Astron. Astrophys. 326, 741-750 (1997)

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He I D3 absorption and its relation to rotation and activity
in G and K dwarfs ^*

S.H. Saar ¹, J. Huovelin ², R.A. Osten ^{1, 3} and A.G. Shcherbakov <sup>4

1</sup> Harvard-Smithsonian Center for Astrophysics, MS 58, 60 Garden St., Cambridge, MA 02138, USA
² Observatory and Astrophysics Laboratory, P.O. Box 14 (Tähtitorninmäki), FIN-00014 University of Helsinki, Finland
³ Center for Astrophysics and Space Astronomy, Campus Box 389, University of Colorado, Boulder, CO 80309, USA
⁴ Crimean Astrophysical Observatory, Russian Academy of Sciences, Nauchny, 334413 Crimea, Ukraine

Received 28 November 1996 / Accepted 23 April 1997

Abstract

We have obtained high resolution, high S/N spectra of the He I D3 line (5876 Å) for 53 stars. Combining these data with previous measurements, we investigate correlations between the flux absorbed by D3, , rotation and other stellar activity indicators for a set of 76 G and K dwarfs. We find that for 4 days. For 4 days, behaviour depends on spectral type, either remaining roughly constant (G stars), decreasing (K stars), or even going into emission (a few late K stars). We study correlations between D3 and chromospheric (Ca II HK), transition region (C IV 1550 Å) and coronal emission, and find, for 4 days, , , and , respectively. Thus, D3 has a response intermediate between Ca II HK and C IV in low to moderate activity stars, consistent with its formation in the upper chromosphere. Our data suggest that the maximum flux absorbed by D3 is erg cm^-2 s^-1  in G stars (equivalent widths 80 mÅ), and about 40% of that value (or 50 mÅ) in K stars, less than predicted by current theoretical models. We discuss the implications of our results for stellar activity and He I line formation, and suggest future avenues of study.

Key words: stars: activity - stars: chromospheres - stars: coronae - stars: late-type

* Based in part on observations obtained at the European Southern Observatory, La Silla, Chile, and at the Crimean Astrophysical Observatory, Nauchny, Ukraine

Send offprint requests to: S. Saar

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Analysis
  • 3.1. Determination of stellar properties and fluxes in D3 and other activity indicators
  • 3.2. Comparison with previous measurements
  • 3.3. Correlations with and rotation
  • 3.4. Correlation of  with other activity fluxes
• 4. Discussion
• 5. Conclusions and future directions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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