2 micron narrowband adaptive optics imaging in the arches cluster

Canada-France-Hawaii-Telescope adaptive optics bonnette images through narr owband filters in the K band are presented for the Arches cluster. Continuu m fluxes, line fluxes, and equivalent widths are derived from high angular resolution images, some nearly diffraction limited, for the well-known mass ive stars in the Arches cluster. Images were obtained in the lines He I 2.0 6 mum, H I Br gamma (2.17 mum), and He II 2.19 mum, as well as continuum po sitions at 2.03, 2.14, and 2.26 mum. In addition, fluxes are presented for H I P alpha (1.87 mum) and a nearby continuum position (1.90 mum) from Hubb le Space Telescope archival data.(2) The 2 mum and P alpha data reveal two new emission-line stars and three fainter candidate emission-line objects. Indications for a spectral change of one object between earlier observation s in 1992-1993 and our data from 1999 are found. The ratio of He II 2.19 mu m to Br gamma emission exhibits a narrow distribution among the stars, sugg esting a narrow evolutionary spread centered predominantly on spectral type s O4 If or Wolf-Rayet stars of the WN7 subtype. From the approximate spectr al types of the identified emission-line stars and comparisons with evoluti onary models we infer a cluster age between similar to2 and 4.5 Myr.