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Astron. Astrophys. 342, L21-L24 (1999)

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Letter to the Editor

Water line emission in low-mass protostars *

C. Ceccarelli 1, E. Caux 2, L. Loinard 3, A. Castets 1, A.G.G.M. Tielens 4, S. Molinari 5, R. Liseau 6, P. Saraceno 7, H. Smith 8 and G. White 9

1 Laboratoire d'Astrophysique de l'Observatoire de Grenoble, B.P. 53X, F-38041 Grenoble Cedex, France
2 CESR-CNES, 9 Avenue de Colonel Roche, F-31029 Toulouse, France
3 Institut de Radio Astronomie Millimetrique, 300 rue de la piscine, F-38406 St. Martin d'Hères, France
4 SRON, P.O. Box 800, 9700 AV, Groningen, The Netherlands
5 IPAC, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
6 Stockholm Observatory, S-133 36 Saltsjobaden, Sweden
7 IFSI - CNR, Via del fosso del cavaliere 100, I-00133 Roma, Italy
8 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
9 Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK

Received 30 November 1998 / Accepted 7 January 1999

Abstract

Using the Long Wavelength Specrometer aboard ISO, we have detected far infrared rotational H2O emission lines in five low-mass young stellar objects in a survey of seven such sources. The total H2O fluxes are well correlated with the 1.3 mm continuum fluxes, but - surprisingly - not with the SiO millimeter emission originating in the outflows, suggesting that the water emission arises in the circumstellar envelopes rather than in the outflows.

In two of the sources, NGC1333-IRAS4 and IRAS16293-2422, we measured about ten H2O lines, and used their fluxes to put stringent constraints on the physical conditions (temperature, density and column density) of the emitting gas. Simple LVG modelling implies that the emission originates in a very small ([FORMULA] AU), dense ([FORMULA] cm-3) and warm ([FORMULA] K) region, with a column density larger than about [FORMULA] cm-2. The detected H2O emission may be well accounted for by thermal emission from a collapsing envelope, and we derive constraints on the acccretion rate and central mass of NGC1333-IRAS4. We also discuss an alternative scenario in which the H2O emission arises in an extremely dense shock very close to the central object, perhaps caused by the interaction of the outflow with the innner regions of the circumstellar envelope.

Key words: stars: formation – ISM: jets and outflows – ISM: individual objects: – infrared: ISM: lines and bands – radiative transfer – ISM: molecules

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Send offprint requests to: C.Ceccarelli

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
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