The Impact of Meridional Circulation On Stellar Butterfly Diagrams and Polar Paps

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Abstract

Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link subsurface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of <= 125 m s(-1) on the latitudinal eruption pattern of magnetic flux tubes and (ii) the influence of the resulting butterfly diagrams on polar magnetic field properties. Prior to their eruption, magnetic flux tubes with low field strengths and initial cross-sections below similar to 300 km experience an enhanced poleward deflection through meridional flows (assumed to be polewards at the top of the convection zone and equatorwards at the bottom). In particular, flux tubes which originate between low and intermediate latitudes within the convective overshoot region are strongly affected. This latitude-dependent poleward deflection of erupting magnetic flux renders the wings of stellar butterfly diagrams distinctively convex. The subsequent evolution of the surface magnetic field shows that the increased number of newly emerging bipoles at higher latitudes promotes the intermingling of opposite polarities of polar magnetic fields. The associated magnetic flux densities are about 20 per cent higher than in the case disregarding the pre-eruptive deflection, which eases the necessity for fast meridional flows predicted by previous investigations. In order to reproduce the observed polar field properties, the rate of the meridional circulation has to be of the order of 100 m s(-1), and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (less than or similar to 50 degrees) must be larger than in the solar case (less than or similar to 35 degrees).

Original languageEnglish
Pages (from-to)1703-1718
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Volume369
Issue number4
DOIs
Publication statusPublished - 11 Jul 2006

Keywords

  • stars : activity
  • stars : imaging
  • stars : interiors
  • stars : magnetic fields
  • stars : rotation
  • stars : spots
  • MAGNETIC-FLUX TUBES
  • SOLAR CONVECTION ZONE
  • CLOSE BINARY STARS
  • ACTIVE REGIONS
  • FLIP-FLOP
  • MAIN-SEQUENCE
  • AB-DORADUS
  • COOL STARS
  • DYNAMO
  • SUN

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