EDP Sciences
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Volume 423, Number 1, August III 2004
Page(s) 155 - 167
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20034147

A&A 423, 155-167 (2004)
DOI: 10.1051/0004-6361:20034147

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

J. P. Maillard1, T. Paumard1, S. R. Stolovy2 and F. Rigaut3

1  Institut d'Astrophysique de Paris (CNRS), 98bis Bd Arago, 75014 Paris, France
    e-mail: maillard@iap.fr
2  Spitzer Science Center, CalTech, MS 220-6, Pasadena, CA 91125, USA
3  Gemini North Headquarter, Hilo, HI 96720, USA

(Received 1 August 2003 / Accepted 8 April 2004)

High spatial resolution observations in the 1 to 3.5 $\mu$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 $\mu$m) and  L bands, HST-NICMOS data in filters between 1.1 and 2.2 $\mu$m, and integral field spectroscopic data from BEAR, an Imaging FTS, in the $\ion{He}{i}$ 2.06 $\mu$m and the Br $\gamma$ 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 $M_{\odot}$. 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 $^{\star}$, 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.

Key words: instrumentation: adaptive optics -- infrared: stars -- Galaxy: center -- stars: Wolf-Rayet

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