NONLINEAR RADIAL PULSATION MODELS FOR EXTREME HELIUM STARS - APPLICATION TO V652 HER (BD-DEGREES-3224)(13)

Opacity Project (OF) and OPAL opacities were used to calculate non-lin ear hydrodynamic models of 0.7-M. radially pulsating extreme helium st ars, having mean (averaged over a pulsation cycle) effective temperatu res and luminosities in the ranges 10(4) K less than or equal to [T-ef f] less than or equal to 5 x 10(4) K and 1282 less than or equal to [L ]/L. less than or equal to 8091 respectively. Separated helium and met al ionizing layers cause local maxima (or bumps) in the depth-dependen t Rosseland mean opacity, which were found to be responsible for two d istinct regions of pulsation instability; they will be referred to as the helium instability region (HeIR) and Z-bump instability region (ZB IR). At [L] < 3000 L. the cooler HeIR and hotter ZBIR are separated by a region of stability where radial pulsations are either unexcited or characterized by a very small radial displacement amplitude (Delta R/ R similar to 10(-3)). At [L] = 1282 L. for example, the stability regi on is bounded by [T-eff](red) approximate to 12 000 K and [T-eff](blue ) approximate to 17 500 K; it becomes narrower with increasing luminos ity, so that both instability regions merge at [L] approximate to 3000 L. where [T-eff](red) approximate to [T-eff](blue) approximate to 15 000 K. All ZBIR models represent fundamental mode radial pulsators; fo r these cases an approximate formula was derived to express the pulsat ion constant (Q) in terms of [L] and the mean mass-radius ratio. BD 13 degrees 3224 (V652 Her) remains the only hot extreme helium star kn own to be a radial pulsator; published photometric and spectroscopic d ata were used to test non-linear radial pulsation models and, indirect ly, new opacities upon which they were based. The best, and remarkably good, agreement between observed and theoretical (radial velocity and luminosity) curves was obtained with M = 0.72 M., [T-eff] = 23 500 K and [L] = 1062 L. when OP opacities (with mixture X = 0.0015, Y = 0.98 287 and Z = 0.01563) were adopted. As a consequence, BD + 13 degrees 3 224 was identified as a ZBIR fundamental mode radial pulsator having a mean surface gravity of log g = 3.7.