Astron. Astrophys. 350, 529-540 (1999)

Strong H₂O and high-J CO emission towards the Class 0 protostar L1448-mm ^*

B. Nisini ^1,2, M. Benedettini ², T. Giannini ^1,2,3, E. Caux ⁴, A.M. Di Giorgio ², R. Liseau ⁵, D. Lorenzetti ^1,2, S. Molinari ⁶, P. Saraceno ², H.A. Smith ⁷, L. Spinoglio ² and G.J. White <sup>8,5

1</sup> Osservatorio Astronomico di Roma, 00040 Monteporzio Catone, Italy
² CNR-Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca Tor Vergata, Via Fosso del Cavaliere, 00133 Roma, Italy
³ Istituto Astronomico, Università La Sapienza, Via Lancisi, 29 00161 Roma, Italy
⁴ CESR, B.P.4346, 31028 Toulouse Cedex 04, France
⁵ Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
⁶ Infrared Processing and Analysis Center, Pasadena, CA 91125, USA
⁷ Harvard - Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, USA
⁸ Physics Department, Queen Mary and Westfield College, University of London, Mile End Road - London E1 4NS, UK

Received 25 January 1999 / Accepted 22 July 1999

Abstract

The spectrum of the Class 0 source L1448-mm has been measured over the wavelength range extending from 6 to 190 µm with the Long Wavelength Spectrometer (LWS) and the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). The far infrared spectrum is dominated by strong emission from gaseous H₂O and from CO transitions with rotational quantum numbers ; in addition, the H₂ pure rotational lines S(3), S(4) and S(5), the OH fundamental line at 119 µm, as well as emission from [OI ] 63 µm and [CII ] 158 µm are also observed. The strong CO and water emission can be consistently explained as originating in a warm gas component at T700-1400 K and (3-50) 10⁴ cm^{- 3}, which fills about 0.2-2% of the 75" LWS field of view (corresponding, assuming a single emitting region, to a physical size of about (3-12)" or (0.5-2) 10^-2 pc at d = 300 pc). We derive an H₂O/CO abundance ratio 5, which, assuming a standard CO/H₂ abundance of 10^-4, corresponds to H₂O/H₂5 10^-4. This value implies that water is enhanced by about a factor 10³ with respect to its expected abundance in the ambient gas. This is consistent with models of warm shocked regions which predict that most of the free atomic oxygen will be rapidly converted into water once the temperature of the post-shocked gas exceeds 300 K. The relatively high density and compact size inferred for this emission may suggest an origin in the shocked region along the molecular jet traced by SiO and EHV CO millimeter line emission. Further support is given by the fact that the observed enhancement in H₂O can be explained by shock conditions similar to those expected to produce the abundant SiO observed in the region. L1448-mm shows the largest water abundance so far observed by ISO amongst young sources displaying outflow activity; we argue that the occurrence of multiple shocks over a relatively short interval of time, like that evidenced in the surroundings of L1448-mm, could have contributed to enrich the molecular jet with a high H₂O column density.

Key words: stars: formation stars: individual: L1448-mm ISM: jets and outflows ISM: molecules infrared: ISM: lines and bands

* 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: Brunella Nisini (bruni@coma.mporzio.astro.it)

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Contents

• 1. Introduction
• 2. Observations and results
  • 2.1. LWS observations
  • 2.2. SWS observations
• 3. Analysis of molecular emission
  • 3.1. CO and H₂
  • 3.2. H₂O and OH
  • 3.3. The N(H₂) column density
• 4. O⁰ and C⁺ emission
• 5. Discussion
  • 5.1. Excitation mechanisms
  • 5.2. Comparison with shock models
  • 5.3. Comparison with other outflow driving sources
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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