arXiv subjects

V. Hénault-Brunet

Publications and source records attributed to V. Hénault-Brunet.

31 records · Page 2Linked to original sources

arXiv2014

The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence

The evolution and fate of very massive stars (VMS) is tightly connected to their mass-loss properties. Their initial and final masses differ significantly as a result of mass loss. VMS have strong stellar winds and extremely high ionising fluxes, which are thought to be critical sources of both mechanical and radiative feedback in giant Hii regions. However, how VMS mass-loss properties change during stellar evolution is poorly understood. In the framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss transition region from optically thin O to denser WNh star winds, thereby testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and WNh stars, an unprecedented sample of stars with the highest masses and luminosities known. We perform a spectral analysis of optical VFTS as well as near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN to obtain stellar and wind parameters. For the first time, we observationally resolve the transition between optically thin O and optically thick WNh star winds. Our results suggest the existence of a kink between both mass-loss regimes, in agreement with recent MC simulations. For the optically thick regime, we confirm the steep dependence on the Eddington factor from previous theoretical and observational studies. The transition occurs on the MS near a luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find that - even when accounting for moderate wind clumping (with f = 0.1) - wind mass-loss rates are enhanced with respect to standard prescriptions currently adopted in stellar evolution calculations. We also show that this results in substantial helium surface enrichment. Based on our spectroscopic analyses, we are able to provide the most accurate ionising fluxes for VMS known to date, confirming the pivotal role of VMS in ionising and shaping their environments.

astro-ph.SR↗

arXiv2013

Long-term evolution of the neutron-star spin period of SXP 1062

The Be/X-ray binary SXP1062 is of especial interest owing to the large spin period of the neutron star, its large spin-down rate, and the association with a supernova remnant constraining its age. This makes the source an important probe for accretion physics. To investigate the long-term evolution of the spin period and associated spectral variations, we performed an XMM-Newton target-of-opportunity observation of SXP1062 during X-ray outburst. Spectral and timing analysis of the XMM-Newton data was compared with previous studies, as well as complementary Swift/XRT monitoring and optical spectroscopy with the SALT telescope were obtained. The spin period was measured to be P_s =(1071.01+-0.16) s on 2012 Oct 14. The X-ray spectrum is similar to that of previous observations. No convincing cyclotron absorption features, which could be indicative for a high magnetic field strength, are found. The high-resolution RGS spectra indicate the presence of emission lines, which may not completely be accounted for by the SNR emission. The comparison of multi-epoch optical spectra suggest an increasing size or density of the decretion disc around the Be star. SXP1062 showed a net spin-down with an average of (2.27+-0.44) s/yr over a baseline of 915 days.

astro-ph.HE↗

arXiv2013

Studying the kinematics of the giant star-forming region 30 Doradus. I. The data

We present high-quality VLT-FLAMES optical spectroscopy of the nebular gas in the giant star-forming region 30 Doradus. In this paper, the first of a series, we introduce our observations and discuss the main kinematic features of 30 Dor, as revealed by the spectroscopy of the ionized gas in the region. The primary data set consists of regular grid of nebular observations, which we used to produce a spectroscopic datacube of 30 Dor, centered on the massive star cluster R136 and covering a field-of-view of 10'x10'. The main emission lines present in the datacube are from Halpha and [NII]6548,6584. The Halpha emission-line profile varies across the region from simple single-peaked emission to complex, multiple-component profiles, suggesting that different physical mechanisms are acting on the excited gas. To analyse the gas kinematics we fit Gaussian profiles to the observed Halpha features. Unexpectedly, the narrowest Halpha profile in our sample lies close to the supernova remnant 30 Dor B. We present maps of the velocity field and velocity dispersion across 30 Dor, finding five previously unclassified expanding structures. These maps highlight the kinematic richness of 30 Dor (e.g. supersonic motions), which will be analysed in future papers.

astro-ph.CO↗

arXiv2013

Discovery of X-ray emission from young suns in the Small Magellanic Cloud

We report the discovery of extended X-ray emission within the young star cluster NGC 602 in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the HII region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low- and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the new-born stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low- and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.

astro-ph.SR↗

arXiv2012

The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136

Detailed studies of resolved young massive star clusters are necessary to determine their dynamical state and evaluate the importance of gas expulsion and early cluster evolution. In an effort to gain insight into the dynamical state of the young massive cluster R136 and obtain the first measurement of its velocity dispersion, we analyse multi-epoch spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the variability, we use the radial velocities of non-variable sources to place an upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within a projected distance of 5 pc from the centre of the cluster. After accounting for the contributions of undetected binaries and measurement errors through Monte Carlo simulations, we conclude that the true velocity dispersion is likely between 4 and 5 km/s given a range of standard assumptions about the binary distribution. This result is consistent with what is expected if the cluster is in virial equilibrium, suggesting that gas expulsion has not altered its dynamics. We find that the velocity dispersion would be ~25 km/s if binaries were not identified and rejected, confirming the importance of the multi-epoch strategy and the risk of interpreting velocity dispersion measurements of unresolved extragalactic young massive clusters.

astro-ph.GA↗

arXiv2012

The VLT-FLAMES Tarantula Survey VI: Evidence for rotation of the young massive cluster R136

Although it has important ramifications for both the formation of star clusters and their subsequent dynamical evolution, rotation remains a largely unexplored characteristic of young star clusters (few Myr). Using multi-epoch spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey, we search for rotation of the young massive cluster R136. From the radial velocities of 36 apparently single O-type stars within a projected radius of 10 pc from the centre of the cluster, we find evidence, at the 95% confidence level, for rotation of the cluster as a whole. We use a maximum likelihood method to fit simple rotation curves to our data and find a typical rotational velocity of ~3 km/s. When compared to the low velocity dispersion of R136, our result suggests that star clusters may form with at least ~20% of the kinetic energy in rotation.

astro-ph.GA↗

arXiv2012

A rare early-type star revealed in the Wing of the Small Magellanic Cloud

Sk 183 is the visually-brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere four. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46+/-2 kK, a low mass-loss rate of ~10^-7 Msun yr^-1, and a spectroscopic mass of 46^+9_-8 Msun (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionising photons. This suggests Sk 183 is the primary influence on the star formation along the inner edge of the nebula.

astro-ph.SR↗

arXiv2012

The VLT-FLAMES Tarantula Survey IV: Candidates for isolated high-mass star formation in 30 Doradus

Whether massive stars can occasionally form in relative isolation or if they require a large cluster of lower-mass stars around them is a key test in the differentiation of star formation theories as well as how the initial mass function of stars is sampled. Previous attempts to find O-type stars that formed in isolation were hindered by the possibility that such stars are merely runaways from clusters, i.e., their current isolation does not reflect their birth conditions. We introduce a new method to find O-type stars that are not affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and additional high resolution imaging we have identified stars that satisfy the following constraints: 1) they are O-type stars that are not detected to be part of a binary system based on RV time series analysis; 2) they are designated spectral type O7 or earlier ; 3) their velocities are within 1σof the mean of OB-type stars in the 30 Doradus region, i.e. they are not runaways along our line-of-sight; 4) the projected surface density of stars does not increase within 3 pc towards the O-star (no evidence for clusters); 5) their sight lines are associated with gaseous and/or dusty filaments in the ISM, and 6) if a second candidate is found in the direction of the same filament with which the target is associated, both are required to have similar velocities. With these criteria, we have identified 15 stars in the 30 Doradus region, which are strong candidates for being high-mass stars that have formed in isolation. Additionally, we employed extensive MC stellar cluster simulations to confirm that our results rule out the presence of clusters around the candidates. Eleven of these are classified as Vz stars, possibly associated with the zero-age main sequence. We include a newly discovered W-R star as a candidate, although it does not meet all of the above criteria.

astro-ph.SR↗

arXiv2011

The VLT-FLAMES Tarantula Survey: The fastest rotating O-type star and shortest period LMC pulsar - remnants of a supernova disrupted binary?

We present a spectroscopic analysis of an extremely rapidly rotating late O-type star, VFTS102, observed during a spectroscopic survey of 30 Doradus. VFTS102 has a projected rotational velocity larger than 500\kms\ and probably as large as 600\kms; as such it would appear to be the most rapidly rotating massive star currently identified. Its radial velocity differs by 40\kms\ from the mean for 30 Doradus, suggesting that it is a runaway. VFTS102 lies 12 pcs from the X-ray pulsar PSR J0537-6910 in the tail of its X-ray diffuse emission. We suggest that these objects originated from a binary system with the rotational and radial velocities of VFTS102 resulting from mass transfer from the progenitor of PSR J0537-691 and the supernova explosion respectively.

astro-ph.SR↗

arXiv2011

Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the Wing of the SMC

We report on a new Be/X-ray pulsar binary located in the Wing of the Small Magellanic Cloud (SMC). The strong pulsed X-ray source was discovered with the Chandra and XMM-Newton X-ray observatories. The X-ray pulse period of 1062 s is consistently determined from both Chandra and XMM-Newton observations, revealing one of the slowest rotating X-ray pulsars known in the SMC. The optical counterpart of the X-ray source is the emission-line star 2dFS 3831. Its B0-0.5(III)e+ spectral type is determined from VLT-FLAMES and 2dF optical spectroscopy, establishing the system as a Be/X-ray binary (Be-XRB). The hard X-ray spectrum is well fitted by a power-law with additional thermal and blackbody components, the latter reminiscent of persistent Be-XRBs. This system is the first evidence of a recent supernova in the low density surroundings of NGC 602. We detect a shell nebula around 2dFS 3831 in H-alpha and [O III] images and conclude that it is most likely a supernova remnant. If it is linked to the supernova explosion that created this new X-ray pulsar, its kinematic age of (2-4)x10^4 yr provides a constraint on the age of the pulsar.

astro-ph.SR↗

arXiv2011

The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136

VFTS 682 is located in an active star-forming region, at a projected distance of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud. It was previously reported as a candidate young stellar object, and more recently spectroscopically revealed as a hydrogen-rich Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as its intrinsic luminosity, and to investigate the origin of VFTS 682. To this purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the non-LTE stellar atmosphere code CMFGEN, as well as the spectral energy distribution from complementary optical and infrared photometry. We find the extinction properties to be highly peculiar (RV ~4.7), and obtain a surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 2.5kK might be explained by chemically homogeneous evolution - suggested to be the key process in the path towards long gamma-ray bursts. Lightcurves of the object show variability at the 10% level on a timescale of years. Such changes are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of Luminous Blue Variables. Finally, we discuss two possibilities for the origin of VFTS 682: (i) the star either formed in situ, which would have profound implications for the formation mechanism of massive stars, or (ii) VFTS 682 is a slow runaway star that originated from the dense cluster R136, which would make it the most massive runaway known to date.

astro-ph.SR↗

arXiv2011

New Constraints on the Origin of the Short-Term Cyclical Variability of the Wolf-Rayet Star WR 46

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 λλ1032, 1038 doublet P Cygni profile and in the SVI λλ933, 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.

astro-ph.SR↗

arXiv2011

The VLT-FLAMES Tarantula Survey II: R139 revealed as a massive binary system

We report the discovery that R139 in 30 Doradus is a massive spectroscopic binary system. Multi-epoch optical spectroscopy of R139 was obtained as part of the VLT-FLAMES Tarantula Survey, revealing a double-lined system. The two components are of similar spectral types; the primary exhibits strong C III 4650 emission and is classified as an O6.5 Iafc supergiant, while the secondary is an O6 Iaf supergiant. The radial-velocity variations indicate a highly eccentric orbit with a period of 153.9 days. Photometry obtained with the Faulkes Telescope South shows no evidence for significant variability within an 18 month period. The orbital solution yields lower mass limits for the components of M1sin^3 i = 78 \pm 8 Msun and M2sin^3 i = 66 \pm 7 Msun. As R139 appears to be the most massive binary system known to contain two evolved Of supergiants, it will provide an excellent test for atmospheric and evolutionary models.

astro-ph.SR↗