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Football molecule explains 100-year-old astronomical problem

Exactly a century ago, astronomers observed the first two diffuse interstellar bands (DIBs) in space. A DIB manifests itself as a colour of light that is missing in the radiation from stars behind an interstellar cloud. Although about 500 DIBs are yet discovered, they remained inexplicable until recently. An international team of astronomers with the help of the Hubble Space Telescope has now proven that some of these DIBs are caused by C60+, a molecule in the shape of a football.

Absorbing football

When you look from the Earth to the light of stars that are behind interstellar clouds of light-years in size, you spot them immediately: the diffuse interstellar bands. ‘Something’ in these clouds absorbs light in a large number of different colours, from the ultraviolet to the near infrared part of the spectrum. Sometimes weak, sometimes strong. It doesn’t matter in which direction you aim your telescope; DIBs seem to be everywhere. ‘But the cause of these bands remained a big mystery’, says Martin Cordiner (NASA Goddard). Cordiner is the principal investigator of the Hubble Space Telescope Project, which recently cleared up all doubt and proved that C60+ is a DIB carrier, or in other words: is responsible for some of the DIB-signals. C60+ is the ion of a molecule consisting of 60 carbon atoms, which are exactly the shape of a football.

Exotic structures

In the Netherlands, Harold Linnartz (Leiden Observatory), Pascale Ehrenfreund (Leiden Observatory/DLR) and Bernard Foing (ESTEC) are involved in the investigation. In the mid-1990s, Ehrenfreund and Foing already suspected a possible DIB carrier, but there was insufficient laboratory data to prove this. 'In the lab we try to create the molecules that could be responsible for a DIB in space', explains Linnartz. 'Because the conditions there are so different from those on earth, we have to make these molecules on the spot. These are ions, radicals and other exotic chemical structures, which do survive in space but in normal circumstances do not survive on earth'. The researchers measure the spectra of the molecules produced. They then compare these spectra with the measured DIB signals.

Weak bands

In 2015 and 2016, laboratories in Basel and Innsbruck provided the first usable C60+ spectra,' says Linnartz. 'Of the five bands found, two strong bands exactly matched the data from space. They were unable to assign the weaker tyres with sufficient certainty at the time. We've managed to do this now.

The researchers used the Hubble Space Telescope. C60+ absorbs light in a wavelength range in which water also absorbs a lot of light. Observing from Earth with a telescope therefore provides data that is difficult to interpret, because the water in our atmosphere disturbs the signal. Another astronomical team claimed earlier that you could also distinguish the weaker links in the data disturbed by water, but those claims were not confirmed by other astronomers', says Cordiner. 'But by measuring above the Earth's atmosphere, you can bypass that problem.' A number of tricks were needed, because at 27 years old the Hubble Space Telescope is already an old lady who can't do everything (anymore). In a paper published in the Astrophysical Journal on 22 April, the scientists describe their new detection method, with which the weaker bands have now also been assigned. 

Nieuwe missie

Linnartz: 'It's great, because for the first time we now know for sure which molecule contributes to the DIB spectrum.' But according to him, that answer is also confusing. There is a lot of radiation in the clouds we are investigating. That is why we have always assumed that large complex molecules, such as C60, do not occur there. Indeed, until recently, the largest known molecule in this type of cloud was C3, which is no less than 20 times smaller. It is now a matter of finding out what exotic chemical processes take place in these clouds. Linnartz explains why this is important: 'On the one hand to unravel which foreign molecules are the carriers of the other DIBs, but also because the material from which these clouds are composed ultimately provides the building blocks of new stars and planets, such as our earth.'

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