28 research outputs found
VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136
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 MNRA
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
A Spectroscopic Survey of WNL Stars in the LMC: General Properties and Binary Status
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 MNRA
Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest
Precision astrometry at microarcsecond accuracy has application to a wide
range of astrophysical problems. This paper is a study of the science questions
that can be addressed using an instrument that delivers parallaxes at about 4
microarcsec on targets as faint as V = 20, differential accuracy of 0.6
microarcsec on bright targets, and with flexible scheduling. The science topics
are drawn primarily from the Team Key Projects, selected in 2000, for the Space
Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities
of this mission to illustrate the importance of the next level of astrometric
precision in modern astrophysics. SIM PlanetQuest is currently in the detailed
design phase, having completed all of the enabling technologies needed for the
flight instrument in 2005. It will be the first space-based long baseline
Michelson interferometer designed for precision astrometry. SIM will contribute
strongly to many astronomical fields including stellar and galactic
astrophysics, planetary systems around nearby stars, and the study of quasar
and AGN nuclei. SIM will search for planets with masses as small as an Earth
orbiting in the `habitable zone' around the nearest stars using differential
astrometry, and could discover many dozen if Earth-like planets are common. It
will be the most capable instrument for detecting planets around young stars,
thereby providing insights into how planetary systems are born and how they
evolve with time. SIM will observe significant numbers of very high- and
low-mass stars, providing stellar masses to 1%, the accuracy needed to
challenge physical models. Using precision proper motion measurements, SIM will
probe the galactic mass distribution and the formation and evolution of the
Galactic halo. (abridged)Comment: 54 pages, 28 figures, uses emulateapj. Submitted to PAS
WR20a: a massive cornerstone binary system comprising two extreme early-type stars
We analyse spectroscopic observations of WR20a revealing that this star is a
massive early-type binary system with a most probable orbital period of \sim
3.675 days. Our spectra indicate that both components are most likely of WN6ha
or O3If^*/WN6ha spectral type. The orbital solution for a period of 3.675 days
yields extremely large minimum masses of 70.7 \pm 4.0 and 68.8 \pm 3.8
M_{\odot} for the two stars. These properties make WR20a a cornerstone system
for the study of massive star evolution.Comment: 5 pages, 3 figures, accepted by A&A Letter
A Changing Wind Collision
We report on the first detection of a global change in the X-ray emitting properties of a wind–wind collision, thanks to XMM-Newton observations of the massive Small Magellenic Cloud (SMC) system HD 5980. While its light curve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the light-curve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind–wind collision, considering the varying wind properties of the eruptive component in HD 5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions
A first orbital solution for the very massive 30 Dor main-sequence WN6h+O binary R145
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 MNRA
The Eddington factor as the key to understand the winds of the most massive stars. Evidence for a Gamma-dependence of Wolf-Rayet type mass loss
The most massive stars are thought to be hydrogen-rich Wolf-Rayet stars of
late spectral subtype (WNh stars). In previous theoretical studies the enhanced
mass loss of these stars has been attributed to their proximity to the
Eddington limit. Here we investigate observed trends in the mass-loss
properties of such young, very massive stars. We derive theoretical
mass-luminosity relations for very massive stars, based on a large grid of
stellar structure models. Using these relations, we estimate Eddington factors
for a sample of stars, under different assumptions of their evolutionary
status. We evaluate the resulting mass-loss relations, and compare them with
theoretical predictions. We find observational evidence that the mass loss in
the WR regime is dominated by the Eddington parameter Gamma_e, which has
important consequences for the way we understand Wolf-Rayet stars and their
mass loss. In addition, we derive wind masses that support the picture that the
WNh stars in young stellar clusters are very massive, hydrogen-burning stars.
Our findings suggest that the proximity to the Eddington limit is the physical
reason for the onset of Wolf-Rayet type mass loss. This means that, e.g. in
stellar evolution models, the Wolf-Rayet stage should be identified by large
Eddington parameters, instead of a helium-enriched surface composition. The
latter is most likely only a consequence of strong mass loss, in combination
with internal mixing. For very massive stars, the enhanced Gamma-dependent mass
loss is responsible for the formation of late WNh subtypes with high hydrogen
surface abundances, partly close to solar. Because mass loss dominates the
evolution of very massive stars, we expect a strong impact of this effect on
their end products, in particular on the potential formation of black holes,
and Gamma-Ray Bursts, as well as the observed upper mass limit of stars
A Variant of TNFR2-Fc Fusion Protein Exhibits Improved Efficacy in Treating Experimental Rheumatoid Arthritis
Etanercept, a TNF receptor 2-Fc fusion protein, is currently being used for the treatment of rheumatoid arthritis (RA). However, 25% to 38% of patients show no response which is suspected to be partially due to insufficient affinity of this protein to TNFα. By using computational protein design, we found that residue W89 and E92 of TNFR2 were critical for ligand binding. Among several mutants tested, W89Y/E92N displayed 1.49-fold higher neutralizing activity to TNFα, as compared to that of Etanercept. Surface plasmon resonance (SPR) based binding assay revealed that the equilibrium dissociation constant of W89Y/E92N to TNFα was 3.65-fold higher than that of Etanercept. In a rat model of collagen-induced arthritis (CIA), W89Y/E92N showed a significantly better ability than Etanercept in reducing paw swelling and improvement of arthritic joint histopathologically. These data demonstrate that W89Y/E92N is potentially a better candidate with improved efficacy in treating RA and other autoimmune diseases