Transfer of line radiation in a magnetic field

Jefferies, J. T.; Lites, B. W.; Skumanich, A.

doi:10.1086/167762

Cited by 129 publications

(83 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The LSD procedure is adequate in the weak-field approximation, if one only uses lines that have the same shape as one another, and assumes wavelength-independent limb darkening. The weak-field approximation assumes that the Zeeman splitting is a fraction of the Doppler width (Jefferies et al 1989), which is indeed the case for HD 96446. The mask was first computed using a Kurucz ATLAS 9 atmosphere model with T eff = 22 000 K and log g = 4.0.…”

Section: Observations and Methodsmentioning

confidence: 96%

HD 96446: a puzzle for current models of magnetospheres?

Neiner

Landstreet

Alécian

et al. 2012

A&A

View full text Add to dashboard Cite

Context. Oblique magnetic dipole fields have been detected in Bp stars for several decades, and more recently also in normal massive stars. In the past decade, it has been established that stellar magnetospheres form through the channelling and confinement of an outflowing stellar wind by the stellar magnetic field. This explains specific properties of magnetic massive stars, such as their rotationally modulated photometric light curve, Hα emission, UV spectra, and X-ray emission. Aims. In the framework of the MiMeS (Magnetism in Massive Stars) project, four HARPSpol observations of the magnetic Bp star HD 96446 have been obtained. HD 96446 is very similar to σ Ori E, the prototype of centrifugally supported rigidly rotating magnetospheres (CM) and is therefore a perfect target to study the validity of this model. Methods. We first updated the basic parameters of HD 96446 and studied its spectral variability. We then analysed the HARPSpol spectropolarimetric observations using the LSD (Least-Squares Deconvolution) technique to derive the longitudinal magnetic field and Zeeman signatures in various types of lines. With LTE spectrum modelling, we derived constraints on the field modulus, the rotational velocity, and the inclination angle, and measured non-solar abundances of several elements which we checked with NLTE modelling. Finally, we calculated the magnetic confinement and Alfvén and Kepler radii from the stellar magnetic field and rotation properties, and we examined the various types of magnetospheres that may be present around HD 96446. Results. We find radial velocity variations with a period around 2.23 h, that we attribute to β Cep-type p-mode pulsations. We detect clear direct magnetic Stokes V signatures with slightly varying values of the longitudinal magnetic field, typical of an oblique dipole rotator, and show that these signatures are not much perturbed by the radial velocity variations. The magnetic confinement parameter and Alfvén radius in the centrifugally supported, rigidly-rotating magnetosphere (CM) model points towards the presence of confined material in the magnetosphere. However, HD 96446 does not present signatures of the presence of such confined material, such as Hα emission. Conclusions. We conclude that, even though HD 96446 fulfills all criteria to host a CM with confined material, it does not. The rotation period must be significantly revised, or another model of magnetosphere with a leakage mechanism will need to be developed to explain the magnetic environment of this star.

show abstract

Section: Observations and Methodsmentioning

confidence: 96%

HD 96446: a puzzle for current models of magnetospheres?

Neiner

Landstreet

Alécian

et al. 2012

A&A

View full text Add to dashboard Cite

show abstract

“…Then, a proportionality exists between the observed local Stokes V profile and the spectral derivative of the observed local Stokes I profile, which can be derived from the polarized transfer equation using a first-order Taylor expansion of the line profile function (e.g., Jefferies et al 1989). This proportionality or linear relation can be written as…”

Section: Disk-integrated Weak-field Approximationmentioning

confidence: 99%

Detecting and quantifying stellar magnetic fields

Carroll¹,

Strassmeier²

2014

A&A

View full text Add to dashboard Cite

Context. In recent years, we have seen a rapidly growing number of stellar magnetic field detections for various types of stars. Many of these magnetic fields are estimated from spectropolarimetric observations (Stokes V) by using the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of this method rapidly deteriorates with increasing noise and thus calls for a more robust procedure that combines signal detection and field estimation. Aims. We introduce an estimation method that provides not only the effective or mean longitudinal magnetic field from an observed Stokes V profile but also uses the net absolute polarization of the profile to obtain an estimate of the apparent (i.e., velocity resolved) absolute longitudinal magnetic field. Methods. By combining the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able to decompose observed Stokes profiles with an overcomplete dictionary of wavelet-basis functions to reliably reconstruct the observed Stokes profiles in the presence of noise. The elementary wave functions of the sparse reconstruction process were utilized to estimate the effective longitudinal magnetic field and the apparent absolute longitudinal magnetic field. A multiresolution analysis complements the OMP algorithm to provide a robust detection and estimation method.Results. An extensive Monte-Carlo simulation confirms the reliability and accuracy of the magnetic OMP approach where a mean error of under 2% is found. Its full potential is obtained for heavily noise-corrupted Stokes profiles with signal-to-noise variance ratios down to unity. In this case a conventional COG method yields a mean error for the effective longitudinal magnetic field of up to 50%, whereas the OMP method gives a maximum error of 18%. It is, moreover, shown that even in the case of very small residual noise on a level between 10 −3 and 10 −5 , a regime reached by current multiline reconstruction techniques, the conventional COG method incorrectly interprets a large portion of the residual noise as a magnetic field, with values of up to 100 G. The magnetic OMP method, on the other hand, remains largely unaffected by the noise, regardless of the noise level the maximum error is no greater than 0.7 G.

show abstract

“…To a first order approximation, the circularly polarized Stokes V signal should scale with Φ L , whereas the linear polarization L = Q 2 + U 2 should scale with Φ 2 T (Jefferies et al 1989). From the observed profiles, we derived the maximal linear and circular polarization of the 1564.8 nm line (L max and V max , see Sect.…”

Section: Reliability Of Proxies In Magnetic Flux Determinationmentioning

confidence: 99%

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Beck¹,

Rezaei²

2009

A&A

View full text Add to dashboard Cite

Context. Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun. Aims. We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 μm, where the sensitivity to magnetic fields is higher than in the visible. Methods. We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10 −4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux. Results. We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (γ > 45 • ) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable. Conclusions. During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating.

show abstract

Transfer of line radiation in a magnetic field

Cited by 129 publications

References 0 publications

HD 96446: a puzzle for current models of magnetospheres?

HD 96446: a puzzle for current models of magnetospheres?

Detecting and quantifying stellar magnetic fields

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Contact Info

Product

Resources

About