Astron. Astrophys. 346, 617-625 (1999)

ISO-LWS observations of Herbig Ae/Be stars ^*

II. Molecular lines

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

1</sup> Istituto Astronomico, Università La Sapienza, via Lancisi 29, I-00161 Roma, Italy
² Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio, Italy
³ Istituto di Fisica Spazio Interplanetario - CNR Area Ricerca Tor Vergata, via Fosso del Cavaliere, I-00133 Roma, Italy
⁴ Agenzia Spaziale Italiana, via di Villa Patrizi 13, I-00161 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, CA, USA
¹¹ Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy
¹² Harvard-Smithsonian Centre for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 25 November 1998 / Accepted 11 March 1999

Abstract

We present the first ISO-LWS observations of the molecular FIR lines in 3 out of a sample of 11 Herbig Ae/Be stars (HAEBE), namely IRAS12496-7650, RCrA and LkH234. High-J rotational CO lines (from = 14 to = 19) have been observed in all the spectra, while two (at 79 µm and 84 µm) and three OH lines (at 71 µm, 79 µm and 84 µm) were detected in LkH234 and RCrA respectively.

For all sources the molecular emission has been consistently fitted with a Large Velocity Gradient (LVG) model and it results originated in a warm (T 200 K) and dense ( 10⁵ cm^-3) gas located in very compact regions having diameters of few hundreds of AU.

These three sources are those with the highest density among the stars of the sample; this suggests that the molecular emission arises in regions showing density peaks.

By comparing the observed cooling ratios with model predictions, we find that the FUV radiation from the central source (or from a more embedded companion) is the most likely responsible for the line excitation. At least for the sources where OH has been observed, the contribution of shocks to the line emission can be reasonably ruled out because of the absence in the spectra of any water vapour lines, in contrast with the predictions for molecular emission coming from warm shocked environments.

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: T. Giannini (teresa@coma.mporzio.astro.it)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Results
• 4. Line emission fitting
  • 4.1. IRAS 12496-7650
  • 4.2. RCrA
  • 4.3. LkH234
• 5. Discussion
  • 5.1. Occurence of the molecular emission among Herbig Ae/Be stars
  • 5.2. Excitation mechanisms for the molecular emission
• 6. Conclusions
• References

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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