars play a vital role in the ecology of the Universe, manufacturing the elements that make up our planet and the life on it. Their intense explosions at the ends of their lives release these elements into space, triggering the formation of the next generation of stars and planets. These observations will also provide summer internship projects for undergraduates. The project includes the construction and free distribution of college-level exercises in astronomy. The investigator team will use three complementary approaches. First, they will measure the luminosities and chemical composition of representative samples of nitrogen-rich Wolf-Rayet stars (WN) and compare these to those predicted by the evolutionary models. Second, the nature of the enigmatic "WN3/O3" stars will be investigated. These are either archetypes of stripped binaries or a missing link between O stars and WRs. Third, they will conduct a systematic survey for O-type stars in the Large and Small Magellanic Clouds. The relative number of O and WR stars as a function of mass (luminosity) is one of the most fundamental diagnostics of the origins of WRs. Together these three programs will put numbers and uncertainties on the fraction of binaries formed by each mechanism. The authors will also construct a high-resolution spectral atlas of 1000 bright stars covering a wide range of spectral types and luminosity classes. This atlas is intended as a heuristic resource available not only for astronomers but also for astronomy students. It will be used in undergraduate lab exercises at Northern Arizona University. These spectra and teaching units will then be made freely available to everyone through a website.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.