| 1. |
- Weigelt, G., et al.
(author)
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VLTI-MATISSE chromatic aperture-synthesis imaging of eta Carinae's stellar wind across the Br alpha line Periastron passage observations in February 2020
- 2021
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652
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Journal article (peer-reviewed)abstract
- Context. Eta Carinae is a highly eccentric, massive binary system (semimajor axis similar to 15.5 au) with powerful stellar winds and a phase-dependent wind-wind collision (WWC) zone. The primary star, eta Car A, is a luminous blue variable (LBV); the secondary, eta Car B, is a Wolf-Rayet or O star with a faster but less dense wind. Aperture-synthesis imaging allows us to study the mass loss from the enigmatic LBV eta Car. Understanding LBVs is a crucial step toward improving our knowledge about massive stars and their evolution. Aims. Our aim is to study the intensity distribution and kinematics of eta Car's WWC zone. Methods. Using the VLTI-MATISSE mid-infrared interferometry instrument, we perform Br alpha imaging of eta Car's distorted wind. Results. We present the first VLTI-MATISSE aperture-synthesis images of eta Car A's stellar windin several spectral channels distributed across the Br alpha 4.052 mu m line (spectral resolving power R similar to 960). Our observations were performed close to periastron passage in February 2020 (orbital phase similar to 14.0022). The reconstructed iso-velocity images show the dependence of the primary stellar wind on wavelength or line-of-sight (LOS) velocity with a spatial resolution of 6 mas (similar to 14 au). The radius of the faintest outer wind regions is similar to 26 mas (similar to 60 au). At several negative LOS velocities, the primary stellar wind is less extended to the northwest than in other directions. This asymmetry is most likely caused by the WWC. Therefore, we see both the velocity field of the undisturbed primary wind and the WWC cavity. In continuum spectral channels, the primary star wind is more compact than in line channels. A fit of the observed continuum visibilities with the visibilities of a stellar wind CMFGEN model (CMFGEN is an atmosphere code developed to model the spectra of a variety of objects) provides a full width at half maximum fit diameter of the primary stellar wind of 2.84 +/- 0.06 mas (6.54 +/- 0.14 au). We comparethe derived intensity distributions with the CMFGEN stellar wind model and hydrodynamic WWC models.
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| 2. |
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| 3. |
- Rivinius, T., et al.
(author)
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Basic parameters and properties of the rapidly rotating magnetic helium-strong B star HR 7355
- 2013
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 429:1, s. 177-188
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Journal article (peer-reviewed)abstract
- The spectral and magnetic properties and variability of the B2Vnp emission-line magnetosphere star HR 7355 were analysed. The object rotates at almost 90 per cent of the critical value, meaning it is a magnetic star for which oblateness and gravity darkening effects cannot be ignored any longer. A detailed modelling of the photospheric parameters indicates that the star is significantly cooler than suggested by the B2 spectral type, with T-eff = 17 500 K atypically cool for a star with a helium-enriched surface. The spectroscopic variability of helium and metal lines due to the photospheric abundance pattern is far more complex than a largely dipolar, oblique magnetic field of about 11-12 kG may suggest. Doppler imaging shows that globally the most He-enriched areas coincide with the magnetic poles and metal-enriched areas with the magnetic equator. While most of the stellar surface is helium enriched with respect to the solar value, some isolated patches are depleted. The stellar wind in the circumstellar environment is governed by the magnetic field, i. e. the stellar magnetosphere is rigidly corotating with the star. The magnetosphere of HR 7355 is similar to the well known sigma Ori E: the gas trapped in the magnetospheric clouds is fairly dense, and at the limit to being optically thick in the hydrogen emission. Apart from a different magnetic obliquity, HR 7355 and the more recently identified HR 5907 have virtually identical stellar and magnetic parameters.
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| 4. |
- Shultz, M. E., et al.
(author)
-
MOBSTER-V. Discovery of a magnetic companion star to the magnetic beta Cep pulsator HD 156424
- 2021
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 504:4, s. 4850-4864
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Journal article (peer-reviewed)abstract
- HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H alpha emission. We have analysed the Transiting Exoplanet Survery Satellite (TESS) light curve, and find that it is a beta Cep pulsator with 11 detectable frequencies, 4 of which are independent p-modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of similar to years; significant differences in the frequencies determined from TESS and RV data sets are consistent with a light-time effect from orbital motion. Close examination of the star's spectrum reveals the presence of a spectroscopic companion, however, as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of least-squares deconvolution profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with < B-z > about +1.5 kG as compared to < B-z > similar to -0.8 kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H alpha emission appears to originate around HD 156424B. Using H alpha, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.
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| 5. |
- Shultz, M. E., et al.
(author)
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MOBSTER - VI. The crucial influence of rotation on the radio magnetospheres of hot stars
- 2022
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 513:1, s. 1429-1448
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Journal article (peer-reviewed)abstract
- Numerous magnetic hot stars exhibit gyrosynchrotron radio emission. The source electrons were previously thought to be accelerated to relativistic velocities in the current sheet formed in the middle magnetosphere by the wind opening magnetic field lines. However, a lack of dependence of radio luminosity on the wind power, and a strong dependence on rotation, has recently challenged this paradigm. We have collected all radio measurements of magnetic early-type stars available in the literature. When constraints on the magnetic field and/or the rotational period are not available, we have determined these using previously unpublished spectropolarimetric and photometric data. The result is the largest sample of magnetic stars with radio observations that has yet been analysed: 131 stars with rotational and magnetic constraints, of which 50 are radio-bright. We confirm an obvious dependence of gyrosynchrotron radiation on rotation, and furthermore find that accounting for rotation neatly separates stars with and without detected radio emission. There is a close correlation between H alpha emission strength and radio luminosity. These factors suggest that radio emission may be explained by the same mechanism responsible for H alpha emission from centrifugal magnetospheres, i.e. centrifugal breakout (CBO), however, while the H alpha-emitting magnetosphere probes the cool plasma before breakout, radio emission is a consequence of electrons accelerated in centrifugally driven magnetic reconnection.
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| 6. |
- Shultz, Matthew E., et al.
(author)
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The magnetic early B-type stars I : magnetometry and rotation
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 475:4, s. 5144-5178
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Journal article (peer-reviewed)abstract
- The rotational and magnetic properties of many magnetic hot stars are poorly characterized, therefore the Magnetism in Massive Stars and Binarity and Magnetic Interactions in various classes of Stars collaborations have collected extensive high-dispersion spectropo-larimetric data sets of these targets. We present longitudinal magnetic field measurements < B-z > for 52 early B-type stars (B5-B0), with which we attempt to determine their rotational periods P-rot. Supplemented with high-resolution spectroscopy, low-resolution Dominion As-trophysical Observatory circular spectropolarimetry, and archival Hipparcos photometry, we determined P-rot for 10 stars, leaving only five stars for which P-rot could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of P-rot is very similar to that observed for the cooler Ap/Bp stars. We also measured v sin i and v(mac) for all stars. Comparison to non-magnetic stars shows that v sin i is much lower for magnetic stars, an expected consequence of magnetic braking. We also find evidence that v(mac) is lower for magnetic stars. Least-squares deconvolution profiles extracted using single-element masks revealed widespread, systematic discrepancies in < B-z > between different elements: this effect is apparent only for chemically peculiar stars, suggesting it is a consequence of chemical spots. Sinusoidal fits to H line < B-z > measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in six stars for which this has not previously been reported; however, in all six cases, the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.
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| 7. |
- Shultz, M. E., et al.
(author)
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The magnetic early B-type stars - III. A main-sequence magnetic, rotational, and magnetospheric biography
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 490:1, s. 274-295
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Journal article (peer-reviewed)abstract
- Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into centrifugal magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in H alpha - the principal diagnostic of these structures - are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with Ha emission originating in a CM are (1) rapid rotators, (2) strongly magnetic, and (3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.
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| 8. |
- Shultz, M. E., et al.
(author)
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The magnetic early B-type stars - IV. Breakout or leakage? : H α emission as a diagnostic of plasma transport in centrifugal magnetospheres
- 2020
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 499:4, s. 5379-5395
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Journal article (peer-reviewed)abstract
- Rapidly rotating early-type stars with strong magnetic fields frequently show H alpha emission originating in centrifugal magnetospheres (CMs), circumstellar structures in which centrifugal support due to magnetically enforced corotation of the magnetically confined plasma enables it to accumulate to high densities. It is not currently known whether the CM plasma escapes via centrifugal breakout (CB), or by an unidentified leakage mechanism. We have conducted the first comprehensive examination of the H alpha emission properties of all stars currently known to display CM-pattern emission. We find that the onset of emission is dependent primarily on the area of the CM, which can be predicted simply by the value B-K of the magnetic field at the Kepler corotation radius R-K. Emission strength is strongly sensitive to both CM area and B-K. Emission onset and strength are not dependent on effective temperature, luminosity, or mass-loss rate. These results all favour a CB scenario; however, the lack of intrinsic variability in any CM diagnostics indicates that CB must be an essentially continuous process, i.e. it effectively acts as a leakage mechanism. We also show that the emission profile shapes are approximately scale-invariant, i.e. they are broadly similar across a wide range of emission strengths and stellar parameters. While the radius of maximum emission correlates closely as expected to R-K, it is always larger, contradicting models that predict that emission should peak at R-K.
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| 9. |
- Shultz, Matthew, et al.
(author)
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HD 156324 : a tidally locked magnetic triple spectroscopic binary with a disrupted magnetosphere
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 475:1, s. 839-852
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Journal article (peer-reviewed)abstract
- HD 156324 is an SB3 (B2VIB5VIB5V) system in the Sco OB4 association. The He-strong primary possesses both a strong magnetic field and Ha emission believed to originate in its centrifugal magnetosphere. We analyse a large spectroscopic and high-resolution spectropolarimetric data set. The radial velocities (RVs) indicate that the system is composed of two subsystems, which we designate A and B. Period analysis of the RVs of the three components yields orbital periods P-orb = 1.5806(1) d for the Aa and Ab components, and P-orb = 6.67(2) d for the B component, a PGa star. Period analysis of the longitudinal magnetic field (B,) and H alpha equivalent widths, which should both be sensitive to the rotational period Prot of the magnetic Aa component, both yield 1.58 d. Since P-orb = P-rot Aa and Ab must be tidally locked. Consistent with this, the orbit is circularized, and the rotational and orbital inclinations are identical within uncertainty, as are the semimajor axis and the Kepler corotation radius. The star's Ha emission morphology differs markedly from both theoretical and observational expectations in that there is only one, rather than two, emission peaks. We propose that this unusual morphology may be a consequence of modification of the gravitocentrifugal potential by the presence of the close stellar companion. We also obtain upper limits on the magnetic dipole strength B-d for the Ab and B components, respectively, finding B-d < 2.6 and <0.7 kG.
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| 10. |
- Shultz, Matthew, et al.
(author)
-
HD 156324 : A Tidally Locked Magnetic SB3 With an Orbitally Disrupted Centrifugal Magnetosphere
- 2018
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In: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842. ; 48:1, s. 298-299
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Journal article (peer-reviewed)abstract
- Period analysis of radial velocity, equivalent width, and magnetic measurements of the SB3 system HD 156324 yield identical results in all cases, indicating the system is tidally locked with orbital and rotational periods of 1.58 d. Its H alpha emission profile exhibits marked morphological departures from the usual pattern observed amongst magnetic B-type stars, which can plausibly be ascribed to tidal disruption of the gravitocentrifugal potential.
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| 11. |
- Shultz, Matthew, et al.
(author)
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NU Ori : a hierarchical triple system with a strongly magnetic B-type star
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 482:3, s. 3950-3965
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Journal article (peer-reviewed)abstract
- NU Ori is a massive spectroscopic and visual binary in the Orion Nebula Cluster, with four components: Aa, Ab, B, and C. The B0.5 primary (Aa) is one of the most massive B-type stars reported to host a magnetic field. We report the detection of a spectroscopic contribution from the C component in high-resolution ESPaDOnS spectra, which is also detected in a Very Large Telescope Interferometer data set. Radial velocity (RV) measurements of the inner binary (designated Aab) yield an orbital period of 14.3027(7) d. The orbit of the third component (designated C) was constrained using both RVs and interferometry. We find C to be on a mildly eccentric 476(1) d orbit. Thanks to spectral disentangling of mean line profiles obtained via least-squares deconvolution, we show that the Zeeman Stokes V signature is clearly associated with C, rather than Aa as previously assumed. The physical parameters of the stars were constrained using both orbital and evolutionary models, yielding M-Aa = 14.9 +/- 0.5 M-circle dot, M-Ab = 3.9 +/- 0.7 M-circle dot, and M-C = 7.8 +/- 0.7 M-circle dot. The rotational period obtained from longitudinal magnetic field (B-z) measurements is P-rot = 1.09468(7)d, consistent with previous results. Modelling of (B-z) indicates a surface dipole magnetic field strength of similar to 8 kG. NU Ori C has a magnetic field strength, rotational velocity, and luminosity similar to many other stars exhibiting magnetospheric Ha emission, and we find marginal evidence of emission at the expected level (similar to 1 per cent of the continuum).
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| 12. |
- Shultz, Matthew, et al.
(author)
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Rotation, Emission, & Evolution of the Magnetic Early B-type Stars
- 2018
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In: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842. ; 48:1, s. 175-179
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Journal article (peer-reviewed)abstract
- We report the results of the first population study of 51 magnetic early B-type stars, based upon a large database of high-resolution spectropolarimetry assembled by the MiMeS and BinaMIcS collaborations. Utilizing these data, rotational periods were determined for all but 5 of the sample stars. This enabled us to determine dipole oblique rotator model parameters, rotational parameters, and magnetospheric parameters. We find that the ratio of the Alfven radius to the Kepler corotation radius is highly predictive of whether or not a star displays Ha emission from a Centrifugal Magnetosphere (CM), as expected from theoretical considerations. We also find that CM host stars are systematically younger than the general population, as expected given that CM emission requires rapid rotation and a strong magnetic field, and a strong magnetic field will lead to rapid magnetic braking. We conclude that emission-line magnetic early B-type stars are, almost without exception, strongly magnetized, rapidly rotating, and young.
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| 13. |
- Shultz, Matthew, et al.
(author)
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The pulsating magnetosphere of the extremely slowly rotating magnetic beta Cep star xi(1) CMa
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 471:2, s. 2286-2310
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Journal article (peer-reviewed)abstract
- xi(1) CMa is a monoperiodically pulsating, magnetic beta Cep star with magnetospheric X-ray emission that, uniquely amongst magnetic stars, is clearly modulated with the star's pulsation period. The rotational period P-rot has yet to be identified, with multiple competing claims in the literature. We present an analysis of a large ESPaDOnS data set with a 9 yr baseline. The longitudinal magnetic field < B-z > shows a significant annual variation, suggesting that Prot is at least of the order of decades. The possibility that the star's Ha emission originates around a classical Be companion star is explored and rejected based upon Very Large Telescope Interferometer AMBER and PIONIER interferometry, indicating that the emission must instead originate in the star's magnetosphere and should therefore also be modulated with Prot. Period analysis of Ha equivalent widths measured from ESPaDOnS and CORALIE spectra indicates P-rot > 30 yr. All evidence thus supports that xi(1) CMa is a very slowly rotating magnetic star hosting a dynamical magnetosphere. Ha also shows evidence for modulation with the pulsation period, a phenomenon that we show cannot be explained by variability of the underlying photospheric line profile, i.e. it may reflect changes in the quantity and distribution of magnetically confined plasma in the circumstellar environment. In comparison to other magnetic stars with similar stellar properties, xi(1) CMa is by far the most slowly rotating magnetic B-type star, is the only slowly rotating B-type star with a magnetosphere detectable in Ha ( and thus, the coolest star with an optically detectable dynamical magnetosphere), and is the only known early-type magnetic star with Ha emission modulated by both pulsation and rotation.
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| 14. |
- Shultz, Matthew, et al.
(author)
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The pulsationally modulated radial crossover signature of the slowly rotating magnetic B-type star xi(1) CMa
- 2018
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In: Monthly Notices of the Royal Astronomical Society: Letters. - : Oxford University Press (OUP). - 1745-3925 .- 1745-3933. ; 478:1, s. L39-L43
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Journal article (peer-reviewed)abstract
- We report the latest set of spectropolarimetric observations of the magnetic beta Cep star xi 1 CMa. The new observations confirm the long-period model of Shultz et al. (2017), who proposed a rotational period of about 30 years and predicted that in 2018 the star should pass through a magnetic null. In perfect agreement with this projection, all longitudinal magnetic field < B-z > measurements are close to 0 G. Remarkably, individual Stokes V profiles all display a crossover signature, which is consistent with < B-z > similar to 0 but is not expected when v sin i similar to 0. The crossover signatures furthermore exhibit pulsationally modulated amplitude and sign variations. We show that these unexpected phenomena can all be explained by a 'radial crossover' effect related to the star's radial pulsations, together with an important deviation of the global field topology from a purely dipolar structure, that we explore via a dipole+ quadrupole configuration as the simplest non-dipolar field.
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| 15. |
- Wade, G. A., et al.
(author)
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Magnetic B stars observed with BRITE : Spots, magnetospheres, binarity, and pulsations
- 2017
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In: Second brite-constellation science conference. - Warsaw : POLISH ASTRONOMICAL SOC. - 9788393827961 ; , s. 94-100
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Conference paper (peer-reviewed)abstract
- Magnetic B-type stars exhibit photometric variability due to diverse causes, and consequently on a variety of timescales. In this paper we describe interpretation of BRITE photometry and related ground-based observations of four magnetic B-type systems: epsilon Lupi, tau Sco, a Cen and epsilon CMa.
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| 16. |
- Wade, G. A., et al.
(author)
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The MiMeS survey of magnetism in massive stars : introduction and overview
- 2016
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:1, s. 2-22
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Journal article (other academic/artistic)abstract
- The MiMeS (Magnetism in Massive Stars) project is a large-scale, high-resolution, sensitive spectropolarimetric investigation of the magnetic properties of O- and early B-type stars. Initiated in 2008 and completed in 2013, the project was supported by three Large Program allocations, as well as various programmes initiated by independent principal investigators, and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) at the Canada-France-Hawaii Telescope, Narval at the Telescope Bernard Lyot and HARPSpol at the European Southern Observatory La Silla 3.6 m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.
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