Armagh Observatory Preprint Series No. xxx

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Cyclic and secular variation in the temperatures and radii of extreme helium stars

Authors: C.S.Jeffery, R.L.C.Starling, P.W.Hill, D.Pollacco

Journal:  MNRAS  (2001) 321, 111-130

Abstract. The ultraviolet properties of seventeen 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 chi^2-minimization procedure, seventy measurements of effective temperature (Teff), angular diameters (theta) and interstellar extinction (E_bv) 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 Teff and E_bv become degenerate, and in the case of the cooler stars with large E_bv, where the total flux is no longer dominated by the ultraviolet.

 The behaviour of twelve helium stars was examined over an interval exceeding ten years. The surfaces of four stars (HD168476, HD160641, BD-9 4395 and BD-1 3438) were found to be heating at rates between 20 and 120 K/yr, 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 ~0.9 Msolar 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: HD168476, BD+1 4381, LS IV-1 2) was examined over a short interval in 1995. All three showed changes in Teff and theta 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 Teff and surface gravities established previously, stellar luminosities and masses were thus obtained directly from observation. In the case of HD168476 the mass is 0.94+/-0.68 Msolar. Assuming a similar gravity for LS IV-1 2 based on its neutral helium line profiles, its mass becomes 0.79+/-0.46 Msolar. The theta amplitude for BD+1 4381 appears to be overestimated by the IUE measurements and leads to a nonsense result. These first direct measurements of luminous extreme helium star masses agree well with previous estimates from stellar structure and pulsation theory.

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