Rotationally Modulated g-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_sun, {ω}/{ω_c}∼ 0.88) 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 ell = 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.

Based on data from the MOST satellite, a Canadian Space Agency mission operated by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia, with assistance from the University of Vienna, Austria.