Other
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993pasp..105.1367l&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.105, p.1367
Other
2
Scientific paper
In order to study the structure and formation history of the Galaxy, low-resolution spectra were obtained of 302 nearby ab-type RR Lyrae variables selected from the General Catalog of Variable Stars. Accurate abundances and radial velocities were derived from the spectra, and distances were computed from apparent magnitudes and reddenings taken from the literature. An abrupt change in the kinematics is seen at [Fe/H] = -1.0. Below this value, the slow Galactic rotation and broad velocity dispersions of the halo are apparent (V_phi = 17 +/- 14 km s^-1 and sigma_r : sigma_phi : sigma_z = 160 +/- 15 : 112 +/- 10 : 99 +/- 10 km s^-1 for [Fe/H] < -1.3), while above this value, the stars have the fast rotation and small dispersions characteristic of the thick disk (V_phi = 186 +/- 10 km s^-1 and sigma_r : sigma_phi : sigma_z = 57 +/- 12 : 35 +/- 9 : 39 +/- 10 km s^-1 for -1.0 < [Fe/H] < -0.45). The transition from disk to halo is more abrupt than in local samples of subdwarf and giant stars, probably because a difference between the horizontal-branch morphologies of the halo and disk populations affects their relative productions of RR Lyraes as a function of abundance. The kinematic properties and spatial distribution of the stars with [Fe/H] > -0.45 are consistent with their being a mixture of the thick disk and old thin disk populations, or part of an extended disk exhibiting a continuum in kinematic properties. The rotational velocity and velocity dispersion were observed to be independent of abundance in the halo, in agreement with the subdwarf samples of Carney et al. (1990) and of Ryan & Norris (1991), further supporting chaotic pictures of halo formation such as the fragment accretion picture of Searle & Zinn (1978). The space density of the local halo RR Lyraes is larger than predicted from distant samples of RR Lyraes (Preston et al. 1991), suggesting that either the halo is flatter (b/a ~ 0.5) than Preston estimated (Green & Morrison 1992), or a two-component halo exists. The rotational velocities of the most metal-poor stars ([Fe/H] < -1.9) in this and other non-kinematically-selected samples are prograde, and are not influenced by metal-poor disk stars as proposed by Majewski (1992). It is suggested that the two-component, Old/Younger Halo model recently proposed by Zinn may account for Majewski's observation of retrograde halo rotation at large Z-heights, where the Younger Halo would dominate. Finally, the metallicity distribution of the field halo RR Lyraes is observed to be broader than those of the RR Lyraes in the inner and outer halo globular clusters. This result is discussed with regard to the relative spreads in horizontal-branch morphology of the field and cluster populations; if age is the dominant "second parameter", a fraction of the halo field stars may be younger than the youngest halo clusters by 1-2 Gyr. (SECTION: Dissertation Summaries)
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