Classification of the B-Type Stars
Abstract
The Draper system of classification is reviewed. Classificajion methods based on line intensities and on line ratios are distinguished, and objections to the former are raised. The principles to be observed and the procedure to be followed in using line I ratios based on measured intensities are discussed. Seven ratios, most of which involve numerous lines of several elements, are set up, which apparently give a good index of average characteristics. Usually three ratios are applicable in any one case and their accordance is good. The stars observed having been reclassified, intensity-type and in- I tensity-luminosity variations are examined for each atom. The very smooth progression of type of maximum with ionization potential substantiates the ionization-temperature I basis of the classification. I For stars belonging to the main sequence, hydrogen shows a steady, fivefold increase in total absorption in passing from the earliest types to class A. Very luminous stars re- tain roughly the same low hydrogen intensity in all subtypes, and in class 0 the two I groups are indistinguishable. The real central intensities in sharp-line stars decrease I regularly from 6o per cent at 08 to 20 per cent at Ao. An absolute-magnitude effect similar to that for hydrogen is found for all helium lines. This observation, together with the intensity variations with serial number in the I two diffuse series, supports the view that the lines are subject to Stark effect. A two- I fold increase occurs between 08 and B i in the singlet-triplet ratio for the diffuse series, but no luminosity effect on this ratio is detected. The evidence is not against a similar I fading-out of the sharp singlets with respect to the sharp triplets as the temperature in- creases. I Ten diagrams are given coimectirig type and intensity for metallic atoms. N+, Si+, Mg+ show a very pronounced luminosity effect, the lines being stronger in the brighter I stars. C+ alone shows no such influence. For N+ and 0+ the gradient effect is examined by grouping stars and treating the line intensities statistically. There is a progressive in- crease in gradient with luminosity. No appreciable difference is found between stars I with sharp and nebulous lines, nor between types earlier and later than the maximum I for 0~. I A statistical discussion of the proper motions and radial velocities of 56 stars with measured interstellar K lines establishes the linear relation: I K = K-line intensity = 0.37 equivalent angstroms per kiloparsec. Thus the expressio
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 1936
- DOI:
- 10.1086/143729
- Bibcode:
- 1936ApJ....83..305W