Theory and evidence of global Rossby waves in upper main-sequence stars: r-mode oscillations in many Kepler stars. Issue 2 (17th November 2017)

• Record Type: Journal Article
• Authors: Saio, Hideyuki; Kurtz, Donald W; Murphy, Simon J; Antoci, Victoria L; Lee, Umin
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 474:Issue 2(2018)
• Page Start: 2774
• Page End: 2786
• Publication Date: 2017-11-17
• Subjects: binaries: eclipsing -- stars: early-type -- stars: oscillations -- stars: rotation -- starspots -- stars: variables: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stx2962
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: Asteroseismic inference from pressure modes (p modes) and buoyancy, or gravity, modes (g modes) is ubiquitous for stars across the Hertzsprung–Russell diagram. Until now, however, discussion of r modes (global Rossby waves) has been rare. Here we derive the expected frequency ranges of r modes in the observational frame by considering the visibility of these modes. We find that the frequencies of r modes of azimuthal order m appear as groups at slightly lower frequency than m times the rotation frequency. Comparing the visibility curves for r modes with Fourier amplitude spectra of Kepler light curves of upper main-sequence B, A, and F stars, we find that r modes are present in many γ Dor stars (as first discovered by Van Reeth et al.), spotted stars, and so-called heartbeat stars, which are highly eccentric binary stars. We also find a signature of r modes in a frequently bursting Be star observed by Kepler . In the amplitude spectra of moderately to rapidly rotating γ Dor stars, r-mode frequency groups appear at lower frequency than prograde g-mode frequency groups, while in the amplitude spectra of spotted early A to B stars, groups of symmetric (with respect to the equator) r-mode frequencies appear just below the frequency of a structured peak that we suggest represents an approximate stellar rotation rate. In many heartbeat stars, a group of frequencies can be fitted with symmetric m = 1 r modes, which can be used to obtain rotation frequencies of these stars.