Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976ap%26ss..45..125s&link_type=abstract
Astrophysics and Space Science, vol. 45, Nov. 1976, p. 125-132.
Astronomy and Astrophysics
Astrophysics
Hot Stars, Intergalactic Media, Stellar Temperature, Ultraviolet Radiation, White Dwarf Stars, Cosmic Rays, Gas Temperature, Hubble Diagram, Hydrogen Clouds, Hydroxyl Emission, Photoionization, Surface Temperature, Thermodynamics
Scientific paper
The effect of ionizing radiation from UV stars (hot prewhite dwarfs) on the intergalactic medium (IGM) is investigated. If the UV stars are powered only by gravitational contraction, they radiate most of their energy at a typical surface temperature of 150,000 K, which produces a very highly ionized IGM in which the elements carbon, nitrogen, and oxygen are left with only one or two electrons. This results in these elements being very inefficient coolants. The gas is cooled principally by free-free emission and the collisional ionization of hydrogen and helium. For a typical UV star temperature of 150,000 K, the temperature of the ionized gas in the IGM is 120,000 K for a Hubble constant of 75 km/s per Mpc and a hydrogen density 1 millionth per cu cm. Heating by cosmic rays and X-rays is insignificant in the IGM except perhaps in H I clouds because when a hydrogen atom recombines in the IGM, it is far more likely to be reionized by a UV-star photon than by either of the other two types of particles due to the greater space density of UV-star photons and their appreciably larger ionization cross sections. If the UV stars radiate a substantial fraction of their energy in a helium-burning stage in which they have surface temperatures of about 50,000 K, the temperature of the IGM could be lowered to about 50,000 K.
Hills Jack G.
Sonnanstine A. E.
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