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Large carbon molecules travel in the interstellar medium

A team of IRAP 1 astronomers with their collaborator from the Observatory of Cagliari (Italy) has demonstrated that close to stars, the fullerene molecule, or C60, is ionized into C60+. This confirms the presence of C60+ in the interstellar medium and proves that these species are in the gas phase and do not lay on dust grains. For about 30 years, astronomers have considered, following the PAH 2hypothesis, that large carbon molecules in the gas phase play a major role in the chemistry and physics of cosmic environments. This new work on C60+ provides direct evidence of the validity of this hypothesis and are published on Jan. 22 in the journal Astronomy & Astrophysics.

Right figure: C60 molecule superimposed on the image taken with the Hubble telescope of the nebula NGC 7023 in which the C60+ ion has been detected. Credit: NASA & ESA / L. Cadars & O. Berné

In the 1980s, H. W. Kroto and his colleagues simulated in the laboratory the formation of carbon molecules in the envelopes of stars. They discovered a particular molecule containing exactly 60 carbon atoms, C60, whose structure looks like that of a football 3. With the increasing use of C60, particularly in the field of nanotechnology, H. W. Kroto, R. F. Curl and R. E. Smalley received the Nobel Prize in Chemistry in 1996 for their discovery. This work then motivated the search for C60 in space as a responsible for the mysterious interstellar diffuse bands (DIB) 4. In the 1990s, B.H. Foing and P. Ehrenfreund published the discovery of two new DIB at 0.9577 and 0.9632 microns, detected at the Observatoire de Haute Provence 5. Based on the laboratory data available at that time, they attributed them to the C60+ ion.

More recently, the progresses made by Infrared Space Astronomy allowed the detection of C60 in the vicinity of young stars and evolved stars, through the observation of four emission bands at 7.0, 8.5, 17.4 and 19.0 microns with NASA's 6 , 7Spitzer Space Telescope. However, the detection of C60 and C60+ raises a number of questions. The identification of the two DIB remains controversial due to a lack of accurate laboratory data. Indeed, the study of the spectroscopic properties of C60+ today remains a challenge for both experimentalists and theorists. Also, the analysis of the infrared spectrum of C60 led to the suggestion that C60 molecules could be trapped on grains 8. The team at IRAP detected new bands at 6.4, 7.1, 8.2 and 10.5 microns in the Spitzer data. Relying on new theoretical calculations, the authors attribute these bands to the C60+ ion and show that this ion is present in the gaseous form in the interstellar medium.

 

Figure above: The diffuse interstellar bands. Credit: P. Jenniskens & F. X. Désert

Note(s): 

  1. Institut de Recherche en Astrophysique et Planétologie (IRAP-CNRS/Université Toulouse 3)
  2. The PAH hypothesis was proposed in 1984 by A. Léger and J.-L. Puget then by an American team to explain a series of bands observed in emission in the mid-IR. According to this hypothesis, this emission is due to a family of large carbon molecules of the family of polycyclic aromatic hydrocarbons (PAH), which are in the gas phase and heated stochastically by ultraviolet photons coming from stars 8.
  3. Kroto H. W., Heath J. R., Obrien S. C., Curl R. F., Smalley R. E., C60: Buckminsterfullerene, Nature 318 (1985), 162-163
  4. Diffuse interstellar bands (DIB) are observed since 1922. They consist in a series of absorption bands, mainly in the visible domain, and are well known to astronomers as signatures of interstellar matter. The number of DIB detected so far is several hundreds. Large carbon molecules such as fullerenes and polycyclic aromatic hydrogenated (PAHs) in the gas phase are prime candidates to account for these DIB but no definitive identification has been made ​​to date.
  5. Foing, B. H.; Ehrenfreund, P., Detection of two interstellar absorption bands coincident with spectral features of C60+, Nature 369 (1994), 296-298
  6. Sellgren K., Werner M. W., Ingalls J. G., Smith J. D. T., Carleton T. M., Joblin C., C60 in Reflection Nebulae, Astrophys. J.  Lett. 722 (2010), L54-L57
  7. Cami J., Bernard-Salas J., Peeters E., Malek S.E., Detection of C60 and C70 in a Young Planetary Nebula, Science 329 (2010), 1180-1182
  8. Léger A., d'Hendecourt L., and Défourneau D., Physics of IR emission by interstellar PAHs, Astron. & Astrophys. 216, (1989), 148-164

Source(s): 

Interstellar C60+, O. Berne, G. Mulas et C. Joblin, Astronomy & Astrophysics, 550, L4, 2013.

Contact(s):

  • Christine Joblin, IRAP (CNRS/Université Paul Sabatier-Toulouse III)
    christine.joblin@irap.omp.eu, 05 61 55 86 01
  • Olivier Berné, IRAP (CNRS/Université Paul Sabatier-Toulouse III)
    olivier.berne@irap.omp.eu, 05 61 55 87 55
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