Rotationally-modulated g-modes in the rapidly-rotating delta Scuti star Rasalhague (alpha Ophiuchi)

TL;DR

This study presents high-precision photometry of the rapidly rotating delta Scuti star Rasalhague, revealing 57 pulsation modes, including a family of g-modes modulated by the star's rotation, with implications for internal stellar dynamics.

Contribution

First detailed asteroseismic analysis of Rasalhague showing rotation-modulated g-modes and linear mode spacing, suggesting prograde Kelvin waves in a rapidly rotating star.

Findings

Identification of 57 pulsation modes above granulation spectrum.

Discovery of rotation-modulated low-frequency g-modes.

Mode spacing indicates prograde Kelvin wave characteristics.

Abstract

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 delta cuti variable star Rasalhague (alpha Oph, A5IV, 2.18 Msun, omega/omega_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 per day. Remarkably, we have also discovered that the fast rotation period of 14.5~hours 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.