The lowest mass stars in the Milky Way galaxy, K and M type stars ("cool dwarfs") are the most abundant, long-lived stars in the galaxy. It is important for astronomers to be able to precisely measure their ages in order to help answer the questions: "What is the fossil record of the assembly of our galaxy from the first stars to the present?" and "How diverse are planetary systems?" Cool dwarfs are sufficiently long-lived that they carry a record of star formation and chemical evolution over the entire history of the galaxy. Furthermore, Earth-sized transiting planets in the habitable zones of M dwarfs are easier to detect than planets orbiting Sun-like stars, but the ages of their host stars are much more difficult to measure. A research collaboration between the University of Hawai'i and Boston University will develop stellar rotation-based tools to infer precise ages for old (>4 billion year-old) K and M type stars. They will do this by studying wide binary star systems containing both a white dwarf (WD) star and a star still on the main sequence (MS) of stellar evolution. Rotation can be measured in the main-sequence star, and an independent age of both stars can be inferred using the white dwarf. The investigators will also directly engage high-school students in scientific research through the HISTAR program at the University of Hawai'i. Students will have the opportunity to experience ownership over an individual stellar system, while working to contribute to the broader goal of building a timeline of galactic events and planetary system evolution. Students will work with data obtained on the summits of Hawaii's mountains, and interact with colleagues at Boston University.
The spin-down of stars with time is a promising age-dating tool, but can only be used if the relationship between age and rotation can be properly calibrated. Currently, a primary limitation is the lack of empirical anchors of known age for these old, low-mass stars. This project will leverage wide WD+MS binaries to provide rotation and age calibrators. Total white dwarf ages are the combination of the time spent cooling as a dead core and the progenitor system's hydrogen and helium burning lifetimes. Both quantities are readily inferred from existing models and initial-final mass relations. These binaries provide the leverage to extend period-age relations out to the age of the galactic disk. Gaia DR2 has enabled the identification over one thousand bright (G < 15 mag), nearby (< 200 pc) WD+MS systems widely separated enough to have never interacted. First, the researchers will use ground-based archival light curves to measure rotation periods of the main-sequence companions and spectroscopically characterize the white dwarfs to determine total system ages. In total, it will develop a sample of >50-100 calibration systems. Second, this proposal will explore systematic uncertainties in the ability of white dwarfs to deliver reliable old ages by studying coeval pairs of wide WD+WD binaries. Finally, it will confront existing models for stellar spin down with empirical anchors, and develop the first calibrated period-age relation for old (>4 Gyr), low-mass (<0.8 solar-mass) stars.
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.
