|Reference : On the interpretation of the emission lines in stars of early spectral class|
|Scientific journals : Article|
|Physical, chemical, mathematical & earth Sciences : Space science, astronomy & astrophysics|
|On the interpretation of the emission lines in stars of early spectral class|
|Struve, Otto [Yerkes Observatory > > > > > >]|
|Swings, Polydore [Université de Liège - ULg > Institut d'Astrophysique > > >]|
|University of Chicago Press|
|Yes (verified by ORBi)|
|[en] The emission-line spectra.-The presence of bright Fe II in the spectra of Be stars suggests that lines of other elements requiring similar conditions of excitation may also be present. A search in the spectra of several stars yielded definite evidence of the presence of Mg II and Si II; Sc II, Cr II, and Ni II are probably present; forbidden Fe II is uncertain. No lines belonging to Ti II coµld be identified, with certainty. The width of the bright line Mg II 4481 greatly exceeds those of the emission lines of H, and in one star the lines of Fe II are definitely wider than those of H. Attributing the origin of the bright lines to a rotating shell of gas, and neglecting support by radiation pressure, we find that the effective distance of Mg II from the center is about seven times the radius of the star, while that of H is ten times the radius of the star. A rough computation of the density of the shell gives 2.5 x 10-13 gr/cm3. The bright lines of Mg II and of Si II are always weak. He I and Ti II are rarely seen in emission.
<br />Classification of bright-line spectra.-The published data on Be and Oe stars were collected and arranged according to the Harvard spectral class as derived from the absorption lines. The character of the emission lines shows a definite progression with spectral class. The degrees of effective excitation of the bright and dark lines are not very different in the Oe's but they differ by more than one spectral class in the Be's. There is relatively little dispersion within any given spectral subdivision. This would indicate, on the rotational hypothesis, that the product of the density of the shell and the dilution factor is approximately constant within a given subdivision, but that it varies as a function of spectral type.
<br />A new Be star.-Data are given for the Be star 60 Cygni (α 20h57m7 δ +45°46'), which has variable hydrogen lines.
<br />Variations in the spectra of Be stars.-Variations in the hydrogen lines of 31 Pegasi (α 22h16m6 δ +11°42') are described. The Be star 31 o Aquarii (α 21h58m1 δ -2°38') is found to have broad and hazy absorption lines of He I and Mg II, and sharp and narrow lines of Fe II. The latter are probably variable. This star is similar to the Be star Є Capricorni; and the suggestion is advanced that the hazy lines are broadened by rotation, while the narrow lines originate through absorption in the same rotating gaseous shell which is believed to give rise to the bright hydrogen lines and to their narrow central absorptions.
<br />The spectrum of 17 Leporis.-This spectrum seems to be related to spectra of the P Cygni type : 17 Leporis has bright Hβ in the normal position, accompanied by a strong absorption line on its violet side. Preliminary measurements of one plate give: Mg II +14 km/sec, Fe II and Ti II -64 km/sec, H -93 km/sec.
<br />The rotational hypothesis of the origin of bright lines.-A summary is given in support of the hypothesis that the bright lines originate in a rotating shell or ring of gas, and that the dispersion in line widths is mainly due to the effect of inclination.
<br />Objections to the rotational hypothesis are discussed and found not to be of a sufficiently serious nature to outweigh the positive evidence. The rotational hypothesis is adopted as the one which fits the observed facts better than any other theory thus far proposed.
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