EDP Sciences
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Volume 366, Number 1, January IV 2001
Page(s) 106 - 120
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20000192

A&A 366, 106-120 (2001)
DOI: 10.1051/0004-6361:20000192

Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster

A. Moneti1, S. Stolovy2, J. A. D. L. Blommaert3, D. F. Figer4 and F. Najarro5

1  Institut d'Astrophysique, 98bis Blvd. Arago, 75014 Paris, France
2  CalTech, Astronomy Dept., 105-24, Pasadena, CA 91125, USA
3  ISO Data Centre, PO Box 50727, 28080 Madrid, Spain
4  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
5  Instituto Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain

(Received 25 September 2000 / Accepted 26 October 2000)

In an attempt to determine the nature of the enigmatic cocoon stars in the Quintuplet Cluster, we have obtained mid-infrared imaging and spectrophotometry of the cluster, using the CAM and SWS instruments on ISO, using SpectroCam-10 on the Palomar 5 m telescope, and NICMOS on HST. The spectra show smooth continua with various dust and ice absorption features. These features are all consistent with an interstellar origin, and there is no clear evidence for any circumstellar contribution to these features. We find no spectral line or feature that could elucidate the nature of these sources. Detailed modeling of the silicate absorption features shows that they are best reproduced by the $\mu$ Cep profile, which is typical of the interstellar medium, with $\tau_{\rm sil} \simeq 2.9$. The high spatial resolution mid-IR images show that three of the five cocoon stars have spatially extended and asymmetric envelopes, with diameters of $\sim$ 20 000AUs. A reddening law similar to that of Lutz (1999) but with silicate features based on the $\mu$ Cep profile and normalized to our value of $\tau_{\rm sil}$ is used to deredden the observed spectrophotometry. The dereddened energy distributions are characterised by temperatures of 750-925 K, somewhat cooler than determined from near IR data alone. Models of optically thin and geometrically thick dust shells, as used by Williams et al. (1987) for very dusty, late-type WC stars, reproduce the observed SEDs from 4 to $17 \mu\rm m$, and imply shell luminosities of $\log (L/L_\odot) \simeq 4.5$-4.9 for the brightest four components. An analysis of the various suggestions proposed to explain the nature of the cocoon stars reveals serious problems with all the hypotheses, and the nature of these sources remains an enigma.

Key words: galactic centre -- quintuplet cluster -- extinction -- ISO

Offprint request: A. Moneti

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