Organic matterial surrounding a star
15 October 1998
A huge ring of organic matter surrounding a young star has been observed by the European Space Agency's ISO space telescope. This is a kind of structure never detected before. With this finding ISO shows again a clear example of how the stars and their environment work as nature's chemical factories: not only is water being produced there - as ISO demonstrated -, but complex organic molecules are also present; these molecules are, essentially, the basic building blocks of all living organisms.The radius of the observed ring is 4400 times the distance between the Earth and the Sun; it surrounds a young star called HD 97300, located in the Chamaeleon constellation about 600 light-years away from the Earth.
However, main author Ralf Siebenmorgen, from the ISO Science Operations Centre in Villafranca (Madrid, Spain), points out that although the ISO image indeed shows an elliptical ring surrounding a central star, deeper analysis of the data suggests a shell-like structure, an empty sphere rather than a two dimensional ring.
The molecules in it are large, made of several hundreds of atoms of carbon and hydrogen; they are very likely to be the so-called Polycyclic Aromatic Hydrocarbons (PAH), that is, chains of atoms of carbon and hydrogen that have already been detected in many environments in our Galaxy, for example on the surface of comets or scattered in the Interstellar Medium - as well as in extragalactic objects.
The young central star HD 97300 belongs to a type of stars known to be brighter than average when observed at infrared wavelengths, an outstanding feature that astronomers have been studying with a programme of infrared observations with ISO. The discovery of the giant ring of organic matter is an unexpected result of this campaign. Of all stars targeted for the programme only HD 97300 shows a ring of organic matter, although this fact doesn't necessarily prove the other stars not to have it.
The observations were made with the infrared camera on-board ISO, ISOCAM, which can also take spectra and thus provide information about the objects' chemical composition. The scientists knew that the ring in HD 97300 was made of dust, but to identify specific components in it they had to apply a theoretical model relating dust chemical composition with dust spectra. The observed spectra fitted surprisingly well with the model-predicted spectra for a dust made of large organic molecules.
The origin of the ring
The mere presence of 'organic' dust in itself gave a hint about the origin of the ring, PAH molecules being very common in the interstellar environment, the ring must be made of interstellar matter rather than of ejected material from the central star.
Also, the ring's morphology suggests that its birth was due to the interaction of the star with the surrounding matter - as a consequence, for instance, of a strong stellar wind sweeping away interstellar material.
As Siebenmorgen explains, "The ring coincides with the inner wall of a three-dimensional almost empty cavity, as if the ring resulted from the interaction of a stellar wind with the surrounding environment. In this hypothesis, the material in the ring is swept-up gas."
Since there is no evidence of a stellar wind at the present time, the ring must actually be the remnant of a mass-loss episode which has died out very recently. This wind giving off material from the star could have reached velocities of hundreds of kilometres per second.
The discovery will be published in the journal Astronomy and Astrophysics. Further observations should produce more accurate measurements of this event and, it is hoped, also an explanation to the question of the origin of the ring.
