On the Radio Emission of Hydrogen Nebulae

Abstract

SummaryThe interpretation of radio-frequency observations of H II regions is discussed with particular regard for the possible effects of random variations in the electron density and electron temperature through the nebulae. It is shown that such variations serve to alter the optical depth and that the conventional definition of the" emission measure " requires modification if it is to be considered an observable quantity. The radio emission of Stromgren spheres is discussed, and a means of determining their electron temperatures is described. An empirical method for the determination of Stromgren's constant defining the ionized volume as a function of the spectral type and luminosity of the exciting star is described.1. INTRonUCTION A number of galactic H II regions have been observed at radio frequencies.The observations published to date cover frequencies ranging from 19·7 to 9375 Mc/s. S-qch observations can provide information about the temperatures and densities of the nebulae, and about the far ultraviolet radiation of the stars exciting them. The objective of the present paper is to examine in some detail the problem of deriving physical data on the nebulae from radio observations;The H II regions constitute a special class of radio source characterized by their spectra, which are "fiat" except at the lower frequencies where they become optically thick. That is, their fiux densities are nearly constant over a very wide range of frequencies. This is what is expected if the nebulae are radiating by the thermal process of free-free transitions in an ionized gas. They are readily distinguished from the" non-thermal" radio sources, whose spectra show a strong frequency dependence.The radio emission of H II regions is well understood theoretically (e.g. Piddington 1951). Discussions of the nebulae as radio sources have generally assumed for simplicity that the objects are uniform throughout, although it has been known that a non-uniform distribution of the nebular gas would tend to increase the radio emission because the emissivity of an ionized gas depends on the square of the electron density. In the present paper we shall give particular attention to the consequences of such a non-uniform distribution.Section II is devoted to a discussion of the directly observable properties of H II regions. Section III considers the effect of density and temperature variations on the optical depth, and the magnitude of the effect is estimated for some particular nebular models. A modification of the definition of the term

Is the Dust Cloud Around Lambda Orionis a Ring or a Shell, or Both?

“…The H II region is thought to be a good example of a classical Strömgren sphere, which is spherically symmetric and centered on λ Ori (Coulson et al 1978;Maddalena & Morris 1987). Many observational studies have found dark clouds external to this H II region; the clouds appear to be circularly symmetric, with a large angular extent of about 8° (Wade 1957(Wade , 1958Courtès 1972;Coulson et al 1978;Maddalena & Morris 1987;Malone et al 1987;Zhang et al 1989;Lang et al 2000) However, there has been a debate as to whether the symmetric appearance of the cloud is a projection effect of a "spherical" shell or is due to a "toroidal" ring. Wade (1957) found an expanding, dense shell of neutral hydrogen surrounding the H II region.…”

Thermal galactic sources

“…In the meter wavelength range T , is, in general, of the order of or greater than T , and Mills, Little, and Sheridan[SIfound from their observations a t 85 Mc that T , has values in the range of 6000°K to ll,OOO°K. -Another possibility suggested by i$Tade[9] makes use of (2) which can be computed as a function of temperature from observations and then compared with the theoretical values on the righthand side. This method makes use of the fact that the In A values are not very sensitive to temperature [A is the logarithmic argument of (3)].…”