EAPSI: Using Seismic Waves to Probe the Structure of Colliding Stars

Leiner Emily, Madison WI

Investigators

Abstract

Blue stragglers are stars that have recently gained mass by colliding with a passing star, or accreting material from a binary companion. Similar interactions are responsible for creating many other interesting objects, including the Type 1a supernovae we use to measure the expansion of the universe. To better understand these processes, astronomers need reliable measurements of the mass and internal structure of mass-transfer and collision products like blue stragglers. Such measurements only recently became possible using a new technique called asteroseismology. Similar to seismologists using earthquakes to study the interior of the Earth, astronomers use seismic waves that propagate inside stars to probe their interior structures. These wave patterns can be detected as small variations in a star's brightness. NASA's Kepler mission has monitored the brightness of thousands of stars for four years, providing a groundbreaking data set to study these seismic waves. This project will study a sample of blue stragglers observed by Kepler, using their seismic wave patterns to determine their masses, rotation rates, and internal structures. This research will be done in collaboration with Dr. Dennis Stello at the University of New South Wales, one of a handful of experts on asteroseismology in the world. Two key uncertainties in the physics of collisions and binary mass transfer are the mass transfer efficiency and the angular momentum evolution. In combination with detailed stellar modeling, asteroseismology can shed light on mass transfer efficiency by providing precise mass measurements of blue stragglers, a constraint that has been largely unavailable to modelers. Some blue stragglers display mixed p- and g- mode oscillations, allowing separate measurements of core and envelope properties. For this project, measuring a core and an envelope rotation rate will illuminate how much angular momentum is accreted at the blue straggler surface, and if it is transported down into the stellar core. This technique has already offered many insights into angular momentum transport in normal stars. This project will be one of the first efforts to measure these properties in non-standard stars like blue stragglers, and promises to offer more insight into the angular momentum evolution of mass-transfer and collision products. This award, under the East Asia and Pacific Summer Institutes program, supports summer research by a U.S. graduate student and is jointly funded by NSF and the Australian Academy of Science.