PAHs molecules and heating of the interstellar gas

Until now it has remained difficult to account for the rather high temperatures seen in many diffuse interstellar clouds. Various heating mechanisms have been considered: photoionization of minor species, ionization of H by cosmic rays, and photoelectric effect on small grains. Yet all these processes are either too weak or efficient under too restricting conditions to balance the observed cooling rates. A major heat source is thus still missing in the thermal balance of the diffuse gas. Using photoionization cross sections measured in the lab, it was shown that in order to balance the observed cooling rates in cold diffuse clouds (T approx. 80 K) the PAHs would have to contain 15 percent of the cosmic abundance of carbon. This value does not contradict the former estimation of 6 percent deduced from the IR emission bands since this latter is to be taken as a lower limit. Further, it was estimated that the contribution to the heating rate due to PAH's in a warm HI cloud, assuming the same PAH abundance as for a cold HI cloud, would represent a significant fraction of the value required to keep the medium in thermal balance. Thus, photoionization of PAHs might well be a major heat source for the cold and warm HI media

Tunnelling investigation of the phonon spectrum of superconducting Be implications for an eventual enhancement of Tc

Beryllium is an exciting superconductor in that the theoretical upper limit for its Tc is 140 K. We discuss here the possibility of increasing the critical temperature of the vapour quenched phase whose Tc is already 9.5 K. Tunnelling into films of this phase indicates that it is a weak coupling superconductor (λ < 0.6). Consequently, its Tc could be greatly enhanced, although experimental attemps have failed up to now.Le béryllium est un supraconducteur tout particulièrement intéressant car la théorie situe la limite supérieure de sa Tc à 140 K. Partant de la phase du Be dont la Tc est déjà de 9,5 K, nous discutons la possibilité d'augmenter cette température critique. Une étude par effet tunnel de cette phase nous conduit à penser qu'il s'agit bien d'un supraconducteur à couplage faible (λ<0,6) qui est donc susceptible de présenter des élévations importantes de sa Tc

Tunnelling investigation of the phonon spectrum of superconducting Be implications for an eventual enhancement of Tc

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