Abstract

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 5m 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 τsil ≃ 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 τsil 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. (Refer to PDF file for exact formulas).

Publication Date

1-4-2001

Comments

Also archived in: arXiv:astro-ph/0010558 v1 27 Oct 2000 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.ISSN:1432-0746 Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type

Article

Department, Program, or Center

School of Physics and Astronomy (COS)

Campus

RIT – Main Campus

Share

COinS