arXiv subjects

the MiMeS Collaboration

Publications and source records attributed to the MiMeS Collaboration.

At least 37 records · Page 2Linked to original sources

arXiv2013

An Analysis of the Rapidly Rotating Bp Star HD 133880

HD 133880 is a rapidly rotating Bp star and host to one of the strongest magnetic fields of any Bp star. A member of the Upper Centaurus Lupus association, it is a star with a well-determined age of 16 Myr. Twelve new spectra obtained from the FEROS, ESPaDOnS, and HARPS instruments, provide sufficient material from which to re-evaluate the magnetic field and obtain a first approximation to the atmospheric abundance distributions of various elements. Using the new magnetic field measurements and optical photometry together with previously published data, we refine the period of HD 133880 to P = 0.877476 \pm 0.000009 days. The magnetic field structure was characterised by a colinear multipole expansion from the observed variations of the longitudinal and surface fields with rotational phase. This simple axisymmetric magnetic field model is based on a predominantly quadrupolar component that roughly describes the field variations. Using spectrum synthesis, we derived mean abundances for O, Mg, Si, Ti, Cr, Fe, Nd and Pr. All elements, except Mg (which has a uniform distribution), are overabundant compared to the Sun and are more abundant in the negative than in the positive magnetic hemisphere. In contrast to most Bp stars the abundance of O in HD 133880 is overabundant compared to the solar abundance ratio. In studying the Halpha and Paschen lines in the optical spectra we could not unambiguously detect information about the magnetosphere of HD 133880. However, radio emission data at both 3 and 6 cm suggests that the magnetospheric plasma is held in rigid rotation with the star by the magnetic field and further supported against collapse by the rapid rotation. Subtle differences in the shapes of the optically thick radio light curves at 3 and 6 cm suggest that the large-scale magnetic field is not fully axisymmetric at large distances from the star.

astro-ph.SR↗

arXiv2013

On magnetic inhibition of photospheric macro-turbulence generated in the iron-bump opacity zone of O-stars

Massive, hot OB-stars show clear evidence of strong macroscopic broadening (in addition to rotation) in their photospheric spectral lines. This paper examines the occurrence of such "macro-turbulence" in slowly rotating O-stars with strong, organised surface magnetic fields. Focusing on the CIV 5811A line, we find evidence for significant macro-turbulent broadening in all stars except NGC1624-2, which also has (by far) the strongest magnetic field. Instead, the very sharp CIV lines in NGC1624-2 are dominated by magnetic Zeeman broadening, from which we estimate a dipolar field of approximately 20 kG. By contrast, magnetic broadening is negligible in the other stars (due to their weaker field strengths, on order 1 kG), and their CIV profiles are typically very broad and similar to corresponding lines observed in non-magnetic O-stars. Quantifying this by an isotropic, Gaussian macro-turbulence, we derive vmac = 2.2 (+- 0.9/2.2) km/s for NGC1624-2, and vmac = 20-65 km/s for the rest of the magnetic sample. We use these observational results to test the hypothesis that the field can stabilise the atmosphere and suppress the generation of macro-turbulence down to stellar layers where the magnetic pressure PB and the gas pressure Pg are comparable. Using a simple grey atmosphere to estimate the temperature T0 at which PB = Pg, we find that T0 > Teff for all investigated magnetic stars, but that T0 reaches the ~160000 K layers associated with the iron opacity-bump in hot stars only for NGC1624-2. This is consistent with the view that the responsible physical mechanism for photospheric O-star macro-turbulence may be stellar gravity-mode oscillations excited by sub-surface convection zones, and suggests that a sufficiently strong magnetic field can suppress such iron-bump generated convection and associated pulsational excitation.

astro-ph.SR↗

arXiv2012

A Magnetic Confinement vs. Rotation Classification of Massive-Star Magnetospheres

Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement (which sets the Alfvén radius RA) and stellar rotation (which sets the Kepler co-rotation radius RK) provides a useful organisation of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with Teff > 16 kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of RK vs. RA. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with RA > RK, vs. dynamical magnetospheres (DM), with RK > RA. We show how key observational diagnostics, like the presence and characteristics of Halpha emission, depend on a star's position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with Halpha emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall timescale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown timescales. [abbreviated]

astro-ph.SR↗

arXiv2012

Discovery of a magnetic field in the rapidly-rotating O-type secondary of the colliding-wind binary HD 47129 (Plaskett's star)

We report the detection of a strong, organized magnetic field in the secondary component of the massive O8III/I+O7.5V/III double-lined spectroscopic binary system HD 47129 (Plaskett's star), in the context of the Magnetism in Massive Stars (MiMeS) survey. Eight independent Stokes $V$ observations were acquired using the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope and the Narval spectropolarimeter at the Télescope Bernard Lyot. Using Least-Squares Deconvolution we obtain definite detections of signal in Stokes $V$ in 3 observations. No significant signal is detected in the diagnostic null ($N$) spectra. The Zeeman signatures are broad and track the radial velocity of the secondary component; we therefore conclude that the rapidly-rotating secondary component is the magnetized star. Correcting the polarized spectra for the line and continuum of the (sharp-lined) primary, we measured the longitudinal magnetic field from each observation. The longitudinal field of the secondary is variable and exhibits extreme values of $-810\pm 150$ G and $+680\pm 190$ G, implying a minimum surface dipole polar strength of $2850\pm 500$ G. In contrast, we derive an upper limit ($3σ$) to the primary's surface magnetic field of 230 G. The combination of a strong magnetic field and rapid rotation leads us to conclude that the secondary hosts a centrifugal magnetosphere fed through a magnetically confined wind. We revisit the properties of the optical line profiles and X-ray emission - previously interpreted as a consequence of colliding stellar winds - in this context. We conclude that HD 47129 represents a heretofore unique stellar system - a close, massive binary with a rapidly rotating, magnetized component - that will be a rich target for further study.

astro-ph.SR↗

arXiv2012

NGC 1624-2: A slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field

This paper presents a first observational investigation of the faint Of?cp star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric properties. Modeling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main sequence star of mass M {\simeq} 30 M{\odot}, and infer an effective temperature of 35 {\pm} 2 kK and log g = 4.0 {\pm} 0.2. Based on an extensive time series of optical spectral observations we report significant variability of a large number of spectral lines, and infer a unique period of 157.99 {\pm} 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong - 5.35 {\pm} 0.5 kG - longitudinal magnetic field , coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus of 14 {\pm} 1 kG, consistent with a surface dipole of polar strength >~ 20 kG. This is the largest magnetic field ever detected in an O-type star, and the first report of Zeeman splitting of Stokes I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of O iii, which we propose may form in the close magnetosphere of the star. We analyze archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency log Lx/Lbol = -6.4, a factor of 4 larger than the canonical value for O-type stars and comparable to that of the young magnetic O-type star θ1 Ori C and other Of?p stars. Finally, we examine the probable magnetospheric properties of the star, reporting in particular very strong magnetic confinement of the stellar wind, with η* {\simeq} 1.5 {\times} 10^4, and a very large Alfven radius, RAlf = 11.4 R*.

astro-ph.SR↗

arXiv2012

Discovery of a magnetic field in the early B-type star sigma Lupi

Magnetic early B-type stars are rare. To identify them, we use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurring symmetric about the approximate rest wavelength. Probable magnetic candidates are targets for follow-up spectropolarimetry to search for a magnetic field. From the UV wind line variability the B1/B2V star sigma Lupi emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The longitudinal component of the magnetic field integrated over the visible surface of the star was determined with the Least-Squares Deconvolution method. The UV line variations of sigma Lupi are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a varying longitudinal magnetic field with amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The EW variations of the UV lines, the magnetic and the optical-line variations are consistent with the photometric period of 3.02 d, which we identify with the rotation period of the star. Additional observations with ESPaDOnS attached to the CFHT confirmed this discovery, and allowed the determination of a precise magnetic period. Analysis revealed that sigma Lupi is a He-strong star, with an enhanced N abundance and an underabundance of C, and has a chemically spotted surface. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission when a magnetic pole points towards the Earth. The 3.01972(43) d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.

astro-ph.SR↗

arXiv2012

Investigating the spectroscopic, magnetic and circumstellar variability of the O9 subgiant star HD 57682

The O9IV star HD 57682, discovered to be magnetic within the context of the MiMeS survey in 2009, is one of only eight convincingly detected magnetic O-type stars. Among this select group, it stands out due to its sharp-lined photospheric spectrum. Since its discovery, the MiMeS Collaboration has continued to obtain spectroscopic and magnetic observations in order to refine our knowledge of its magnetic field strength and geometry, rotational period, and spectral properties and variability. In this paper we report new ESPaDOnS spectropolarimetric observations of HD 57682, which are combined with previously published ESPaDOnS data and archival Hα spectroscopy. This dataset is used to determine the rotational period (63.5708 \pm 0.0057 d), refine the longitudinal magnetic field variation and magnetic geometry (dipole surface field strength of 880\pm50 G and magnetic obliquity of 79\pm4\circ as measured from the magnetic longitudinal field variations, assuming an inclination of 60\circ), and examine the phase variation of various lines. In particular, we demonstrate that the Hα equivalent width undergoes a double-wave variation during a single rotation of the star, consistent with the derived magnetic geometry. We group the variable lines into two classes: those that, like Hα, exhibit non-sinusoidal variability, often with multiple maxima during the rotation cycle, and those that vary essentially sinusoidally. Based on our modelling of the Hα emission, we show that the variability is consistent with emission being generated from an optically thick, flattened distribution of magnetically-confined plasma that is roughly distributed about the magnetic equator. Finally, we discuss our findings in the magnetospheric framework proposed in our earlier study.

astro-ph.SR↗

arXiv2012

The MiMeS Survey of Magnetism in Massive Stars

The Magnetism in Massive Stars (MiMeS) survey represents a high precision systematic search for magnetic fields in hot, massive OB stars. To date, MiMeS Large Programs (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO3.6) and associated PI programs (FORS@VLT) have yielded nearly 1200 circular spectropolarimetric observations of over 350 OB stars. Within this sample, 20 stars are detected as magnetic. Follow-up observations of new detections reveals (i) a large diversity of magnetic properties, (ii) evidence for magnetic wind confinement in optical spectra of all magnetic O stars, (iii) the presence of strong, organized magnetic fields in all known Galactic Of?p stars, and (iv) a complete absence of magnetic fields in classical Be stars.

astro-ph.SR↗

arXiv2012

A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612

The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line emission on a 538-day period. We show here that its variable Halpha emission can be well reproduced by the rotational phase variation of synthetic spectra computed directly from full radiation magneto-hydrodynamical simulations of a magnetically confined wind. In slow rotators such as HD191612, wind material on closed magnetic field loops falls back to the star, but the transient suspension of material within the loops leads to a statistically overdense, low velocity region around the magnetic equator, causing the spectral variations. We contrast such "dynamical magnetospheres" (DMs) with the more steady-state "centrifugal magnetospheres" of stars with rapid rotation, and discuss the prospects of using this DM paradigm to explain periodic line emission from also other non-rapidly rotating magnetic massive stars.

astro-ph.SR↗

arXiv2012

Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations

Recent spectropolarimetric studies of 7 SPB and $β$ Cep stars have suggested that photospheric magnetic fields are more common in B-type pulsators than in the general population of B stars, suggesting a significant connection between magnetic and pulsational phenomena. We present an analysis of new and previously published spectropolarimetric observations of these stars. New Stokes $V$ observations obtained with the high-resolution ESPaDOnS and Narval instruments confirm the presence of a magnetic field in one of the stars ($ε$ Lup), but find no evidence of magnetism in 5 others. A re-analysis of the published longitudinal field measurements obtained with the low-resolution FORS1/2 spectropolarimeters finds that the measurements of all stars show more scatter from zero than can be attributed to Gaussian noise, suggesting the presence of a signal and/or systematic under-estimation of error bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO archive demonstrates that small changes in reduction procedure lead to substantial changes in the inferred longitudinal field, and substantially reduces the number of field detections at the 3$σ$ level. Furthermore, we find that the published periods are not unique solutions to the time series of either the original or the revised FORS1/2 data. We conclude that the reported field detections, proposed periods and field geometry models for $α$ Pyx, 15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction procedures, and that magnetic fields at the reported strength are no more common in SPB/$β$ Cep stars than in the general population of B stars.

astro-ph.SR↗

arXiv2011

Magnetospheres of massive stars across the EM spectrum

Magnetic massive stars -- which are being discovered with increasing frequency -- represent a new category of wind-shaping mechanism for O and B stars. Magnetic channeling of these stars' radiation-driven winds, the Magnetically Confined Wind Shock paradigm, leads to the formation of a shock-heated magnetosphere, which can radiate X-rays, modify UV resonance lines, and create disks of Halpha emitting material. The dynamical properties of these magnetospheres are well understood from a theoretical point of view as an interplay between the magnetic wind confinement and rotation. However, the manifestations of magnetospheres across the spectrum may be more complex and various than first anticipated. On the other hand, recent advances in modeling these magnetospheres provide a key to better understand massive star winds in general. We will summarize the coordinated observational, theoretical, and modeling efforts from the Magnetism in Massive Star Project, addressing key outstanding questions regarding magnetosphere manifestations across the spectral domain.

astro-ph.SR↗

arXiv2011

Magnetic Fields of Slowly Pulsating B Stars and β Cep Variables: Comparing Results from FORS1/2 and ESPaDOnS

Recently announced magnetic models for four SPB and β Cep stars, along with magnetic detections for two additional stars, have potentially doubled the number of known magnetic SPB and βCep stars (see Grunhut et al., these proceedings). We have reanalyzed the published data and re-reduced archival low resolution spectropolarimetry collected with the FORS1/2 instruments at VLT on which the models were based, and compare them with high resolution data from the ESPaDOnS spectropolarimeter at CFHT, investigating previously noted inconsistencies between results from the two instruments.

astro-ph.SR↗

arXiv2011

The incidence of magnetic fields in massive stars: An overview of the MiMeS Survey Component

With only a handful of known magnetic massive stars, there is a troubling deficit in the scope of our knowledge of the influence of magnetic fields on stellar evolution, and almost no empirical basis for understanding how fields modify mass loss and rotation in massive stars. Most remarkably, there is still no solid consensus regarding the origin physics of these fields - whether they are fossil remnants, or produced by contemporaneous dynamos, or some combination of these mechanisms. This article will present an overview of the Survey Component of the MiMeS Large Programs, the primary goal of which is to search for Zeeman signatures in the circular polarimetry of massive stars (stars with spectral types B3 and hotter) that were previously unknown to host any magnetic field. To date, the MiMeS collaboration has collected more than 550 high-resolution spectropolarimetric observations with ESPaDOnS and Narval of nearly 170 different stars, from which we have detected 14 new magnetic stars.

astro-ph.SR↗

arXiv2011

The spectral variability and magnetic field characteristics of the Of?p star HD 148937

We report magnetic and spectroscopic observations and modeling of the Of?p star HD 148937 within the context of the MiMeS LP at the CFHT. Thirty-two high signal-to-noise ratio circularly polarised (Stokes V) spectra and 13 unpolarised (Stokes I) spectra of HD 148937 were acquired in 2009 and 2010. A definite detection of a Stokes V Zeeman signature is obtained in the grand mean of all observations (in both LSD mean profiles and individual spectral lines). The longitudinal magnetic field inferred from the Stokes V LSD profiles is consistently negative, in contrast to the essentially zero field strength measured from the diagnostic null profiles. A period search of equivalent width measurements confirms the previously-reported 7.03 d variability period. The variation of equivalent widths is not strictly periodic: we present evidence for evolution of the amount or distribution of circumstellar plasma. Interpreting the 7.03 d period as the stellar rotational period within the context of the ORM, we have phased the equivalent widths and longitudinal field measurements. The longitudinal field measurements show a weak sinusoidal variation of constant sign, with extrema out of phase with the Hα variation by about 0.25 cycles. The inferred magnetic configuration confirms the suggestion of Nazé et al (2010), who proposed that the weaker variability of HD 148937 as compared to other members of this class is a consequence of the stellar geometry. Based on the derived magnetic properties and published wind characteristics, we find a wind magnetic confinement parameter η\ast \simeq 20 and rotation parameter W = 0.12, supporting a picture in which the Halpha emission and other line variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channeled wind. (Abridged.)

astro-ph.SR↗

arXiv2011

HR 5907: Discovery of the most rapidly rotating magnetic B-type star by the MiMeS Collaboration

We report the discovery and analysis of a very strong magnetic field in the rapidly rotating early B-type star HR 5907, based on observations obtained as part of the Magnetism in Massive Stars (MiMeS) project. We infer a rotation period of 0.508276 +0.000015/-0.000012 d from photometric and Hα EW measurements, making this the shortest period, non-degenerate, magnetic massive star known to date. From the comparison of IUE UV and optical spectroscopy with LTE BRUCE/KYLIE models we find a solid-angle integrated, uniform black-body temperature of 17 000 \pm 1000 K, a projected rotational velocity of 290 \pm 10 km/s, an equatorial radius of 3.1 \pm 0.2 R_sun, a stellar mass of 5.5 \pm 0.5 M_sun, and an inclination angle of the rotation axis to our line-of-sight of 70 \pm 10\circ. Our measurements of the longitudinal magnetic field, which vary between -500 and -2000 G, phase coherently with the rotation period and imply a surface dipole field strength of \sim15.7 kG. On the other hand, from fits to mean Least-Squares Deconvolved Stokes V line profiles we infer a dipole field strength of \sim10.4 kG. This disagreement may result from a magnetic configuration more complex than our model, and/or from the non-uniform helium surface abundance distribution. In either case we obtain a magnetic obliquity nearly aligned with the rotation axis (β = 7+2/-1\circ). Our optical spectroscopy also shows weak variability in carbon, silicon and nitrogen lines. The emission variability in hydrogen Balmer and Paschen lines indicates the presence of a dense, highly structured magnetosphere, interpreted as a centrifugally supported, magnetically confined circumstellar disk.

astro-ph.SR↗

arXiv2011

Revisiting the Rigidly Rotating Magnetosphere model for sigma Ori E. I. Observations and Data Analysis

We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star sigma 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 (2005), applied to the specific case of sigma Ori E by Townsend et al. (2005). This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness, and Halpha emission. We analyze 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 A and 6678 A lines are fit poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al. (2005). These results challenge the offset dipole field configuration assumed in the application of the RRM model to sigma Ori E, and indicate that future models of its magnetic field should also include complex, higher-order components.

astro-ph.SR↗

arXiv2011

Discovery of the magnetic field of the B1/B2V star σLupi

In our search for new magnetic massive stars we use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurring in a velocity range symmetric around zero. Our aim is to obtain follow-up spectropolarimetry to search for a magnetic field in magnetic candidate stars. We quantify UV wind line variability, and analyse its time behaviour. The B1/B2V star sigma Lup emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The stellar wind line variations of sigma Lup are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a longitudinal magnetic field with varying strength and amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic and the optical line variations are consistent with the well-known photometric period of 3.02 days, which we identify with the rotation period of the star. Additional observations with ESPaDOnS at CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period. Further analysis revealed that $σ$ Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a spotted surface. We conclude that sigma Lup is a magnetic oblique rotator, and is a He-strong star. It is the 4th B star for which a magnetic field is discovered from studying only its wind variability. Like in the other magnetic B stars the wind emission originates in the magnetic equator, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.02 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field. A full paper will be submitted to A&A.

astro-ph.SR↗

arXiv2011

The Incidence of Magnetic Fields in Massive Stars: An Overview of the MiMeS Survey Component

astro-ph.SR↗