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Astron. Astrophys. 346, 604-616 (1999)


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ISO-LWS observations of Herbig Ae/Be stars *

I. Fine structure lines

D. Lorenzetti 1, E. Tommasi 2, T. Giannini 1,3,4, B. Nisini 4, M. Benedettini 4, S. Pezzuto 4, F. Strafella 5, M. Barlow 6, P.E. Clegg 7, M. Cohen 8, A.M. DiGiorgio 4, R. Liseau 9, S. Molinari 10, F. Palla 11, P. Saraceno 4, H.A. Smith 12, L. Spinoglio 4 and G.J. White 7

1 Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio, Italy
2 Agenzia Spaziale Italiana, via di Villa Patrizi 13, I-00161 Roma, Italy
3 Istituto Astronomico, Università La Sapienza, via Lancisi 29, I-00161 Roma, Italy
4 Istituto di Fisica Spazio Interplanetario - CNR Area Ricerca Tor Vergata, via Fosso del Cavaliere, I-00133 Roma, Italy
5 Dipartimento di Fisica, Università di Lecce, I-73100 Lecce, Italy
6 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
7 Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK
8 Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA
9 Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
10 IPAC/Caltech, MS 100-22, Pasadena, California, USA
11 Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy
12 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 25 November 1998 / Accepted 11 March 1999

Abstract

We present the results of the first spectrophotometric survey of a sample of eleven Herbig Ae/Be stars (HAEBE) obtained with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The [OI] 63µm and the [CII] 158µm lines are observed in all the investigated sources, while the [OI] 145µm transition, due to its relative faintness, sometimes remains undetected. By comparing line intensity ratios with model predictions, photodissociation, due to the UV photons from the central star, results the dominating excitation mechanism although contributions of C-shocks to the [OI] emission cannot be ruled out. A clear example for the presence of a photodissociation region (PDR) illuminated by an HAEBE is shown by LWS spectroscopic mapping of NGC 7129. Some diagnostic probes of the radiation field and density are provided for the objects in our sample: these substantially agree with the known characteristics of both the star and its circumstellar environment, although the observed ratio [OI]63/[OI]145 tends to be smaller than predicted by PDR models. The most likely explanation for this behaviour is self-absorption at 63µm by cold atomic oxygen. Fine structure lines of the ionised species [OIII], [NII] were detected whenever the star had a spectral type of B0 or earlier; in particular, around the star CoD-42o11721, besides a compact HII region, evidence is given for an extended low electron density ionised region. Finally, molecular line emission is associated with stars powering a CO outflow, and clumpy PDR models, better than C-shock models, predict for them relative cooling (CO vs OI and CO vs OH) similar to the observed ones.

Key words: stars: circumstellar matter – stars: pre-main sequence – infrared: ISM: lines and bands – infrared: stars

* Based on observations with ISO, 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)

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999

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