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A.-N. Chené,
A. F. J. Moffat,
C. Cameron,
R. Fahed,
R. C. Gamen,
L. Lefèvre,
J. F. Rowe, N. St-louis,
V. Muntean,
A. De La Chevrotière,
D. B. Guenther,
R. Kuschnig,
J. M. Matthews,
S. M. Rucinski,
D. Sasselov,
and W. W. Weiss
[show abstract]
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ABSTRACT: A 30 day contiguous photometric run with the Microvariability and Oscillations of STars (MOST) satellite on the WN5-6b star WR 110 (HD 165688) reveals a fundamental periodicity of P = 4.08 ± 0.55 days along with a number of harmonics at periods P/n, with n 2, 3, 4, 5, and 6, and a few other possible stray periodicities and/or stochastic variability on timescales longer than about a day. Spectroscopic radial velocity studies fail to reveal any plausible companion with a period in this range. Therefore, we conjecture that the observed light-curve cusps of amplitude ~0.01 mag that recur at a 4.08 day timescale may arise in the inner parts, or at the base, of a corotating interaction region (CIR) seen in emission as it rotates around with the star at constant angular velocity. The hard X-ray component seen in WR 110 could then be a result of a high velocity component of the CIR shock interacting with the ambient wind at several stellar radii. Given that most hot, luminous stars showing CIRs have two CIR arms, it is possible that either the fundamental period is 8.2 days or, more likely in the case of WR 110, there is indeed a second weaker CIR arm for P = 4.08 days, that occurs ~two-thirds of a rotation period after the main CIR. If this interpretation is correct, WR 110 therefore joins the ranks with three other single WR stars, all WN, with confirmed CIR rotation periods (WR 1, WR 6, and WR 134), albeit with WR 110 having by far the lowest amplitude photometric modulation. This illustrates the power of being able to secure intense, continuous high-precision photometry from space-based platforms such as MOST. It also opens the door to revealing low-amplitude photometric variations in other WN stars, where previous attempts have failed. If all WN stars have CIRs at some level, this could be important for revealing sources of magnetism or pulsation in addition to rotation periods.
The Astrophysical Journal 06/2011; 735(1):34. · 6.02 Impact Factor
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[show abstract]
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ABSTRACT: Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8
stars in general and WR123 in particular? A dedicated spectroscopic campaign
targets the ten-hour period previously found in the high-precision photometric
data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a
series of high signal-to-noise, mid-resolution spectra from several sites in
the {\lambda}{\lambda} 4000 - 6940 A^{\circ} domain. We also followed the star
with occasional broadband (Johnson V) photometry. The acquired spectroscopy
allowed a detailed study of spectral variability on timescales from \sim 5
minutes to months. Results. We find that all observed spectral lines of a given
chemical element tend to show similar variations and that there is a good
correlation between the lines of different elements, without any significant
time delays, save the strong absorption components of the Hei lines, which tend
to vary differently from the emission parts. We find a single sustained
periodicity, P \sim 9.8 h, which is likely related to the relatively stable
pulsations found in MOST photometry obtained one year later. In addition,
seemingly stochastic, large-amplitude variations are also seen in all spectral
lines on timescales of several hours to several days.
04/2011;
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ABSTRACT: This study is the second part of a survey searching for large-scale spectroscopic variability in apparently single Wolf-Rayet (WR) stars. In a previous paper (Paper I), we described and characterized the spectroscopic variability level of 25 WR stars observable from the northern hemisphere and found 3 new candidates presenting large-scale wind variability, potentially origi-nating from large-scale structures named Co-rotating Interaction Regions (CIRs). In this second paper, we discuss an additional 39 stars observable from the southern hemisphere. For each star in our sample, we obtained 4–5 high-resolution spectra with a signal-to-noise ratio of ∼100 and determined its variability level using the approach described in Paper I. In total, 10 new stars are found to show large-scale spectral variability of which 7 present CIR-type changes (WR 8, WR 44, WR55, WR 58, WR 61, WR 63, WR 100). Of the remaining stars, 20 were found to show small-amplitude changes and 9 were found to show no spectral variability as far as can be concluded from the data in hand. Also, we discuss the spectroscopic variability level of all single galactic WR stars that are brighter than v ∼ 12.5, and some WR stars with 12.5 < v ≤ 13.5; i.e. all the stars presented in our two papers and 4 more stars for which spectra have already been published in the literature. We find that 23/68 stars (33.8%) present large-scale variability, but only 12/54 stars (∼22.1%) are potentially of CIR-type. Also, we find 31/68 stars (45.6%) that only show small-scale variability, most likely due to clumping in the wind. Finally, no spectral variability is detected based on the data in hand for 14/68 (20.6%) stars. Interestingly, the variability with the highest amplitude also have the widest mean velocity dispersion.
04/2011;
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[show abstract]
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ABSTRACT: The Wolf-Rayet star WR 46 is known to exhibit a very complex variability
pattern on relatively short time scales of a few hours. Periodic but
intermittent radial velocity shifts of optical lines as well as multiple
photometric periods have been found in the past. Non-radial pulsations, rapid
rotational modulation or the presence of a putative low-mass companion have
been proposed to explain the short-term behaviour. In an effort to unveil its
true nature, we observed WR 46 with FUSE (Far Ultraviolet Spectroscopic
Explorer) over several short-term variability cycles. We found significant
variations on a time scale of ~8 hours in the far-ultraviolet (FUV) continuum,
in the blue edge of the absorption trough of the OVI {\lambda}{\lambda}1032,
1038 doublet P Cygni profile and in the SVI {\lambda}{\lambda}933, 944 P Cygni
absorption profile. We complemented these observations with X-ray and UV
light-curves and an X-ray spectrum from archival XMM-Newton (X-ray Multi-Mirror
Mission - Newton Space Telescope) data. The X-ray and UV light-curves show
variations on a time scale similar to the variability found in the FUV. We
discuss our results in the context of the different scenarios suggested to
explain the short-term variability of this object and reiterate that non-radial
pulsations is the most likely to occur.
04/2011;
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[show abstract]
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ABSTRACT: We present the results of an intensive photometric and spectroscopic monitoring campaign of the WN4 Wolf-Rayet (WR) star WR 1 = HD 4004. Our broadband V photometry covering a timespan of 91 days shows variability with a period of P = 16.9+0.6 –0.3 days. The same period is also found in our spectral data. The light curve is non-sinusoidal with hints of a gradual change in its shape as a function of time. The photometric variations nevertheless remain coherent over several cycles and we estimate that the coherence timescale of the light curve is of the order of 60 days. The spectroscopy shows large-scale line-profile variability which can be interpreted as excess emission peaks moving from one side of the profile to the other on a timescale of several days. Although we cannot unequivocally exclude the unlikely possibility that WR 1 is a binary, we propose that the nature of the variability we have found strongly suggests that it is due to the presence in the wind of the WR star of large-scale structures, most likely corotating interaction regions (CIRs), which are predicted to arise in inherently unstable radiatively driven winds when they are perturbed at their base. We also suggest that variability observed in WR 6, WR 134, and WR 137 is of the same nature. Finally, assuming that the period of CIRs is related to the rotational period, we estimate the rotation rate of the four stars for which sufficient monitoring has been carried out, i.e., v rot = 6.5, 40, 70, and 275 km s–1 for WR 1, WR 6, WR 134, and WR 137, respectively.
The Astrophysical Journal 05/2010; 716(2):929. · 6.02 Impact Factor
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ABSTRACT: Theoretical work suggests that strange-mode pulsations (SMPs) are
present in the envelope of hot and luminous stars with a large
luminosity-to-mass ratio, where the thermal timescale is short compared
to the dynamical timescale, and where radiation pressure dominates
(Glatzel et al. 1993). The most violent SMPs are expected in classical
Wolf-Rayet (WR) stars, i.e the bare, compact helium-burning cores of
evolved massive stars (Glatzel et al. 1999), where SMPs manifest
themselves in cyclic photometric variability with periods ranging from
minutes to hours. However, these variabilities are expected to be
epoch-dependent. Here we report on our attempts to detect SMPs in
several WR stars using rapid, high-precision photometry.
Communications in Asteroseismology 06/2009; 158:214.
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[show abstract]
[hide abstract]
ABSTRACT: We present the results of a systematic search for large-scale spectroscopic
variability in apparently single Wolf-Rayet stars brighter than ~12.5. In this
first paper we characterize the various forms of variability detected and
distinguish several separate groups. For each star in our sample, we obtained
4-5 high-resolution spectra with a signal-to-noise ratio ~100. Our ultimate
goal is to identify new candidates presenting variability that potentially
comes from Co-rotating Interaction Regions (CIR).
Out of a sample of 25 stars, 10 were found to display large-scale changes of
which 4 are of CIR-type (WR1, WR115, WR120 and WR134). The star WR134 was
already known to show such changes from previous studies. Three WN8 stars
present a different type of large-scale variability and we believe deserve a
group of their own. Also, all three WC9d stars in our sample present
large-scale variability, but it remains to be checked if these are binaries, as
many dust-making WR stars are double. Finally, of the remaining stars, 10 were
found to show small-amplitude spectral changes which we attribute to normal
line-profile variability due to inhomogeneities in the wind, and 5 were found
to show no spectral variability, as far as can be concluded from the data in
hand.
Follow-up studies are required to identify potential periods for our
candidates showing CIR-type changes and eventually estimate a rotation rate for
these WR stars.
05/2009;
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[show abstract]
[hide abstract]
ABSTRACT: Using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of the very young (~1 Myr), massive and dense cluster R136, in the Large Magellanic Cloud (LMC). We also de-archived available images that were obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS), and extracted high-quality, differential photometry of our target stars to check for any variability related to binary motion. Previous studies, relying on spatially unresolved, integrated, optical spectroscopy, had reported that one of these stars was likely to be a 4.377-day binary. Our study set out to identify the culprit and any other short-period system among our targets. However, none displays significant photometric variability, and only one star, BAT99-112 (R136c), located on the outer fringe of R136, displays a marginal variability in its radial velocities; we tentatively report an 8.2-day period. The binary status of BAT99-112 is supported by the fact that it is one of the brightest X-ray sources among all known WR stars in the LMC, consistent with it being a colliding-wind system. Follow-up observations have been proposed to confirm the orbital period of this potentially very massive system. Comment: 9 pages, 6 figures; accepted for publication in MNRAS
05/2009;
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D. Massa,
W. Fullerton,
J. S. Nichols,
S. P. Owocki,
R. K. Prinja, N. St-Louis,
A. J. Willis,
B. Altner,
C. T. Bolton,
J. P. Cassinelli, [......],
S. R. McCandliss,
J. Moffat,
T. Nugis,
J. Puls,
C. Robert,
R. E. Schulte-Ladbeck,
L. J. Smith,
M. A. Smith,
W. L. Waldron,
and R. L. White
[show abstract]
[hide abstract]
ABSTRACT: Wind variability in OB stars may be ubiquitous, and a connection between projected stellar rotation velocity and wind activity is well established. However, the origin of this connection is unknown. To probe the nature of the rotation connection, several of the attendees at the workshop on Instability and Variability of Hot-Star Winds drafted an IUE observing proposal. The goal of this program was to follow three stars for several rotations to determine whether the rotation connection is correlative or causal. The stars selected for monitoring all have rotation periods ≤5 days. They were HD 50896 (WN5), HD 64760 (B0.5 Ib), and HD 66811 [ζ Pup; O4 If(n)]. During 16 days of nearly continuous observations in 1995 January (dubbed the "MEGA" campaign), 444 high-dispersion IUE spectra of these stars were obtained. This Letter presents an overview of the results of the MEGA campaign and provides an introduction to the three following Letters, which discuss the results for each star.
The Astrophysical Journal 01/2009; 452(1):L53. · 6.02 Impact Factor
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[hide abstract]
ABSTRACT: We report the results of a spectroscopic and polarimetric study of the massive, hydrogen-rich WN6h stars R144 (HD 38282 = BAT99-118 = Brey 89) and R145 (HDE 269928 = BAT99-119 = Brey 90) in the LMC. Both stars have been suspected to be binaries by previous studies (R144: Schnurr et al. 2008b; R145: Moffat 1989). We have combined radial-velocity (RV) data from these two studies with previously unpublished polarimetric data. For R145, we were able to establish, for the first time, an orbital period of 158.8 days, along with the full set of orbital parameters, including the inclination angle i, which was found to be i = 38 \pm 9 deg. By applying a modified version of the shift-and-add method developed by Demers et al. (2002), we were able to isolate the spectral signature of the very faint-line companion star. With the RV amplitudes of both components in R145, we were thus able to estimate their absolute masses. We find minimum masses M_WR sin^{3}i = (116 \pm 33) M_sol and M_O sin^{3}i = (48 \pm 20)$ M_sol for the WR and the O component, respectively. Thus, if the low inclination angle were correct, resulting absolute masses of the components would be at least 300 and 125 M_sol, respectively. However, such high masses are not supported by brightness considerations when R145 is compared to systems with known, very high masses such as NGC3603-A1 or WR20a. An inclination angle close to 90 degrees would remedy the situation, but is excluded by the currently available data. More and better data are thus required to firmly establish the nature of this puzzling, yet potentially very massive and important system. As to R144, however, the combined data sets are not sufficient to find any periodicity. Comment: 15 pages, 13 figures; accepted for publication by MNRAS
01/2009;
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L. Lefèvre,
S. V. Marchenko,
A. F. J. Moffat,
A. N. Chené,
S. R. Smith, N. St-Louis,
J. M. Matthews,
R. Kuschnig,
D. B. Guenther,
C. A. Poteet,
S. M. Rucinski,
D. Sasselov,
G. A. H. Walker,
and W. W. Weiss
[show abstract]
[hide abstract]
ABSTRACT: We present the results of intensive visual-broadband photometric monitoring of the highly variable WN8 Wolf-Rayet star WR 123, obtained by the MOST (Microvariability and Oscillations of STars) satellite. This first Canadian astronomical space telescope observed WR 123 for 38 days nonstop during 2004 June and July. Fourier analysis shows that no periodic signal is stable for more than several days in the low-frequency domain (f < 1 day-1), where most of the stochastic power is contained. Also, no significant variability is seen in the high-frequency domain (10 day-1 < f < 1400 day-1) down to the level of 0.2 mmag, an order of magnitude lower than theoretical predictions for strange-mode pulsations. On the other hand, there seems to be a relatively stable 9.8 hr periodic signal present throughout the whole run. This period is probably too short to represent the axial rotation of the star, unless it is related to multiple substructures equidistantly spread along the stellar equator. It is also too short to be orbital in nature; it is more likely to be related to pulsational instablilities (although with a much longer period than expected), thus finally revealing a possible fundamental driver behind the highly variable wind of this object, and others of similar type.
The Astrophysical Journal 12/2008; 634(1):L109. · 6.02 Impact Factor
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[hide abstract]
ABSTRACT: In this paper we present unfiltered and multiband (i.e., UBVRI) polarimetric observations of the short-period Wolf-Rayet binary CQ Cep. Using the basic assumptions of an optically thin, corotating envelope and pointlike sources (i.e., BME78 assumptions), we determined the orbital parameters of the system (i.e., i = 99° ± 1° and Ω = 76° ± 2° at the 2 σ level) with an accuracy many times better than any previous work. Residual non-BME78 variability around phase 0.0 was present in our data, which we associate with the polarimetric eclipse of the dense central parts of the Wolf-Rayet (W-R) wind by the orbiting O star. We attribute the observed phase lag of -0.15 between our residuals and those expected for a standard polarimetric eclipse to a wind-wind interaction (WWI) region distorted by Coriolis forces using the model presented by Marchenko et al. This model was also able to explain the strong wavelength dependence of the polarimetric amplitudes in our multiband observations. Our analysis also reveals important epoch-dependent departures of the matter distribution from spherical symmetry that were not related to the orbital plane and therefore cannot be the result of tidal interaction. We conclude that binarity is not playing an important role in driving the wind of the W-R star in CQ Cep and contributing to the observed nonspherical matter distribution. On the other hand, this asymmetry could be explained by a rotationally induced disk misaligned with the orbital plane.
The Astrophysical Journal 12/2008; 623(2):1092. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortest period known in our Galaxy. To examine the circumstellar matter distribution and to better constraint the orbital parameters and mass-loss rate of the W-R star, we obtained broadband and multiband (i.e., UBVRI) linear polarization observations of the system. Our analysis of the phase-locked polarimetric modulation confirms the high orbital inclination of the system (i.e., i = 65°). Using the orbital solution of Lewis et al. (1993), we obtain masses of 33.9 and 23.9 M☉ for the O and W-R stars, respectively, which agree with their spectral types. A simple polarimetric model accounting for finite stellar size effects allowed us to derive a mass-loss rate for the W-R star of (0.3-0.5) × 10-5 M☉ yr-1. This result was remarkably independent of the model's input parameters and favors an earlier spectral type for the W-R component (i.e., WN4). Finally, using our multiband observations, we fitted and subtracted from our data the interstellar polarization. The resulting constant intrinsic polarization of 3%-4% is misaligned in relation to the orbital plane (i.e., θCIP = 26° vs. Ω = 75°) and is the highest intrinsic polarization ever observed for a W-R star. This misalignment points toward a rotational (or magnetic) origin for the asymmetry and contradicts the most recent evolutionary models for massive stars (Meynet & Maeder 2003) that predict spherically symmetric winds during the W-R phase (i.e., CIP = 0%).
The Astrophysical Journal 12/2008; 640(2):995. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf–Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This survey concludes the decade-long effort of the Montréal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the six crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial velocity (RV) variability in order to identify the short- to intermediate-period (P≲ 200 d) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries is classical, helium-burning WNL star, but rather superluminous, hence extremely massive, hydrogen-burning object, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group.
Monthly Notices of the Royal Astronomical Society 09/2008; 389(2):806 - 828. · 4.90 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This survey concludes the decade-long effort of the Montr\'eal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the 6 crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial-velocity (RV) variability in order to identify the short- to intermediate-period ($P \la 200$ days) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries are classical, helium-burning WNL stars, but rather superluminous, hence extremely massive, hydrogen-burning objects, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group. Comment: 25 pages, 11 figures; accepted for MNRAS
06/2008;
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[show abstract]
[hide abstract]
ABSTRACT: Using VLT/SINFONI, we have obtained repeated AO-assisted, NIR spectroscopy of the three central WN6ha stars in the core of the very young (~1 Myr), massive and dense Galactic cluster NGC3603. One of these stars, NGC3603-A1, is a known 3.77-day, double-eclipsing binary, while another one, NGC3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-day binary, although only the WN6ha component is visible in our spectra; therefore we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M_1 = (116 \pm 31) Mo for the primary and M_2 = (89 \pm 16) Mo for the secondary component of A1. While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 Mo, respectively; therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far. Comment: 5 pages, 4 figures; accepted for MNRAS Letters
06/2008;
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A. F. J. Moffat,
S. V. Marchenko,
L. Lefèvre,
A.-N. Chené, N. St-Louis,
B. E. Zhilyaev,
C. Aerts,
H. Saio,
G. A. H. Walker,
J. M. Matthews,
R. Kuschnig,
C. Cameron,
J. F. Rowe,
D. B. Guenther,
S. M. Rucinski,
D. Sasselov,
W. W. Weiss
[show abstract]
[hide abstract]
ABSTRACT: It has now been three years since the first Canadian space telescope
MOST (Microvariability and Oscillations of STars) continues to make its
unique mark in stellar asteroseismology, exoplanetology and other
studies of high-precision photometric variability. Among massive stars,
three OB, three later-type B stars and two Wolf-Rayet stars have been
studied so far with unprecedented precision and time coverage in samples
with of order 100 000 data points collected without a break over several
weeks. Of particular interest are: a first clear pulsation period of P =
9.8h has been found in a WR star (WR123, WN8); no short periods between
a minute and an hour have been seen in either of the two WR stars
observed (WR123; WR103, WC9d) to the 0.2 mmag level, although these
stars both exhibit numerous short-lived oscillations mostly with periods
longer than a day, which must be related to stellar pulsations; g-mode
pulsations were detected in a blue supergiant; non-radial g-mode
pulsations may be excited in all classical Be stars and thus may play a
pivotal rôle in the Be-star mass-ejection process. A review of all
the massive-star results to date from MOST and their implications are
presented.
05/2008; 388:29.
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C. Foellmi,
G. Koenigsberger,
L. Georgiev,
O. Toledano,
S. V. Marchenko,
P. Massey,
T. H. Dall,
A. F. J. Moffat,
N. Morrell,
M. Corcoran,
A. Kaufer,
Y. Naze,
J. Pittard, N. St-Louis,
A. Fullerton,
D. Massa,
A. M. T. Pollock
[show abstract]
[hide abstract]
ABSTRACT: We present the results of optical wavelength observations of the unusual SMC eclipsing binary system HD 5980 obtained in 1999 and 2004--2005. Radial velocity curves for the erupting LBV/WR object (star A) and its close WR-like companion (star B) are obtained by deblending the variable emission-line profiles of N IV and N V lines under the simplistic assumption that these lines originate primarily in the winds of star A and star B. The derived masses M_A=58--79 Mo and M_B=51--67 Mo, are more consistent with the stars' location near the top of the HRD than previous estimates. The presence of a wind-wind interaction region is inferred from the orbital phase-dependent behavior of He I P Cygni absorption components. The emission-line intensities continued with the declining trend previously seen in UV spectra. The behavior of the photospheric absorption lines is consistent with the results of Schweickhardt (2002) who concludes that the third object in the combined spectrum, star C, is also a binary system with P(starC)~96.5 days, e=0.83. The data used in this paper will be made publicly available for further analysis. Comment: 48 pages, 26 figures
11/2007;
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[show abstract]
[hide abstract]
ABSTRACT: In the past, different authors have suspected that many linearly
polarized standard stars are actually variable objects. This conclusion
has been challenged, at least for some of these observations. We revisit
this subject and show that while some care is definitely needed, at
least some of the stars suspected to vary in fact do vary. Therefore
they should no longer be used as polarized standard stars for precision
work. A statistical test, better than those used in the past and based
on the cumulative distribution function of the polarization data, is
presented in some detail.
03/2007; 364:529.
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[show abstract]
[hide abstract]
ABSTRACT: We present a preliminary analysis of FUSE observations for three
Wolf-Rayet (WR) binaries in the Magellanic Clouds: Sand 1 (SMC), Brey 22
(LMC) and Brey 32 (LMC). Phase-dependent variability is detected for
these systems which we attribute to selective wind eclipses and excess
emission from the shock cone that is formed when the two massive winds
in the system collide. For Sand 1, profile fits yield an orbital
inclination of i˜40°, a total cone opening-angle of
˜80-90° and a streaming velocity along the shock cone of
˜3000 kms-1. For Brey 22 and Brey 32, two systems with
almost the same spectral type, similar changes are observed but because
the orbital periods are very different for these two binaries, some
notable differences in their variability patterns are found.
05/2006; 348:121.
