Magnetic fields of HgMn stars

Abstract

Context. The frequent presence of weak magnetic fields on the surface of spotted late-B stars with HgMn peculiarity in binary systems has been controversial during the two last decades. Recent studies of magnetic fields in these stars using the least-squares deconvolution (LSD) technique have failed to detect magnetic fields, indicating an upper limit on the longitudinal field between 8 and 15 G. In these LSD studies, assumptions were made that all spectral lines are identical in shape and can be described by a scaled mean profile. Aims. We re-analyse the available spectropolarimetric material by applying the moment technique on spectral lines of inhomogeneously distributed elements separately. Furthermore, we present new determinations of the mean longitudinal magnetic field for the HgMn star HD 65949 and the hotter analog of HgMn stars, the PGa star HD 19400, using FORS 2 installed at the VLT. We also give new measurements of the eclipsing system AR Aur with a primary star of HgMn peculiarity, which were obtained with the SOFIN spectropolarimeter installed at the Nordic Optical Telescope. Methods. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for eight HgMn stars and one normal and one superficially normal B-type star obtained in 2010. Out of this sample, three HgMn stars belong to spectroscopic double-lined systems. The application of the moment technique to the HARPS and SOFIN spectra allowed us to study the presence of the longitudinal magnetic field, the crossover effect, and quadratic magnetic fields. Results for the HgMn star HD 65949 and the PGa star HD 19400 are based on a linear regression analysis of low-resolution spectra obtained with FORS 2 in spectropolarimetric mode. Results. Our measurements of the magnetic field with the moment technique using spectral lines of several elements separately reveal the presence of a weak longitudinal magnetic field, a quadratic magnetic field, and the crossover effect on the surface of several HgMn stars as well as normal and superficially normal B-type stars. Furthermore, our analysis suggests the existence of intriguing correlations between the strength of the magnetic field, abundance anomalies, and binary properties. The results are discussed in the context of possible mechanisms responsible for the development of the element patches and complex magnetic fields on the surface of late B-type stars.

Abundance analysis, spectral variability, and search for the presence of a magnetic field in the typical PGa star HD 19400

“…A few polarimetric spectra of HD 19400 were previously obtained with FORS 1 (Hubrig et al 2006b), and most re- cently with FORS 2 on Antu (UT1) from 2011 May to 2012 January (Hubrig et al 2012) at the rather low resolution of ∼2000. The magnetic field measurements from these earlier data are presented in Table 4 together with the modified Julian date of mid-exposure.…”

The orbit of the mercury-manganese binary 41 Eridani

“…The elements of the five-day apparent orbit were derived and, when combined with those of the spectroscopic orbit (Hubrig et al 2012), delivered the stellar masses with an accuracy of 2% and the orbital parallax with an accuracy of 1%. The diameters of the nearly identical components, while barely re- solved, were consistent with estimates based on fitting the spectral energy distribution.…”

“…• to make the epoch refer to conjunction and required a very small adjustment of 0.0013 days to the value quoted by Hubrig et al (2012).…”

“…To explain the abundance anomalies in the atmospheres of HgMn stars, radiatively driven selective diffusion is most often invoked. However, since parameter-free diffusion models appear unable to reproduce the observed abundance anomalies, other mechanisms are expected to contribute, such as anisotropic stellar winds, membership in binary and multiple systems, and the presence of tangled magnetic fields (e.g., Hubrig & Mathys 1995;Hubrig et al 2012).…”

“…Although strong large-scale magnetic fields have not generally been found in HgMn stars, it has never been ruled out that these stars might have tangled magnetic fields with only very weak net longitudinal components (Hubrig et al , 2012. The studies of Hubrig et al also suggested the existence of in-triguing correlations between magnetic field, abundance anomalies, and binary properties.…”