The composition of circumstellar gas and dust in 51 Oph^*

¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge MA 02138, USA
² Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
⁴ SRON, PO Box 800, 9700 AV Groningen, The Netherlands

Corresponding author: M. E. van den Ancker, mvandenancker@cfa.harvard.edu

Received: 2 January 2001
Accepted: 14 February 2001

Abstract

We analyze ISO archive data of the nearby bright emission-line star 51 Oph, previously classified as a proto-planetary system similar to β Pic. The infrared spectrum reveals the presence of gas-phase emission bands of hot (~850 K) CO, CO₂, H₂O and NO. In addition to this, partially crystalline silicate dust is present. The solid-state bands and the energy distribution are indicative of dust that has formed recently, rather than of debris dust. The presence of hot molecular gas and the composition of the circumstellar dust are highly unusual for a young star and are reminiscent of what is found around evolved (AGB) stars, although we exclude the possibility of 51 Oph belonging to this group. We suggest several explanations for the nature of 51 Oph, including a recent episode of mass loss from a Be star, and the recent destruction of a planet-sized body around a young star.