scholarly journals Broadband Linear Polarization in Ap Stars

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.

scholarly journals Magnetic fields of rapidly oscillating Ap stars

1993 ◽  
Vol 139 ◽  
pp. 132-132
Author(s):  
G. Mathys
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Zeeman Effect ◽  
Rotation Frequency ◽  
Spectral Lines ◽  
Driving Mechanism ◽  
Line Splitting ◽  
Ap Stars ◽  
The Magnetic Field

Magnetic field appears to play a major role in the pulsations of rapidly oscillating Ap (roAp) stars. Understanding of the behaviour of these objects thus requires knowledge of their magnetic field. Such knowledge is in particular essential to interpret the modulation of the amplitude of the photometric variations (with a frequency very close to the rotation frequency of the star) and to understand the driving mechanism of the pulsation. Therefore, a systematic programme of study of the magnetic field of roAp stars has been started, of which preliminary (and still very partial) results are presented here.Magnetic fields of Ap stars can be diagnosed from the Zeeman effect that they induced in spectral lines either from the observation of line-splitting in high-resolution unpolarized spectra (which only occurs in favourable circumstances) or from the observation of circular polarization of the lines in medium- to high-resolution spectra.

scholarly journals Some Properties of Magnetic Variables

1971 ◽  
Vol 15 ◽  
pp. 59-62 ◽  
Author(s):  
Karl D. Rakoš
Keyword(s):  
Magnetic Field ◽  
Light Curve ◽  
Spectral Lines ◽  
Light Curves ◽  
Magnetic Pole ◽  
Yellow Light ◽  
The Past ◽  
Oblique Rotator ◽  
Ap Stars ◽  
The Magnetic Field

It is certain, that the mechanism causing variations of the magnetic field and spectral lines in Ap stars must also cause variations in their luminosities. The light curves are synchronous with the magnetic variations and usually the maximum of the positive magnetic field strength coincides with the minimum of the light curve. In the past the oblique rotator theory was not able to explain easily such brightness change. There is no simple reason to suppose, that the brightness of the surface of a star would increase or decrease at one magnetic pole only. Since that time a few stars were found with some indications for secondary minima and maxima in the light curves, but the first established double wave in a light curve was recently found by H. M. MAITZEN and K. D. RAKOš in HD 125 248 (1970), see Figure 1. It is a very exciting result, only the light curve in yellow light shows two maxima and two minima. The light curves in blue and ultraviolet are very smooth and show no evidence for secondary waves.

scholarly journals Pulsation of Rotating Magnetic Stars

1993 ◽  
Vol 139 ◽  
pp. 134-134
Author(s):  
H. Shibahashi ◽  
M. Takata
Keyword(s):  
Magnetic Field ◽  
Fine Structure ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Dipole Field ◽  
Dipole Mode ◽  
Magnetic Stars ◽  
Ap Stars ◽  
The Magnetic Field ◽  
Split Frequency

Recently, one of the rapidly oscillating Ap stars, HR 3831, has been found to have an equally split frequency septuplet, though its oscillation seems to be essentially an axisymmetric dipole mode with respect to the magnetic axis which is oblique to the rotation axis (Kurtz et al. 1992; Kurtz 1992). In order to explain this fine structure, we investigate oscillations of obliquely rotating magnetic stars by taking account of the perturbations due to the magnetic fields and the rotation. We suppose that the star is rigidly rotating and that the magnetic field is a dipole field and its axis is oblique to the rotation axis. We treat the effects of the rotation and of the magnetic field as perturbations. In doing so, we suppose that the rotation of the star is slow enough so that the effect of the rotation on oscillations is smaller than that of the magnetic field.

scholarly journals Broadband Measurements of the Transverse Magnetic Field of Cool Ap Stars

1993 ◽  
Vol 138 ◽  
pp. 274-278 ◽  
Author(s):  
J.-L. Leroy ◽  
J.D. Landstreet ◽  
E. Landi degl’Innocenti ◽  
M. Landolfi
Keyword(s):  
Magnetic Field ◽  
Linear Polarization ◽  
Rotation Axis ◽  
Zeeman Effect ◽  
Transverse Magnetic ◽  
Large Field ◽  
Longitudinal Field ◽  
Axis Of Rotation ◽  
The Mean ◽  
Ap Stars

AbstractObservations of variable broadband linear polarization in magnetic Ap stars (due to the transverse Zeeman effect), when combined with measurements of the mean longitudinal field Bɩ can in some cases allow one to determine the angles i and β (which describe the inclination of the stellar axis of rotation and the obliquity of the magnetic axis to the rotation axis) much more accurately than these angles can be determined from observations of Bɩ alone. Such variable intrinsic linear polarization has been observed for a number of stars; the effect is generally detectable only in cool Ap stars of unusually large field strength. We discuss the data and simple modelling for the stars HD 24712 = HR 1217, HD 137909 = β CrB, and HD 62140 = 49 Cam.

scholarly journals Modeling surface magnetic fields in stars with radiative envelopes

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.

scholarly journals roAp stars

1995 ◽  
Vol 10 ◽  
pp. 338-340
Author(s):  
D. Kurtz ◽  
P. Martinez
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Rotation Axis ◽  
Zeeman Effect ◽  
Spectral Lines ◽  
Effective Magnetic Field ◽  
Surface Magnetic Field ◽  
Ap Stars

Among the A stars there is a subclass of peculiar stars, the Ap stars, which show strongly enhanced spectral lines of the Fe peak, rare earth and lanthanide elements. These stars have global surface magnetic fields several orders of magnitude larger than that of the Sun, 0.3 to 30 kGauss is the measured range. For stars with the strongest magnetic fields, the spectral lines are split by the Zeeman Effect and the surface magnetic field strength can be measured. Generally, though, the magnetic fields are not strong enough for the magnetic splitting to exceed other sources of line broadening. In these cases residual polarization differences between the red and blue wings of the spectral lines give a measure of the effective magnetic field strength - the integral of the longitudinal component of the global magnetic field over the visible hemisphere, weighted by limb-darkening. In the Ap stars the effective magnetic field strengths vary with rotation. This is well understood in terms of the oblique rotator model in which the magnetic axis is oblique to the rotation axis, so that the magnetic field is seen from varying aspect with rotation.

scholarly journals Virtual and spurious surface structure on Ap stars

1996 ◽  
Vol 176 ◽  
pp. 61-68 ◽  
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?

Polarized Radiation from AM Herculis Stars

1983 ◽  
Vol 72 ◽  
pp. 217-221
Author(s):  
P. E. Barrett ◽  
G. Chanmugam
Keyword(s):  
Magnetic Field ◽  
Circular Polarization ◽  
Linear Polarization ◽  
Ad Hoc ◽  
Photon Scattering ◽  
Cyclotron Radiation ◽  
The Magnetic Field ◽  
Striking Effect ◽  
General Reduction ◽  
Polarized Radiation

ABSTRACTCalculations of polarized cyclotron radiation from magnetized hot plasmas are modified to include the effects of collisions and photon scattering. These effects produce a general reduction in both the linear and circular polarization. The background unpolarized flux which had previously been invoked, in an ad hoc manner for the collisionless case, is no longer required to bring agreement with observations. The most striking effect is at small viewing angles with the magnetic field, where the fractional circular polarization is reduced from nearly 100% to close to 0%. The calculations also show that the shape of the linear polarization pulse is essentially unchanged.

scholarly journals Strong variable linear polarization in the cool active star II Peg

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.

REMOVAL OF THE DIFFERENCE BETWEEN CHARACTERISTIC AND REAL AGE IN THE MAGNETAR MODEL FROM THE ANALYSIS OF SPECTRAL PROPERTIES OF AXPs, SGRs AND DRQNSs

2003 ◽  
Vol 12 (08) ◽  
pp. 1333-1362 ◽  
Author(s):  
OKTAY H. GUSEINOV ◽  
ÖZGÜR TAŞKIN ◽  
EFE YAZGAN ◽  
SEVINÇ TAGIEVA
Keyword(s):  
Magnetic Field ◽  
Neutron Stars ◽  
Observational Data ◽  
Transition Period ◽  
Rotation Axis ◽  
Period Change ◽  
Spin Period ◽  
Long Time ◽  
The Difference ◽  
The Magnetic Field

We put together many observational data of SGRs and AXPs and analyze them with the main purpose to remove contradiction between the real age of these objects and their characteristic times of period change. This work indicates that SGRs and AXPs are neutron stars with magnetic fields up to ~3×1014 G at birth, which is less than the possible value in the existing magnetar model. These neutron stars undergo star-quakes and reconnection of magnetic field occurs from time to time. As a result of these processes plasma is ejected from the NS and propeller mechanism starts to work. Due to the propeller effect [Formula: see text] increases and τ decreases. Indeed, high [Formula: see text] values are observed in SGRs and in half of the AXPs. Then, for a long time NS looses its activity, its [Formula: see text] decreases, τ increases and rapid cooling begins. Each NS stage (AXP, SGR, dim) may occur once or several times until the spin period of the neutron star becomes P >10–12 s . At the transition period to the SGR and AXP stages, the magnetic field component perpendicular to the rotation axis may increase up to 2–3 times. Observational data and mainly the data of AXP1E1048-5937 and DRQNS RX J1308.8+2127 support this idea.

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