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

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 or similar to 2.8 cycles day(-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-eff approximate to 4.41 and 15 M-circle dot at log Teff approximate to 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.