A search for magnetic fields in the variable HgMn star α Andromedae

¹ Physics Dept., Royal Military College of Canada, PO Box 17000, Station Forces, K7K 4B4, Kingston, Canada e-mail: Gregg.Wade@rmc.ca
² Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, Toulouse, France
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁴ Department of Physics & Astronomy, The University of Western Ontario, London, N6A 3K7, Ontario, Canada
⁵ Herzberg Institute of Astrophysics, 5071 West Saanich Road, V9E 2E7, Victoria, Canada
⁶ Obs. de Paris LESIA, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁷ Department of Physics, Queen's University, K7L 3N6, Kingston, Canada

Received: 10 November 2005
Accepted: 23 January 2006

Abstract

Context.The chemically peculiar HgMn stars are a class of Bp stars which have historically been found to be both non-magnetic and non-variable. Remarkably, it has recently been demonstrated that the bright, well-studied HgMn star α And exhibits clear Hg ii line profile variations indicative of a non-uniform surface distribution of this element.

Aims.With this work, we have conducted an extensive search for magnetic fields in the photosphere of α And.

Methods.We have acquired new circular polarisation spectra with the MuSiCoS and ESPaDOnS spectropolarimeters. We have also obtained FORS1 circular polarisation spectra from the ESO Archive, and considered all previously published magnetic data. This extensive dataset has been used to systematically test for the presence of magnetic fields in the photosphere of α And. We have also examined the high-resolution spectra for line profile variability.

Results.The polarimetric and magnetic data provide no convincing evidence for photospheric magnetic fields. The highest-S/N phase- and velocity-resolved Stokes V profiles, obtained with ESPaDOnS, allow us to place a  upper limit of about 100 G on the possible presence of any undetected pure dipolar, quadrupolar or octupolar surface magnetic fields (and just 50 G for fields with significant obliquity). We also consider and dismiss the possible existence of more complex fossil and dynamo-generated fields, and discuss the implications of these results for explaining the non-uniform surface distribution of Hg. The very high-quality ESPaDOnS spectra have allowed us to confidently detect variability of Hg ii , and . The profile variability of the Hg ii lines is strong, and similar to that of the Hg ii  line. On the other hand, variability of other lines (e.g. Mn, Fe) is much weaker, and appears to be attributable to orbital modulation, continuum normalisation differences and weak, variable fringing.