HIFI has already resulted in more than 60 scientific articles

How exactly are new stars born? Using the Dutch molecule hunter HIFI, astronomers have been trying to answer that question for the past 18 months. The results are highly promising: to date no fewer than 62 scientific articles have been published thanks to HIFI’s observations. For example, using the space instrument astronomers have discovered two new types of water as well as hydrogen fluoride in space.

Normally, the formation of stars and planets remains hidden from our view, but thanks to Herschel we can now see which processes are taking place behind the dust clouds. (ESA, PACS & SPIRE)

Astronomers know where new stars are born: in dense dark clouds. Yet the birth of stars and planets normally remains hidden from our view, so we still do not know exactly how this happens. For example, why are far more stars formed in some clouds than in others? And why are low-mass stars formed sometimes and high-mass stars at other times? Thanks to the ESA space telescope Herschel, with HIFI onboard, we can now study the processes that take place in dust clouds by observing light in the far infrared. This is particularly important for high-mass stars. These exert an enormous, and mostly destructive, influence on their surroundings. It is therefore hardly surprising that astronomers would like to know more about where these ‘beasts’ come from.

Molecule hunter HIFI is one of the three scientific instruments onboard Herschel. Herschel can determine the total amount of far infrared light using the instruments PACS and SPIRE, or it can extricate the radiation from specific molecules using HIFI, the space instrument constructed under the leadership of SRON Netherlands Institute for Space Research.

Herschel not only makes photos of stars and nebulae, but can also produce images of a larger part of the sky. This image shows galaxies, each of which contains billions of stars. (ESA, SPIRE)

Water in clouds
Water is one of the most important molecules in space and because telescopes on earth cannot see this, it is HIFI’s main objective. It plays a role in all phases of the dynamic process of star and planet formation. In dense dark clouds water (in gaseous form) is responsible for cooling as a result of which these clouds contract faster into new stars. Young stars have a disc of dust particles around them, and water (in the form of an ice layer on the dust particles) acts as a glue, enabling the dust particles to stick together so that first of all blocks of rock and ultimately planets arise. On the surface of these planets, water is present in liquid form that acts as a transport medium, bringing together the organic molecules that are the building blocks of life.

SRON researcher Floris van der Tak: "Using HIFI, astronomers have now established that the quantity of water in dense dark clouds varies immensely. Not only between clouds but also within clouds. A lot of water has been found in the vicinity of some young stars, which indicates that these have warmed up their surroundings considerably. As a result of this, any further star formation in these clouds will be difficult, as warm clouds cannot contract easily."

New forms of water

The Orion nebula, with a HIFI spectrum of water superimposed on it. (ESA, HEXOS)

A second, unexpected result was the discovery of two new forms of water: charged water (OH+ and H2O+) and heavy water (D2O). The existence of charged water points to UV radiation: it does not occur naturally on earth but it can be produced in a laboratory with a strong radiation source. Van der Tak: “In dense dark clouds UV radiation cannot be seen directly and therefore OH+ and H2O+ are useful indicators for this invisible radiation. In our galaxy, the sources of UV radiation in gas clouds are young high-mass stars. The radiation they emit destroys their surroundings with the result that no further star formation takes place."

The discovery of heavy water (D2O), which is only formed at very low temperatures, means that the dense dark clouds where new stars are now being born must have been particularly cold in a previous stage (less than 10 K). The difference in rest energy between H2O and D2O is very small and only becomes significant at a very low temperatures. As it is only formed at low temperatures, heavy water renders very cold gas and dust visible. That is particularly useful, as gas and dust emit little radiation at very low temperatures and are easily overshadowed by warmer material in the foreground and background.

Other new molecules
Besides these new forms of water, HIFI has also discovered other new molecules in space. The most important of these is probably the molecule hydrogen fluoride (HF), which is most prevalent in very rarefied clouds.