WFIRST ultra-precise astrometry II: Asteroseismology

Abstract

WFIRST microlensing observations will return high-precision parallaxes, σ(π) ≲ 0.3 μas, for the roughly 1 million stars with H < 14 in its 2.8 deg 2 field toward the Galactic bulge. Combined with its 40,000 epochs of high precision photometry (∼ 0.7 mmag at H vega = 14 and ∼ 0.1 mmag at H = 8), this will yield a wealth of asteroseismic data of giant stars, primarily in the Galactic bulge but including a substantial fraction of disk stars at all Galactocentric radii interior to the Sun. For brighter stars, the astrometric data will yield an external check on the radii derived from the two asteroseismic parameters, the large-frequency separation 〈∆〉 nl › and the frequency of maximum oscillation power ν max, while for the fainter ones, it will enable a mass measurement from the single measurable asteroseismic parameter ν max. Simulations based on Kepler data indicate that WFIRST will be capable of detecting oscillations in stars from slightly less luminous than the red clump to the tip of the red giant branch, yielding roughly 1 million detections.