Cartography of the magnetic fields and chemical spots of Ap stars

AbstractWe will introduce a project using Magnetic Doppler Imaging (MDI) to create assumption-free vector magnetic field maps and chemical surface structure maps of chemically peculiar A and B type (or Ap) stars. We are exploiting the latest generation of spectropolarimeters (NARVAL at the Pic du Midi observatory, and ESPaDOnS at the Canada-France-Hawaii telescope), to obtain high-resolution time series of Stokes IQUV spectra of a selection of Ap stars. The spectra have superior signal-to-noise ratio, resolution and wavelength coverage to those used previously. This combined with the ground-breaking inversion techniques introduced by Kochukhov et al. (2002) results in maps which represent the state-of-the-art in the field of stellar cartography. These maps will allow us to better understand the links between the magnetic field and the physical processes leading to the formation of chemical structures in the photosphere and allow us to address questions surrounding the detailed magnetic field geometry of Ap stars.

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High-resolution spectropolarimetry of κ Cet: A proxy for the young Sun

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314001902 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 142-143

Author(s):

J. D. do Nascimento ◽

P. Petit ◽

M. Castro ◽

G. F. Porto de Mello ◽

S. V. Jeffers ◽

...

Keyword(s):

Magnetic Field ◽

High Resolution ◽

Atmospheric Model ◽

Longitudinal Component ◽

Least Square ◽

Doppler Imaging ◽

Inversion Method ◽

Radial Magnetic Field ◽

Field Geometry ◽

The Magnetic Field

Abstractκ1 Cet (HD 20630, HIP 15457, d = 9.16 pc, V = 4.84) is a dwarf star approximately 30 light-years away in the equatorial constellation of Cetus. Among the solar proxies studied in the Sun in Time, κ1 Cet stands out as potentially having a mass very close to solar and a young age. On this study, we monitored the magnetic field and the chromospheric activity from the Ca II H & K lines of κ1 Cet. We used the technique of Least-Square-Deconvolution (LSD, Donati et al. 1997) by simultaneously extracting the information contained in all 8,000 photospheric lines of the echelogram (for a linelist matching an atmospheric model of spectral type K1). To reconstruct a reliable magnetic map and characterize the surface differential rotation of κ1 Cet we used 14 exposures spread over 2 months, in order to cover at least two rotational cycles (Prot ~9.2 days). The Least Square deconvolution (LSD) technique was applied to detect the Zeeman signature of the magnetic field in each of our 14 observations and to measure its longitudinal component. In order to reconstruct the magnetic field geometry of κ1 Cet, we applied the Zeeman Doppler Imaging (ZDI) inversion method. ZDI revealed a structure in the radial magnetic field consisting of a polar magnetic spot. On this study, we present the fisrt look results of a high-resolution spectropolarimetric campaign to characterize the activity and the magnetic fields of this young solar proxy.

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Magnetic field topology and chemical spot distributions of the Ap star HD 119419

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731914 ◽

2018 ◽

Vol 609 ◽

pp. A88 ◽

Cited By ~ 4

Author(s):

N. Rusomarov ◽

O. Kochukhov ◽

A. Lundin

Keyword(s):

Magnetic Field ◽

Open Cluster ◽

Doppler Imaging ◽

Clear Correlation ◽

Stokes Vector ◽

Chemical Abundance ◽

Surface Field ◽

Local Strength ◽

Field Geometry ◽

The Magnetic Field

Context. Analysis of high-resolution spectropolarimetric time-series observations of early-type magnetic stars is currently the most advanced method of obtaining detailed information on their surface magnetic field topologies and horizontal spot distributions. Aims. In this study we analyse a new set of high-quality full Stokes vector observations of the magnetic Ap star HD 119419 – a member of the 14 Myr old Lower Cen-Cru association – for the purpose of studying the surface field topology and mapping the chemical abundance spots. Methods. We made use of the circular and linear polarisation data collected for HD 119419 with the HARPSpol instrument at the ESO 3.6-m telescope. These observations were analysed with a multi-line magnetic diagnostic technique and modelled in detail with a Magnetic Doppler imaging (MDI) code. Results. We present a new set of high-precision mean longitudinal magnetic field measurements and derive a revised stellar rotational period by comparing our measurements with the literature data. We also redetermine the basic stellar atmospheric parameters. Our four Stokes parameter magnetic inversions reveal a moderately complex surface field topology with a mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal dipolar component dominates the magnetic energy spectrum of the surface field in HD 119419. However, significant contributions of the higher-order spherical harmonic components are also present. We show that the dipole plus quadrupole part of the reconstructed field geometry is incapable of reproducing the observed amplitudes and shapes of the Stokes Q and U profiles. The chemical abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with the magnetic field geometry, are characterised by large abundance gradients and a lack of clear correlation with the magnetic field structure. Conclusions. This full Stokes vector analysis of HD 119419 extends the modern hot-star magnetic mapping investigations to an open cluster Ap star with a well-determined age. Further, MDI studies of cluster members will allow us to study the field topologies and chemical abundance spots as a function of stellar age.

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Modeling surface magnetic fields in stars with radiative envelopes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002312 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 290-299

Author(s):

Oleg Kochukhov

Keyword(s):

Magnetic Field ◽

High Resolution ◽

Magnetic Fields ◽

Stellar Atmospheres ◽

Spectral Lines ◽

Doppler Imaging ◽

Magnetic Field Mapping ◽

Ap Stars ◽

Surface Fields ◽

The Magnetic Field

AbstractStars with radiative envelopes, specifically the upper main sequence chemically peculiar (Ap) stars, were among the first objects outside our solar system for which surface magnetic fields have been detected. Currently magnetic Ap stars remains the only class of stars for which high-resolution measurements of both linear and circular polarization in individual spectral lines are feasible. Consequently, these stars provide unique opportunities to study the physics of polarized radiative transfer in stellar atmospheres, to analyze in detail stellar magnetic field topologies and their relation to starspots, and to test different methodologies of stellar magnetic field mapping. Here I present an overview of different approaches to modeling the surface fields in magnetic A- and B-type stars. In particular, I summarize the ongoing efforts to interpret high-resolution full Stokes vector spectra of these stars using magnetic Doppler imaging. These studies reveal an unexpected complexity of the magnetic field geometries in some Ap stars.

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Virtual and spurious surface structure on Ap stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900083108 ◽

1996 ◽

Vol 176 ◽

pp. 61-68 ◽

Cited By ~ 2

Author(s):

M.J. Stift

Keyword(s):

Magnetic Field ◽

Surface Structure ◽

Surface Composition ◽

Rotation Axis ◽

Diffusion Theory ◽

Line Strength ◽

Doppler Imaging ◽

Zero Field ◽

Ap Stars ◽

The Magnetic Field

Modelling of magnetic Ap type stars has a long and distinguished history. The Oblique Rotator Model (ORM) – a dipole inside the star, its axis not aligned with the rotation axis – proposed by Babcock (1949a) provides a simple yet flexible enough paradigm for the modelling both of the magnetic and the spectral line variations of these stars. Deutsch (1958) developed a method to derive surface composition distributions from magnetic field measurements in conjunction with line strength variations but subsequent investigators concentrated either on the magnetic field or on the abundance distributions. Hardly ever was the question of consistency between field and composition mapping addressed – Landstreet (1988) constitutes the exception. In abundance mapping, Doppler imaging (Vogt et al. 1987) has meanwhile replaced most other approaches and is credited with fairly reliable results. But can one really carry out such mapping, as done by Hatzes (these proceedings) without accounting for the magnetic field and can these zero-field abundance maps and their relation to the magnetic configuration be compared to the predictions of diffusion theory? Did Landstreet ever have a real chance of disentangling magnetic and abundance effects using intensity (Stokes I) profiles only? What is the probability of obtaining spurious surface structure from intensity Doppler imaging of Ap stars?

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Determination of Luminosity, Atmospheric Structure, and Magnetic Geometry from Studies of the Pulsation in roAp Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100021035 ◽

1993 ◽

Vol 138 ◽

pp. 561-576

Author(s):

D. W. Kurtz ◽

Peter Martinez

Keyword(s):

Magnetic Field ◽

High Speed ◽

Current Knowledge ◽

Dipole Mode ◽

Frequency Spectra ◽

Star Models ◽

High Speed Photometry ◽

Field Geometry ◽

Ap Stars ◽

The Magnetic Field

AbstractFor several of the rapidly oscillating Ap stars the best luminosity estimates available come from the asteroseismological interpretation of their pulsational frequency spectra. We give a list of the 23 currently known roAp stars along with their Strömgren photometric indices, Teff estimated from the Hβ index, and luminosity estimated asteroseismologically. In one case, ϒ Equ, we have an asteroseismological luminosity and a parallax luminosity which are in good agreement. Some of the roAp stars pulsate with frequencies greater than the critical frequency calculated for standard A-star models. This plus multi-colour high-speed photometry of HR 3831 indicate that the temperature gradients in the atmospheres of these stars are substantially steeper than in standard A-star models. We advocate a fine analysis of HR 3831 to see if there is consistency with the pulsational conclusions about T-τ in this star. Further fine analyses and multi-colour pulsational analyses on other roAp stars are then called for. The pulsation mode in HR 3831 can be decomposed into primarily an axisymmetric dipole mode with small radial, quadrupole and octupole perturbations. If the magnetic field is governing the distortion of this mode from a purely dipole mode, then the pulsation can be used to infer the magnetic field geometry. Comments on our current knowledge of all 23 roAp stars are made.

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Magnetic Properties of Rapidly Oscillating Ap Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900238849 ◽

1998 ◽

Vol 185 ◽

pp. 313-314

Author(s):

G. Mathys ◽

S. Hubrig

Keyword(s):

Magnetic Field ◽

Magnetic Properties ◽

Longitudinal Magnetic Field ◽

Rotation Frequency ◽

Pulsation Amplitude ◽

Field Geometry ◽

The Mean ◽

Ap Stars ◽

The Magnetic Field ◽

Maximum Pulsation

Rapidly oscillating Ap stars generally pulsate in multiple modes, characterized by different frequencies. The amplitudes of these modes may furthermore be modulated with the rotation frequency of the star. For the two roAp stars whose magnetic fields have been sufficiently studied, the maximum pulsation amplitude coincides in phase with one of the extrema of the mean longitudinal magnetic field. Two interpretations of this property have been proposed: the oblique pulsator model, according to which the pulsation modes are aligned with the magnetic axis of the star, and the spotted pulsator model, which assumes that the pulsation modes are symmetric about the rotation axis of the star, and that pulsation amplitude modulation is due to the inhomogeneity of the stellar surface (which, itself, is related to the magnetic field geometry). At present, no definite choice between these two models can be made, though the oblique pulsator model is often preferred.

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Strong variable linear polarization in the cool active star II Peg

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002518 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 369-372

Author(s):

Lisa Rosén ◽

Oleg Kochukhov ◽

Gregg A. Wade

Keyword(s):

Magnetic Field ◽

Circular Polarization ◽

Linear Polarization ◽

Transverse Component ◽

Doppler Imaging ◽

Stokes Parameters ◽

Limited Information ◽

Magnetic Imaging ◽

Field Geometry ◽

The Magnetic Field

AbstractMagnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. This provides limited information about the magnetic field geometry since circular polarization is only sensitive to the line-of-sight component of the magnetic field. Reconstructions of the magnetic field topology will therefore not be completely trustworthy when only circular polarization is used. On the other hand, linear polarization is sensitive to the transverse component of the magnetic field. By including linear polarization in the reconstruction the quality of the reconstructed magnetic map is dramatically improved. For that reason, we wanted to identify cool stars for which linear polarization could be detected at a level sufficient for magnetic imaging. Four active RS CVn binaries, II Peg, HR 1099, IM Peg, and σ Gem were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. Mean polarization profiles in all four Stokes parameters were derived using the multi-line technique of least-squares deconvolution (LSD). Not only was linear polarization successfully detected in all four stars in at least one observation, but also, II Peg showed an extraordinarily strong linear polarization signature throughout all observations. This qualifies II Peg as the first promising target for magnetic Doppler imaging in all four Stokes parameters and, at the same time, suggests that other such targets can possibly be identified.

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Broadband Linear Polarization in Ap Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100020662 ◽

1993 ◽

Vol 138 ◽

pp. 305-309

Author(s):

Marco Landolfi ◽

Egidio Landi Degl’Innocenti ◽

Maurizio Landi Degl’Innocenti ◽

Jean-Louis Leroy ◽

Stefano Bagnulo

Keyword(s):

Magnetic Field ◽

Circular Polarization ◽

Linear Polarization ◽

Rotation Axis ◽

Spectral Lines ◽

Line Of Sight ◽

Long Time ◽

Rotating Star ◽

Ap Stars ◽

The Magnetic Field

AbstractBroadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

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Circumstellar environment of 55 Cancri

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937186 ◽

2020 ◽

Vol 633 ◽

pp. A48 ◽

Cited By ~ 6

Author(s):

C. P. Folsom ◽

D. Ó Fionnagáin ◽

L. Fossati ◽

A. A. Vidotto ◽

C. Moutou ◽

...

Keyword(s):

Magnetic Field ◽

Stellar Wind ◽

Large Scale ◽

Rotation Axis ◽

Doppler Imaging ◽

Magnetic Pole ◽

Average Strength ◽

Detailed Understanding ◽

The Magnetic Field ◽

Circumstellar Environment

Context. 55 Cancri hosts five known exoplanets, most notably the hot super-Earth 55 Cnc e, which is one of the hottest known transiting super-Earths. Aims. Because of the short orbital separation and host star brightness, 55 Cnc e provides one of the best opportunities for studying star-planet interactions (SPIs). We aim to understand possible SPIs in this system, which requires a detailed understanding of the stellar magnetic field and wind impinging on the planet. Methods. Using spectropolarimetric observations and Zeeman Doppler Imaging, we derived a map of the large-scale stellar magnetic field. We then simulated the stellar wind starting from the magnetic field map, using a 3D magneto-hydrodynamic model. Results. The map of the large-scale stellar magnetic field we derive has an average strength of 3.4 G. The field has a mostly dipolar geometry; the dipole is tilted by 90° with respect to the rotation axis and the dipolar strength is 5.8 G at the magnetic pole. The wind simulations based on this magnetic geometry lead us to conclude that 55 Cnc e orbits inside the Alfvén surface of the stellar wind, implying that effects from the planet on the wind can propagate back to the stellar surface and result in SPI.

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Magnetic properties of cobalt microwires measured by piezoresistive cantilever magnetometry

Nanofabrication ◽

10.2478/nanofab-2014-0008 ◽

2014 ◽

Vol 1 (1) ◽

Cited By ~ 3

Author(s):

G. Tosolini ◽

J. M. Michalik ◽

R. Córdoba ◽

J. M. de Teresa ◽

F. Pérez-Murano ◽

...

Keyword(s):

Magnetic Field ◽

Room Temperature ◽

Signal To Noise Ratio ◽

Static Deflection ◽

Magnetic Structures ◽

Dual Beam ◽

Piezoresistive Cantilevers ◽

Good Signal ◽

The Magnetic Field

AbstractWe present the magnetic characterization of cobalt wires grown by focused electron beam-induced deposition (FEBID) and studied using static piezoresistive cantilever magnetometry. We have used previously developed high force sensitive submicron-thick silicon piezoresistive cantilevers. High quality polycrystalline cobalt microwires have been grown by FEBID onto the free end of the cantilevers using dual beam equipment. In the presence of an external magnetic field, the magnetic cobalt wires become magnetized, which leads to the magnetic field dependent static deflection of the cantilevers. We show that the piezoresistive signal from the cantilevers, corresponding to a maximum force of about 1 nN, can be measured as a function of the applied magnetic field with a good signal to noise ratio at room temperature. The results highlight the flexibility of the FEBID technique for the growth of magnetic structures on specific substrates, in this case piezoresistive cantilevers.

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