Comparing SWAS and ISO observations of water in outflows

¹ CNR-Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere 100, 00133 Roma, Italy
² Department of Physics and Astronomy, UCL, Gower St., London WC1E 6BT, UK
³ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio, Italy
⁴ National Astronomy and Ionosphere Center, Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853-6801, USA

Corresponding author: M. Benedettini, milena@ifsi.rm.cnr.it

Received: 24 April 2002
Accepted: 27 August 2002

Abstract

We present a detailed comparison of SWAS and ISO observations of H₂O  emission in a sample of outflows. By taking into consideration the different methods used to derive the fractional water abundance, we find that, despite the initial apparent discrepancies, there is satisfactory agreement between ISO and SWAS results for the outflows observed by both satellites. Such an agreement is reached by assuming that most of the detected water comes from a single region smaller than both the ISO and SWAS beam apertures and considering temperatures higher ( 100 K) and density lower (n(H₂) < 2  10⁶ cm^-3) than previously adopted. The ratio between the integrated flux of the ortho-H₂O 2₁₂–1₀₁ line at 179.5 μm  observed by ISO and the fundamental ortho-H₂O 1₁₀–1₀₁ line at 538.3 μm  observed by SWAS is always smaller (never by more than a factor of 7) than the expected ratio. We propose that the fundamental ortho-H₂O transition has a contribution from a cooler gas component to which SWAS is sensitive but which is not traced by ISO. We conclude that, while ISO and SWAS observations are consistent with chemical models predicting a significant enhancement of water vapor abundance in outflow regions, the explanation for the low water abundance derived from SWAS observations of the cold ISM is still unclear.