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

Moneti, A. and Stolovy, S. and Blommaert, J. A. D. L. and Figer, D. F. and Najarro, F. (2001) Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster. Astronomy & Astrophysics, 366 (1). pp. 106-120. ISSN 0004-6361. doi:10.1051/0004-6361:20000192.

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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 μ Cep profile, which is typical of the interstellar medium, with τ_(sil) ≃ 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 ∼ 20 000AUs. A reddening law similar to that of Lutz (1999) but with silicate features based on the μ Cep profile and normalized to our value of τ_(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 µm, and imply shell luminosities of log (L/L_⊙ ≃ 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.

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Additional Information:© 2001 ESO. Received: 25 September 2000. Accepted: 26 October 2000. We would like to thank T. Hayward and T. Herter for assistance in obtaining and reducing the SpectroCam-10 data and Cornell University for providing access to the Hale telescope. S.R.S. acknowledges support by NASA grant NAG2-207 and NSF grant AST-9218038. NASA grants NAGW-2551 and NAGW-2870 supported the development of SpectroCam-10 and the detector.
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Subject Keywords:galactic centre - quintuplet cluster - extinction - ISO
Issue or Number:1
Record Number:CaltechAUTHORS:20180307-071857262
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Official Citation:Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster A. Moneti, S. Stolovy, J. A. D. L. Blommaert, D. F. Figer and F. Najarro A&A, 366 1 (2001) 106-120 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85154
Deposited By: Ruth Sustaita
Deposited On:07 Mar 2018 17:50
Last Modified:15 Nov 2021 20:26

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