2004A&A...423..155M

High spatial resolution observations in the 1 to 3.5µm region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the H, Kp (2.12/0.4µm) and L bands, HST-NICMOS data in filters between 1.1 and 2.2µm, and integral field spectroscopic data from BEAR, an Imaging FTS, in the HeI 2.06µm and the Brγ line regions. Analysis of all these data provides a completely new picture of the main component, IRS 13E, which appears as a cluster of seven individual stars within a projected diameter of ∼0.5" (0.02pc). The brightest sources, 13E1, 13E2, 13E3 which is detected as a binary, and 13E4, are all massive stars of different type. The star 13E1 is a luminous, blue object, with no detected emission line. 13E2 and 13E4 are two hot, high-mass emission line stars, 13E2 being at the WR stage and 13E4 a massive O-type star. In contrast, 13E3A and B are extremely red objects, proposed as other examples of dusty WR stars, like IRS 21 (Tanner et al., 2002ApJ...575..860T). All these sources have a common westward proper motion (Ott et al., 2003Msngr.111....1O) indicating they are bounded. Two other sources, detected after deconvolution of the AO images in the H and Kp bands, are also identified. One, that we call 13E5, is a red source similar to 13E3A and B, while the other one, 13E6, is probably a main sequence O star in front of the cluster. Considering this exceptional concentration of comoving massive hot stars, IRS 13E is proposed as the remaining core of a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) (Portegies Zwart & McMillan, 2002ApJ...576..899P) of ∼1300M_☉. This detection plays in favor of a scenario, first suggested by Gerhard (2001ApJ...546L..39G), in which the helium stars and the other hot stars in the central parsec originate from the stripping of a massive cluster formed several tens of pc from the center. This cluster would have spiraled towards SgrA^*, and IRS 13E would be its remnant. Furthermore, IRS 13E might be the second black hole needed according to a model by Hansen & Milosavljevic (2003ApJ...593L..77H) to drag massive main-sequence stars, in the required timescale, very close to the massive black hole. The detection of a discrete X-ray emission (Baganoff et al., 2003ApJ...591..891B) at the IRS 13 position (within the positional accuracy) is examined in this context.