Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988a%26a...205..257b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 205, no. 1-2, Oct. 1988, p. 257-266.
Astronomy and Astrophysics
Astrophysics
8
Density Distribution, Interstellar Space, Molecular Clouds, Pressure Distribution, Supernova Remnants, Abundance, Electron Density (Concentration), Hydrogen, Molecular Gases, Radio Emission, Star Formation
Scientific paper
The electron density in the recombination region of large supernova remnants (SNR's), and the maximum size observed in these objects, indicate that there is a well defined density distribution of the medium where SNR's predominantly expand. This distribution is similar to the one observed in the molecular gas, namely, a steep rise from 4 to 5 kpc and a negative gradient beyond this point. It is argued that the most likely component of the interstellar medium into which SNR's expand, is the warm interstellar medium (WISM). The temperature gradient obtained from H II regions is probably due to the galactic abundance gradient. Thus, it is produced by a general property of the galaxy, and not by an effect concerning only H II regions. Hence, it is assumed that this temperature gradient can also be associated to the WISM. The thermal pressure of this medium is obtained from this temperature gradient and the density distribution derived from the SNR's data. The thermal pressure distribution is again similar to that of H2. The density and thermal pressure distribution of the WISM above the galactic plane, is predicted using an approximation to models for the galactic potential. The thickness of this medium increases rapidly from 80 parsec at 4 kpc from the galactic center, to 220 parsec at the solar circle and 530 parsec at a distance of 15 kpc. The scale height distribution predicted for this component, is markedly different to the one observed in H I and H2 within 10 kpc. From this point and beyond, the observed thickness of the H I intercloud component is comparable to the one predicted here for the WISM. Finally, thermal pressure is compared with a critical pressure, above which the conversion of diffuse into molecular clouds is viable. It is found that the thermal pressure of the WISM exceeds this critical value within the solar circle but not beyond, accounting for the strong concentration of molecular clouds towards the galactic center. It is also shown that, within this region, the thickness of the molecular and diffuse cloud layer is smaller than the height above the plane at which thermal pressure is equal to the critical pressure. This can also account for the relatively thin layer occupied by the molecular gas about the galactic plane. It might also explain why diffuse clouds have a constant scale height and z-velocity in spite of a varying galactic potential, since their distribution is being determined by the processes through which these are transformed into molecular clouds.
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