Are there tangled magnetic fields on HgMn stars?^⋆

¹ Department Physics and Astronomy Uppsala University, Box 516, 75120 Uppsala, Sweden
e-mail: oleg.kochukhov@fysast.uu.se
² Institute of Astrophysics, Georg-August University, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005, USA
⁴ Sterrewacht Leiden, Universiteit Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁵ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21211, USA

Received: 13 March 2013
Accepted: 21 April 2013

Abstract

Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects.

Aims. We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections of weak longitudinal magnetic fields in HgMn stars based on the moment technique.

Methods. Our spectrum synthesis code calculated the Zeeman broadening of metal lines in HARPS spectra, assuming an unstructured, turbulent magnetic field. A simple line formation model with a homogeneous radial field distribution was applied to assess compatibility between previous longitudinal field measurements and the observed mean circular polarization signatures.

Results. Our analysis of the Zeeman broadening of magnetically sensitive spectral lines reveals no evidence of tangled magnetic fields in any of the studied HgMn or normal stars. We infer upper limits of 200–700 G for the mean magnetic field modulus – much smaller than the field strengths implied by studies based on differential magnetic line intensification and quadratic field diagnostics. The new HARPSpol longitudinal field measurements for the extreme HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero at a precision of 3–6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD 11753 shows that the recent moment technique measurements retrieved from the same data are incompatible with the lack of circular polarization signatures in the spectrum of this star.

Conclusions. We conclude that there is no evidence for substantial tangled magnetic fields on the surfaces of studied HgMn stars. We cannot independently confirm the presence of very strong quadratic or marginal longitudinal fields for these stars, so results from the moment technique are likely to be spurious.