The Extraordinary Complex Magnetic Field of the Helium-Strong Star Hd 37776

Kochukhov, Oleg; Lundin, Andreas; Romanyuk, I. I.; Kudryavtsev, D. O.

doi:10.1088/0004-637x/726/1/24

Cited by 97 publications

(101 citation statements)

References 48 publications

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“…In this paper we study the He i 6678 Å line. Its circular polarisation shows a variability pattern very similar to the He i 5876 Å line analysed by Kochukhov et al (2011) using lower quality data.…”

Section: Observational Datamentioning

confidence: 55%

“…The high-resolution circular polarisation spectra of HD 37776 analysed here enable a straightforward verification of the ZDI results of Kochukhov et al (2011) using the cumulative Stokes V profile diagnostic method. Figure 9 shows the Stokes V and CSV spectra centred on the He i 6678 Å line.…”

Section: He-strong Bp Star Hd 37776mentioning

confidence: 98%

“…Indeed, Bohlender (1994) and Khokhlova et al (2000) derived quadrupolar magnetic geometry models for HD 37776 based on fitting the longitudinal field curve and Stokes V profile variation, respectively. However, using a more extensive set of phase-resolved spectropolarimetric observations, Kochukhov et al (2011) showed that the circular polarisation spectra of HD 37776 cannot be fitted with an axisymmetric quadrupolar field structure. Instead, the ZDI inversion carried out in that study suggested a complex field distribution comprising a series of magnetic spots with alternating positive and negative polarity.…”

Section: He-strong Bp Star Hd 37776mentioning

confidence: 99%

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Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

Kochukhov

2015

A&A

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Information about stellar magnetic field topologies is obtained primarily from high-resolution circular polarisation (Stokes V) observations. Because of their generally complex morphologies, the stellar Stokes V profiles are usually interpreted with elaborate inversion techniques such as Zeeman Doppler imaging (ZDI). Here we further develop a new method for interpreting circular polarisation signatures in spectral lines using cumulative Stokes V profiles (anti-derivative of Stokes V). This method is complimentary to ZDI and can be applied to validate the inversion results or when the available observational data are insufficient for an inversion. Based on the rigorous treatment of polarised line formation in the weak-field regime, we show that, for rapidly rotating stars, the cumulative Stokes V profiles contain information about the spatially resolved longitudinal magnetic field density. Rotational modulation of these profiles can be employed for a simple, qualitative characterisation of the stellar magnetic field topologies. We apply this diagnostic method to the archival observations of the weak-line T Tauri star V410 Tau and Bp He-strong star HD 37776. We show that the magnetic field in V410 Tau is dominated by an azimuthal component, in agreement with the ZDI map that we recover from the same data set. For HD 37776 the cumulative Stokes V profile variation indicates the presence of multiple regions of positive and negative field polarity. This behaviour agrees with the ZDI results, but contradicts the popular hypothesis that the magnetic field of this star is dominated by an axisymmetric quadrupolar component.

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Section: Observational Datamentioning

confidence: 55%

Section: He-strong Bp Star Hd 37776mentioning

confidence: 98%

Section: He-strong Bp Star Hd 37776mentioning

confidence: 99%

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Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

Kochukhov

2015

A&A

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“…Compared to the sort of mapping done by Kochukhov et al (2004Kochukhov et al ( , 2011, who map field vector and element distributions over the surface with ∼20 • spatial resolution in both surface coordinates, our models, in general, represent only a very rough approximation. However, the data set required for detailed mapping is far more extensive than the few, irregularly distributed, mostly I spectra, obtained with a variety of resolving powers, and covering a large range in SNR, that are available at present for HD 318107.…”

Section: Modelling Techniquementioning

confidence: 99%

Magnetic field and atmospheric chemical abundances of the magnetic Ap star HD 318107

Bailey

Landstreet

Bagnulo³

et al. 2011

A&A

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Context.A new generation of powerful and efficient spectropolarimeters has recently been used to provide the first sample of magnetic Ap stars of accurately known ages. Modelling of these data offer the possibility of significant new insights into the physics and main sequence evolution of these remarkable stars. Aims. New spectra have been obtained with the ESPaDOnS spectropolarimeter, and are supplemented with unpolarised spectra from the ESO UVES, UVES-FLAMES, and HARPS spectrographs, of the very peculiar large-field magnetic Ap star HD 318107, a member of the open cluster NGC 6405 and thus a star with a well-determined age. The available data provide sufficient material with which to re-analyse the first-order model of the magnetic field geometry and to derive chemical abundances of Si, Ti, Fe, Nd, Pr, Mg, Cr, Mn, O, and Ca. Methods. The models were obtained using ZEEMAN, a program which synthesises spectral line profiles for stars that have magnetic fields. The magnetic field structure was modelled with a low-order colinear multipole expansion, using coefficients derived from the observed variations of the field strength with rotation phase. The abundances of several elements were determined using spectral synthesis. After experiments with a very simple model of uniform abundance on each of three rings of equal width in co-latitude and symmetric about the assumed magnetic axis, we decided to model the spectra assuming uniform abundances of each element over the stellar surface. Results. The new magnetic field measurements allow us to refine the rotation period of HD 318107 to P = 9.7088 ± 0.0007 days. Appropriate magnetic field model parameters were found that very coarsely describe the (apparently rather complex) field moment variations. Spectrum synthesis leads to the derivation of mean abundances for the elements Mg, Si, Ca, Ti, Cr, Fe, Nd, and Pr. All of these elements except for Mg and Ca are strongly overabundant compared to the solar abundance ratios. There is considerable evidence of non-uniformity, for example in the different values of B z found using lines of different elements. Conclusions. The present data set, while limited, is nevertheless sufficient to provide a useful first-order assessment of both the magnetic and surface abundance properties of HD 318107, making it one of the very few magnetic Ap stars of well-known age for which both of these properties have been studied.

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“…But even these studies frequently require deviations from the canonical oblique dipolar topology to reproduce observations. In A&A 574, A79 (2015) some cases, only a moderate distortion of a dipole is advocated , while in other situations (so far limited to early-B magnetic stars) a complex non-dipolar field is clearly evident from observations (Donati et al 2006;Kochukhov et al 2011). To summarise, the current paucity of detailed polarisation line profile studies of Ap stars does not allow one to draw any firm conclusions about the systematic variation of the surface field morphology with the field strength or with key stellar parameters, such as mass and age.…”

Section: Introductionmentioning

confidence: 99%

Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049

Kochukhov

Rusomarov

Valenti

et al. 2015

A&A

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Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Aims. Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of ∼30 kG, one of the strongest known for any non-degenerate star. Methods. We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field. Results. Based on high-precision measurements of the mean magnetic field modulus, we refined the rotational period of HD 75049 to P rot = 4.048267 ± 0.000036 d. We also derived basic stellar parameters, T eff = 10 250 ± 250 K and log g = 4.3 ± 0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Of the chemical elements, Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole. Conclusions. The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediatemass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths.

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The Extraordinary Complex Magnetic Field of the Helium-Strong Star Hd 37776

Cited by 97 publications

References 48 publications

Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles

Magnetic field and atmospheric chemical abundances of the magnetic Ap star HD 318107

Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049

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