Spectrophotometric Measurements of Early-Type Stars*: IV. Results for Stars of Types O6-B0

Abstract

Measures are given of the equivalent widths of thirty-five lines in the visible region of the spectra of sixteen stars of types O6–B0. These equivalent widths are derived by integrating the unsmoothed measures of intensity between specific wave-length limits and therefore include, in addition to the main line, the effect of any blend lines. The method of blend analysis is described and blending contributions are tabulated for all lines. The subsequent discussion of the results is made for the blend-free equivalent widths. The variation of line strength with spectral type and luminosity is investigated, incorporating the Edinburgh B1 results (references II and III) so as to embrace the range O6–B1 in spectral type. Three luminosity criteria are introduced which are relatively insensitive to temperature. This is done by forming the ratios of the strengths of spectral lines which are differently sensitive to luminosity but which are produced by the same ion. The three criteria are defined by He I [4472 + (4144 + 4388)/2]/He I (5876 + 6678); He II 4686/He II (4200 + 4542 + 5412) and C III 5696/C III 4647–51. Absolute magnitudes are derived from the strength of H γ by using Petrie's (H γ , Mv ) calibration (reference 19). This calibration is only applicable to spectral types O9 and later and curves relating the luminosity criteria in terms of absolute magnitude are derived for the later spectral subtypes. The early O stars, for which spectroscopic absolute magnitudes have not yet been derived, are of particular interest and the fit of the measures of five such stars (O6–O8) into the general scheme shown by the later subtypes, was examined. This was most conveniently done by reading off the late O star calibration curves, the absolute magnitudes corresponding to the values of the luminosity criteria (including H γ ) in the early O stars. Good agreement is shown by the values of “ absolute magnitude ” thus obtained and is taken to mean that the four luminosity criteria are governed by one general parameter–the effective surface gravity (or electron pressure). This is correlated with absolute magnitude in the later O stars but the relation must change or disappear in the early O stars. The results are consistent with the idea that the atmospheres of the early O stars become mechanically unstable. One of the stars investigated (λ Cep) is an Of star and thus the Of and “ normal ” O stars can be placed into a continuous two parameter sequence. Measures are made of the emission features due to He II 4686, N III 4634–2 and C III 5696, whose strengths show a strong luminosity progression. The presence of these lines in emission is the characteristic feature of Of stars, and He II 4686 and N III 4634–42 have been detected here as late as O9.5 (in the supergiant α Cam) and C III 5696 as late as B0 (in the supergiant є Orion). In general, the profiles of the emission features seem to be composite in nature, comprising a central, relatively sharp emission line superimposed on a faint but very extensive emission band. The detection of this broad emission is believed to be new and is accomplished by means of a precise method of interpolating the continuum between regions of continuous spectrum. The extent of the emission features is of the same order as observed in Wolf–Rayet stars and probably indicates a common mechanism. An analysis of the profiles on Beal's ejection Hypothesis gave ejection velocities of the order of 1500 km/sec.