ROTATIONALLY MODULATEDg-MODES IN THE RAPIDLY ROTATING δ SCUTI STAR RASALHAGUE (α OPHIUCHI)

Despite a century of remarkable progress in understanding stellar interiors, we know surprisingly little about the inner workings of stars spinning near their critical limit. New interferometric imaging of these so-called rapid rotators combined with breakthroughs in asteroseismology promise to lift this veil and probe the strongly latitude-dependent photospheric characteristics and even reveal the internal angular momentum distribution of these luminous objects. Here, we report the first high-precision photometry on the low-amplitude δ Scuti variable star Rasalhague (α Oph, A5IV, 2.18 M ☉ , ) based on 30 continuous days of monitoring using the MOST satellite. We have identified 57 ± 1 distinct pulsation modes above a stochastic granulation spectrum with a cutoff of ∼26 cycles day −1 . Remarkably, we have also discovered that the fast rotation period of 14.5 hr modulates low-frequency modes (1–10 day periods) that we identify as a rich family of g -modes (| m | up to 7). The spacing of the g -modes is surprisingly linear considering Coriolis forces are expected to strongly distort the mode spectrum, suggesting we are seeing prograde "equatorial Kelvin" waves (modes ℓ = m ). We emphasize the unique aspects of Rasalhague motivating future detailed asteroseismic modeling—a source with a precisely measured parallax distance, photospheric oblateness, latitude temperature structure, and whose low-mass companion provides an astrometric orbit for precise mass determinations.