Weak magnetic fields in Ap/Bp stars

Aurière, M.; Wade, G. A.; Silvester, J.; Lignières, F.; Bagnulo, S.; Bale, Karen; Dintrans, B.; Donati, J. F.; Folsom, C. P.; Gruberbauer, M.; Hui-Bon-Hoa, A.; Jeffers, S. V.; Johnson, N.; Landstreet, J. D.; Lèbre, A.; Lueftinger, T.; Marsden, S. C.; Mouillet, D.; Naseri, Shahrokh; Paletou, F.; Petit, P.; Power, J.; Rincon, F.; Strasser, S.; Toqué, N.

doi:10.1051/0004-6361:20078189

Cited by 300 publications

(376 citation statements)

References 75 publications

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“…It is already clearly demonstrated that magnetic fields are detected in essentially every Ap star if high enough precision is reached (Aurière et al 2007). Therefore, the reliability of the detection of weak fields reported in these papers will not greatly change what we know about the occurrence of magnetic Ap and Bp stars.…”

Section: The Incidence Of Magnetic Fields In Various Classes Of Starsmentioning

confidence: 73%

Magnetic field measurements and their uncertainties: the FORS1 legacy

Bagnulo

Landstreet

Fossati

et al. 2012

A&A

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147

277

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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: The Incidence Of Magnetic Fields In Various Classes Of Starsmentioning

confidence: 73%

Magnetic field measurements and their uncertainties: the FORS1 legacy

Bagnulo

Landstreet

Fossati

et al. 2012

A&A

Self Cite

147

277

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“…More than 300 spectra of such stars were obtained in polarimetric mode with FORS1. Such stars, if correctly identified by classification, are known to possess magnetic fields that can almost always be detected at the level of B z ∼ 10 2 G or more (Aurière et al 2007). B z , often called the mean longitudinal field, is the mean value of the line-of-sight component of the magnetic field, averaged over the visible stellar hemisphere, and is the signature of a magnetic field to which a low-resolution spectropolarimeter is sensitive.…”

Section: Introductionmentioning

confidence: 99%

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

Landstreet

Bagnulo

Fossati³

2014

A&A

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Context. The ESO archive of FORS1 spectropolarimetric observations may be used to create a homogeneous database of magnetic field measurements. However, no systematic comparison of FORS field measurements to those obtained with other instruments has been undertaken so far. Aims. We exploit the FORS archive of circular spectropolarimetric data to examine in a general way how reliable and accurate field detections obtained with FORS are. Methods. We examine the observations of Ap and Bp stars, on the grounds that almost all of the unambiguous detections of magnetic fields in the FORS1 archive are in these kinds of stars. We assess the overall quality of the FORS1 magnetic data by examining the consistency of field detections with what is known from previous measurements obtained with other instruments, and we look at patterns of internal consistency. Results. FORS1 magnetic measurements are fully consistent with those made with other instruments, and the internal consistency of the data is excellent. However, it is important to recognise that each choice of grism and wavelength window constitutes a distinct instrumental measuring system, and that simultaneous field measurements in different instrumental systems may produce field strength values that differ up to 20%, or more. Furthermore, we found that field measurements using hydrogen lines only yield results that meaningfully reflect the field strength as sampled specifically by lines of hydrogen for stars with effective temperatures above about 9000 K. Conclusions. In general the magnetic field measurements of Ap and Bp stars obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1 provides an extremely useful example that offers valuable lessons for field measurements with other low-resolution Cassegrain spectropolarimeters.

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“…Wade et al 2000;Aurière et al 2007) while fields with large-scale components can be excluded above a level of a few Gauss in other types of stars belonging to the same mass domain (e.g. Shorlin et al 2002;Wade et al 2006;Aurière et al 2007Aurière et al , 2010.…”

Section: Introductionmentioning

confidence: 99%

The rapid rotation and complex magnetic field geometry of Vega

Petit

Lignières

Wade

et al. 2010

A&A

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106

144

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Context. The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields. Aims. We aim to confirm the field detection reported by Lignières et al. (2009, A&A, 500, L41) and provide additional observational constraints about the field characteristics, by modelling the large-scale magnetic geometry of the star and by investigating a possible seasonal variability of the reconstructed field topology. Methods. We analyse a total of 799 high-resolution circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. Using about 1100 spectral lines, we employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20 000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. Results. We confirm the detection of circularly polarized signatures in the mean line profiles. The signal shows up in four independent data sets acquired with both NARVAL and ESPaDOnS. The amplitude of the polarized signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 ± 0.008 d. The reconstruction of the magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. Conclusions. The repeated observational evidence that Vega possesses a weak photospheric magnetic field strongly suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars now investigatable with the current generation of stellar spectropolarimeters.

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Weak magnetic fields in Ap/Bp stars

Cited by 300 publications

References 75 publications

Magnetic field measurements and their uncertainties: the FORS1 legacy

Magnetic field measurements and their uncertainties: the FORS1 legacy

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

The rapid rotation and complex magnetic field geometry of Vega

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