ISO provides first measurements of cold water in the Milky Way
11 April 2001The search for water in space goes on. Using ESA's Infrared Space Observatory (ISO), Spanish and Italian astronomers have for the first time measured the total amount of water in cold regions of our galaxy. This is especially interesting because these regions are the birthplace of stars like the Sun, and Solar Systems like our own. These new measurements show that water is more abundant than expected - in fact it is the third most abundant molecule in the regions which were studied.
The mean temperature of the water in these cold regions is -263oC. The researchers also determined how much water is in gaseous form and how much is 'frozen'. This has implications for the study of newborn planetary systems, since the water vapour and ices will end up in gaseous planets, planetary atmospheres and solid bodies like comets. As a result of this research, astronomers now know how much of each of these ingredients is available when a new planetary system is in the making.
Chemical reactions producing water are common in space. The existence of huge amounts of water in many different regions was discovered by ISO four years ago. In the Orion nebula, for instance, ISO found enough water to fill the Earth's oceans thousands of times over. And yet that was only the tip of the iceberg. Astronomers from the Consejo Superior de Investigaciones Cientmficas (CSIC) in Madrid, Spain, have continued the exploration, focusing for the first time on regions where low-mass stars like our Sun are born.
These regions are called 'quiescent' or 'cold' clouds, because they don't form massive stars and hence lack strong internal heat sources. They are at a mean temperature of -263oC, just 10 degrees above absolute zero. This is a disadvantage when it comes to searching for all the water available in the cloud: only part of it can be detected. The 'solid' water, or ices as they are usually called, are detected relatively easily in cold regions, but water vapour is elusive. In quiescent clouds the water vapour does not emit radiation detectable by the telescopes, precisely because of the low temperature and density of the cloud. On the other hand, water in liquid form does not exist in space because the temperature and pressure conditions are not suitable.
Consequently, only ices had been detected so far in cold clouds. But astronomers knew that water vapour should also be there, even if in small amounts. It was impossible to estimate the global amount of water in cold clouds, and its relative abundance as compared with other molecules, unless water vapour could be found.
As Italian astronomer Andrea Moneti explains, "in cold regions you expect to find most of the water forming ices because water vapour condenses on cold dust grains, much as it does on car roofs and windows in the winter. In warmer regions, on the contrary, the stars heat the environment and the ice on the dust grains evaporates - as when the Sun makes the frost evaporate off your car. So the rule is: the colder the cloud, the less water vapour. But we expected that there had to be at least some water vapour in quiescent clouds, only it had not been detected."
To search for the cold water vapour the team tried a different strategy than that used for the ices. They knew that if the light from a far-away object goes through some water vapour on its way to Earth, the water vapour will leave a 'chemical fingerprint' on that light. Astronomers decided to search for the fingerprint in the light coming from two regions in the galactic centre. These regions were selected because light emitted from there passes through several cold clouds on its way to Earth.
Cold water is also very abundant
Moneti and his colleagues used data stored in the ISO Archive, and were lucky. They detected the presence of both water vapour and solid ices in the cold regions. Now for the first time they can estimate the global amount of water available in these places and its relative abundance as compared with other molecules. Water turns out to be very abundant.
"We find that in cold regions there is as much water (ice and vapour) as in the very active star-forming regions. The important result is that after molecular hydrogen and carbon monoxide, water is the most abundant molecule," says Moneti, who was at the Spanish CSIC when the work was done and is currently at the Institut d'Astrophysique, in Paris.
According to this result, in a cold cloud with a thousand times the mass of the Sun the amount of water (ice and vapour) is equivalent to about a hundred Jupiter masses. Its average temperature is that of the cloud, -263oC. Astronomers estimate that there are millions of cold clouds in the Milky Way.
The fact that cold and warm clouds have the same amount of water is surprising, because some hypotheses suggested that the molecule was best preserved by processes happening exclusively in warm clouds. This ISO finding therefore gives new insights into the question of how water is formed and preserved in space.
Ice versus vapour
The team has also found that 99% of the water is ice, condensed on cold dust grains, while only 1% is in gaseous form. Knowing this percentage has important implications for the study of newborn planetary systems: it provides new data to help understand the role of water in the formation of planets and comets. Although the process is not yet fully understood, a simplified description is that part of the ice remains unprocessed and ends up in comets, while the rest turns into vapour and is used, together with the original water vapour, to make planetary atmospheres and gaseous planets.
As Moneti explains, "When a protoplanetary system is formed around a low-mass star, most of the gas and dust that does not form the central star will eventually find its way to the planets, while part of it will form comets. It is not yet clear how the water contained in the gas and in the grains contributes to form planetary atmospheres, but it is clear that a lot of processing occurs in the planets. On the other hand, we are fairly certain that comets are made of relatively unprocessed material: comets are essentially large, dirty snowballs, and we believe that the 'snow' that makes the comets shares many characteristics with the water ice in cold molecular clouds."
Astronomers expect that future infrared space telescopes, like ESA's Herschel, due to be launched in 2007, will penetrate even more deeply into the cold and dark regions where star, and eventually planet formation, first begins.
The paper explaining this discovery is accepted for publication in the Astrophysical Journal Letters, available at http://arXiv.org/abs/astro-ph/0012292
The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations.
For more information contact:
Andrea Moneti (Institut d'Astrophysique, Paris)
+33 1 4432 8067
Leo Metcalfe, ISO Project Scientist (ESA, Villafranca Satellite Tracking Station, Spain)
ESA Science Communication Service