Astronomers have long speculated that water lines play an important role in cooling the interstellar medium and allowing stars to form. They have been unable to verify this before because the lines are completed absorbed by water vapour in the Earth's atmosphere; even at aircraft or balloon altitudes, they have proved impossible to detect. And although maser emission by water and hydroxyl molecules have been detected previously from various sources, the physical properties and the quantity of the water cannot be determined from such observations because the water is in a peculiar state and not in equilibrium with its surroundings.
The Long-Wavelength Spectrometer (LWS) has observed water-vapour lines in a variety of sources including regions of star-formation, evolved stars and planetary nebulae. These observations constitute a breakthrough in the investigations of inter- and circum-stellar material and illustrate the power of ISO to provide important new data. As well as telling us about the energy balance of the ISM, these new studies give us more insight to the complex chemistry which is taking place in interstellar space and the atmospheres of evolved stars.
The LWS' first - and surprising - sight of water vapour was in the outflow of material from NGC 7027, a carbon-rich planetary nebula at a distance of 800 pc (2600 light-years) in the constellation of Cygnus. It was expected that any oxygen in the outflow would have been mopped up by carbon to form carbon monoxide which is, indeed, seen in the LWS spectrum. But lines of water vapour are clearly visible along with those of its progenitor hydroxyl radical. The LWS astronomers speculate that the water may be formed by reactions between ions and molecules at the interface between the ionised and neutral gas around NGC 7027; alternatively, it is left over from an early stage of evolution, before large quantities of carbon had been dredged up from the star's interior by convection.
More recent observations have shown water vapour to be abundant in evolved stars including, appropriately enough, the variable cool, oxygen-rich red giant star W Hydrae in the constellation of Hydra, the Water-monster. A whole forest of rotational lines is observed in this source, variously estimated to be 100 parsecs (325 light-years) away. These have been used by the LWS team to model conditions in the outflow from the star and to estimate the rate at which it is losing mass. Comparison with observations made with the Short-Wavelength spectrometer will allow these models to be refined. According to Professor Mike Barlow, of University College London, leading the team analysing these data, "The detection by the LWS of more than 30 water lines in the spectrum of W Hya will provide vastly more information about how water molecules are formed and excited in the gas which flows out from these stars into interstellar space."
Molecules and dust formed in the atmospheres of stars in the later stages of their evolution seed the interstellar medium from which new stars are formed. In this dense and cold medium water is predominantly in its solid form of ice which, if heated sublimes directly into vapour. Such heating can be provided by supersonic jet streams which emanate from stellar objects in the process of formation. The Herbig-Haro object HH 54 is such a shock-heated clump of gas at a distance of 250 pc (800 light-years) i n the southern constellation of Chamaeleon, and it emits the water lines in abundance. Using these lines, together with complementary lines of species such as the carbon monoxide molecule and the hydroxyl radical (also observed with the LWS) it will be possible to model better the conditions under which stars form, one of the main aims of a programme, lead by Professor Paolo Saraceno of CNR-Istituto di Fisica dello Spazio Interplanetario in Frascati, Italy. According to Associate Professor Rene Liseau of Stockholm Observatory, who has analysed these data together with his Italian colleagues, "Without exaggeration, these discoveries will be of utmost importance for our understanding of the highly complex chemistry and physics which govern the formation of stars and planets."
The water vapour lines discussed so far are emission lines, that is they result from transitions between molecules from a higher to lower energy states. The LWS has also detected such lines in absorption, the molecules in the interstellar medium absorbing energy from the continuum spectra of background sources. These observations tell us that water is quite abundant in the interstellar medium and helps us understand the conditions in the cooler material surrounding protostars.
The Long-Wavelength Spectrometer is one of four instruments in the European Space Agency's Infrared Astronomy Satellite ISO. It covers the wavelength range 43 to 198 micrometers at either moderate (around 200) or high (about 10,000) spectral resolution. It was designed by a consortium of scientists and engineers from Canada, France, Italy, Sweden, the UK and the USA. Laboratories in France, Italy and the UK provided hardware and software, funded by national agencies including CNES, CNR and PPARC. Additional funding of individual scientists is provided by these agencies as well as NSERC and NASA.
For more information, contact:
| Professor Peter Clegg | Telephone: +44-(0)171-975-5038 |
|---|---|
| E-Mail: mailto:p.e.clegg@qmw.ac.uk |
18 June 1996