Non-linear radial pulsation models for extreme helium stars: application to V652 Her (BD + 13° 3224) Free

Opacity Project (OP) and OPAL opacities were used to calculate non-linear hydrodynamic models of 0.7-M₂₂₉₉ radially pulsating extreme helium stars, having mean (averaged over a pulsation cycle) effective temperatures and luminosities in the ranges 10⁴K≤ ‹ T_eff› ≤ 5 × 10⁴K and 1282 ≤ ‹L› /L₂₂₉₉ ≤ 8091 respectively. Separated helium and metal ionizing layers cause local maxima (or bumps) in the depth-dependent Rosseland mean opacity, which were found to be responsible for two distinct regions of pulsation instability; they will be referred to as the helium instability region (HeIR) and Z-bump instability region (ZBIR). 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 (ΔR/R ∼ 10⁻³). At ‹ L› =1282L_⊙ for example, the stability region is bounded by ‹T_eff›_red ≈ 12000 K and ‹T_eff›_blue ≈ 17500 K; it becomes narrower with increasing luminosity, so that both instability regions merge at ‹L› ≈ 3000 L_⊙ where ‹T_eff›_red≈ ‹T_eff›_blue≈15 000 K. All ZBIR models represent fundamental mode radial pulsators; for these cases an approximate formula was derived to express the pulsation constant (Q) in terms of ‹L and the mean mass-radius ratio.

BD + 13°3224 (V652 Her) remains the only hot extreme helium star known to be a radial pulsator; published photometric and spectroscopic data were used to test non-linear radial pulsation models and, indirectly, 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 = 23500K and ‹L= 1062 L_⊙ when OP opacities (with mixture X=0.0015, Y=0.98287 and Z= 0.01563) were adopted. As a consequence, BD +13°3224 was identified as a ZBIR fundamental mode radial pulsator having a mean surface gravity of logg =3.7.