2001MNRAS.321..111J

The ultraviolet properties of 17 extreme helium stars have been examined using 150 IUE spectra. Combining short-wave and long-wave image pairs and using a grid of hydrogen-deficient model atmospheres and a χ² minimization procedure, 70 measurements of effective temperature (T_eff), angular diameters (θ) and interstellar extinction {formmu1} were obtained. In most cases, these were in good agreement with previous measurements, but there are some ambiguities in the case of the hotter stars, where the solutions for T_eff and {formmu2} become degenerate, and in the case of the cooler stars with large {formmu3}, where the total flux is no longer dominated by the ultraviolet.

The behaviour of 12 helium stars was examined over an interval exceeding 10yr. The surfaces of four stars (HD 168476, HD 160641, BD -9°4395 and BD -1°3438) were found to be heating at rates between 20 and 120K.yr^–1, in remarkable agreement with theoretical predictions. This result provides the first direct evidence that extreme helium stars are helium shell-burning stars of up to ∼0.9M_☉ contracting towards the white dwarf sequence. Low-luminosity helium stars do not show a detectable contraction, also in agreement with theory, although one, BD +10°2179, may be expanding.

The short-term behaviour of three variable helium stars (PV Tel variables: HD 168476, BD +1°4381, LSIV -1°2) was examined over a short interval in 1995. All three showed changes in T_eff and θ on periods consistent with previous observations. Near-simultaneous radial velocity (v) measurements were used to establish the total change in radius, with some reservations concerning the adopted periods.

Subsequently, measurements of the stellar radii and distances could be derived. With T_eff and surface gravities established previously, stellar luminosities and masses were thus obtained directly from observation. In the case of HD 168476, the mass is {formmu4} Assuming a similar gravity for LSIV -1°2 based on its neutral helium line profiles, its mass becomes {formmu5} The θ amplitude for BD +1°4381 appears to be overestimated by the IUE measurements and leads to a nonsensical result. These first direct measurements of luminous extreme helium star masses agree well with previous estimates from stellar structure and pulsation theory.