Astron. Astrophys. 355, 1087-1097 (2000)

Distribution of starspots on cool stars

II. Pre-main-sequence and ZAMS stars between 0.4  and 1.7 

Th. Granzer ¹, M. Schüssler ², P. Caligari ² and K.G. Strassmeier <sup>1

1</sup> Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Vienna, Austria (granzer,strassmeier@astro.univie.ac.at)
² Kiepenheuer Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany (msch,cale@kis.uni-freiburg.de)

Received 15 June 1999 / Accepted 27 September 1999

Abstract

We study the dynamics of magnetic flux tubes in young stars with masses between 0.4 and 1.7 and for rotation rates between 0.25 and 63 . The resulting latitudinal emergence patterns at the stellar surface are compared with observed distributions of starspots in stellar latitude. The stellar models considered cover the range of evolutionary stages from shortly after the Hayashi phase down to the ZAMS, i.e. from the classical and weak-line T Tauri stars to the -Persei stars. We use numerical simulations to follow the evolution of magnetic flux tubes from their origin at the bottom of the convection zone up to near-surface layers. We find a strong increase of emergence latitude with increasing rotation rate, a moderate decrease with increasing stellar mass, and a stronger decrease with stellar age. At very early evolutionary stages, when the central radiative zone is still quite small, we find magnetic flux emergence both in low latitudes as well as in the polar areas. High-latitude emergence is predicted in slightly more evolved pre-main-sequence and young main-sequence stars, but truly polar spots on these stars require an additional transport mechanism acting after magnetic flux emergence at the surface, probably meridional circulation or poleward slip of the `anchored' part of the erupted flux tubes. We discuss our findings in the light of the results obtained from Doppler-imaging studies.

Key words: Magnetohydrodynamics (MHD) stars: rotation stars: pre-main sequence stars: magnetic fields stars: activity

Send offprint requests to: Th. Granzer

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Contents

• 1. Introduction
• 2. Stellar models
• 3. Model assumptions
  • 3.1. Critical field strength
  • 3.2. Magnetic flux
  • 3.3. Internal rotation profile
  • 3.4. The stellar-surface limit
• 4. Patterns of flux emergence: the `standard' cases
  • 4.1. Dependence on rotation rate and evolutionary state
• 5. Parameter study
  • 5.1. Initial location of the flux tube
  • 5.2. Amount of magnetic flux
  • 5.3. Growth time of the instability
  • 5.4. Stellar structure
• 6. Comparison with observations and discussion
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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