Abundances and Kinematics of K Giants in the Galactic Nuclear Bulge

Citation

Rich, Robert Michael (1986) Abundances and Kinematics of K Giants in the Galactic Nuclear Bulge. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zyq4-ta43. https://resolver.caltech.edu/CaltechETD:etd-09102008-152942

Abstract

Spectroscopy and photometry has been obtained for 100 K giants in Baade's Window at l = 1°, b = -4°. For a galactocentric distance 7.5 kpc the line of sight passes 522 pc below the nucleus. The abundance distribution of the nuclear bulge K giants has been derived relative to 45 stars of known abundance. The abundances run from -1 to nearly +1 dex, with a peak at 0.3 dex, or twice the solar abundance. Of the 88 stars with derived abundances, 22% exceeded the abundance of the most metal rich local K giants; 50% exceeded the solar abundance, and 10% were metal poor (< -0.6 dex).

Radial velocities have been measured for 53 nuclear bulge K giants which also have derived abundances. Their velocity dispersion is 104 km/sec. The mean velocity is -19 ± 14 km/sec; within 1σ of the solar II velocity of -10 km/sec. When this sample is divided into 3 subsets based on the abundances, the subset of 21 stars > 0.3 dex has σ = 92 ± 14 km/sec and the metal poor subset of 16 stars < -0.3 dex has σ = 126 ± 22 km/sec. An intermediate set of 16 stars has σ = 97 ± 17 km/sec. The most metal rich stars may have a bulk velocity of -38 ± 14 km/sec, 1σ less than the -19 km/sec of the metal poor stars.

The abundance distribution function is found to be fit very well by the simple model of chemical evolution with complete gas consumption.

The smaller velocity dispersion for the metal rich stars can be interpreted as supporting a steep power law for their spatial distribution ρ ~ r^-7. The metal rich stars may belong to a special central component of the Galaxy, which cuts off completely at 1kpc.

No evidence was found that either the metal rich or metal poor stars follow the galactic rotation curve; hence neither population appears to be rotation supported.

Optical and infrared photometry is presented for nuclear bulge K giants. The colors of these stars are shown to be too hot for their derived metal abundances.

Analysis of color-magnitude diagrams in bulge fields at -4° (Baade's Window) and -8° shows that there cannot be a 1 Gyr old population of main sequence stars in the galactic bulge. A turnoff population is detected at -8°, and comparison with isochrones indicates that the population is older than 5 Gyr. For the first time, there is a clear indication of a horizontal branch, or "globular cluster feature" in the luminosity function of the galactic bulge.

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