Astron. Astrophys. 330, 685-695 (1998)

Doppler imaging of stellar surface structure

VI. HD 129333 = EK Draconis: a stellar analog of the active young Sun

K.G. Strassmeier ^{* 1} and J.B. Rice <sup>2

1</sup> Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Austria (strassmeier@astro.ast.univie.ac.at)
² Department of Physics, Brandon University, Brandon, Manitoba R7A 6A9, Canada (rice@BrandonU.ca)

Received 3 September 1997 / Accepted 29 September 1997

Abstract

We present the first Doppler images of the single G1.5V-star EK Draconis from high-resolution CFHT spectra obtained in 1995. EK Dra is an important target for investigating the evolution of stellar magnetism because it resembles the rapidly-rotating young Sun. From the inversion of a total of 12 spectral lines we reconstruct several cool spots at low and medium latitudes but the dominating feature is located consistently at a latitude of 70-80^o , thus, at far higher latitudes than where our Sun shows spots. In fact, our data indicate that this feature could be an appendage of a larger polar cap-like spot as seen on other rapidly-rotating stars, however, due to the small of just 17.3 0.4 km s^-1 the cap's existence remains inconclusive. Spot temperatures between 1200 K and 400 K are recovered. We have used the non-parametrized convective-flux description of Canuto & Mazzitelli for our input model atmospheres and found good agreement with local line profiles computed from standard mixing-length atmospheres.

New phase-resolved UBVRI data spanning the 1996/97 observing season indicate a surprisingly short photometric period of 2.599 0.001 days, which is different by 7% from the longest seasonal period found so far but in good agreement with the long-term photometric period of 2.605 days. We interpret these differences as being due to differential surface rotation and adopt the long-term photometric period to be the rotation period of EK Dra. Our data also confirms the continuing decline of the mean brightness seen since 1994 and we suspect the high-latitude feature to be the cause.

Key words: stars: activity of stars: imaging stars: individual: EK Dra stars: late-type starspots

* Visiting Astronomer, Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii

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Contents

• 1. Introduction
• 2. Observations
• 3. Photometric monitoring
  • 3.1. The light-curve variability
  • 3.2. The rotational period
• 4. Multiline Doppler imaging
  • 4.1. The inversion code TEMP MAP
  • 4.2. The input model atmospheres
  • 4.3. Numerical simulations with =17.5 km s^-1
  • 4.4. Doppler imaging of EK Dra
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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