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Interstellar dust consists of small grains of solid matter observed in the interstellar medium (ISM). Dust grains are produced initially in the envelopes of stars by the condensation of atoms and molecules that cool down as they flow outward. After the grains are injected into the ISM, various mechanisms can cause their destruction. It has been estimated that the rate of destruction of the grains in the ISM is faster than the rate of their production by stars. Observations, however, reveal that dust populations survive in the ISM, hence the hypothesis of a mechanism that re-forms the grains in situ. Remarkably, this mechanism would take place at the very low temperatures, between 10 and 100 K, that prevail locally. It is proposed that grains are re-formed by adsorbing atoms and molecules present in the interstellar gas phase, as depicted in fig. 1a. Only specific species become chemically bound, thus contributing to the re-formation of the grains, while the other species return eventually to the gas phase. The energy necessary to the desorption of the latter is possibly provided by interstellar radiations or by the binding chemical reactions of re-formation.

As interstellar dust is mostly composed of silicate grains and carbonaceous grains, our group studies experimentally the formation of these materials under conditions that mimic those of the re-formation of grains in the ISM. The reactions that are susceptible to bind atoms and molecules at very low temperatures are studied by inserting the reactants in superfluid helium nanodroplets [1], which have a temperature of 0.37 K (see this project). The formation of entire grains by the accretion of cold atoms and molecules is attempted by warming up and evaporating neon matrices doped with the species of interest [1-3]. The process, illustrated with fig. 1b, is monitored using UV/vis and IR spectroscopy and the grains synthesized in the experiments are further examined by electron microscopy.

Our experiments have shown that silicate grains can be re-formed at cryogenic temperatures [1-3] and that carbon molecules condense into a solid at the same low temperatures [4]. The current works concern the condensation of species comprising both silicon oxide molecules and carbon molecules. The process leads to the formation of distinct carbonaceous grains and silicate particles [5].

 

accretion principle

Fig. 1. Principle of accretion of atoms and molecules into a solid refractory material (a) in the interstellar medium (ISM) and (b) in an evaporating doped Ne matrix.

 

References:

[1] S. A. Krasnokutski, G. Rouillé, C. Jäger, F. Huisken, S. Zhukovska, and Th. Henning: Formation of silicon oxide grains at low temperature, Astrophys. J. 782, 15/1-15/10 (2014). [DOI][arXiv.org]
[2] G. Rouillé, S. A. Krasnokutski, M. Krebsz, C. Jäger, F. Huisken, and Th. Henning: Cosmic dust formation at cryogenic temperatures, in: The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments, edited by A. Andersen, M. Baes, H. Gomez, C. Kemper, and D. Watson, PoS(LCDU 2013)047. [DOI]
[3] G. Rouillé, C. Jäger, S. A. Krasnokutski, M. Krebsz, and Th. Henning: Cold condensation of dust in the ISM, Faraday Discuss. 168, 449-460 (2014). [DOI][arXiv.org]
[4] D. Fulvio, S. Góbi, C. Jäger, Á. Kereszturi, and Th. Henning: Laboratory experiments on the low-temperature formation of carbonaceous grains in the ISM, Astrophys. J. Suppl. Ser. 233, 14/1-14/11 (2017). [DOI][arXiv.org]
[5] Th. Henning, C. Jäger, G. Rouillé, D. Fulvio, and S. A. Krasnokutski: Dust formation at cryogenic temperatures, in: Astrochemistry VII: Through the Cosmos from Galaxies to Planets, edited by M. Cunningham, T. Millar, and Y. Aikawa, Proceedings IAU Symposium No. 332, 312-319 (2017). [DOI]

 

For more information contact Dr. Cornelia Jäger.

Related funding(s): DFG JA 2107/2-2.

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