Proposal type : Open Time
Scientific category : Interstellar_Matter
Title :
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A STUDY OF WATER EMISSION IN MOLECULAR CLOUDS : THE EXTENT OF WATER VAPOR EMISSION IN GALACTIC MOLECULAR CLOUDS
PI : J. CERNICHARO
Instituto de Estructura de la Materia
Dpto. Física Molecular
Serrano 123
28006 Madrid
e-mail : cerni@astro.iem.csic.es
Tel: 91-5901611
Fax: 91-5855184
Keywords :
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abundances
diffuse interstellar medium
line formation
molecular clouds
Abstract :
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For many years the extent of water emission and its abundance in
molecular clouds have been key problems in the study of the
interstellar medium -ISM-. Although all chemical models predict a high
water abundance, ground-based observations of non-masering rotational
transitions of water are practically impossible. The first detected 22
GHz transition of water, which has been observed in many star forming
regions, circumstellar envelopes and external galaxies, is in fact a
maser emission which probes small volumes of gas characterized by
extreme physical conditions. Recently, Cernicharo et al. (1990, 1994)
succeeded in observing at 183 GHz the weakly masering 313-220
transition of water in many molecular clouds opening a new window to
study water in the ISM. In particular, it was shown for the first time
that water emission in Orion is spatially extended. The derived
abundances demonstrate that water is one of the most abundant molecular
species in warm molecular clouds. However, the masering nature of the
313-220 transition limits its use as a diagnostic of the physical
conditions of the gas and the excellent weather conditions required to
observe at 183 GHz clearly restricts its measurement.
ISO is the only instrument in the next decade which will allow to
observe at high spatial resolution and sensitivity the thermal emission
of water vapor in the ISM. The angular resolution of ISO in the
far-infrared together with the sensitivity of the LWS FP spectrometer
will enable to map and resolve the large scale distribution of water in
nearby molecular clouds. We propose to map several nearby molecular
clouds in two lines of H2O connecting low-lying energy levels. Other
transitions of water and its rare HH18O isotope will be made towards
selected positions in these molecular clouds. The proposed observations
will provide fundamental data on the distribution of water in molecular
clouds and will result in a major contribution to our knowledge of
water and its role in the interstellar medium.
Publications :
"Widespread water vapour absorption in SgrB2". Cernicharo et al. (1997). Accepted for Astronomy and Astrophysics. page 1 , page 2 , page 3 and page 4 . [Full text].
See also: ESA PRESSE N. 12-97.
Significant Results :
Figure 1 in Cernicharo et al. (1997): Raster map obtained with the LWS grating spectra around the 2$_{12}-1_{01}$ transition of water vapor at 179.5~$\rm \mu m$. The offset positions are given in arcsec in each box of the raster and the central position corresponds to: 17$^h$ 47$^m$ 20.4$^s$, -28$^o$ 23$^{\prime}$ 32.2$^{\prime\prime}$ (J~2000). The X axis corresponds to wavelength (from 168 to 196~$\rm \mu m$) and the Y axis to the line over continuum ratio (e$^{-{\tau}}$). The spectrum of the central position is shown before the correction of the interference pattern (continuous line) in (a) and after the correction in (b) where the continuous line represents the adopted continuum level. The bottom left insert (c) shows the resulting spectrum in the central position divided by the continuum together with line identifications. The top right insert (d) shows the averaged water spectrum for offsets 270$^{\prime\prime}$, 180$^{\prime\prime}$ and 90$^{\prime\prime}$ (E, W, N and S) %%% on too much here together with the spectrum of the central position for comparison. The bottom right insert (e) shows the ortho water energy levels for energies below 200~K. The allowed radiative transitions are indicated by arrows with the numbers right and left of them indicating the wavelength in microns (left) and the Einstein coefficients in units of 0.01~s (right).
Updated: May 23, 1997.
