Spectrophotometric Measurements of Early-Type Stars*: V. Results and Discussion for 20 Stars of MK Type B2
Year: 1958
Type: article
Abstract: Measures of the equivalent widths of 69 absorption and emission lines and bands have been measured in the spectra of 20 stars of MK type B2. Every possible effort has been made to eliminate systematic errors. The methods used, which are almost entirely numerical and free from subjective estimates, are described. The results obtained are strictly comparable with those in previous papers in the series. Where appropriate, allowances for blends have been made, the methods used being described. A quantitative measure ϕ L , similar to that used in earlier papers in this series is derived for all 20 stars, this measure being based chiefly on the strengths of N II , O II and Si III absorption lines. Comparison with other results shows that ϕ L is closely correlated with luminosity and that it is in Some ways superior to the equivalent width of H γ as a measure of luminosity, in particular when it is applied to dwarf stars with emission characteristics. Certain luminosity-sensitive relations between the strengths of lines of neutral Helium are shown to be correlated with ϕ L and also with the strength of the Balmer line H γ . A smooth trend towards emission with increasing luminosity is shown for H α and H β in the normal stars. At any one luminosity, mean wing profiles for H α , H β and H γ are shown to have the same profiles within the limits of experimental error. In addition to Mg II 4481 previously reported, the absorption lines of Si II and C II are found to be relatively in sensitive to luminosity. Behaviour of individual lines with luminosity throughout the measured spectrum is described. Emission phenomena in four B2 dwarfs are examined, and it is shown that when emission occurs in H α , real or incipient emission may also occur in any of He I , He II , Fe II , Mg II and C II . It is shown that N II , O II and Si III and therefore ϕ L are not affected by emission and thus ϕ L as an indicator of luminosity for these stars is not affected. By use of ϕ L , an emission profile of H γ in υ Cygni is obtained. Some comparisons are made between these stars and ϕ Persei. The stars treated here divide conveniently into four groups—supergiant, giant, dwarf and dwarf with Balmer emission. In the Atlas at the end of the paper are given mean spectra of the supergiant, dwarf and emission dwarf stars for wavelength regions 4125–4977 and 5620–6780 A. They are presented so that comparisons can conveniently be made between supergiant and dwarf and between the two types of dwarf stars. These spectra, though of smaller resolution than might be desirable have high accuracy in intensity, the standard error of the individual plotted point being 0.5 per cent. Measures are given of numerous interstellar lines and bands, together with a profile of the band at 4430 A. The relationship between interstellar line absorption and distance modulus is given, and would appear to place all three stars of class Ia beyond the absorption associated with our galactic spiral arm. Although their distance moduli extend to 2600 parsecs there is no indication of the increase in absorption which might be expected in a further spiral arm. An absorption band centred on 4890 A, similar in shape to that at 6180 A (which was reported earlier from this Observatory), and to that at 4430 A, has been detected. No measures of this band can be given yet.
Authors: H. E. Butler, H. Seddon
Institution: Royal Observatory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2004277311