Letter to the Editor

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

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.