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The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared

Abstract
High spatial resolution observations in the 1 to 3.5 m region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the  H, Kp (2.12/0.4 m) and  L bands, HST-NICMOS data in filters between 1.1 and 2.2 m, and integral field spectroscopic data from BEAR, an Imaging FTS, in the  2.06 m and the Br  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´´ (0.02 pc). 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. 2002). All these sources have a common westward proper motion (Ott et al. 2003) 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 2002) of ~1300 . This detection plays in favor of a scenario, first suggested by Gerhard (2001), 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 (2003) 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. 2003) at the IRS 13 position (within the positional accuracy) is examined in this context.