EDP Sciences
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Volume 395, Number 2, November IV 2002
Page(s) 535 - 540
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021113

A&A 395, 535-540 (2002)
DOI: 10.1051/0004-6361:20021113

Temperature and gravity of the pulsating extreme helium star LSS 3184 ( BX Cir ) through its pulsation cycle

V. M. Woolf and C. S. Jeffery

Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland
    e-mail: vmw@astro.washington.edu,csj@star.arm.ac.uk

(Received 6 May 2002 / Accepted 30 July 2002)

We report the analysis of optical spectra of the extreme helium star LSS 3184 (BX Cir) to determine its effective temperature and gravity throughout its pulsation cycle. The spectra were also used to measure its chemical abundances.

We report rest gravity, $\log g = 3.38 \pm 0.02$, and a chemical abundance mixture consistent with those reported earlier in a study using an optical spectrum with lower spectral resolution and a lower signal to noise ratio. Our analysis decreases the upper limit for the H abundance to ${\rm H < 6.0}$ (mass fraction $< 7.1 \times 10^{-7} $). Our gravity corresponds to stellar mass $M = 0.47 \pm 0.03~M_\odot$.

We find that the effective $\log g$ varies through the pulsation cycle with an amplitude of 0.28 dex. The effective gravity is smaller than the rest gravity except when the star is very near its minimum radius. The change in effective gravity is primarily caused by acceleration of the stellar surface.

Based on the optical spectra, we find the temperature varies with an amplitude of 3450 K. We find a time averaged mean temperature, $23\,390 \pm 90$ K, consistent with that found in the earlier optical spectrum study. The mean temperature is 1750 K hotter than that found using combined ultraviolet spectra and  V and  R photometry and the variation amplitude is larger. This discrepancy is similar to that found for the extreme helium star V652 Her .

Key words: stars: chemically peculiar -- stars: oscillations -- stars: variables -- stars: individual: LSS 3184 -- stars: atmospheres

Offprint request: V. M. Woolf, vmw@star.arm.ac.uk

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