The search for magnetic fields in mercury-manganese stars

AbstractMercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.

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Magnetic main sequence stars as progenitors of blue supergiants

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314007224 ◽

2014 ◽

Vol 9 (S307) ◽

pp. 391-392

Author(s):

I. Petermann ◽

N. Castro ◽

N. Langer

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Main Sequence ◽

Mass Range ◽

Main Sequence Stars ◽

Core Mass ◽

Hydrogen Burning ◽

Sequence Band ◽

Convective Core ◽

The Right

AbstractBlue supergiants (BSGs) to the right the main sequence band in the HR diagram can not be reproduced by standard stellar evolution calculations. We investigate whether a reduced convective core mass due to strong internal magnetic fields during the main sequence might be able to recover this population of stars. We perform calculations with a reduced mass of the hydrogen burning convective core of stars in the mass range 3–30 M⊙ in a parametric way, which indeed lead to BSGs. It is expected that these BSGs would still show large scale magnetic fields in the order of 10 G.

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Impacts of radiative accelerations on solar-like oscillating main-sequence stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833361 ◽

2018 ◽

Vol 618 ◽

pp. A10 ◽

Cited By ~ 14

Author(s):

M. Deal ◽

G. Alecian ◽

Y. Lebreton ◽

M. J. Goupil ◽

J. P. Marques ◽

...

Keyword(s):

Seismic Data ◽

Main Sequence ◽

Chemical Elements ◽

Mass Range ◽

Companion Paper ◽

Transport Processes ◽

Atomic Diffusion ◽

Parameter Method ◽

Main Sequence Stars ◽

Gravitational Settling

Context. Chemical element transport processes are among the crucial physical processes needed for precise stellar modelling. Atomic diffusion by gravitational settling is usually taken into account, and is essential for helioseismic studies. On the other hand, radiative accelerations are rarely accounted for, act differently on the various chemical elements, and can strongly counteract gravity in some stellar mass domains. The resulting variations in the abundance profiles may significantly affect the structure of the star. Aims. The aim of this study is to determine whether radiative accelerations impact the structure of solar-like oscillating main-sequence stars observed by asteroseismic space missions. Methods. We implemented the calculation of radiative accelerations operating on C, N, O, Ne, Na, Mg, Al, Si, S, Ca, and Fe in the CESTAM code using the single-valued parameter method. We built and compared several grids of stellar models including gravitational settling, some with and others without radiative accelerations. We considered masses in the range [0.9, 1.5] M⊙ and three values of the metallicity around the solar value. For each metallicity we determined the mass range where differences between models due to radiative accelerations exceed the uncertainties of global seismic parameters of the Kepler Legacy sample or expected for PLATO observations. Results. We found that radiative accelerations may not be neglected for stellar masses higher than 1.1 M⊙ at solar metallicity. The difference in age due to their inclusion in models can reach 9% for the more massive stars of our grids. We estimated that the percentage of the PLATO core program stars whose modelling would require radiative accelerations ranges between 33% and 58% depending on the precision of the seismic data. Conclusions. We conclude that in the context of Kepler, TESS, and PLATO missions which provide (or will provide) high-quality seismic data, radiative accelerations can have a significant effect when properly inferring the properties of solar-like oscillators. This is particularly important for age inferences. However, the net effect for each individual star results from the competition between atomic diffusion including radiative accelerations and other internal transport processes. Rotationally induced transport processes for instance are believed to reduce the effects of atomic diffusion. This will be investigated in a forthcoming companion paper.

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The spots on Ap stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311015328 ◽

2010 ◽

Vol 6 (S273) ◽

pp. 249-255 ◽

Cited By ~ 4

Author(s):

Oleg Kochukhov

Keyword(s):

Magnetic Fields ◽

Chemical Elements ◽

Stellar Atmospheres ◽

Atomic Diffusion ◽

Magnetic Field Mapping ◽

Current State ◽

Spotted Stars ◽

New Type ◽

Formation And Evolution ◽

Ap Stars

AbstractThe upper main sequence magnetic chemically peculiar (Ap) stars exhibit a non-uniform distribution of chemical elements across their surfaces and with height in their atmospheres. These inhomogeneities, responsible for the conspicuous photometric and spectroscopic variation of Ap stars, are believed to be produced by atomic diffusion operating in the stellar atmospheres stabilized by multi-kG magnetic fields. Here I present an overview of the current state-of-the-art in understanding Ap-star spots and their relation to magnetic fields. In particular, I highlight recent 3-D chemical spot structure studies and summarize magnetic field mapping results based on the inversion of the full Stokes vector spectropolarimetric observations. I also discuss a puzzling new type of spotted stars, HgMn stars, in which the formation and evolution of heavy element spots is driven by a poorly understood mechanism, apparently unrelated to magnetic fields.

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Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900035191 ◽

1988 ◽

Vol 132 ◽

pp. 309-312

Author(s):

David A. Bohlender ◽

J. D. Landstreet

Keyword(s):

Magnetic Field ◽

High Temperature ◽

Magnetic Fields ◽

Main Sequence ◽

Line Profiles ◽

Main Sequence Stars ◽

Hot Stars ◽

Surface Magnetic Field ◽

Peculiar Stars ◽

Ap Stars

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

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Magnetic Star Models with Toroidal Component of Field

International Astronomical Union Colloquium ◽

10.1017/s0252921100080398 ◽

1976 ◽

Vol 32 ◽

pp. 25-27

Author(s):

D.L. Moss

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Field Component ◽

Main Sequence ◽

Toroidal Field ◽

Magnetic Star ◽

Main Sequence Stars ◽

Star Models ◽

Rotation Axes ◽

Poloidal Magnetic Fields

A variety of models of main sequence stars of a few solar masses containing large scale magnetic fields have been published in the last few years. Most of them have contained purely poloidal magnetic fields (with magnetic and rotation axes aligned). Those containing a toroidal field component were either polytropic models, or, if more realistic, the toroidal field was not well behaved at infinity.

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Magnetic Fields and Stellar Evolution

Symposium - International Astronomical Union ◽

10.1017/s0074180900074003 ◽

1981 ◽

Vol 93 ◽

pp. 257-272 ◽

Cited By ~ 1

Author(s):

L. Mestel

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Main Sequence ◽

Late Type ◽

Main Sequence Stars ◽

Stellar Objects ◽

Cool Stars ◽

Solar Type ◽

Early Type Stars ◽

M Stars

Magnetic fields are now observed or inferred in a wide variety of stellar objects. The class of early-type stars with strong large-scale fields extends from types B to F, with effective fields from 300 gauss up to several x 104 gauss (Borra and Landstreet 1980). Fields between 4 × 106 and 108 gauss have been inferred in a small percentage of white dwarfs, and of over 1012 gauss in neutron stars. Some Cepheids show measurable fields. Evidence has built up of solar-type activity in late-type stars. The pioneering work by Wilson (1978) on Ca activity has shown convincingly the occurrence of periodicity reminiscent of the solar cycle in a number of G, K and M stars. Ca II emission appears to be a good predictor of simultaneous X-ray emission from hot coronae around cool stars (Vaiana 1979, Mewe and Zwaan 1980). Fields of some 2 × 103 gauss have been reported in two late-type main sequence stars (Robinson, Worden and Harvey 1980).

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Spectroscopy and Geography of Magnetic Ap Stars: Implications for Structure

Symposium - International Astronomical Union ◽

10.1017/s0074180900227344 ◽

1991 ◽

Vol 145 ◽

pp. 161-171

Author(s):

J. D. Landstreet

Keyword(s):

Large Scale ◽

Main Sequence ◽

Meridional Circulation ◽

Field Structure ◽

Surface Chemical ◽

Main Sequence Stars ◽

Chemical Abundances ◽

B Stars ◽

Uniform Distributions ◽

Ap Stars

Some main sequence A and B stars have strong, ordered magnetic fields. These Ap and Bp stars usually have anomalous chemical abundances, and often rather non-uniform distributions of at least some elements (e.g. He, Ca, Ti, Cr) over their surfaces. Maps of magnetic field structure may provide a means of testing theories of how large coherent fields form, and of probing large-scale hydrodynamic flows (meridional circulation) inside stars. Maps of distributions of various elements can help to elucidate the mechanisms, such as diffusion under competing influences of gravity and radiation, turbulence, meridional circulation, mass loss, and perhaps accretion from the interstellar medium, that lead to the distinctive abundances and surface abundance distributions. This kind of mapping is important as an aid to understanding how the Ap and Bp stars develop. It is even more important because the processes involved in producing Ap and Bp stars probably have significant effects on surface chemical abundances of “normal” upper main sequence stars, and so understanding the relevant physics is essential to correctly relating observed surface chemistry to stellar and galactic evolution. In this paper, efforts to map field and abundance structures in Ap stars are reviewed, and some of the principal results obtained thus far are discussed.

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The dichotomy between strong and ultra-weak magnetic fields among intermediate-mass stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002440 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 338-347 ◽

Cited By ~ 5

Author(s):

François Lignières ◽

Pascal Petit ◽

Michel Aurière ◽

Gregg A. Wade ◽

Torsten Böhm

Keyword(s):

Magnetic Fields ◽

Stellar Evolution ◽

Large Scale ◽

Field Measurements ◽

Mass Range ◽

Mass Star ◽

Rotating Stars ◽

Intermediate Mass ◽

Weak Magnetic Fields ◽

Chemically Peculiar Stars

AbstractUntil recently, the detection of magnetic fields at the surface of intermediate-mass main-sequence stars has been limited to Ap/Bp stars, a class of chemically peculiar stars. This class represents no more than 5-10% of the stars in this mass range. This small fraction is not explained by the fossil field paradigm that describes the Ap/Bp type magnetism as a remnant of an early phase of the star-life. Also, the limitation of the field measurements to a small and special group of stars is obviously a problem to study the effect of the magnetic fields on the stellar evolution of a typical intermediate-mass star.Thanks to the improved sensitivity of a new generation of spectropolarimeters, a lower bound to the magnetic fields of Ap/Bp stars, a two orders of magnitude desert in the longitudinal magnetic field and a new type of sub-gauss magnetism first discovered on Vega have been identified. These advances provide new clues to understand the origin of intermediate-mass magnetism as well as its influence on stellar evolution. In particular, a scenario has been proposed whereby the magnetic dichotomy between Ap/Bp and Vega-like magnetism originate from the bifurcation between stable and unstable large scale magnetic configurations in differentially rotating stars. In this paper, we review these recent observational findings and discuss this scenario.

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The Boundary between the Magnetic Fields of Ap Stars and the Fields of Solar-type Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100079884 ◽

1991 ◽

Vol 130 ◽

pp. 342-346

Author(s):

John D. Landstreet

Keyword(s):

Magnetic Fields ◽

Main Sequence ◽

Convection Zone ◽

Frequency Of Occurrence ◽

Main Sequence Stars ◽

Chemically Peculiar ◽

Solar Type ◽

F Stars ◽

Ap Stars

AbstractThe boundary between Ap-type magnetic fields and the magnetic fields of solar-type stars occurs near Te ~ 7000K, about where deep envelope convection develops in main sequence stars. This seems natural for solar-type stars, in which the field is generated by the convection zone. However, among magnetic Ap stars the frequency of occurrence declines from about 10% of all A stars near A0 to about 1% near F0. It is not clear what produces this decline in frequency, but the convection zone is probably not responsible. In fact, it seems likely that if global fossil fields occur in main sequence F stars, such fields should be detectable even if the stars having them are not chemically peculiar.

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The role of magnetic fields in pre-main sequence stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314001653 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 25-37 ◽

Cited By ~ 6

Author(s):

Gaitee A. J. Hussain ◽

Evelyne Alecian

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Main Sequence ◽

Circumstellar Disks ◽

T Tauri Stars ◽

Main Sequence Stars ◽

T Tauri ◽

First Results ◽

Pms Stars

AbstractStrong, kilo-Gauss, magnetic fields are required to explain a range of observational properties in young, accreting pre-main sequence (PMS) systems. We review the techniques used to detect magnetic fields in PMS stars. Key results from a long running campaign aimed at characterising the large scale magnetic fields in accreting T Tauri stars are presented. Maps of surface magnetic flux in these systems can be used to build 3-D models exploring the role of magnetic fields and the efficiency with which magnetic fields can channel accretion from circumstellar disks on to young stars. Long-term variability in T Tauri star magnetic fields strongly point to a dynamo origin of the magnetic fields. Studies are underway to quantify how changes in magnetic fields affect their accretion properties. We also present the first results from a new programme that investigates the evolution of magnetic fields in intermediate mass (1.5–3M⊙) pre-main sequence stars as they evolve from being convective T Tauri stars to fully radiative Herbig AeBe stars.

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