A data set of adaptive optics images has been analyzed in order to study the effects of angular anisoplanatism and to characterize Mauna Kea nighttime turbulence. The data set consists of a selection of Galactic center images obtained with the adaptive optics instrument Hokupa'a and the near-infrared camera QUIRC on the Gemini-North 8 m telescope. Using the Strehl ratio and the FWHM as tracers for anisoplanatism in the images, it is possible to draw conclusions about the effective turbulence height by model-fitting synthetic h data. From the relatively small statistical sample, we obtain a median km, with 10th and 90th percentiles h p 3.5 being 2.2 and 5.6 km. The implications of these findings for a turbulence-conjugated adaptive optics system are addressed. We also find the Zernike modal equivalent of Hokupa'a to be , i.e., on the average slightly N ≈ 18 better than 4 radial degrees.

High spatial resolution, near-infrared observations of the Galactic Center source, close to Sgr A , known historically as IRS13, are presented. These observations include ground-based adaptive optics images in the H, K' and L bands, HST-NICMOS observations in filters between 1.1 and 2.2 µm, and spectroimaging data in the He I 2.06 µm line and the Brγ line. Analysis of all these data has made possible the resolution of the main component, IRS 13E, into a cluster of seven individual stars within a projected diameter of ∼ 0.5 (0.02 pc), and to build their SED. The main sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are hot stars of different nature. 13E2 and 13E4 are emission line stars. The spectral type of the various members goes from O5I to WR, including dusty WRs like IRS 21 (Tanner et al. 2002). All these sources have a common westward proper motion. Two weaker sources, 13E5 and 13E6, are also detected within the compact cluster, with 13E5 proposed as another dusty WR and 13E6 as a O5V star. An extended halo seen around the cluster, part of the mini-spiral of dust is particularly enhanced in the L band. It is interpreted as a contribution of the scattered light from the inner cluster and the thermal emission from the dust. IRS 13E is proposed to be the remaining core of a massive, young star cluster which was disrupted in the vicinity of Sgr A , and hence, the possible source of the young stars in the central parsec, from the helium stars to the S stars.

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