A. F. J. Moffat

Early-type stars in the core of the young open cluster Westerlund 2 ⋆⋆⋆

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link bet ween the cluster and the very massive Wolf-Rayet binary WR 20a as well as the Hii complex RCW 49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund 2 are used to search for light variability and to establish the spectral types of the early-type stars in the c luster core. Results. The first light curves of the eclipsing binary WR 20a in B and V filters are analysed and a distance of 8 kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometri c variability above 1% at the 1-σ level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significant ly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR 20a, indicating that WR 20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0± 1.4 kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund 2 does not provide enough ionizing photons to account for the radio emission of the RCW 49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW 49.

MOST Detects g-Modes in the Be Star HD 163868*

We have extracted a 37 day light curve with a precision of 0.0012 mag per point for the Microvariability and Oscillations of Stars (MOST) guide star, HD 163868 (B5 Ve). Its rich frequency spectrum resembles that of a slowly pulsating B (SPB) star but, being a rapid rotator, we designate it SPBe. The 60 most significant periods lie in three distinct groups centered on 8 days and 14 and 7 hr. We demonstrate that the 14 and 7 hr periods can be modeled by two swarms of high-order, prograde sectorial g-modes (m = -1, -2), which are destabilized by the iron opacity bump. Our model also predicts a group of r-modes with periods near 2.3 days, which correspond to frequencies observed in the tail of the 8 day group. The remaining periodicities, between 7 and 11 days, cannot be explained by unstable modes in our model.

The Hipparcos distance determination of the Wolf-Rayet system gamma(2) Velorum (WC8+O) and its ramifications

Hipparcos parallax measurements give a distance to the Wolf-Rayet WC8 + O spectroscopic binary gamma(2) Vel of d = 258(-31)(+41) pc and a distance to the O4I(n)f star zeta Pup of d = 429(-77)(+120) pc. Adopting for gamma(2) Vel an interstellar extinction of A(v) = 0.06 mag, this implies an absolute magnitude M-v = -5.4 mag for the WC8 + O binary system. Given that the binary components have a magnitude difference Delta m = 1.4 mag, we derive M-v(WC8) = -3.7 and M-v(O) = -5.0 mag. The latter indicates an 08.5III rather than an O9I companion, as was adopted during the last 25 years. Apparently gamma(2) Vel is not a member of, but a foreground object before the open cluster Cr173 and the association Vel OB2. Given a re-assessment of the distance of the Gum Nebula, gamma(2) Vel is still one of its ionizing sources, while zeta Pup appears to be located at the back of the Gum Nebula. Consequences of the Hipparcos distance determination of gamma(2) Vel for its mass, mass loss rate, luminosities at various wavelengths, and, briefly, its association with the Gum Nebula, are discussed

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 t he faint Of?p star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric pro perties. Modeling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main sequence star of mass M≃ 30 M⊙, and infer an effective temperature of 35± 2 kK and log g = 4.0± 0.2. Based on an extensive time series of optical spectral observation s we report significant variability of a large number of spectral lines, and infer a unique period of 157.99± 0.94 d which we interpret as the rotational period of the star. We report the detec tion of a very strong - 5.35± 0.5 kG - longitudinal magnetic field h Bzi , coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus h Bi of 14± 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 Stoke s I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of Oiii, 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 effi ciency log Lx/Lbol =−6.4, a factor of 4 larger than the canonical value for O-type sta rs and comparable to that of the young magnetic O-type starθ 1 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∗≃ 1.5× 10 4 , and a very large Alfven radius, RAlf = 11.4 R∗.

Polarization variability among Wolf-Rayet stars. V - Linear polarization of the bright Cygnus stars and an anticorrelation of variability with wind speed

This paper presents polarimetric data for seven of the eight bright WR stars in Cygnus. Six of the stars show only random, low-amplitude modulation on time scales of hours to days. One of these behaves as expected for a long-period WR + O system with an elliptical orbit. The eighth star is the only short-period WR + O binary in the sample. It is confirmed that the degree of random, intrinsic scatter in polarization is correlated with spectral subclass and terminal wind velocity. It is suggested that this is caused by the presence of propagating blobs which form, survive, and/or grow more easily in slower winds. Two models are proposed to explain the origin of the blobs. 93 refs.

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 (Plas- ketts star), in the context of the Magnetism in Massive Star s (MiMeS) survey. Eight inde- pendent Stokes V observations were acquired using the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope and the Narval spectropolarimeter at the Telescope 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 di agnostic 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 co mponent is the magnetized star. Cor- recting the polarized spectra for the line and continuum of the (sharp-lined) primary, we mea- sured the longitudinal magnetic field from each observation . The longitudinal field of the secondary is variable and exhibits extreme values of−810± 150 G and +680± 190 G, im- plying a minimum surface dipole polar strength of 2850± 500 G. In contrast, we derive an upper limit (3σ) to the primarys surface magnetic field of 230 G. The combina tion of a strong magnetic field and rapid rotation leads us to conclude that th e secondary hosts a centrifugal magnetosphere fed through a magnetically confined wind. We r evisit the properties of the op- tical line profiles and X-ray emission - previously interpre ted 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 rotat ing, magnetized component - that will be a rich target for further study.

Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data

The MOST (Microvariability and Oscillations of Stars) satellite obtains ultraprecise photometry from space with high sampling rates and duty cycles. Astronomical photometry or imaging missions in low Earth orbits, like MOST, are especially sensitive to scattered light from Earthshine, and all these missions have a common need to extract target information from voluminous data cubes. They consist of upwards of hundreds of thousands of two-dimensional CCD frames (or subrasters) containing from hundreds to millions of pixels each, where the target information, superposed on background and instrumental effects, is contained only in a subset of pixels (Fabry Images, defocused images, mini-spectra). We describe a novel reduction technique for such data cubes: resolving linear correlations of target and background pixel intensities. This step-wise multiple linear regression removes only those target variations which are also detected in the background. The advantage of regression analysis versus background subtraction is the appropriate scaling, taking into account that the amount of contamination may differ from pixel to pixel. The multivariate solution for all pairs of target/background pixels is minimally invasive of the raw photometry while being very effective in reducing contamination due to, e.g. stray light. The technique is tested and demonstrated with both simulated oscillation signals and real MOST photometry.

High-resolution X-ray imaging of the colliding wind shock in WR 147

We analyze new high-resolution Chandra X-ray images of the Wolf-Rayet binary system WR147. This system contains a WN8 star with an early-type companion located 0:6 00 to its north, and is the only known early- type binary with a separation on the sky large enough for the wind-wind collision between the stars to currently be resolved at X-ray energies. The 5 ksec Chandra HRC-I image provides the rst direct evidence for spatially extended X-ray emission in an early-type binary system. The X-ray emission peaks close to the position of the radio bow shock and north of the WN8 star. A deeper X-ray image is needed to accurately determine the degree of spatial extension, to exactly align the X-ray and optical/radio frames, and to determine whether part of the detected X-ray emission arises in the individual stellar winds. Simulated X-ray images of the wind-wind collision have a FWHM consistent with the data, and maximum likelihood ts suggest that a deeper observation may also constrain the inclination and wind momentum ratio of this system. However, as the WR wind dominates the colliding wind X-ray emission it appears unlikely that _ M OB and v1OB can be separately determined from X-ray observations. We also note an inconsistency between numerical and analytical estimates of the X-ray luminosity ratio of the stronger and weaker wind components, and conclude that the analytical results are in error.

Asteroseismology of Hybrid Pulsators Made Possible: Simultaneous MOST Space Photometry and Ground-Based Spectroscopy of γ Peg

We have acquired simultaneous high-precision space photometry and radial velocities of the bright hybrid β Cep/Slowly Pulsating B pulsator γ Peg. Frequency analyses reveal the presence of six gravity (g) modes of high radial order together with eight low-order β Cep oscillations in both data sets. Mode identification shows

Polarization variability among Wolf-Rayet stars. VI. Linear polarimetry of the eclipsing binary V444 Cygni

Precision broad-band blue light monitoring in linear polarization of the bright Cygnus W-R + O system V444 Cyg is reported. Analysis of the data yields an orbital inclination i = 78.5 deg, in good agreement with previous estimates based on polarimetry and on the analysis of light curves. In addition, V444 Cyg shows rapid polarimetric modulation around phase 0.5, when the O star companion eclipses the scattering electrons located primarily in the dense inner W-R wind. Simple modeling of the polarization eclipse based on a spherical electron envelope around the W-R star yields a satisfactory fit to the data. This model also supports the small value of the W-R core radius previously proposed by other authors. 35 refs.