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Astron. Astrophys. 323, 442-448 (1997)


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Research Note

The nature of the unusual source IRAS 18530+0817 *

H.J. Walker 1, V. Tsikoudi 2, C.A. Clayton 1, T. Geballe 3, D.H. Wooden 4 and H.M. Butner 5

1 CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK
2 University of Ioannina, Dept. of Physics, 45 110 Ioannina, Greece
3 Joint Astronomy Centre, 660 N A'ohoku Place, Hilo, Hawaii 96720, USA
4 NASA Ames Research Center, Moffett Field, CA 94035, USA
5 Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA

Received 15 March 1996 / Accepted 15 January 1997

Abstract

Infrared photometry and spectroscopy of the unassociated (infrared-only) IRAS source, IRAS 18530+0817, indicate that it is a highly unusual evolved star surrounded by a dense envelope of cool gas and dust. The IRAS data reveal a large infrared excess and significant flux variability in the 12 - 25 µm region, as well as a prominent, silicate emission feature at 8.5 - 11.5 µm. The shape of the feature also appears to vary; in 1993 it was apparently attenuated by an absorption with a sharp edge at 9.5µm, which may be circumstellar or photospheric in origin. At other times the silicate feature appears to be self-absorbed by varying amounts. Near-infrared spectra of IRAS 18530+0817 are dominated by steam absorptions, with the K-band spectrum showing only a narrow band of relatively unabsorbed continuum at 2.15 - 2.29 µm. The object appears to possess the strongest stellar photospheric H2 O absorptions yet observed. We tentatively conclude that the central star of IRAS 18530+0817 is a late-type, O-rich, AGB star (a Mira variable) surrounded by an envelope of O-rich dust.

Key words: circumstellar matter - stars: IRAS 18530+0817 - stars: evolution - stars: mass-loss - stars: AGB and post-AGB - infrared: stars

* Based on observations from IRAS, UKIRT, IRTF and SAAO.

Send offprint requests to: H. J. Walker

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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