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D. Figer, J. MacKenty, M. Robberto, K. Smith (STScI), F. Najarro (CSIC), R. Kudritzki (UH)
We report the discovery of an extraordinarily massive young cluster of stars in the Galaxy, having an inferred total initial cluster mass comparable to those inferred for the most massive young clusters in the Galaxy. Using IRMOS, 2MASS, and Spitzer observations we conclude that there are 14 red supergiants in the cluster, compared with 5 in what was previously the richest Galactic cluster of such stars. We infer spectral types from near-infrared spectra that reveal deep CO bandhead absorption that can only be fit by red supergiants. We also identify a gap of \Delta Ks~4 magnitudes between the stars and the bulk of the other stars in the region that can only be fit by models if the brightest stars in the cluster are red supergiants. We make an independent estimate of the distance by associating an OH maser with the envelope of one of the stars. The velocity of the maser, along with the Galactic rotation curve, suggests that the cluster distance is 5.8 Kpc. Given our estimate of the extinction to the stars, and this distance, we conclude that the bright cluster stars can only be red supergiants. We also identify a ``yellow'' supergiant of G6I type in the cluster. Assuming a Salpeter IMF, we infer an initial cluster mass of >20,000 M\sun for cluster ages of 8-18 Myr. Continuing with these assumptions, we find 80% of the intial mass and 99% of the number of stars remain at the present time.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.