NASA Technical Reports Server (NTRS) : PAHs pdf

NASA Technical Reports Server (NTRS) : PAHs

More Book Details

Description of the Book:

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

• Creator/s: NASA Technical Reports Server (NTRS
• Date: 12/1/1989
• Year: 1989
• Book Topics/Themes: NASA Technical Reports Server (NTRS), CYCLIC HYDROCARBONS, INFRARED SPECTRA, INTERSTELLAR GAS, IONIZATION CROSS SECTIONS, MOLECULAR INTERACTIONS, PHOTOIONIZATION, STELLAR TEMPERATURE, ABUNDANCE, COSMIC DUST, GAS HEATING, HEAT SOURCES, HEATING, INTERSTELLAR MATTER, MOLECULAR CLOUDS, PHOTOELECTRIC EFFECT, RECOMBINATION REACTIONS, VIBRATIONAL SPECTRA, Verstraete, Laurent, Leger, Alain, Dhendecourt, Louis B., Dutuit, O., Defourneau, D

An excerpt captured from the PDF book