| 1. |
- Wade, G. A., et al.
(författare)
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The MiMeS survey of magnetism in massive stars : introduction and overview
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:1, s. 2-22
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)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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| 2. |
- Shultz, M. E., et al.
(författare)
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MOBSTER - VI. The crucial influence of rotation on the radio magnetospheres of hot stars
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 513:1, s. 1429-1448
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Tidskriftsartikel (refereegranskat)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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| 3. |
- David-Uraz, A., et al.
(författare)
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Magnetic OB[A] Stars with TESS : probing their Evolutionary and Rotational properties (MOBSTER) - I. First-light observations of known magnetic B and A stars
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 487:1, s. 304-317
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of 19 previously known magnetic O, B, and A stars observed in 2-min cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS.
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| 4. |
- Groh, J. H., et al.
(författare)
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Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
- 2010
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 517
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Tidskriftsartikel (refereegranskat)abstract
- We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using the CRIRES spectrograph mounted on the 8m UT 1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He I lambda 10833 extending up to -1900 km s(-1) across the 2009.0 spectroscopic event. Analysis of archival Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet and optical data identifies a similar high-velocity absorption (up to -2100 km s(-1)) in the ultraviolet resonance lines of Si IV lambda lambda 1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si II lambda lambda 1527, 1533 and CII lambda lambda 1334, 1335, show absorption only up to -1200 km s(-1), indicating that the absorption with velocities -1200 to -2100 km s(-1) originates in a region markedly more rapidly moving and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989-2009) also show high-velocity absorption in He I lambda 10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than -900 km s(-1) is present in the phase range 0.976 <= phi <= 1.023 of the spectroscopic cycle, but absent in spectra taken at phi <= 0.947 and phi >= 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We propose that the high-velocity absorption component originates in shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is along the line-of-sight to the primary star only if the binary system is oriented in the sky such that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega similar to 240 degrees-270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i = 40 degrees-60 degrees.
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| 5. |
- Fletcher, C. L., et al.
(författare)
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Investigating the Magnetospheres of Rapidly Rotating B-type Stars
- 2017
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Ingår i: Lives And Death-Throes Of Massive Stars. - 9781107170063 ; , s. 369-372
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Konferensbidrag (refereegranskat)abstract
- Recent spectropolarimetric surveys of bright, hot stars have found that similar to 10% of OB-type stars contain strong (mostly dipolar) surface magnetic fields (similar to kG). The prominent paradigm describing the interaction between the stellar winds and the surface magnetic field is the magnetically confined wind shock (MCWS) model. In this model, the stellar wind plasma is forced to move along the closed field loops of the magnetic field, colliding at the magnetic equator, and creating a shock. As the shocked material cools radiatively it will emit X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and characterizing the hot wind material confined by the magnetic fields of these stars. Some B-type stars are found to have very short rotational periods. The effects of the rapid rotation on the X-ray production within the magnetosphere have yet to be explored in detail. The added centrifugal force due to rapid rotation is predicted to cause faster wind outflows along the field lines, leading to higher shock temperatures and harder X-rays. However, this is not observed in all rapidly rotating magnetic B-type stars. In order to address this from a theoretical point of view, we use the X-ray Analytical Dynamical Magnetosphere (XADM) model, originally developed for slow rotators, with an implementation of new rapid rotational physics. Using X-ray spectroscopy from ESA's XMM-Newton space telescope, we observed 5 rapidly rotating B-types stars to add to the previous list of observations. Comparing the observed X-ray luminosity and hardness ratio to that predicted by the XADM allows us to determine the role the added centrifugal force plays in the magnetospheric X-ray emission of these stars.
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| 6. |
- Oksala, M. E., et al.
(författare)
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Revisiting the Rigidly Rotating Magnetosphere model for sigma Ori E-I : Observations and data analysis
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 419:2, s. 959-970
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Tidskriftsartikel (refereegranskat)abstract
- We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star s Ori E with both the Narval and ESPaDOnS spectropolarimeters. The aim of these observations is to test, with modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki, applied to the specific case of s Ori E by Townsend, Owocki & Groote. This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness and Ha emission. We analyse new spectroscopy, including H i, He i, C ii, Si iii and Fe iii lines, confirming the diversity of variability in photospheric lines, as well as the double S-wave variation of circumstellar hydrogen. Using the multiline analysis method of least-squares deconvolution (LSD), new, more precise longitudinal magnetic field measurements reveal a substantial variance between the shapes of the observed and RRM model time-varying field. The phase-resolved Stokes V profiles of He i 5876 and 6678 angstrom lines are fitted poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al.. These results challenge the offset dipole field configuration assumed in the application of the RRM model to s Ori E, and indicate that future models of its magnetic field should also include complex, higher order components.
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| 7. |
- Oksala, M. E., et al.
(författare)
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Revisiting the rigidly rotating magnetosphere model for sigma Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451:2, s. 2015-2029
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Tidskriftsartikel (refereegranskat)abstract
- The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma Ori E by Townsend, Owocki & Groote triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength B-d = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic Ha emission and broad-band photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.
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| 8. |
- Shultz, M. E., et al.
(författare)
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Detection of an extremely strong magnetic field in the double-degenerate binary merger product HD 144941
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 507:1, s. 1283-1295
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Tidskriftsartikel (refereegranskat)abstract
- HD 144941 is an extreme He (EHe) star, a rare class of subdwarf OB star formed from the merger of two white dwarf (WD) stars. Uniquely amongst EHe stars, its light curve has been reported to be modulated entirely by rotation, suggesting the presence of a magnetic field. Here, we report the first high-resolution spectropolarimetric observations of HD 144941, in which we detect an extremely strong magnetic field both in circular polarization (with a line-of-sight magnetic field averaged over the stellar disc < B-z > similar to -8 kG) and in Zeeman splitting of spectral lines (yielding a magnetic modulus of < B > similar to 17 kG). We also report for the first time weak H alpha emission consistent with an origin and a centrifugal magnetosphere. HD 144941's atmospheric parameters could be consistent with either a subdwarf or a main-sequence (MS) star, and its surface abundances are neither similar to other EHe stars nor to He-strong magnetic stars. However, its H alpha emission properties can only be reproduced if its mass is around 1 M-circle dot, indicating that it must be a post-MS object. Since there is no indication of binarity, it is unlikely to be a stripped star, and was therefore most likely produced in a WD merger. HD 144941 is therefore further evidence that mergers are a viable pathway for the generation of fossil magnetic fields.
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| 9. |
- Shultz, M. E., et al.
(författare)
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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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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 499:4, s. 5379-5395
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Tidskriftsartikel (refereegranskat)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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