Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........13w&link_type=abstract
Thesis (PH.D.)--MICHIGAN STATE UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-10, Section: B, page: 5
Physics
11
Galactic Halo
Scientific paper
An understanding of the kinematic properties of the Galactic halo is an essential component to the ultimate understanding of the Galactic formation history. The blue field horizontal branch stars (FHB) are excellent stellar probes with which to explore these properties. The FHB stars are both intrinsically luminous, which allows kinematic exploration beyond the confines of the local solar neighborhood, and numerous, which is important for good statistical results. In the past, however, isolating a pure sample of FHB stars from their main sequence, A-star, counterparts has proven to be a difficult and highly uncertain task leaving the FHB kinematic results open to skepticism. This dissertation explores the kinematic properties of an FHB sample identified through the use of a new spectral calibration technique. Medium resolution (1-2 A) spectroscopic and UBV photometric observations have been completed for 1213 FHB and main sequence A-type stars (FHB/A) drawn from the candidates in the HK objective prism surveys of Beers et al. A new spectral calibration was developed using synthetic spectra and colors to calculate the physical parameters for the program stars. The corresponding parameter uncertainties of sigma Teff= +/-250 K, sigma log g=+/-0.25, and sigma[ Fe/H]= +/-0.30, are sufficient to push the stellar class misidentifications below 10%. A statistical kinematic analysis of the FHB stars indicates the halo is composed of at least two components. The dominant component is flattened with a vertical velocity dispersion of ~70 km/s and has a mild pro-grade systemic rotation of ~ 20 km/s. The second appears to have a more isotropic velocity ellipsoid and a systemic counter-rotation of {~-90} km/s. The kinematics of the metal-poor main sequence A-stars (BMP) indicate an isotropic velocity ellipsoid of ~90 km/s and a systemic rotation of ~90 km/s. These values are different from either the halo or thick-disk populations. The kinematic evidence from the FHB stars strongly suggests that the outer halo of the Galaxy formed from accretion of discrete fragments while the inner halo kinematics are consistent with a contracted origin. The BMP kinematics are consistent with an origin from one or more delayed accretion events.
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