MOST detects g- and p-modes in the B supergiant HD 163899 (B2 Ib/II )

Abstract

The Microvariability and Oscillations of Stars (MOST ) satellite observed the B supergiant HD 163899 (B2 Ib/ II ) for 37 days as a guide star and detected 48 frequencies [less than tilde] 2.8 cycles day [superscript minus 1] with amplitudes of a few millimagnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between 20 M[circle dot] at log T[subscript eff][approximately equal] 4.41 and 15 M[circle dot] at log T[subscript eff] [approximately equal] 4.36 are roughly consistent with the observed frequency range. Excitation by the Fe bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The MOST light curve of HD 163899 shows that such a reflection of g-modes actually occurs and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from [alpha] Cyg variables. Such g-modes have great potential for asteroseismology.