Comparison of the Paschen and Balmer Series of Hydrogen Lines in Stellar Spectra

Introduction. The structure of the hydrogen spectrum, and previous observations of the Paschen series are briefly described. The behavior of the Paschen lines m = 12 to m = 14, recently photographed at Mount Wilson in various types of stellar spectra resembles in general that of the Balmer lines in the same types. The Paschen series is very conspicuous in the c stars β Orionis and α Cygni . Photometric data. Measurements of structure and intensity have been made by the usual photometric methods on numerous lines in both hydrogen series in the spectra of α Leonis B8n, α Lyrae AO, α Canis Majoris AO, β Orionis cB8, and α Cygni cA2. Structure of lines. Most lines not affected by the overlapping of neighboring lines have contours of the simple exponential form, although in α Leonis the centers are flattened. In the broad-line stars α Leonis and α Lyrae considerable departure from the exponential form is exhibited by the wings of lines near the heads of the series. In β Orionis, α Cygni, α Leonis, and the emission-line stars γ Cassiopeiae and P Cygni the shapes of Balmer and Paschen lines are similar, with the dimensions proportional to wavelength; but this is not true of α Lyrae in whose spectrum the Balmer lines have relatively more intense wings. Central intensities. Measured values for numerous lines are given in Table VI. In general the central intensities of Paschen lines are less than those of the corresponding Balmer lines. In α Leonis the Paschen series is relatively intense and has a slow decrement. In β Orionis and α Cygni both series exhibit slow decrements and little overlapping. Problem of overlapping lines. The shapes as well as the central intensities of many lines, particularly in the Balmer series, are probably seriously modified by the overlapping wings of adjacent lines. The problem is to find the shapes and intensities of the "true" or original lines which by their mutual interaction produce the observed curve. A physically reasonable sequence of lines has been found which yields close approximations to the observed shapes and intensities in α Lyrae but gives too great reduction of the general intensity of the whole spectrum toward the head of the series. This difficulty which appears to be of a rather general nature is briefly discussed. Total line absorption. Measured values for lines of both series in α Cygni, β Orionis, and α Leonis are given in Table VII. From these data Unsold's equation gives the numbers of atoms in the second and third levels. The numbers computed for a given level from various lines differ systematically, indicating that the theory is incomplete. The maximum numbers, used in Boltzmann's equation for thermal equilibrium give reasonable values of the temperature of the absorbing hydrogen. Intensities of emission lines. Several bright lines in both series were measured in γ Cassiopeiae and P Cygni. The relative importance of induced and spontaneous emission is discussed. The photospheric temperatures computed from the intensities of pairs of lines having a common upper level depend on the assumed distribution of the atoms with respect to azimuthal quantum number. Assuming distribution corresponding to thermal equilibrium leads to reasonable temperatures for both stars. The possibility of determining the amount of space absorption by comparison with other data is pointed out.