Astron. Astrophys. 321, 177-188 (1997)

Magnetic loops on rapidly rotating stars

Y.C. Unruh and M. Jardine

School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK

Received 9 May 1996 / Accepted 19 September 1996

Abstract

We present models for the thermal and mechanical equilibria of slender magnetic loops on rapidly-rotating stars. These loops are embedded within an arcade located on the stellar equator. The loop properties are governed principally by the specified base pressure and conductive flux. While rapid rotation is important in determining the pressure structure within the loop, its main effect on the loop shape is through its influence on the base values of magnetic field strength, pressure, and conductive flux. We have compared our models with observations of the temperatures and X-ray fluxes of stars with a range of rotation rates. By using the observed variation of the differential emission measure with rotation rate and allowing the base magnetic field strength to scale as () we can explain the variation of the temperature and X-ray flux of the slower rotators. For the most rapid rotators, however (approximately ) it appears that a single value of q for all rotation rates cannot explain the observations and some other mechanism is needed to explain the saturation in the X-ray flux. We have also investigated the effect of using a heating function that is proportional to the density and one that falls off exponentially with height. With the parameters that we can calculate from these models it is not possible to discriminate between these two types of heating.

Key words: MHD stars: magnetic fields stars: coronae

Send offprint requests to: Y.C. Unruh

Contents

• 1. Introduction
• 2. Loop model
  • 2.1. The coordinate system
  • 2.2. The equation of motion
  • 2.3. Energy balance
• 3. Details of the model
  • 3.1. The surrounding plasma
  • 3.2. The loop equilibrium
  • 3.3. The boundary values
• 4. Numerical results
  • 4.1. Method
  • 4.2. Loop solutions
  • 4.3. Over-pressured hot loops
  • 4.4. Over-pressured cool loops
  • 4.5. Under-pressured and hot loops
  • 4.6. Under-pressured and cool loops
  • 4.7. The conductive flux and heating at the base
  • 4.8. The magnetic field strength and scale height
• 5. Importance of the rotational velocity
  • 5.1. Results
• 6. Summary and discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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