The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC

[Abridged] We have studied the optical spectra of 28 O- and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star-forming region N11. Stellar parameters are determined using an automated fitting method, combining the stellar atmosphere code FASTWIND with the genetic-algorithm optimisation routine PIKAIA. Results for stars in the LH9 and LH10 associations of N11 are consistent with a sequential star formation scenario, in which activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2, of which the hottest is found to be ~49-54 kK (cf. ~45-46 kK for O3 stars). The masses of helium-enriched dwarfs and giants are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally-enhanced mixing, leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. For bright giants and supergiants no such mass-discrepancy is found, implying that these stars follow tracks of modestly (or non-)rotating objects. Stellar mass-loss properties were found to be intermediate to those found in massive stars in the Galaxy and the SMC, and comparisons with theoretical predictions at LMC metallicity yielded good agreement over the luminosity range of our targets, i.e. 5.0 < log L/L(sun) < 6.1

The effect of stellar rotation on colour-magnitude diagrams: on the apparent presence of multiple populations in intermediate age stellar clusters

A significant number of intermediate age clusters (1 − 2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bi-modal or extended main sequence turn offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred Myr, which would call into question the long held notion that clusters are simple stellar populations. Here we show that stellar rotation in stars with masses between 1.2 − 1.7 M can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20-50% of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low mass stars (<1M⊙) are not expected to be rapid rotators

Tides in asynchronous binary systems

Context. Stellar oscillations are excited in non-synchronously rotating stars in binary systems due to the tidal forces. Tangential components of the tides can drive a shear flow which behaves as a differentially forced rotating structure in a stratified outer medium. Aims. The aims of this paper are to show that our single-layer approximation for the calculation of the forced oscillations yields results that are consistent with the predictions for the synchronization timescales in circular orbits, τsync ∼ a6, thus providing a simplified means of computing the energy dissipation rates, ˙E . Furthermore, by calibrating our model results to fit the relationship between synchronization timescales and orbital separation, we are able to constrain the value of the kinematical viscosity parameter, ν. Methods. We compute the values of ˙E for a set of 5 M + 4 M model binary systems with different orbital separations, a, and use these to estimate the synchronization timescales. Results. The resulting τsynch vs. a relation is comparable to that of Zahn (1977, A&A, 57, 383) for convective envelopes, providing a calibration method for the values of ν. For the 4 + 5 M binary modeled in this paper, ν is in the range 0.0015–0.0043 R2 /day for orbital periods in the range 2.5–25 d. In addition, ˙E is found to decrease by ∼2 orders of magnitude as synchronization is approached, implying that binary systems may approach synchronization relatively quickly but that it takes a much longer timescale to actually attain this condition. Conclusions. The relevance of these results is threefold: 1) our model allows an estimate for the numerical value of ν under arbitrary conditions in the binary system; 2) it can be used to calculate the energy dissipation rates throughout the orbital cycle for any value of eccentricity and stellar rotational velocity; and 3) it provides values of the tangential component of the velocity perturbation at any time throughout the orbit and predicts the location on the stellar surface where the largest shear instabilities may be occurring. We suggest that one of the possible implication of the asymmetric distribution of ˙E over the stellar surface is the generation of localized regions of enhanced surface activity

On the mass–radius relation of hot stellar systems

Most globular clusters have half-mass radii of a few pc with no apparent correlation with their masses. This is different from elliptical galaxies, for which the Faber–Jackson relation suggests a strong positive correlation between mass and radius. Objects that are somewhat in between globular clusters and low-mass galaxies, such as ultracompact dwarf galaxies, have a mass–radius relation consistent with the extension of the relation for bright ellipticals. Here we show that at an age of 10 Gyr a break in the mass–radius relation at∼106M is established because objects below this mass, i.e. globular clusters, have undergone expansion driven by stellar evolution and hard binaries. From numerical simulations we find that the combined energy production of these two effects in the core comes into balance with the flux of energy that is conducted across the half-mass radius by relaxation. An important property of this ‘balanced’ evolution is that the cluster half-mass radius is independent of its initial value and is a function of the number of bound stars and the age only. It is therefore not possible to infer the initial mass–radius relation of globular clusters, and we can only conclude that the present day properties are consistent with the hypothesis that all hot stellar systems formed with the same mass–radius relation and that globular clusters have moved away from this relation because of a Hubble time of stellar and dynamical evolution

The photometric evolution of dissolving star clusters. I. First predictions

The broad-band photometric evolution of unresolved star clusters is calculated in a simplified way, including the preferential loss of low-mass stars due to mass segregation. The stellar mass function of a cluster evolves due to three effects: (a) the evolution of the massive stars reduces their number; (b) tidal effects before cluster-wide mass segregation reduce the mass function homogeneously, i.e. independently of the stellar mass; (c) after mass segregation has finished, tidal effects preferentially remove the lowest-mass stars from the cluster. These effects result in a narrowing of the stellar mass range. These effects are described quantitatively, following the results of N-body simulations, and are taken into account in the calculation of the photometric history, based on the galev cluster evolution models for solar metallicity and a Salpeter mass function. We find the following results: (1) During the first ∼40% of the lifetime of a cluster its colour evolution is adequately described by the standard galev models (without mass segregation) but the cluster becomes fainter due to the loss of stars by tidal effects. (2) Between ∼40 and ∼80% of its lifetime (independent of the total lifetime), the cluster becomes bluer due to the loss of low-mass stars. This will result in an underestimate of the age of clusters if standard cluster evolution models are used. (3) After ∼80% of the total lifetime of a cluster it will rapidly become redder. This will result in an overestimate of the age of clusters if standard cluster evolution models are used. (4) Clusters with mass segregation and the preferential loss of low-mass stars evolve along almost the same tracks in colour–colour diagrams as clusters without mass segregation. Only if the total lifetime of clusters can be estimated can the colours be used to give reliable age estimates. (5) The changes in the colour evolution of unresolved clusters due to the preferential loss of low-mass stars will affect the determination of the star formation histories of galaxies if they are derived from clusters that have a total lifetime of less than about 30 Gyr. (6) The preferential loss of low-mass stars might explain the presence of old (∼13 Gyr) clusters in NGC 4365 which are photometrically disguised as intermediate-age clusters (2–5 Gyr), if the expected total lifetime of these clusters is between 16 and 32 Gyr

Counterstreaming magnetized plasmas with kappa distributions - II. Perpendicular wave propagation

The analysis of the stability and the dispersion properties of a counterstreaming plasma system with kappa distributions are extended here with the investigation of perpendicular instabilities. Purely growing filamentation (Weibel-like) modes propagating perpendicular to the background magnetic field can be excited in streaming plasmas with or without an excess of parallel temperature. In this case, however, the effect of suprathermal tails of kappa populations is opposite to that obtained for parallel waves: the growth rates can be higher and the instability faster than for Maxwellian plasmas. The unstable wavenumbers also extend to a markedly larger broadband making this instability more likely to occur in space plasmas with anisotropic distributions of kappa-type. The filamentation instability of counterstreaming magnetized plasmas could provide a plausible mechanism for the origin of two-dimensional transverse magnetic fluctuations detected at different altitudes in the solar wind

Wolf-Rayet stars as gamma-ray burst progenitors

It became clear in the last few years that long gamma-ray bursts are associated with the endpoints of massive star evolution. They occur in star forming regions at cosmological distances (Jakobsson et al., 2005), and are associated with supernova-type energies. The collapsar model explains gamma-ray burst formation via the collapse of a rapidly rotation massive iron core into a black hole (Woosley, 1993). The short time scale of gamma-ray emission requires a compact stellar size object, of the order of light seconds. This constraint leaves only massive Wolf–Rayet stars as possible progenitors. However, this poses a difficulty: Wolf–Rayet stars in the local universe are known to have strong stellar winds (Nugis et al., 1998 T. Nugis, P.A. Crowther and A.J. Willis, A&A 333 (1998), p. 956. View Record in Scopus | Cited By in Scopus (73)Nugis et al., 1998), which lead to a rapid spin-down (Langer, 1998) – in agreement with the absence of signatures of rapid rotation in the Galactic Wolf–Rayet sample. An additional obstacle for forming a rapidly rotating Wolf–Rayet star in the course of single star evolution is the shear between core and envelope generated by the former’s contraction and the latter’s expansion after the main sequence. Magnetic torques are expected to lead to a strong coupling and related core spin down ([Spruit, 2002], [Heger et al., 2005] and [Petrovic et al., 2005]). Indeed, such coupling is required to understand the slow rotation of young neutron stars (Ott et al., 2006) and white dwarfs (Berger et al., 2005). This implies that single stars which, during their evolution, become a supergiant, i.e. obtain a massive extended envelope, will not be suitable GRB progenitors, even if they end their life as Wolf–Rayet star

Star cluster disruption by giant molecular clouds

We investigate encounters between giant molecular clouds (GMCs) and star clusters. We propose a single expression for the energy gain of a cluster due to an encounter with a GMC, valid for all encounter distances and GMC properties. This relation is verified with N-body simulations of cluster–GMC encounters, where the GMC is represented by a moving analytical potential. Excellent agreement is found between the simulations and the analytical work for fractional energy gains of ΔE/|E0| <10 , where |E0| is the initial total cluster energy. The fractional mass loss from the cluster scales with the fractional energy gain as (ΔM/M0) =f(ΔE/|E0|) , where f≃ 0.25 . This is because a fraction 1 −f of the injected energy goes to the velocities of escaping stars, that are higher than the escape velocity. We therefore suggest that the disruption time of clusters, tdis, is best defined as the time needed to bring the cluster mass to zero, instead of the time needed to inject the initial cluster energy. We derive an expression for tdis based on the mass loss from the simulations, taking into account the effect of gravitational focusing by the GMC. Assuming spatially homogeneous distributions of clusters and GMCs with a relative velocity dispersion of σcn, we find that clusters lose most of their mass in relatively close encounters with high relative velocities (∼2σcn) . The disruption time depends on the cluster mass (Mc) and half-mass radius (rh) as tdis= 2.0 S(Mc/104 M⊙)(3.75 pc/rh)3 Gyr , with S≡ 1 for the solar neighbourhood and S scales with the surface density of individual GMCs (Σn) and the global GMC density (ρn) as S∝ (Σnρn)−1 . Combined with the observed relation between rh and Mc, that is, rh∝Mλc, tdis depends on Mc as tdis∝Mγc . The index γ is then defined as γ= 1 − 3λ . The observed shallow relation between cluster radius and mass (e.g. λ≃ 0.1 ), makes the value of the index γ= 0.7 similar to that found from observations and from simulations of clusters dissolving in tidal fields (γ≃ 0.62) . The constant of 2.0 Gyr, which is the disruption time of a 104 M⊙ cluster in the solar neighbourhood, is about a factor of 3.5 shorter than that found from earlier simulations of clusters dissolving under the combined effect of Galactic tidal field and stellar evolution. It is somewhat higher than the observationally determined value of 1.3 Gyr. It suggests, however, that the combined effect of tidal field and encounters with GMCs can explain the lack of old open clusters in the solar neighbourhood. GMC encounters can also explain the (very) short disruption time that was observed for star clusters in the central region of M51, since there ρn is an order of magnitude higher than that in the solar neighbourhoo

Supernovae from massive AGB stars

We present new computations of the final fate of massive AGB-stars. These stars form ONeMg cores after a phase of carbon burning and are called Super AGB stars (SAGB). Detailed stellar evolutionary models until the thermally pulsing AGB were computed using three di erent stellar evolution codes. The subsequent evolution was modeled by a synthetic code with di erent options for mass loss rate and dredge-up e ciency. We find a range of initial masses between 9:0M and 9:25M for which we expect an SAGB star to explode as an electron capture supernova. Our models allow a detailed assessment of the envelope properties of electron-capture supernova progenitors. SAGB stars with lower initial masses are the progenitors of ONeMg white dwarf, while more massive stars ignite (o -center) neon burning and follow the classical core-collapse path

Optical identification of IGR J19140+0951

IGR J19140+0951 was discovered by INTEGRAL in 2003 in the 4–100 keV band. Observations with INTEGRAL and RXTE provide a tentative identification as a high-mass X-ray binary (HMXB) with a neutron star as accretor. However, an optical counterpart was thus far not established, nor was the presence of a pulsar which is commonly observed in HMXBs. We observed IGR J19140+0951 with Chandra and find the source to be active at a similar flux as previous measurements. The lightcurve shows a marginally significant oscillation at 6.5 ks which requires confirmation. We determine a sub-arcsecond position from the Chandra data and identify the heavily reddened optical counterpart 2MASS 19140422+0952577 in the 2MASS catalog. Optical follow-up observations with theWilliam Herschel Telescope at La Palma exhibit a continuum spectrum coming out of extinction above 7000 Å without strong absorption or emission features. V, I and Ks band photometry point to an optical counterpart with an extinction of AV = 11 ± 2. The extinction is consistent with the interstellar value. None of the data reject the suspicion that IGR J19140+0951 is an HMXB with additional circumstellar obscuration around the accretor