A DRIVING MECHANISM FOR THE NEWLY DISCOVERED CLASS OF PULSATING SUBDWARF B-STAR

We present new calculations that strongly reinforce the idea-originall y proposed by Charpinet et al.-that pulsation modes are driven through an opacity bump due to a local enhancement of the iron abundance in t he envelopes of sdB stars. Our improved models incorporate nonuniform iron abundance distributions obtained through the condition of diffusi ve equilibrium between gravitational settling and radiative levitation . They also include special Rosseland opacity tables that take into ac count the large variations of the iron abundance about the cosmic valu e that are predicted by equilibrium radiative levitation theory. For r epresentative models with M = 0.48 M. and log g = 5.8, we find strong instabilities for low-order radial and nonradial (p and f) pulsation m odes in the range 36,500 K greater than or similar to T-eff greater th an or similar to 29,000 K. The four pulsating sdB stars currently know n all have effective temperatures in that range. In addition, one of o ur models with T-eff = 34,000 K has a band of unstable modes with peri ods in the range 116-195 s, in excellent agreement with those of the k nown pulsators. We therefore claim that our proposed iron bump mechani sm provides a natural explanation for the instabilities found in the n ewly discovered class of pulsating sdB stars.