Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani

Abstract

Context. According to our current understanding, a subclass of the upper main-sequence chemically peculiar stars, called mercurymanganese (HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were recently discovered on their surfaces. At the same time, no global magnetic fields stronger than 1-100 G are detected by systematic studies. Aims. The goals of our study are to search for a magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar surfaces of both components. Methods. Our analysis is based on high-quality spectropolarimetric time-series observations obtained during 10 consecutive nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search we employed a least-squares deconvolution (LSD). We used spectral disentangling to measure radial velocities and study the line profile variability. Chemical spot geometry was reconstructed using multi-line Doppler imaging. Results. We report a non-detection of magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field. Circular polarization profiles also do not indicate any signatures of complex surface magnetic fields. For a simple dipolar field configuration we estimated an upper limit of the polar field strength to be 60-70 G. For the HgMn component we found variability in spectral lines of Ti, Ba, Y, and Sr with the rotational period equal to the orbital one. The surface maps of these elements reconstructed with the Doppler imaging technique show a relative underabundance on the hemisphere facing the secondary component. The contrast of chemical inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.

Magnetic field measurements and their uncertainties: the FORS1 legacy

“…Numerous investigations of magnetism in such stars (e.g. Borra & Landstreet 1980;Glagolevskij et al 1985;Shorlin et al 2002;Aurière et al 2010;Makaganiuk et al 2011) have found no convincing evidence for fields in this class of stars with uncertainties often in the range of a few G up to a few tens of G. Furthermore, α And has been specifically investigated with the MuSiCoS (13 observations, typical σ ∼ 30-60 G) and ESPaDOnS (5 observations, typical σ ∼ 6-19 G) spectropolarimeters without any significant detection of a nonzero longitudinal field or of any Stokes V signature indicative of a non-zero field ).…”

B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid nebula, one of the youngest star forming regions

“…For the second software package we used the LSD technique (Donati et al 1997;Kochukhov et al 2010). The details of the analysis procedure can be found in Makaganiuk et al (2011) and Fossati et al (2013). About 170 lines were used in the line mask, but a test with the 80 lines used for the SVD program package did not show any significant difference in the results.…”

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy

“…The Hg ii λ3984 line is variable in most HgMn stars, but there are exceptions (66 Eri A; Makaganiuk et al 2011b). Conversely, there are cases where the variations are dramatic for the Hg lines, but are (if any) below the detection limit for lines of other elements (α And; Wade et al 2006).…”

“…To allow for a more robust normalisation of Hg ii λ3984, the red wing of H was used to define pseudocontinuum regions. (Hubrig et al 2006(Hubrig et al , 2011Savanov et al 2009), 66 Eri A (Makaganiuk et al 2011b), α And A (Adelman et al 2002;Kochukhov et al 2007), and φ Phe (Makaganiuk et al 2012;Korhonen et al 2013). All the stars discussed here are slow or moderately fast rotators with equatorial velocities below 55 km s −1 and rotation periods in the range 2-10 days.…”

“…The diversity of behaviours observed and the very limited number of objects investigated in detail obviously do not allow us to anticipate what spot morphology might be expected for HD 45975. Furthermore, it should be kept in mind that the spot distribution may change on a monthly/yearly timescale Hubrig et al 2010Hubrig et al , 2012Korhonen et al 2013), and that the presence of a close companion might have a profound effect on the spot morphology (e.g., Hubrig et al 2006;Makaganiuk et al 2011b). However, the sinusoidal character of the photometric and spectroscopic changes suggest a rather simple spot configuration at the time of the observations.…”