Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996pasp..108..726w&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.108, p.726
Astronomy and Astrophysics
Astronomy
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, counter-parts 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. Follow up medium resolution (1-2 A), low S/N (10 < S/N < 20) spectroscopy and UBV photometry have been obtained 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. (1988, ApJS, 67, 461). This data was used to determine stellar radial velocities, metal abundance, and to separate the FHB/A candidates, by exploiting the inherent differences in surface gravity between the two classes. To determine the stellar physical parameters, an 833 point calibration grid was constructed spanning the range of T_eff, log g and [Fe/H] for the program objects. The grid includes the widths of H-delta, H-gamma and the CaII K line, determined from synthetic spectra, and the Kurucz Atlas9 color relations. A calibration technique was then developed which gives varying weights to the observables in accord with their dependency on the physical parameters. Comparison to standard stars, noise-injected synthetic spectra, and globular cluster HB stars indicate an uncertainty in the derived physical parameters of sigma T_eff = +/- 250 K, sigma log g = +/- 0.25, and sigma [Fe/H] = +/- 0.30. Results from the noisy synthetic data place class misidentifications at less than 8%. A statistical analysis of the FHB star kinematics indicate the halo is composed of at least two components. The locally dominant component shows flattening in the vertical velocity dispersion and an overall velocity ellipsoid of (sigma_r: sigma_phi: sigma_theta) = (124 +/- 32:163 +/- 35:62 +/- 23) km/s. This component has a prograde systemic rotation of V_rot = 40 +/- 17 km/s. The second component, which begins to dominate at |Z| > 4 kpc, and [Fe/H] < -1.6, has a more spherical velocity ellipsoid of (sigma_r: sigma_phi: sigma_theta) = (86 +/- 43:146 +/- 54:122 +/- 33) km/s. Also, the second component is in systemic counter-rotation with V_rot = -93 +/- 36 km/s. It is expected that the rather large rotational velocity dispersion, which is found in both results, is in part due to mixing of the two components. There is only marginal kinematic evidence for the existence of a metal-weak thick disk in the FHB sample. The kinematics of the metal-poor ([Fe/H] < -1.0) main sequence A-stars (BMP) indicate an isotropic velocity ellipsoid of ~90 km/s and a systemic rotation of V_rot = 85 +/- 18 km/s. These values are uniquely different from either the halo or thick-disk populations. There is also strong kinematic evidence for a cutoff in the BMP population at metal abundances of [Fe/H] < -2.2. The stars with metallicities below this value have a systemic rotation of V_rot ~0 km/s and are expected to be either misidentified FHB stars or "true" halo blue stragglers. 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 likely formed from a dissipative collapse. The BMP kinematics are consistent with an origin from one or more delayed accretion events. The progenitor fragments of the BMPs are expected to have had a range in metallicty of -1.0 > [Fe/H] > -2.2. (SECTION: Dissertation Summaries)
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