Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

Free access article

Issue		A&A Volume 440, Number 2, September III 2005


Page(s)		L37 - L40
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200500164

A&A 440, L37-L40 (2005)
DOI: 10.1051/0004-6361:200500164

Letter

Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

S. Hubrig¹, N. Nesvacil^{1, 2}, M. Schöller¹, P. North³, G. Mathys¹, D. W. Kurtz⁴, B. Wolff⁵, T. Szeifert¹, M. S. Cunha⁶ and V. G. Elkin⁴

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
e-mail: shubrig@eso.org
² Department of Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
⁴ Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150 Porto, Portugal

(Received 7 June 2005 / Accepted 29 July 2005 )

Abstract
We have discovered an extraordinarily large mean longitudinal magnetic field of 7.5 kG in the ultra-cool low mass Ap star HD 154708 using FORS 1 in spectropolarimetric mode. From UVES spectra, we have measured a mean magnetic field modulus of 24.5 kG. This is the second-largest mean magnetic field modulus ever measured in an Ap star.

Furthermore, it is very likely that this star is one of the coolest and least massive among the Ap stars and is located in the H-R diagram in the same region in which rapidly oscillating Ap stars have been detected. We note that all known roAp stars have much smaller magnetic fields, by at least a factor of three.

Key words: stars: chemically peculiar -- stars: polarization -- stars: magnetic fields -- stars: abundances -- stars: individual: HD 154708

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