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Astron. Astrophys. 357, 1035-1044 (2000)

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Far infrared spectroscopy of FU Ori objects

ISO-LWS observations *

D. Lorenzetti 1,3, T. Giannini 1,2,3, B. Nisini 1, M. Benedettini 3, M. Creech-Eakman 4, G.A. Blake 4, E.F. van Dishoeck 5, M. Cohen 6, R. Liseau 7, S. Molinari 8, S. Pezzuto 3, P. Saraceno 3, H.A. Smith 9, L. Spinoglio 3 and G.J. White 7,10

1 Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio, Italy
2 Istituto Astronomico, Università La Sapienza, Via Lancisi 29, 00161 Roma, Italy
3 Istituto di Fisica Spazio Interplanetario - CNR Area Ricerca Tor Vergata, Via Fosso del Cavaliere, 00133 Roma, Italy
4 Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
5 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
6 Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA
7 Stockholm Observatory, 133 36, Saltsjöbaden, Sweden
8 IPAC/Caltech, MS 100-22, Pasadena, California, USA
9 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
10 Queen Mary & Westfield College, University of London, Mile End Road, London E1 4NS, UK

Received 12 January 2000 / Accepted 7 March 2000

Abstract

We present the results of the first spectrophotometric observations of a sample of FU Ori objects obtained with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The [OI] (63 µm) and the [CII] (158 µm) lines are commonly observed in all spectra (both ON and OFF source). The observational novelty is the presence in most of the sources of the transition of ionised nitrogen [NII] (122 µm), which is not detected in other objects in a similar evolutionary phase. This line probes low ionisation and low density material not easily traced by other lines. Line intensities and intensity ratios are used along with model predictions to infer the prevailing mechanisms for line excitation. To reconcile our far-infrared spectroscopy with previous knowledge of these objects, the simultaneous presence of two components is required: well localised J-shocks, responsible for the [OI] emission, and an extended low density ionised medium produced by UV photons from the disc boundary layer, responsible for the [NII] and [CII] emission. A few molecular lines (CO, OH, H2O) associated with relatively cold and dense peaks are revealed and their intensities are in good agreement with the proposed scenario. Other ionic lines ([OIII] and [NIII]) are detected in two sources in the Cyg OB7 region and likely trace the presence of nearby HII regions.

Key words: stars: circumstellar matter – stars: individual: FU Ori – infrared: ISM: lines and bands – infrared: stars

* ISO is an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA

Send offprint requests to: D. Lorenzetti (dloren@coma.mporzio.astro.it)

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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