The fundamental parameters of the roAp star α Circini

Creators: Bruntt, H.; North, J. R.; Cunha, M.; Brandão, I. M.; Elkin, V. G.; Kurtz, D. W.; Davis, J.; Bedding, T. R.; Jacob, A. P.; Owens, S. M.; Robertson, J. G.; Tango, W. J.; Gameiro, J. F.; Ireland, M. J.; Tuthill, P. G.

Abstract

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙ . We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K . 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 α 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 α Cir.

Additional Information

© 2008 RAS. Accepted 2008 February 25; Received 2008 February 5; in original form 2007 December 10. This research has been jointly funded by the University of Sydney and the Australian Research Council as part of the SUSI project. We acknowledge the support provided by a University of Sydney Postgraduate Award (APJ) and Denison Postgraduate Awards (APJ and SMO). VGE and DWK acknowledge support from the UK STFC. MC is supported by the European Community's FP6, FCT and FEDER (POCI2010) through the HELAS international collaboration and through the projects POCI/CTE-AST/57610/2004 and PTDC/CTE-AST/66181/2006 FCT-Portugal. This research has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. We made use of atomic data compiled in the VALD data base (Kupka et al. 1999). We thank Christian St¨utz for supplying the LLMODELS (Shulyak et al. 2004) we used in the analysis. We wish to thank Brendon Brewer for his assistance with the theory and practicalities of MCMC simulations. SUPPLEMENTARY MATERIAL The following supplementary material is available for this article. Table 5. The atomic number, element name, wavelength, and oscillator strength (log g f ) from the VALD data base for lines used in the abundance analysis. This material is available as part of the online paper from: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365- 2966.2008.13167.x (this link will take you to the article abstract). Please note: Blackwell Publishing are not responsible for the content or functionality of any supplementary materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article. Based on observations with the Sydney University Stellar Interferometer at the Paul Wild Observatory, Narrabri (Australia) and observations collected at the European Southern Observatory, Paranal (Chile), as part of programmes 072.D-0138 and 077.D-0150.

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