* Astronomy

Title: The Fundamental Parameters of Four Massive Eclipsing Binaries in Westerlund 1
Authors: E. Koumpia, A. Z. Bonanos

Only a small number of high mass stars (> 30 Mo) have fundamental parameters measured with high enough accuracy from eclipsing binaries to constrain formation and evolutionary models of massive stars. This work aims to increase this limited sample, by studying the 4 massive eclipsing binary candidates discovered by Bonanos in the young massive cluster Westerlund 1. We present new follow-up echelle spectroscopy of these binaries and models of their light and radial velocity curves. We obtain fundamental parameters (i.e. masses, radii) for the 8 component stars, finding masses that span a range of 10-40 Mo, and contributing accurate fundamental parameters for 1 additional very massive star, the 33 Mo component of W13. WR77o is found to have a ~40 Mo companion, which provides a second dynamical constraint on the mass of the progenitor of the magnetar known in the cluster. We also use W13 to estimate the first, direct, eclipsing binary distance to Westerlund 1 and therefore the magnetar, and find it to be at 4.0 ±0.6 kpc. Our results confirm previous evidence for a high mass for the progenitor of the magnetar. In addition, the availability of eclipsing binaries with accurate parameters opens the way for direct, independent, high precision eclipsing binary distance measurements to Westerlund 1.

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Title: Discovery of extended VHE gamma-ray emission from the vicinity of the young massive stellar cluster Westerlund 1 

Authors: HESS Collaboration: A. Abramowski, F. Acero, F. Aharonian, A.G. Akhperjanian, G. Anton, A. Balzer, A. Barnacka, U. Barres de Almeida, Y. Becherini, J. Becker, B. Behera, K. Bernlöhr, E. Birsin, J. Biteau, A. Bochow, C. Boisson, J. Bolmont, P. Bordas, J. Brucker, F. Brun, P. Brun, T. Bulik, I. Büsching, S. Carrigan, S. Casanova, M. Cerruti, P.M. Chadwick, A. Charbonnier, R.C.G. Chaves, A. Cheesebrough, L.-M. Chounet, A.C. Clapson, G. Coignet, G. Cologna, J. Conrad, M. Dalton, M.K. Daniel, I.D. Davids, B. Degrange, C. Deil, H.J. Dickinson, A. Djannati-Ataï, W. Domainko, L.O'C. Drury, F. Dubois, G. Dubus, K. Dutson, J. Dyks, M. Dyrda, K. Egberts, P. Eger, P. Espigat, L. Fallon, C. Farnier, S. Fegan, F. Feinstein, M.V. Fernandes, A. Fiasson, G. Fontaine, A. Förster, M. Fübling, et al. (139 additional authors not shown)

Results obtained in very-high-energy (VHE; E > 100 GeV) \gamma-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from \gamma-ray induced particle cascades in the Earth's atmosphere is used to search for VHE \gamma\ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. The detection of the degree-scale extended VHE \gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S. observations performed between 2004 and 2008. The VHE \gamma-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20\sigma. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of \Gamma=2.19 ±0.08_{stat} ±0.20_{sys} and a flux normalisation at 1 TeV of \Phi_0 = (9.0 ±1.4_{stat} ±1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2 TeV amounts to (5.2 ±0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident with HESS J1646-458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648-4611 could be viable to explain parts of the VHE \gamma-ray emission of HESS J1646-458.

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How Much Mass Makes a Black Hole?

Using ESO's Very Large Telescope, European astronomers have for the first time demonstrated that a magnetar - an unusual type of neutron star - was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar. This now raises a fundamental question: just how massive does a star really have to be to become a black hole?
To reach their conclusions, the astronomers looked in detail at the extraordinary star cluster Westerlund 1, located 16 000 light-years away in the southern constellation of Ara (the Altar). From previous studies, the astronomers knew that Westerlund 1 was the closest super star cluster known, containing hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two thousand times the diameter of the Sun (as large as the orbit of Saturn).
The astronomers studied the stars that belong to the eclipsing double system W13 in Westerlund 1 using the fact that, in such a system, masses can be directly determined from the motions of the stars.
By comparison with these stars, they found that the star that became the magnetar must have been at least 40 times the mass of the Sun. This proves for the first time that magnetars can evolve from stars so massive we would normally expect them to form black holes. The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars and those above 25 solar masses would produce black holes.
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Title: The most massive progenitors of neutron stars: CXO J164710.2-455216
Authors: K.Belczynski (LANL), R.Taam (NU)

The evolution leading to the formation of a neutron star in the very young Westerlund 1 star cluster is investigated. The turnoff mass has been estimated to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single neutron star and potentially a magnetar. Since this source was argued to be a member of the cluster, the neutron star progenitor must have been very massive (M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are generally believed to form black holes (rather than neutron stars), the existence of this object poses a challenge for understanding massive star evolution. We point out while single star progenitors below M_zams < 20 Msun form neutron stars, binary evolution completely changes the progenitor mass range. In particular, we demonstrate that mass loss in Roche lobe overflow enables stars as massive as 50-80 Msun, under favourable conditions, to form neutron stars. If the very high observed binary fraction of massive stars in Westerlund 1 (> 70 percent) is considered, it is natural that CXO J164710.2-455216 was formed in a binary which was disrupted in a supernova explosion such that it is now found as a single neutron star. Hence, the existence of a neutron star in a given stellar population does not necessarily place stringent constraints on progenitor mass when binary interactions are considered. It is concluded that the existence of a neutron star in Westerlund 1 cluster is fully consistent with the generally accepted framework of stellar evolution.

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