A VLT/FLAMES survey for massive binaries in Westerlund 1: II. Dynamical constraints on magnetar progenitor masses from the eclipsing binary W13

Westerlund 1 is a young, massive Galactic starburst cluster that contains a rich coeval population of Wolf-Rayet stars, hot- and cool-phase transitional supergiants, and a magnetar. We use spectroscopic and photometric observations of the eclipsing double-lined binary W13 to derive dynamical masses for the two components, in order to determine limits for the progenitor masses of the magnetar CXOU J164710.2-455216 and the population of evolved stars in Wd1. W13 has an orbital period of 9.2709+/-0.0015 days and near-contact configuration. The shallow photometric eclipse rules out an inclination greater than 65 degrees, leading to lower limits for the masses of the emission-line optical primary and supergiant optical secondary of 21.4+/-2.6Msun and 32.8+/-4.0Msun respectively, rising to 23.2 +3.3/-3.0Msun and 35.4 +5.0/-4.6 Msun for our best-fit inclination 62 +3/-4 degrees. Comparison with theoretical models of Wolf-Rayet binary evolution suggest the emission-line object had an initial mass in excess of 35Msun, with the most likely model featuring highly non-conservative late-Case-A/Case-B mass transfer and an initial mass in excess of 40Msun. This confirms the high magnetar progenitor mass inferred from its membership in Wd1, and represents the first dynamical constraint on the progenitor mass of any magnetar. The red supergiants in Wd1 must have similar progenitor masses to W13 and are therefore amongst the most massive stars to undergo a red supergiant phase, representing a challenge for population models that suggest stars in this mass range end their redwards evolution as yellow hypergiants. [ABRIDGED]Comment: Accepted for publication in A&A. 8 pages, 5 figures. See also http://www.eso.org/public/news/eso1034/ from noon (CEST) Wed 18th Augus

The eclipsing LMC star OGLE05155332--6925581: a clue for Double Periodic Variables

We investigate the nature of OGLE05155332-6925581, one of the brightest members of the enigmatic group of Double Periodic Variables (DPVs) recently found in the Magellanic Clouds. The modeling of archival orbital light curves (LCs), along with the analysis of the radial velocities suggest that this object is a semi--detached binary with the less massive star transferring matter to the more massive and less evolved star, in an Algol--like configuration. We find evidence for additional orbital variability and H$\alpha$ emission, likely caused by an accretion disc around the primary star. As in the case of $\beta Lyr$ the circumprimary disc seems to be more luminous than the primary, but we do not detect orbital period changes. We find that the LC follows a loop in the color--magnitude diagram during the long cycle; the system is redder when brighter and the rising phase is bluer than during decline. Infrared excess is also present. The source of the long--term periodicity is not eclipsed, indicating its circumbinary origin. Strong asymmetries, discrete absorption components (DACs) and a $\gamma$ shift are new and essential observational properties in the infrared H I lines. The DACs strength and RV follow a saw--teeth pattern during the orbital cycle. We suggest that the system experiences supercycles of mass outflow feeding a circumbinary disc. Mass exchange and mass loss could produce comparable but opposite effects in the orbital period on a long time scale, resulting in a quasi--constancy of this parameter.Comment: submitted to MNRA

Efficiency of mass transfer in massive close binaries, Tests from double-lined eclipsing binaries in the SMC

One of the major uncertainties in close binary evolution is the efficiency of mass transfer beta: the fraction of transferred mass that is accreted by a secondary star. We attempt to constrain the mass-transfer efficiency for short-period massive binaries undergoing case A mass transfer. We present a grid of about 20,000 detailed binary evolution tracks with primary masses 3.5-35 Msun, orbital periods 1-5 days at a metallicity Z=0.004, assuming both conservative and non-conservative mass transfer. We perform a systematic comparison, using least-squares fitting, of the computed models with a sample of 50 double-lined eclipsing binaries in the Small Magellanic Cloud, for which fundamental stellar parameters have been determined. About 60% of the systems are currently undergoing slow mass transfer. In general we find good agreement between our models and the observed detached systems. However, for many of the semi-detached systems the observed temperature ratio is more extreme than our models predict. For the 17 semi-detached systems that we are able to match, we find a large spread in the best fitting mass-transfer efficiency; no single value of beta can explain all systems. We find a hint that initially wider systems tend to fit better to less conservative models. We show the need for more accurate temperature determinations and we find that determinations of surface abundances of nitrogen and carbon can potentially constrain the mass-transfer efficiency further.Comment: Accepted for publication in A&A 15/03/2007, 16 page

The evolution of rotating stars

First, we review the main physical effects to be considered in the building of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The internal rotation law evolves as a result of contraction and expansion, meridional circulation, diffusion processes and mass loss. In turn, differential rotation and mixing exert a feedback on circulation and diffusion, so that a consistent treatment is necessary. We review recent results on the evolution of internal rotation and the surface rotational velocities for stars on the Upper MS, for red giants, supergiants and W-R stars. A fast rotation is enhancing the mass loss by stellar winds and reciprocally high mass loss is removing a lot of angular momentum. The problem of the ``break-up'' or $\Omega$-limit is critically examined in connection with the origin of Be and LBV stars. The effects of rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the blue to red supergiant ratios, the formation of W-R stars, the chemical abundances in massive stars as well as in red giants and AGB stars, are reviewed in relation to recent observations for stars in the Galaxy and Magellanic Clouds. The effects of rotation on the final stages and on the chemical yields are examined, as well as the constraints placed by the periods of pulsars. On the whole, this review points out that stellar evolution is not only a function of mass M and metallicity Z, but of angular velocity $\Omega$ as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and Astrophysics, vol. 38 (2000

The VLT-FLAMES Tarantula Survey

Context. The Tarantula region in the Large Magellanic Cloud (LMC) contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and mass-loss properties. Aims. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. Methods. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model fastwind with the genetic fitting algorithm pikaia to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. The latter has been constrained without taking into account the contribution from macro-turbulent motions to the line broadening. Results. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. The calibration for giants shows a +1kK offset compared to similar Galactic calibrations; a shift of the same magnitude has been reported for dwarfs. The supergiant calibrations, though only based on a handful of stars, do not seem to indicate such an offset. The presence of a strong upturn at spectral type O3 and earlier can also not be confirmed by our data. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by state-of-the-art models. For stars initially more massive than approximately 60 M⊙, the giant phase already appears relatively early on in the evolution; the supergiant phase develops later. Bright giants, however, are not systematically positioned between giants and supergiants at Minit ≳ 25 M⊙. At masses below 60 M⊙, the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at Minit ≲ 18 M⊙. Here, stars classified as late O III and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late O V stars are actually seen. Though we can not exclude that these stars represent a physically distinct group, this behavior may reflect an intricacy in the luminosity classification at late O spectral subtype. Indeed, on the basis of a secondary classification criterion, the relative strength of Si iv to He i absorption lines, these stars would have been assigned a luminosity class IV or V. Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. This outcome is independent of their projected spin rates. The aforementioned five stars present moderate projected rotational velocities (i.e., νesini < 200kms-1) and hence do not agree with current predictions of rotational mixing in main-sequence stars. They may potentially reveal other physics not included in the models such as binary-interaction effects. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are ~0.3 dex higher than earlier results. For stars brighter than 105 L⊙, that is, in the regime of strong stellar winds, the measured (unclumped) mass-loss rates could be considered to be in agreement with line-driven wind predictions if the clump volume filling factors were fV ~ 1/8 to 1/6