EDP Sciences
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Volume 378, Number 1, October IV 2001
Page(s) 146 - 152
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011155

A&A 378, 146-152 (2001)
DOI: 10.1051/0004-6361:20011155

ISO observations of symbiotic stars
I. HM Sge

H. Schild1, S. P. S. Eyres2, 3, A. Salama4 and A. Evans2

1  Institut für Astronomie, ETH-Zentrum, 8092 Zürich, Switzerland
2  Department of Physics, Keele University, Keele, Staffs, ST5 5BG, UK
3  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
4  ISO Data Centre, ESA Astrophysics Division, Villafranca del Castillo, Spain

(Received 2 October 2000 / Accepted 14 August 2001 )

We present ISO SWS and LWS observations of the symbiotic star HM Sge. The infrared emission longward of ~5$ \mu$m is dominated by a silicate dust shell. There is a number of high ionization emission lines superimposed on the dust continuum. The ISO continuum spectra can be fitted with a single, optically thick silicate dust shell but such a model does not fit other known properties of this system, such as the observed $J\!H\!K$ colours, the optical visibility of the red giant and the occurrence of a dust obscuration event. We favour a two component model in which in addition to a partial Mira dust shell, a second dust component is associated with the hot source. This second shell is located at the interface between the Mira dust shell and the dust free region carved out by the hot component. This picture corresponds closely to that deduced independently from radio observations. The two dust components are very different. The dust envelope associated with the Mira is very extended and optically thin, whereas the second dust component is geometrically rather thin but dust rich. The model not only fits the ISO data reasonably well, but is also compatible with other IR properties of HM Sge. The thin dust shell contributes strongly to the dust emission over most of the ISO range. It may be the combination of the two shells that makes a symbiotic a D type. The general agreement of the present models with ISO observations indicate that radiation (rather than collisions) as the main dust heating mechanism is sufficient.

Key words: binaries: symbiotic -- circumstellar matter -- stars: individual: HM Sge -- stars: winds, outflows -- infrared: stars

Offprint request: A. Evans, ae@astro.keele.ac.uk

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