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Astron. Astrophys. 341, 768-783 (1999)

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Measurements of magnetic field strength on T Tauri stars *

Eike W. Guenther 1, Holger Lehmann 1, James P. Emerson 2 and Jürgen Staude 3

1 Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, D-07778 Tautenburg, Germany
2 Department of Physics, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK
3 Astrophysikalisches Institut Potsdam, Sonnenobservatorium Einsteinturm, D-14473 Potsdam, Germany

Received 19 December 1997 / Accepted 29 October 1998

Abstract

We have investigated the magnetic field strength of one weak-line and four classical T Tauri stars. The magnetic field strength is derived from the differential change of the equivalent width of photospheric Fe i lines in the presence of a magnetic field, calculated using a full radiative transfer code. The method was successfully tested by applying it to a non-magnetic solar-type star, and to VY Ari which is believed to have a strong magnetic field. For two of the classical T Tauri stars, we find a product of magnetic field strength and filling factor [FORMULA] kG for T Tau, and [FORMULA] kG for LkCa 15. For the classical T Tauri star UX Tau A and the weak-line T Tauri star LkCa 16 the detection is only marginal, indicating magnetic field strengths of the order of 1 kG and possibly of more than 2 kG, respectively. No field could be detected for the classical T Tauri star GW Ori. For the two classical T Tauri stars for which we have detected a field, we find the filling factors to be larger than [FORMULA], which indicates that the magnetic field covers most of the photosphere. We also show that ignoring a magnetic field can, depending on the lines used, result in errors in effective temperature and underestimates of veiling.

Key words: stars: magnetic fields – stars: pre-main sequence

* Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio de los Roque de los Muchachos of the Instituto de Astrofisica de Canarias

Send offprint requests to: E.W. Guenther

Correspondence to: Thüringer Landessternwarte, D-07778 Tautenburg, Germany

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998
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