The magnetic fields of the helium-weak B stars

Borra, E. F.; Landstreet, J. D.; Thompson, I. B.

doi:10.1086/190889

Cited by 133 publications

(124 citation statements)

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“…Norris (1968) failed to detect a magnetic field stronger than 700 G but a relatively weak field was tentatively detected by Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, The Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Wolff & Morrison (1974) and quantitatively confirmed by Borra et al (1983). Photometric, spectroscopic and magnetic field data all vary on the Norris (1968) period, assumed to be the rotation period of the star.…”

Section: Introductionmentioning

confidence: 85%

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Doppler imaging of the helium-variable star a Centauri

Bohlender¹,

Rice²,

Hechler³

2010

A&A

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Aims. The helium-peculiar star a Cen exhibits interesting line profile variations of elements such as iron, nitrogen and oxygen in addition to its well-known extreme helium variability. The objective of this paper is to use new high signal-to-noise, high-resolution spectra to perform a quantitative measurement of the helium, iron, nitrogen and oxygen abundances of the star and determine the relation of the concentrations of the heavier elements on the surface of the star to the helium concentration and perhaps to the magnetic field orientation. Methods. Doppler images have been created for the elements helium, iron, nitrogen and oxygen using the programs described in earlier papers by Rice and others. An alternative surface abundance mapping code has been used to model the helium line variations after our Doppler imaging of certain individual helium lines produced mediocre results. Results. Doppler imaging of the helium abundance of a Cen confirms the long-known existence of helium-rich and helium-poor hemispheres on the star and we measure a difference of more than two orders of magnitude in helium abundance from one side of the star to the other. Helium is overabundant by a factor of about 5 over much of the helium-rich hemisphere. Of particular note is our discovery that the helium-poor hemisphere has a very high abundance of 3 He, approximately equal to the 4 He abundance. a Cen is therefore a new member of the small group of helium-3 stars and the first well-established magnetic member of the class. For the three metals investigated here, there are two strong concentrations of abundance near the equator at longitude roughly 135 • consistent with the positive magnetic maximum and two somewhat weaker concentrations of abundance near longitude 315 • on the equator near where the helium concentration is centered and roughly where the negative peak of the magnetic field would be found. Another strong concentration is found near the equator at about longitude 45 • and this is not explainable in terms of any simple symmetry with the helium abundance or the apparent magnetic field main polar locations.

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Section: Introductionmentioning

confidence: 85%

“…The magnetic variations of a Cen measured by Borra et al (1983) ruled out the two-cap or symmetric model of Mihalas (1973) as they found the sinusoidal longitudinal magnetic field curve to be nearly symmetric with extrema of −470 and +430 G.…”

Section: Introductionmentioning

confidence: 97%

Doppler imaging of the helium-variable star a Centauri

Bohlender¹,

Rice²,

Hechler³

2010

A&A

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“…A survey of several such stars some years ago (Borra et al 1983) revealed no fields, with uncertainties of hundreds of G. The large FORS1 survey reported by Hubrig et al (2006b) included two PGa stars observed with much smaller uncertainties. One of these stars, HD 19400, was reported to have a field.…”

Section: Am Hgmn and Pga Starsmentioning

confidence: 99%

Magnetic field measurements and their uncertainties: the FORS1 legacy

Bagnulo

Landstreet

Fossati

et al. 2012

A&A

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147

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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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“…As an estimate of the magnetic field strength we used the rms longitudinal magnetic field strength compiled by Bychkov, Bychkova & Madej (2004). It has been computed from all measurements following the formula of Borra, Landstreet & Thompson (1983): The B l values are presented for each sub-sample separately in Tables 3 and 4 in the second columns and have been preferentially selected from magnetic field measurements carried out using Balmer hydrogen lines. As hydrogen is expected to be homogeneously distributed over the stellar surface, the longitudinal magnetic field measurements should sample the magnetic field fairly uniformly over the observed hemisphere.…”

Section: The Evolution Of the Magnetic Field Strength Across The Mainmentioning

confidence: 99%

Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

Hubrig¹,

North

Schöller

2007

Astron Nachr

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We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0 • in slower rotating stars of mass M > 3 M too, analog to the behaviour of angles β in slowly rotating stars of M < 3 M . The obliquity angle distribution as inferred from the distribution of r-values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle β tends to reach values close to either 90 • or 0 • for M < 3 M . The evolution of the obliquity angle β seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of β with the elapsed time on the main sequence for stars with M > 3 M , no similar trend is found for stars with M < 3 M . However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories.

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The magnetic fields of the helium-weak B stars

Cited by 133 publications

References 0 publications

Doppler imaging of the helium-variable star a Centauri

Doppler imaging of the helium-variable star a Centauri

Magnetic field measurements and their uncertainties: the FORS1 legacy

Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

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