An investigation of certain problems related to the classification and physical properties of faint blue stars

Abstract

Faint blue stars situated out of the galactic plane have been studied using a variety of techniques. Stromgren photometry has been obtained for a number of these stars and classifications derived from the photometry. An interference filter centred at 3775 A near the Balmer discontinuity has been used in conjunction with the Stromgren filters. Observation of Stromgren standard stars, bright B stars and faint blue stars have been used to study the behaviour of the colours (u-38) and (38-b). Reddening-free parameters [u-38] and [38-b] have been evaluated and [38-b] is found to be linearly related to the Crawford H𝛽 index for B-type dwarfs, giants and supergiants. This relationship has been used to derive absolute magnitudes and distances for faint apparently normal B stars. [38-b] allows this work to be done at fainter magnitudes than possible with H𝛽 photometry. A filter pair centred on the HeII 𝜆4686 line has been used to study 0-type stars including 0 subdwarfs. The resulting index is strongly correlated with the equivalent width of the Hell line and has a range of 0ᵐ.13. It is possible to separate sd0, 0 and 0f stars using this index. Although the index is correlated with absolute magnitude for 0 and 0f stars, the detailed behaviour precludes the accurate determination of absolute magnitude for dwarfs and giants. Theoretical indices have been computed from the models of Kudritzki and agree well with the results for stars with high helium abundance. These computations suggest that it is possible to separate sdO and DO stars with this system. The results have also been compared with the work of Auer and Mihalas. 30 A/mm image tube spectra have been used for Ealmer line width measurement and radial velocity determination. The line widths were used to estimate surface gravities for faint blue stars in conjunction with temperatures determined from the Stromgren [u-b] index. The results indicate that 30 A/mm image tube spectra can be used successfully for surface gravity determination. 75 A/mm image tube spectra have been used for spectral classification as a check on the classification provided by Stromgren photometry. The kinematics of certain subdwarfs and main sequence stars at high galactic latitude have been considered. Proper motions have been employed as a check for subluminosity using appropriate absolute magnitudes. Galactic orbits have been computed using the Schmidt model of the galactic force field. Several B-type dwarfs and subdwarfs were found to have been ejected from the galactic plane at high velocity. Thirty percent of the stars previously classified as normal dwarfs were found to be subluminous. The evolutionary implications of these results are discussed.