M. E. Oksala

Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C

We report on four Chandra grating observations of the oblique magnetic rotator � 1 Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star’s 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray–emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hotplasma. We investigate these diagnostics as a function of viewing angle andanalyzetheminconjunctionwithnewMHDsimulationsofthemagneticallychanneledwindshockmechanism on � 1 Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray–emitting plasma with rotation phase. Subject headingg stars: early-type — stars: individual (HD 37022) — stars: magnetic fields — stars: mass loss — stars: rotation — stars: winds, outflows — X-rays: stars Online material: color figure

A magnetic confinement versus 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 Alfv´ en radius RA) and stellar rotation (which sets the Kepler co-rotation radius RK) provides a useful organization 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 versus RA. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with RA > RK ,v ersusdynamical magnetospheres (DM), with RK > RA. We show how key observational diagnostics, like the presence and characteristics of Hα 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 Hα 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 time-scale 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 time-scales. Finally, we discuss how other observational diagnostics, e.g. variability of UV wind lines or X-ray emission, relate to the inferred magnetic properties of these stars, and summarize prospects for future developments in our understanding of massive-star magnetospheres.

The MiMeS survey of magnetism in massive stars: introduction and overview

The Magnetism in Massive Stars (MiMeS) survey represents a highprecision 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) ubiquitous evidence for magnetic wind confinement in optical spectra of all magnetic O stars, and (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.

DISCOVERY OF ROTATIONAL BRAKING IN THE MAGNETIC HELIUM-STRONG STAR SIGMA ORIONIS E

We present new U-band photometry of the magnetic helium-strong star σ Ori E, obtained over 2004–2009 using the SMARTS 0.9 m telescope at Cerro Tololo Inter-American Observatory. When combined with historical measurements, these data constrain the evolution of the star’s 1.19 day rotation period over the past three decades. We are able to rule out a constant period at the pnull = 0.05% level, and instead find that the data are well described (pnull = 99.3%) by a period increasing linearly at a rate of 77 ms per year. This corresponds to a characteristic spin-down time of 1.34 Myr, in good agreement with theoretical predictions based on magnetohydrodynamical simulations of angular momentum loss from magnetic massive stars. We therefore conclude that the observations are consistent with σ Ori E undergoing rotational braking due to its magnetized line-driven wind.

Discovery of a strong magnetic field in the rapidly rotating B2 Vn star HR 7355

We report the detection of a strong, organized magnetic field in the He-variable early B-type star HR 7355 using spectropolarimetric data obtained with ESPaDOnS on the 3.6-m Canada– France–Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is both the most rapidly rotating known main-sequence magnetic star and the most rapidly rotating He-strong star, with v sin i = 300 ± 15km s −1 and a rotational period of 0.521 4404 ± 0.000 0006 d. We have modelled our eight longitudinal magnetic field measurements assuming an oblique dipole magnetic field. Constraining the inclination of the rotation axis to be between 38 ◦ and 86 ◦ , we find the magnetic obliquity angle to be between 30 ◦ and 85 ◦ , and the polar strength of the magnetic field at the stellar surface to be between 13–17 kG. The photometric light curve constructed from HIPPARCOS archival data and new CTIO measurements shows two minima separated by 0.5 in rotational phase and occurring 0.25 cycles before/after the magnetic extrema. This photometric behaviour, coupled with previously reported variable emission of the Hα line (which we confirm), strongly supports the proposal that HR 7355 harbours a structured magnetosphere similar to that in the prototypical He-strong star, σ Ori E.

Revisiting the Rigidly Rotating Magnetosphere model for σ Ori E – I. Observations and data analysis

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.

Probing the ejecta of evolved massive stars in transition - A VLT/SINFONI K-band survey

Massive evolved stars in transition phases, such as Luminous Blue Variables (LBVs), B[e] Supergiants (B[e]SGs), and Yellow Hypergiants (YHGs), are not well understood, and yet crucial steps in determining accurate stellar and galactic evolution models. The circumstellar environments of these stars reveal their mass-loss history, identifying clues to both their individual evolutionary status and the connection between objects of different phases. Here we present a survey of 25 such evolved massive stars (16 B[e]SGs, 6 LBVs, 2 YHGs, and 1 Peculiar Oe star), observed in the K-band with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 8 m telescope. The sample can be split into two categories based on spectral morphology: one group includes all of the B[e]SGs, the Peculiar Oe star, and two of the LBVs, while the other includes the YHGs and the rest of the LBVs. The difference in LBV spectral appearance is due to some objects being in a quiescent phase and some objects being in an active or outburst phase. CO emission features are found in 13 of our targets, with first time detections for MWC 137, LHA 120-S 35, and LHA 115-S 65. From model fits to the CO band heads, the emitting regions appear to be detached from the stellar surface. Each star with ^12CO features also shows ^13CO emission, signaling an evolved nature. Based on the level of ^13C enrichment, we conclude that many of the B[e]SGs are likely in a pre-Red Supergiant phase of their evolution. There appears to be a lower luminosity limit of log L/L_solar = 5.0 below which CO is not detected. The lack of CO features in several high luminosity B[e]SGs and variability in others suggests that they may in fact be LBV candidates, strengthening the connection between these two very similar transition phases.

The MiMeS survey of Magnetism in Massive Stars: magnetic analysis of the O-type stars

We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the Magnetism in Massive Stars (MiMeS) Survey. Mean least-squares deconvolved Stokes I and V line profiles were extracted for each observation, from which we measured the radial velocity, rotational and non-rotational broadening velocities, and longitudinal magnetic field Bl. The investigation of the Stokes I profiles led to the discovery of two new multiline spectroscopic systems (HD 46106, HD 204827) and confirmed the presence of a suspected companion in HD 37041. We present a modified strategy of the least-squares deconvolution technique aimed at optimizing the detection of magnetic signatures while minimizing the detection of spurious signatures in Stokes V. Using this analysis, we confirm the detection of a magnetic field in six targets previously reported as magnetic by the MiMeS collaboration (HD 108, HD 47129A2, HD 57682, HD 148937, CPD-28 2561, and NGC 1624-2), as well as report the presence of signal in Stokes V in three new magnetic candidates (HD 36486, HD 162978, and HD 199579). Overall, we find a magnetic incidence rate of 7 ± 3 per cent, for 108 individual O stars (including all O-type components part of multiline systems), with a median uncertainty of the Bl measurements of about 50 G. An inspection of the data reveals no obvious biases affecting the incidence rate or the preference for detecting magnetic signatures in the magnetic stars. Similar to A- and B-type stars, we find no link between the stars’ physical properties (e.g. Teff, mass, and age) and the presence of a magnetic field. However, the Of?p stars represent a distinct class of magnetic O-type stars.

Molecular emission from GG Carinae’s circumbinary disk

The appearance of the B[e] phenomenon in evolved massive stars such as B[e] supergiants is still a mystery. While these stars are generally found to have disks that are cool and dense enough for efficient molecule and dust condensation, the origin of the disk material is still unclear. We aim at studying the kinematics and origin of the disk in the eccentric binary system GG Car, whose primary component is proposed to be a B[e] supergiant. Based on medium- and high-resolution near-infrared spectra we analyzed the CO-band emission detected from GG Car. The complete CO-band structure delivers information on the density and temperature of the emitting region, and the detectable 13CO bands allow us to constrain the evolutionary phase. In addition, the kinematics of the CO gas can be extracted from the shape of the first 12CO band head. We find that the CO gas is located in a ring surrounding the eccentric binary system, and its kinematics agrees with Keplerian rotation with a velocity, projected to the line of sight, of (80\pm 1) km/s. The CO ring has a column density of (5\pm 3)x10^21 cm^-2 and a temperature of 3200\pm 500 K. In addition, the material is chemically enriched in 13CO, which agrees with the primary component being slightly evolved off the main sequence. We discuss two possible scenarios for the origin of the circumbinary disk: (i) non-conservative Roche lobe overflow, and (ii) the possibility that the progenitor of the primary component could have been a classical Be star. Neither can be firmly excluded, but for Roche lobe overflow to occur, a combination of stellar and orbital parameter extremawould be required.

Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment. Aims. One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. Methods. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e. g., surface abundance peculiarities, Ha emission), which are also of interest for future studies. Results. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Ha emission with respect to the theory of centrifugally-supported magnetospheres.