we propose to use iso sws to measure the abundance of water vapor in photo-dissociation regions and the molecular cloud in their vicinity. such regions are expected to be chemically very active, particularly due to the release of gaseous species from the grain mantles, where molecule synthesis may occur. because of the high dipole moment of h2o, 1.88 debye, the gas density in these regions, 1e4 to 1e6 cm-3, allows very little collisional excitation. however, radiative pumping by the strong dust continuum is possible. the case of h2o is particularly favorable, since the fundamental vibrational transition of the molecule, nu2, happens to be coincident with the 6.2 mu emission feature. the de-excitation produces emission lines between 6.4 and 6.7 mu, in a spectral region of minimum continuum flux density. consequently, these lines show a high contrast on the dust continuum, and can be detected with sws, despite the low resolution of the grating. the intensity of the lines depends only on the h2o column density and the 6.2 mu exciting flux density, so that h2o abundances can be derived. using the lines of h2o and h218o allows to measure water vapor abundances ranging from 1e-9 to 1e-4. both the ortho and para h2o will be probed with this method, and the ortho- to-para abundance ratio, which can be a signature of the h2o formation temperature (grain mantles or gas phase), will be determined. the regions selected are a/ a galactic photodissociation region, m17-sw (autumn launch) or orion bar (spring launch), and b/ a prototypical starburst galaxy, ngc 253 (autumn) or m 82 (spring). for the galactic region, a coarse map covering the interface and the molecular cloud will be performed in the emission lines. concerning the starburst, only the central position will be observed.
Instrument
SWS02
Temporal Coverage
1996-10-13T23:31:02Z/1996-10-13T23:59:18Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
