On the Doppler Broadening of Absorption Lines by Turbulence and by Multiple Interstellar Clouds.

Abstract

view Abstract Citations (9) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Doppler Broadening of Absorption Lines by Turbulence and by Multiple Interstellar Clouds. Huang, Su-Shu Abstract The influence of turbulence on an absorption line is studied in a way similar to that customarily used in treating the influence of thermal motions on the line. A mean line-absorption coefficient for the turbulent absorbing gas is first calculated on the assumption of various distribution functions for eddy velocities. Mathematically, all these cases lead to series expansions in terms of the same functions with different numerical coefficients. As in the case in which there is no turbulence, the mean line-absorption coefficient in a turbulent gas can be separated into two parts: one referring to the core of a line and the other to the wings. It is shown that the effect of turbulence in the wings is negligible; the damping part of the curve of growth is therefore unchanged by its presence. Following a general treatment of the effects of turbulence, line profiles and curves of growth have been computed both for the case of a 4unction distribution of eddy velocities (which is equivalent to a step- function distribution of velocities in the line of sight) and for the case of a Gaussian distribution of eddy velocities. Both kinds of profiles and curves of plotted in units of the pure thermal Doppler width for the same values of mean-square eddy velocities. In the case of the Gaussian distribution of eddy velocities, we can obtain line profiles and curves of growth from the original profiles and curves of growth by a mere change of scale. In the case of the f unction distribution, the calculation can also be simplified, and the series expansion is not required. The result of the numerical computation shows that a weak line that has been broadened by turbulence is much shallower than a strong line. It is also found that the flat portion of the curve of growth derived from the -function distribution is much lower than that derived from the Gaussian distribution. Thus it is concluded that, unless the real distribution function of eddy velocities is known, the value of the turbulent velocity derived from the curve of growth is uncertain. The broadening of an absorption line by multiple interstellar clouds occurs in the same manner as does the broadening by turbulence; hence the line profiles and the curves of growth of interstellar lines are given directly by the present method. Publication: The Astrophysical Journal Pub Date: November 1950 DOI: 10.1086/145355 Bibcode: 1950ApJ...112..399H full text sources ADS | data products SIMBAD (1)