scholarly journals HD 965: An extremely peculiar A star with an extremely long rotation period

2019 ◽  
Vol 629 ◽  
pp. A39 ◽  
Author(s):  
G. Mathys ◽  
I. I. Romanyuk ◽  
S. Hubrig ◽  
D. O. Kudryavtsev ◽  
M. Schöller ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Rotation Period ◽  
Field Measurements ◽  
Special Astrophysical Observatory ◽  
General Context ◽  
Rotation Periods ◽  
Variation Curve ◽  
The Mean ◽  
Magnetic Poles ◽  
Ap Stars

Context. One of the keys to understanding the origin of Ap stars and their significance in the general context of stellar astrophysics is by considering the most extreme properties displayed by some of them. For this reason, HD 965 is particularly interesting as it combines some of the most pronounced chemical peculiarities with one of the longest rotation periods known. Aims. We characterise the variations of the magnetic field of the Ap star HD 965 and derive constraints about its structure. Methods. We combined published measurements of the mean longitudinal field ⟨Bz⟩ of HD 965 with new determinations of this field moment from circular spectropolarimetry obtained at the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus ⟨B⟩, literature data were complemented by the analysis of ESO archive spectra. Results. We present the first determination of the rotation period of HD 965, Prot = (16.5 ± 0.5) yr. The star HD 965 is only the third Ap star with a period longer than ten years for which magnetic field measurements have been obtained over more than a full cycle. The variation curve of ⟨Bz⟩ is well approximated by a cosine wave. Furthermore, ⟨B⟩ does not show any significant variation. The observed behaviour of these field moments is well represented by a simple model consisting of the superposition of collinear dipole, quadrupole, and octupole. The distribution of neodymium over the surface of HD 965 is highly non-uniform. The element appears concentrated around the magnetic poles, especially the negative one. Conclusions. The shape of the longitudinal magnetic variation curve of HD 965 indicates that its magnetic field is essentially symmetric about an axis passing through the centre of the star. Overall, HD 965 appears similar to the bulk of the long-period Ap stars, as far as its magnetic field is concerned.

scholarly journals Variation of the magnetic field of the Ap star HD 50169 over its 29-year rotation period

2019 ◽  
Vol 624 ◽  
pp. A32 ◽  
Author(s):  
G. Mathys ◽  
I. I. Romanyuk ◽  
S. Hubrig ◽  
D. O. Kudryavtsev ◽  
J. D. Landstreet ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Rotation Period ◽  
Field Measurements ◽  
Significant Degree ◽  
Special Astrophysical Observatory ◽  
Stellar Rotation ◽  
The Mean ◽  
Ap Stars ◽  
Academy Of Sciences ◽  
The Magnetic Field

Context. The Ap stars that rotate extremely slowly, with periods of decades to centuries, represent one of the keys to the understanding of the processes leading to the differentiation of stellar rotation. Aims. We characterise the variations of the magnetic field of the Ap star HD 50169 and derive constraints about its structure. Methods. We combined published measurements of the mean longitudinal field ⟨Bz⟩ of HD 50169 with new determinations of this field moment from circular spectropolarimetry obtained at the 6m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus ⟨B⟩, literature data were complemented by the analysis of ESO spectra, both newly acquired and from the archive. Radial velocities were also obtained from these spectra. Results. We present the first determination of the rotation period of HD 50169, Prot = 29.04 ± 0.82 yr. HD 50169 is currently the longest-period Ap star for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of both ⟨Bz⟩ and ⟨B⟩ have a significant degree of anharmonicity, and there is a definite phase shift between their respective extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its orbital elements, and suggest that the secondary is probably a dwarf star of spectral type M. Conclusions. The shapes and mutual phase shifts of the derived magnetic variation curves unquestionably indicate that the magnetic field of HD 50169 is not symmetric about an axis passing through its centre. Overall, HD 50169 appears similar to the bulk of the long-period Ap stars.

scholarly journals The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

2020 ◽  
Vol 636 ◽  
pp. A6 ◽  
Author(s):  
G. Mathys ◽  
V. Khalack ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
Accurate Determination ◽  
Rotation Period ◽  
Field Measurements ◽  
Time Interval ◽  
Valuable Insight ◽  
Radial Pulsation ◽  
Magnetic Field Measurements ◽  
The Mean ◽  
Ap Stars

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, Prot = (3836 ± 30) d, from the analysis of 56 measurements of the mean magnetic field modulus ⟨B⟩ based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field ⟨Bz⟩ spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of ⟨B⟩ and of ⟨Bz⟩ are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accurately determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

scholarly journals A high resolution lithospheric magnetic field model over southern Africa based on a joint inversion of CHAMP, Swarm, WDMAM and ground magnetic field data

2018 ◽  
Author(s):  
Foteini Vervelidou ◽  
Erwan Thébault ◽  
Monika Korte
Keyword(s):  
Magnetic Field ◽  
Southern Africa ◽  
Field Model ◽  
Joint Inversion ◽  
Field Measurements ◽  
Magnetic Field Data ◽  
Magnetic Field Model ◽  
Lithospheric Magnetic Field ◽  
The Mean ◽  
Ground Magnetic

Abstract. We derive a lithospheric magnetic field model up to equivalent Spherical Harmonic degree 1000 over southern Africa. We rely on a joint inversion of satellite, near-surface and ground magnetic field data. The input data set consists of magnetic field vector measurements from the CHAMP satellite, across-track magnetic field differences from the Swarm mission, the World Digital Magnetic Anomaly Map and magnetic field measurements from repeat stations and three local INTERMAGNET observatories. For the inversion scheme, we use the Revised-Spherical Cap Harmonic Analysis (R-SCHA), a regional analysis technique able to deal with magnetic field measurements obtained at different altitudes. The model is carefully assessed and displayed at different altitudes and its spectral content is compared to high resolution global lithospheric field models. By comparing the shape of its spectrum to a statistical power spectrum of Earth's lithospheric magnetic field, we infer the mean magnetic thickness and the mean magnetization over southern Africa.

scholarly journals Pulsar Death at an Advanced Age

2000 ◽  
Vol 177 ◽  
pp. 449-454 ◽  
Author(s):  
Jonathan Arons
Keyword(s):  
Magnetic Field ◽  
Electric Fields ◽  
Rotation Period ◽  
Death Valley ◽  
Particle Beams ◽  
Frame Dragging ◽  
Inverse Compton ◽  
Inertial Frames ◽  
Field Lines ◽  
Magnetic Poles

AbstractI summarize the theory of acceleration of non-neutral particle beams by starvation electric fields along the polar magnetic field lines of rotation powered pulsars, including the effect of dragging of inertial frames which dominates the acceleration of a space charge limited beam. I apply these results to a new calculation of the radio pulsar death line, under the hypotheses that pulsar “death” corresponds to cessation of pair creation over the magnetic polesandthat the magnetic field has a locally dipolar topology. The frame dragging effect in star centered dipole geometry does improve comparison of the theory with observation, but an unacceptably large conflict between obsrvation and theory still persists. Offsetting the dipole improves the comparison, but a fully satisfactory theory requires incorporating magnetic conversion of inverse Compton gamma rays, created by scattering thermal photons from the surface of old neutron stars (t> 108years) kept warm (T≥ 105K) by friction between the rotating core and the crust. The result is a “death valley” for pulsars; offsets of the dipole center from the stellar center in the oldest stars ~ (0.7 − 0.8)R*suffice. The resulting theory predicted the existence of rotation powered pulsars with these advanced ages, a prediction confirmed by the recent discovery that PSR J2144-3933 actually has a rotation period of 8.5 seconds.

scholarly journals The anomalous atmospheric structure of the strongly magnetic Ap star HD 166473

2020 ◽  
Vol 499 (2) ◽  
pp. 2734-2743
Author(s):  
S P Järvinen ◽  
S Hubrig ◽  
G Mathys ◽  
V Khalack ◽  
I Ilyin ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Rare Earth ◽  
Field Strength ◽  
Longitudinal Magnetic Field ◽  
Chemical Elements ◽  
Field Measurements ◽  
Horizontal Distribution ◽  
Spectral Lines ◽  
Surface Distribution ◽  
The Mean

ABSTRACT High-resolution spectropolarimetric observations of the strongly magnetic, superslowly rotating rapidly oscillating Ap star HD 166473 are used to investigate the implications of the presence of a variable strong magnetic field on the vertical and surface horizontal distribution of various chemical elements. The analysis of the calculated least-squares deconvolution Stokes I and V profiles confirms the previously reported detection of non-uniform horizontal surface distribution of several chemical elements. To test the vertical abundance stratification of iron peak and rare earth elements, magnetic field measurements were carried out using spectral lines of these elements belonging to neutral and ionized stages. We find clear indication of the existence of a relation between the magnetic field strength and its orientation and vertical element stratification: magnetic field values obtained for elements in different stages close to the magnetic equator are rather similar, whereas the dispersion in field strengths is remarkably large in the regions close to magnetic field poles. At the phases of negative and positive extrema the mean longitudinal field strength determined from the analysis of the rare-earth element lines is usually stronger than when using Fe and Cr. The strongest mean longitudinal magnetic field, up to −4160 ± 226 G, is detected using the La ii line list at the negative extremum, followed by the measurements using the Pr iii lines with 〈Bz〉=−3740 ± 343 G and the Ce ii lines with 〈Bz〉 = −3372 ± 247 G. The strongest mean longitudinal magnetic field of positive polarity, up to 3584 ± 354 G is detected using the Pr iii lines, followed by the measurement 〈Bz〉 = 2517 ± 249 G using the Ce ii lines.

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.

Calculation of corotation electric field based on Van Allen Probes measurements

2020 ◽  
Author(s):  
Wenlong Liu ◽  
Zhao Zhang
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Large Scale ◽  
Rotation Period ◽  
Field Measurements ◽  
Inner Magnetosphere ◽  
Dipole Magnetic Field ◽  
Van Allen Probes ◽  
Electric Field Measurements ◽  
Euv Imaging

<p>Corotation electric field is important in the inner magnetosphere topology, which was usually calculated by assuming 24h corotation period. However, some studies suggested that plasmasphere corotation lag exists which leads to the decrease of corotation electric field. In this study, we use electric field measurements from Van Allen Probes mission from 2013 to 2017 to statistically calculate the distribution of large-scale electric field in the inner magnetosphere. A new method is subsequently developed to separate corotation electric field from convection electric field. Our research shows electric field is inversely proportional to the square of L, and, with the assumption of dipole magnetic field, the rotation period of plasmasphere is estimated as 27h, consistent to the results by Sandel et al. [2003] and Burch et al. [2004] with EUV imaging of the plasmasphere. Based on the research, a new empirical model of innermagnetospheric corotation electric field was estibalished, which is significant for a more accurate understanding the large-scale electric field in the inner magnetosphere.</p>

scholarly journals Spectroscopic Diagnosis of Magnetic Fields of AP Stars

1998 ◽  
Vol 11 (2) ◽  
pp. 676-678
Author(s):  
G. Mathys
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Weighted Average ◽  
High Dispersion ◽  
Spectroscopic Methods ◽  
Magnetic Vector ◽  
Model Independent ◽  
The Mean ◽  
Ap Stars

Diagnosis of Ap star magnetic fields through spectroscopic methods is a broad topic, which cannot be fully covered within the rather tight limits of this contribution. Accordingly, the latter is devoted to only one particular approach, namely the determination of the mean magnetic field modulus from the observation of resolved magnetically split lines in high-dispersion spectra taken in unpolarized light. This determination is straightforward, mostly approximation free and model independent. The derived quantity, the mean magnetic field modulus, is the line-intensity weighted average over the visible stellar hemisphere of the modulus of the magnetic vector.

scholarly journals Discovery of a young pre-intermediate polar

2021 ◽  
Author(s):  
David J Wilson ◽  
Odette Toloza ◽  
John D Landstreet ◽  
Boris T Gänsicke ◽  
Jeremy J Drake ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
White Dwarf ◽  
White Dwarfs ◽  
Hubble Space Telescope ◽  
Cataclysmic Variables ◽  
Rotation Period ◽  
Field Measurements ◽  
The Magnetic Field

Abstract We present the discovery of a magnetic field on the white dwarf component in the detached post common envelope binary (PCEB) CC Cet. Magnetic white dwarfs in detached PCEBs are extremely rare, in contrast to the high incidence of magnetism in single white dwarfs and cataclysmic variables. We find Zeeman-split absorption lines in both ultraviolet Hubble Space Telescope (HST) spectra and archival optical spectra of CC Cet. Model fits to the lines return a mean magnetic field strength of 〈|B|〉 ≈ 600–700 kG. Differences in the best-fit magnetic field strength between two separate HST observations and the high v sin  i of the lines indicate that the white dwarf is rotating with a period ∼0.5 hours, and that the magnetic field is not axisymmetric about the spin axis. The magnetic field strength and rotation period are consistent with those observed among the intermediate polar class of cataclysmic variable, and we compute stellar evolution models that predict CC Cet will evolve into an intermediate polar in 7–17 Gyr. Among the small number of known PCEBs containing a confirmed magnetic white dwarf, CC Cet is the hottest (and thus youngest), with the weakest field strength, and cannot have formed via the recently proposed crystallisation/spin-up scenario. In addition to the magnetic field measurements, we update the atmospheric parameters of the CC Cet white dwarf via model spectra fits to the HST data and provide a refined orbital period and ephemeris from TESS photometry.

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