Wa. Dziembowski et al., THE OPACITY MECHANISM IN B-TYPE STARS .2. EXCITATION OF HIGH-ORDER G-MODES IN MAIN-SEQUENCE STARS, Monthly Notices of the Royal Astronomical Society, 265(3), 1993, pp. 588-600
We show that the OPAL opacities, in addition to explaining the origin
of the pulsations of beta Cep stars, also predict the existence of a l
arge region in the main-sequence band at lower luminosities, where hig
h-order g-modes of low harmonic degree l are unstable. The excitation
mechanism remains the same, and is due to the usual kappa-effect actin
g in the metal opacity bump (T almost-equal-to 2 X 10(5) K). The new i
nstability domain nearly bridges the gap in spectral types between del
ta Sct and beta Cep stars. The periods of the unstable modes are in th
e range 0.4-3.5 d for l = 1 and 2. We propose that this excitation mec
hanism causes photometric variability in the slowly pulsating B-type s
tars (SPB stars), and perhaps in other B stars for which variability i
n the same period range has been reported. Typically, a large number o
f modes are simultaneously unstable in one model. Most of them have 1
> 2. Such modes are not likely to be detected photometrically, but may
be visible in line profile changes. The excitation of many high-l mod
es in a star may also cause a spurious contribution to the rotational
v sin i values. Sequences of unstable modes at each value of 1 exhibit
a periodically varying departure from equal spacing in period. This f
eature, first noted in white dwarf g-mode spectra (calculated and meas
ured), is in the present case a probe of the region left behind the sh
rinking core (the mu-gradient zone). We discuss prospects for and diff
iculties of SPB-star asteroseismology.