AAS 207th Meeting, 8-12 January 2006
Session 91 Star Clusters Large and Small
Oral, Tuesday, 10:00-11:30am, January 10, 2006, Balcony A

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[91.03] A Neutron Star with a Massive Progenitor in the Star Cluster Westerlund 1

M. P. Muno (UCLA), J. S. Clark (Open University), P. A. Crowther (Sheffield), S. M. Dougherty (Dominion), R. de Grijs (Sheffield), C. Law (Northwestern), S. L. W. McMillan (Drexel), M. R. Morris (UCLA), I. Negueruela (Alicante), D. Pooley (Berkeley), S. Portegies Zwart (Amsterdam), F. Yusef-Zadeh (Northwestern)

We report the discovery of a slow (10.6 s) X-ray pulsar in the young (4±1 Myr) Galactic super star cluster Westerlund 1. The association between the star cluster and the pulsar is important, because we know the cluster contains a coeval population of stars, and that 35M\odot O7 stars are still present on the main sequence. Therefore, we conclude that the progenitor to the neutron star had an initial mass of at least 40M\odot. The progenitor had to have lost 95% of its mass either through stellar winds, binary mass transfer, or during the supernova. Moreover, the pulsar's slow spin period, soft X-ray spectrum, and lack of an infrared counterpart indicates that it is likely to be one of about a dozen magnetars. This is one of only a handful of model-independent constraints on the masses of progenitors to neutron stars, most of which imply that highly magnetized neutron star descend from >30M\odot progenitors. This raises the question of whether magnetic fields play some role in driving the required mass loss.

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