Decomposition of the Milky Way Galaxy: kinematics and abundances
Clues to the formation and evolution of our Milky Way galaxy are embedded in its building blocks - the stars. A systematic study of stellar motions and the composition reveal the origin and evolution of the Galaxy and its present structure. In collaboration with David Lambert and others from the University of Texas, Austin, we embarked on a systematic study of a large number of stars in our Galaxy. Unevolved stars of spectral type F, G and K are well suited for this purpose as they retain the cemical composition of their natal clouds. Kinematic motion of any star can be computed given its astrometric data: parallax and proper motion, and radial velocity.The astrometric data were taken from the European space mission Hipparcos. At present accurate astrometry is available for only nearby stars of magnitudes mv≤10.0. This puts limits on the sample selection. Radial velocity measurements were done using high resolution spectroscopy.
A sample of 400 stars were grouped into thin disk, thick disk and the halo categories based on their kinematic properties: space velocities (U, V, W) and velocity dispersions (σ(U), σ(V), σ(W)). For each star the probability of its belonging to one of the three categories was estimated and a probability >.70 was used as the basic criterion to put the stars in one of the three groups. This is illustrated in the figure. Stars with ambiguous probabilities (<0.7) are excluded from the figure. In the U-V plane one can easily notice larger dispersions in velocities as one scans from top to bottom: thin disk - thick disk - halo.
Thin disk stars are confined to the Galactic plane (within 300pc) and they move with the Galactic disk. Thick disk
stars move slowly and lag behind the thin disk stars by ~40 - 100 kms-1. They have eccentric orbits. They lie around 600 - 1000 pc from the Galactic plane.
Abundances were computed for 27 elements from Carbon to Europium using model stellar atmospheres and the reduced spectral line strengths. Elements chosen for the study form three groups: - elements produced mostly in SNII (massive star explosions), Fe - peak elements mostly produced in SNIa (low mass binary stars), and s-process elements mostly produced in AGB stars. Abundance ratios are compared for the three groups of stars. It is found (a) Thick disk stars are more metal-poor ([Fe/H] ~ -0.6) and older (Age ~10-12 Gyrs) compared to thin disk stars: (b) Ratios of α-elements to Fe ([α/Fe]) are larger for the thick disk stars than the thin disk stars at given overlapping [Fe/H]. This implies that the composition of the thick disk stars is influenced by SN II whose lifetimes are small. (c) For the thick disk [α/ Fe] versus [Fe/H] diagrams show little or no trend suggesting that it formed rapidly within 1-2 Gyrs with very little/no contribution from SNI. (d) Plots of abundance ratios against metallicity show a scatter which is within the measurement errors. This suggests stars in the disk formed from well mixed gas.
Results refute earlier suggestions that the thick disk is a simple extension of thin disk at the metal-poor end or of the halo at its metal-rich end. Instead, results suggest a major merger event in the early epoch of our Galaxy i.e 10-12 Gyrs ago. This is illustrated in Figure 2. Element ratios [Mn/α] and [Fe/α] are shown against [α/H]. The arrow AB is the evolution of thick disk. At point B a major merger of a very metal-poor dwarf galaxy with our Galaxy might have occurred. The resultant metal-rich gas in our Galaxy and the metal-poor gas from the victim might be the fodder for the thin disk star formation (point D). The jump CD can be interpreted due to the delayed SNI (Fe contributor) in the thick disk. The arrow DE (notice the
slope) is the evolution of thin disk with major contributions from SNI products. These results are in concurrence with
the recent predictions from hierarchical ∧–CDM model for structure formation. ( B. E. Reddy)
