A Study of A- and B-Type Stars in the Southern Galactic Halo

Citation

Pier, Jeffrey Ross (1983) A Study of A- and B-Type Stars in the Southern Galactic Halo. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s939-dc28. https://resolver.caltech.edu/CaltechETD:etd-09082008-142312

Abstract

A sample of over 200 stars of spectral types A and B has been selected for study. Drawn from the Preston and Shectman objective-prism survey of the southern galactic halo, the stars comprise a sample of halo objects selected without kinematic bias. Photoelectric UBV photometry has been obtained and the sample stars have apparent V magnitudes mostly in the range 13.5 ≤ V ≤ 15.5. The (U-B)--(B-V) two-color diagram shows that the halo AB stars have colors similar to those of globular cluster blue horizontal-branch (BHB) stars.

Reticon spectroscopy of the sample confirms the BHB status of the large majority of the sample. Interesting non-BHB stars found include a few high luminosity A stars, several stars of low metallicity and apparently normal main-sequence gravities which may be field Population II blue stragglers, and ten metallic-line stars. A few normal A stars have been found as far as 2 to 3 kpc from the plane, but none beyond that distance. It is suggested that the metal-rich early-type population found by other authors belongs to a population of old disk stars with a scale height of over 1 kpc.

Interstellar Ca II K lines are found in many of the high-velocity stars. The interstellar components arise from low-velocity material associated with the disk.

The solar motion of the halo AB stars is -236 ± 39 km s^-1 and the velocity ellipsoid is elongated towards the galactic center throughout the halo. By adding a list of halo objects drawn from other sources and solving for the solar motion a lower limit of -208 ± 20 km s^-1 is set for the circular velocity at the solar radius. The solar motion solution is found to be metallicity-dependent. Anisotropy in the velocity ellipsoid is strongly indicated for the combined halo objects. The presence of this global anisotropy removes one of the arguments favoring a flattened halo. Since the gravitational potential of the disk increases the z component of velocity it is seen that at very early times in the history of the Galaxy the velocities were highly anisotropic (predominantly radial).

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