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The fundamental parameters of the roAp star α Circini

H. Bruntt, J. R. North, M. S. Cunha, I. M. Brand„o, V. G. Elkin, D. W. Kurtz, C. J. Davis, T. R. Bedding, A. Jacob, S. M. Owens, J. G. Robertson, W. J. Tango, J. F. Gameiro, M. J. Ireland, P. G. Tuthill

Abstract
We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of alpha Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, alphaCir being the brightest member of its class.We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066R⊙. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420±170K. This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of alpha Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of alpha Cir.

Monthly Notices of the Royal Astronomical Society
Volume 386, Page 2039
2008 June

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Funda√ß√£o para a Ci√™ncia e a Tecnologia
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