Advertisement

Observations of Magnetic Fields on T Tauri Stars

  • Jeff A. Valenti
  • Christopher M. Johns-Krull
Chapter

Abstract

We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS.

Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1–3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres.

The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS.

Keywords

T Tauri stars magnetospheric accretion infrared spectroscopy Zeeman broadening circular spectropolarimetry 

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Cameron, A.C. and Campbell, C.G.: 1993, AandA 274, 309.Google Scholar
  2. Johns-Krull, C.M., Valenti, J.A., Hatzes, A.P. and Kanaan, A.: 1999a, ApJL 510, L41.ADSCrossRefGoogle Scholar
  3. Johns-Krull, C.M., Valenti, J.A. and Koresco, C.: 1999b, ApJ 516, 900.ADSCrossRefGoogle Scholar
  4. Johns-Krull, C.M. and Gafford, A.D.: 2002, ApJ 583, 685.ADSCrossRefGoogle Scholar
  5. Königl, A.: 1991, ApJL 370, L39.CrossRefGoogle Scholar
  6. Lynden-Bell, D. and Pringle, J.E.: 1974, MNRAS 168, 603.ADSGoogle Scholar
  7. Mandavi, A. and Kenyon, S.J.: 1998, ApJ 497, 342.ADSCrossRefGoogle Scholar
  8. Shu, F.H., Najita, J., Ostriker, E., Wilkin, F., Ruden, S. and Lizano, S.: 1994, Api 429, 781.ADSGoogle Scholar
  9. Uchida, Y.: 1983, in: P.B. Byrne and M. Rodono (eds.), Activity in Red-Dwarf Stars, Reidel, Dordrecht, The Netherlands, p. 625.CrossRefGoogle Scholar
  10. Uchida, Y. and Shibata, K.: 1984, PASJ 36, 105.ADSGoogle Scholar
  11. Valenti, J.A., Basri, G. and Johns-Krull, C.M.: 1993, Ai 106, 2024.ADSGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2004

Authors and Affiliations

  • Jeff A. Valenti
    • 1
  • Christopher M. Johns-Krull
    • 2
  1. 1.Space Telescope Science InstituteBaltimoreUSA
  2. 2.Department of Physics and AstronomyRice UniversityHoustonUSA

Personalised recommendations