This award will support an observational project that will address how close binary stars form and evolve. Various formation scenarios lead to differing expectations about whether the spin axes of the stars are aligned with the orbital axis (for example, the Earth's spin axis is about 23 degrees away from the orbital axis.) The project will perform a statistical analysis on 50 eclipsing binaries discovered in the Kepler survey, and will conduct detailed time-series spectroscopy of a dozen brighter systems. The latter will make use of the Rossiter-McLaughlin effect, which produces changes in the observed spectra of one member of a binary system as the other star passes in front of it. Careful measurement of the effect can determine the projection of the spin-orbit axis misalignment on the sky.

The research project is expected to help interpret the recent discoveries of exoplanets on highly tilted orbits. Integrated with this research program are training opportunities for students and a postdoctoral associate, as well as efforts to increase the public understanding of science.

Project Report

Nearly half of the stars that we can see in the sky are actuallybinary stars: a pair of stars orbiting one another. The distancesbetween the stars in a pair can range widely, from thousands of timesthe size of our entire Solar System, to as small as just a few tenthsof the distance from the Earth to the Sun. The formation of binarystars is one of the outstanding questions in astronomy. Stars areknown to form from the contraction of a galactic gas cloud under theinfluence of its own gravity - but how and when does this cloud decideto divide itself into pieces, forming multiple stars? Thesmall-scale, "tight" binaries are especially interesting and puzzlingbecause somehow the individual stars have managed to achieve a verytight and close orbit while maintaining their separate existence. Inthis research program we addressed the question of close binaryformation by undertaking precise and pioneering measurements of a keyaspect of binary star systems: the relative orientations of thestellar rotation axes and the orbital axes. Some theories predict thatbinary star systems should be neatly aligned, with the stars rotatingin the same way as each other, and with their orbital motion. Othertheories allow for major misalignments. However, measuring therelevant geometries of binary star systems is not straightforward, andrequires careful analysis of the stars' spectra. We performedobservations of about a dozen binary star systems and used the data todetermine the geometry of the stellar rotation and orbit in eachsystem. We found several cases in which the stars are grosslymisaligned with the orbit, confirming that binaries are not alwaysneatly aligned. We have demonstrated through direct observation thatit is risky to assume that the orbital and spin axes are aligned, asis often done in the literature. Furthermore, because misalginmentsare now seen to be somewhat common, it seems clear that furthermeasurements of binary geometries for systems with different types ofstars and orbits will be helpful in understanding the formation andevolution of close binary stars.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Maria Womack
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Massachusetts Institute of Technology
United States
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