Computation of emergent flux and intensities for blanketing-effect models and comparison with observation

Abstract

The methods of constructing a model atmosphere and computing its theoretical spectrum are reviewed, attention being directed to the restrictions which are imposed and to the simplifications which are made in representing the interactions between radiation and matter. One important restriction is that it is assumed that Boltzmann's, Saha's and Kirchhoff's laws are valid throughout the atmosphere. However, observation indicates that these relationships are certainly not valid in those parts of the atmosphere where many of the strong lines which are used for classifying O B stars are formed. More accurate theories of line formation are required to predict the strengths and shapes of lines from the lowest levels and from metastable levels or levels connected to the metastable levels by strong transitions in the spectra of the light elements.

It is pointed out that the constraints radiative equilibrium and hydrostatic equilibrium do not define sufficiently accurately the parts of the atmosphere where many empirically selected classification lines are formed. Here the motion of the gases seems to play a significant role in determining the shape and strength of the absorption lines. Some examples are given of the differences between the line profiles predicted from blanketed and unblanketed early-type models and it is shown that these differences would not be readily detected from the usually observed spectrum in the range 3500 Å to 5000 å. Although line blanketing lowers the effective temperatures of O B models by 2500° to 3000° from the values estimated by means of unblanketed models, the unblanketed models are adequate for interpreting the spectrum between 3500 Å and 5000 Å and deducing abundances from weak lines.