Distances of Southern B-stars and Galactic Structure from H -Luminosities

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Abstract
The equivalent widths of H γ have been determined for 137 Southern B-stars (tabulated in Section 5), using material obtained from the Radcliffe Observatory in Pretoria (on the average 3 spectra per star). Most of the spectrophotometric reductions were carried out at the Dominion Astrophysical Observatory in Victoria, B.C. Detailed analysis, described in Sections 3 and 4, showed that there were no systematic differences, either between different observers, or between the present southern work and the previous investigations of northern stars by Petrie at Victoria. The internal error derived for the present determinations of H γ intensities is 7 per cent per star, equivalent to 0 m .23 in the absolute magnitudes. To convert these absolute magnitudes into stellar distances, a uniform system of star-magnitudes and colour-indices has been established for the southern hemisphere (Section 7). The preceding Section 6 discusses the luminosity-spectrum array. Section 8 provides satisfactory tests of the newly-determined spectroscopic distances, in particular with the help of mean proper-motion parallaxes and, furthermore, by using mean distances derived from galactic-rotation effects on the radial velocities. In the latter case, it is shown (Section 8.4.3), that we may not be justified in using the south “calcium-cloud distances” (i.e. “effective half-distances”, based on the K-line) since, differing from the north, lines of sight in the south mostly run through cloud-free inter-arm regions. Section 9 relates the new material to the structure of the Milky Way, and makes use of recent work on H I -areas, clusters, associations, H II -regions and various proposed Milky-Way arm-patterns. A general colour-excess diagram, illustrating the dependency on galactic longitude, is derived in Section 9.3. Figs. 9, 10 and 11 represent the B-stars and other Milky-Way constituents in their relation to different galactic regions out to about 4, 1.5, and 7 kiloparsecs, respectively. Particular attention is drawn to the occurrence of a conspicuous group of reddened B-stars which are clearly situated between hydrogen arms, lying especially outside the Sagittarius arm. Various significant discrepancies between location of stars, neutral hydrogen, and rotational models of the Galaxy are pointed out. Evolutionary aspects are indicated.

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