Young massive clusters

Westerlund 1, above, is a candidate for the most massive young cluster in the Local Group, with a population of WR stars, red supergiants and yellow hypergiants with a central density of 100,000 solar masses per cubic parsec, and so is the nearest contender for a young globular cluster.

A neutron star CXO J164710.2-455216 has recently been detected in Chandra X-ray observations of Wd1 – see the Chandra press release.

We have re-evaluated the initial masses of the most massive stars in the youngest, highest mass Milky Way (NGC 3603) and LMC (R136) clusters, from which initial masses of 170 and 320 solar masses were obtained, together with at least 185 solar masses for the Arches cluster.

Monte Carlo simulations suggest that each are consistent with an upper mass limit of circa 300 solar masses, a factor of two higher than hitherto believed. We have also observed all the high mass stars in R136 with HST/STIS. Beyond R136, the central cluster of 30 Doradus, we are undertaking the VLT-FLAMES Tarantula survey, a multi-epoch optical spectroscopic study of 800 massive stars in the 30 Doradus region.

We have also obtained Gemini GNIRS spectroscopy of massive stars within the Galactic 1806-20 cluster, from which a downward revision to its distance to 8.7 kpc was obtained Its kinematic distance has hitherto been taken as 15 kpc.

This revision is significant since it leads to a factor of three reduction in the 2004 December outburst of the magnetar (highly magnetised pulsar) SGR 1806-20 – providing it is associated with the young massive cluster – which in turn indicates a reduction in the contamination of BATSE short GRBs from giant outbursts from magnetars. It is significant that both 1806-20 and Westerlund 1 apparently host magnetars, with progenitor masses of about 50 Msun.