No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars

Aurière, M.; Wade, G. A.; Lignières, F.; Hui-Bon-Hoa, A.; Landstreet, J. D.; Iliev, I. Kh.; Donati, J.‐F.; Petit, P.; Roudier, T.; Théado, S.

doi:10.1051/0004-6361/201014848

Cited by 64 publications

(63 citation statements)

References 38 publications

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“…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 ).…”

Section: Repeated Observations Of Non-magnetic Starsmentioning

confidence: 99%

Magnetic field measurements and their uncertainties: the FORS1 legacy

Bagnulo

Landstreet

Fossati

et al. 2012

A&A

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149

278

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Context. During the last decade, the FORS1 instrument of the ESO Very Large Telescope has been extensively used to study stellar magnetism. A number of interesting discoveries of magnetic fields in several classes of stars have been announced, many of which obtained at a ∼3σ level; some of the discoveries are confirmed by measurements obtained with other instruments, some are not. Aims. We investigate the reasons for the discrepancies between the results obtained with FORS1 and those obtained with other instruments. Methods. Using the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determination. We have applied this procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive (except for most of the observations obtained in multi-object spectropolarimetric mode). We have devised and applied a number of consistency checks to our field determinations, and we have compared our results to those previously published in the literature. Results. We find that for high signal-to-noise ratio measurements, photon noise does not account for the full error bars. We discuss how field measurements depend on the specific algorithm adopted for data reduction, and we show that very small instrument flexures, negligible in most of the instrument applications, may be responsible for some spurious field detections in the null profiles. Finally, we find that we are unable to reproduce some results previously published in the literature. Consequently, we do not confirm some important discoveries of magnetic fields obtained with FORS1 and reported in previous publications. Conclusions. Our revised field measurements show that there is no contradiction between the results obtained with the low-resolution spectropolarimeter FORS1 and those obtained with high-resolution spectropolarimeters. FORS1 is an instrument capable of performing reliable magnetic field measurements, provided that the various sources of uncertainties are properly taken into account.

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Section: Repeated Observations Of Non-magnetic Starsmentioning

confidence: 99%

Magnetic field measurements and their uncertainties: the FORS1 legacy

Bagnulo

Landstreet

Fossati

et al. 2012

A&A

Self Cite

149

278

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“…At the same time, all systematic studies of the magnetic field in HgMn stars reported its absence (Shorlin et al 2002;Wade et al 2006;Folsom et al 2010;Aurière et al 2010;Makaganiuk et al 2011b). In the best cases, an upper limit set on the strength of the mean longitudinal magnetic field is a few Gauss (Aurière et al 2010;Makaganiuk et al 2011b).…”

Section: Introductionmentioning

confidence: 95%

Magnetism, chemical spots, and stratification in the HgMn starϕ Phoenicis

Makaganiuk

Kochukhov

Piskunov

et al. 2012

A&A

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Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results. Aims. In this study, we investigate the possible presence of a magnetic field in ϕ Phe (HD 11753) and reconstruct surface distribution of chemical elements that show variability in spectral lines. We also test a hypothesis that a magnetic field is concentrated in chemical spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere. Methods. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructed surface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elements was calculated with the DDAFit program. Results. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field. For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric layers. Conclusions. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in ϕ Phe. This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the spot evolution time-scale reported for ϕ Phe.

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“…For instance, Wade et al (2006) searched for a magnetic field in α And. They analyzed magnetic field measurements obtained with three different polarimeters and concluded that the star has no longitudinal magnetic field stronger than about 6−19 G. Folsom et al (2010) performed a similar analysis for AR Aur, once again finding no field stronger than 20−40 G. Finally, Auriere et al (2010) presented an analysis of a small sample of three bright, sharp-lined HgMn stars, reporting no magnetic field detections at the level of 1−3 G.…”

Section: Introductionmentioning

confidence: 98%

“…First, with the exception of the four stars studied by Wade et al (2006) and Auriere et al (2010), previous work was not particularly precise, possibly missing weak magnetic fields which are still capable of causing chemical spot formation. Second, they included a small number of HgMn stars, often with a strong emphasis on the most slowly rotating ones.…”

Section: Introductionmentioning

confidence: 99%

The search for magnetic fields in mercury-manganese stars

Makaganiuk

Kochukhov

Piskunov

et al. 2010

A&A

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Context. A subclass of the upper main-sequence chemically peculiar stars, mercury-manganese (HgMn) stars were traditionally considered to be non-magnetic, showing no evidence of variability in their spectral line profiles. However, discoveries of chemical inhomogeneities on their surfaces imply that this assumption should be investigated. In particular, spectroscopic time-series of AR Aur, α And, and five other HgMn stars indicate the presence of chemical spots. At the same time, no signatures of global magnetic fields have been detected. Aims. We attempt to understand the physical mechanism that causes the formation of chemical spots in HgMn stars and gain insight into the potential magnetic field properties at their surfaces; we performed a highly sensitive search for magnetic fields for a large set of HgMn stars. Methods. With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6 m-telescope, we obtained highquality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines to ensure significantly greater sensitivity to the presence of magnetic fields, including very weak fields. Results. For the 47 individual objects studied, including six components of SB2 systems, we do not detect any magnetic fields at greater than the 3σ level. The lack of detection in the circular polarization profiles indicates that if strong fields are present on these stars, they must have complex surface topologies. For simple global fields, our detection limits imply upper limits to the fields present of 2−10 Gauss in the best cases. Conclusions. We conclude that HgMn stars lack large-scale magnetic fields, which is typical of spotted magnetic Ap stars, of sufficient strength to form and sustain the chemical spots observed on HgMn stars. Our study confirms that in addition to magnetically altered atomic diffusion, there exists another differentiation mechanism operating in the atmospheres of late-B main sequence stars that can produce compositional inhomogeneities on their surfaces.

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No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars

Cited by 64 publications

References 38 publications

Magnetic field measurements and their uncertainties: the FORS1 legacy

Magnetic field measurements and their uncertainties: the FORS1 legacy

Magnetism, chemical spots, and stratification in the HgMn starϕ Phoenicis

The search for magnetic fields in mercury-manganese stars

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