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Studying interstellar grains in laboratory through an electrostatic storage ring

Physicists have developed an electrostatic storage ring, whose size is only a few tens of centimeters, in order to study in laboratory the properties of interstellar grains or large carbon molecules. This mechanism keeps the particles long enough to reproduce the extreme vacuum and low temperature conditions which prevail in astrophysical environments.

Right Figure : "MINIRING." The visualization of the beam path has been obtained by injecting nitrogen into the vacuum chamber. The blue color comes from the de-excitation of the nitrogen. (credit photo montage G. Montagne)

To isolate the material from any outside disturbance, the researchers store elementary particles into storage rings and trap ions or molecules into electrostatic or magnetic devices. However, these devices are unsuitable when the mass to charge ratio of the molecules or trapped particles becomes too large. For this reason, physicists have been developing for several years a new type of  fully electrostatic storage ring   to retain particles without mass limit. Physicists of the Institut Lumière Matière - ILM (CNRS / Univ. Lyon 1) have just designed and implemented such an instrument, called "MINIRING", which is characterized by a small size, a few tens of centimeters, a relatively low cost, which makes it affordable for a large number of laboratories, without sacrificing technical performances, such as the angular and energetic dispersion of the stored particles. On the other hand, in collaboration with a researcher of the Institut de Recherche en Astrophysique et Planétologie - IRAP (CNRS / Univ. Toulouse 3), this instrument has led scientists to obtain new results on the radiative cooling of the anthracene. This opens new perspectives on the understanding of the life duration and the critical size of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium. This work is the subject of a publication in the journal Physical Review Letters.

 

Figure above : Scheme of the storage ring "MINIRING"

When one wishes to keep particles into a ring, one of the major challenges is the focus of the particle cloud: how to prevent the stored particle from spreading as the number of laps grows, and how to manage that the velocity dispersion is minimal ? To solve this problem, the physicists of Lyon choosed a very special geometry: the trapped particles are going back and forth between two electrostatic mirrors separated by a foot and are taking two separate and distinct paths curves of ten centimeters in the middle of the instrument on the outward and return. The conical electrostatic mirrors chosen by the researchers focus particles which are reflected in both directions while the cylindrical deflecting usually used only act in one direction. With this trick, the number of focusing devices  to insert into the path of the beam is significantly reduced, which allows a very compact ring: this facilitates cooling - up to the 4 kelvin expected - and provides a very good vacuum without the need for a large budget ...

Thanks to this instrument, the researchers have stored and studied the anthracene ion  C14H10+ , polycyclic aromatic hydrocarbons, which are a part of the interstellar medium. After this molecule has been excited with a laser beam, they measured a typical duration of cooling of the order of one hundredth second. This cooling is nearly a hundred times faster than what is expected of the cooling from the emission of infrared radiation by vibrational transitions. This new cooling mechanism, which they attribute to the emission of thermally excited electrons , could have implications in astrophysics and lead to better understand the life duration and the critical size of polycyclic aromatic hydrocarbons in the interstellar medium. The anthracene, a small carbon molecule, is a first step. The team now wants to investigate the cooling of large hydrocarbons and of very small grains.

Further details

 :  Fast radiative cooling of anthracene observed in a compact electrostatic storage ring, S. Martin1, J. Bernard1, R. Brédy1, B. Concina1, C. Joblin2, M. Ji1, C. Ortega1 et L. Chen1, Physical Review Letter (Phys. Rev. Lett. 110, 063003 (2013)

IRAP Researcher : Christine Joblin (christine.joblin@irap.omp.eu)

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