2 Micron Narrowband Adaptive Optics Imaging in the Arches Cluster

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Abstract
Canada-France-Hawaii-Telescope adaptive optics bonnette images through narrowband filters in the K band are presented for the Arches cluster. Continuum fluxes, line fluxes, and equivalent widths are derived from high angular resolution images, some nearly diffraction limited, for the well-known massive stars in the Arches cluster. Images were obtained in the lines He I 2.06 μm, H I Brγ (2.17 μm), and He II 2.19 μm, as well as continuum positions at 2.03, 2.14, and 2.26 μm. In addition, fluxes are presented for H I Pα (1.87 μm) and a nearby continuum position (1.90 μm) from Hubble Space Telescope archival data*. The 2 μm and Pα data reveal two new emission-line stars and three fainter candidate emission-line objects. Indications for a spectral change of one object between earlier observations in 1992–1993 and our data from 1999 are found. The ratio of He II 2.19 μm to Brγ emission exhibits a narrow distribution among the stars, suggesting a narrow evolutionary spread centered predominantly on spectral types O4 If or Wolf-Rayet stars of the WN7 subtype. From the approximate spectral types of the identified emission-line stars and comparisons with evolutionary models we infer a cluster age between ~2 and 4.5 Myr.
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