A statistical analysis of the magnetic structure of CP stars

Bagnulo, S.; Degl'Innocenti, Maurizio; Landolfi, M.; Mathys, G.

doi:10.1051/0004-6361:20021221

Cited by 116 publications

(217 citation statements)

References 20 publications

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“…From these data a surface magnetic field with dipole structure was derived, with the local magnetic field strength varying between 2.1 kG and 4.2 kG. These results agree well with the dipolar model derived by Bagnulo et al (1995) from the phase curves of the longitudinal field and broad-band linear polarization. Abundance distributions of Fe, Nd, and sixteen additional ions were also determined by Lüftinger et al (2010).…”

Section: Hd 24712supporting

confidence: 79%

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Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Rusomarov

Kochukhov

Piskunov

et al. 2013

A&A

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Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter. Aims. The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. Methods. HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300-600 and resolving power exceeding 10 5 . The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements. Results. We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, B z , with an accuracy of 5-10 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous B z measurements with our data allowed us to determine an improved rotational period of the star, P rot = 12.45812 ± 0.00019 d. We also measured the longitudinal magnetic field from the cores of Hα and Hβ lines. The analysis of B z measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our B z and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the Hα core and related it to a non-uniform surface distribution of rare earth elements.

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Section: Hd 24712supporting

confidence: 79%

“…Using magnetic field measurements inferred from the LSD profiles of HD 24712, we derived the canonical model parameters in the same way as done by Bagnulo et al (1995). An χ 2 function was constructed that connects the observed and theoretical values of the longitudinal field and net linear polarization.…”

Section: Global Magnetic Field Parametersmentioning

confidence: 99%

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Rusomarov

Kochukhov

Piskunov

et al. 2013

A&A

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“…(5)), which directs the solution toward a general, non-axisymmetric second-order multipolar expansion of the magnetic field structure. The multipolar model field B mult , calculated by our code at each iteration, is equivalent to the dipole plus quadrupole parameterization of Bagnulo et al (1996). Stokes IV time series of the Ap stars α 2 CVn , HD 24712 (Lüftinger et al 2010) and HD 72106 (Folsom et al 2008) were interpreted using Invers10 in the multipolar regularization mode.…”

Section: Magnetic Inversion Techniquementioning

confidence: 99%

Magnetic Doppler imaging ofα²Canum Venaticorum in all four Stokes parameters

Kochukhov¹,

Wade

2010

A&A

114

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Context. Strong organized magnetic fields have been studied in the upper main sequence chemically peculiar stars for more than half a century. However, only recently have observational methods and numerical techniques become sufficiently mature to allow us to record and interpret high-resolution four Stokes parameter spectra, leading to the first assumption-free magnetic field models of these stars. Aims. Here we present a detailed magnetic Doppler imaging analysis of the spectropolarimetric observations of the prototypical magnetic Ap star α 2 CVn. This is the second star for which the magnetic field topology and horizontal chemical abundance inhomogeneities have been inferred directly from phase-resolved observations of line profiles in all four Stokes parameters, free from the traditional assumption of a low-order multipolar field geometry.Methods. We interpret the rotational modulation of the circular and linear polarization profiles of the strong Fe ii and Cr ii lines in the spectra of α 2 CVn recorded with the MuSiCoS spectropolarimeter. The surface abundance distributions of the two chemical elements and a full vector map of the stellar magnetic field are reconstructed in a self-consistent inversion using our state-of-the-art magnetic Doppler imaging code Invers10. Results. We succeeded in reproducing most of the details of the available spectropolarimetric observations of α 2 CVn with a magnetic map which combines a global dipolar-like field topology with localized spots of higher field intensity. We demonstrate that these small-scale magnetic structures are inevitably required to fit the linear polarization spectra; however, their presence cannot be inferred from the Stokes I and V observations alone. We also found high-contrast surface distributions of Fe and Cr, with both elements showing abundance minima in the region of weaker and topologically simpler magnetic field. Conclusions. Our magnetic Doppler imaging analysis of α 2 CVn and previous results for 53 Cam support the view that the upper main sequence stars can harbour fairly complex surface magnetic fields which resemble oblique dipoles only at the largest spatial scales. Spectra in all four Stokes parameters are absolutely essential to unveil and meaningfully characterize this field complexity in Ap stars. We therefore suggest that understanding magnetism of stars in other parts of the H-R diagram is similarly incomplete without investigation of their linear polarization spectra.

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“…For HD 81009 we used r = 0.64, determined by Landstreet & Mathys (2000). The v sin i values have been measured on our spectra or taken from published spectroscopic studies (Abt 2001;Abt, Levato & Grosso 2002;Bagnulo et al 2002;Balona & Laney 2003;Bohlender, Landstreet & Thompson 1993;Hensberge et al 1979;Landstreet 1970;Landstreet & Mathys 2000;Levato et al 1996;Nielsen & Wahlgren 2002;Royer et al 2002;Wade et al 2000). We could not find in the literature values of v sin i for three stars: HD 54118, HD 73340 and HD 133880.…”

Section: The Magnetic Field Geometrymentioning

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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A statistical analysis of the magnetic structure of CP stars

Cited by 116 publications

References 20 publications

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Magnetic Doppler imaging ofα²Canum Venaticorum in all four Stokes parameters

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

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A statistical analysis of the magnetic structure of CP stars

Cited by 116 publications

References 20 publications

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Magnetic Doppler imaging ofα2Canum Venaticorum in all four Stokes parameters

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

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Product

Resources

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Magnetic Doppler imaging ofα²Canum Venaticorum in all four Stokes parameters