Chemical Composition of Field RR Lyrae Stars as an Indicator of the Evolution of Galactic Subsystems
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The relationships among the chemical and spatial-kinematic properties of field RR Lyrae variables (lyrids) are examined. It is shown that some of them are metal-rich lyrids with thin-disk kinematics and the problem of the existence of these lyrids is discussed. Evidence is presented of a reduction in the upper limit on the mass of the formed stars with increasing metallicity in the thin disk and with better mixing of interstellar matter in this subsystem of the Galaxy. It is found that the lyrids with thick-disk kinematics mostly have metal contents [Fe/H]<–1.0 and high ratios [α/Fe] ≈ 0.4 , while only about 10% of the field dwarfs with the same chemical composition have a so-called “low-metallicity tail.” A sharp drop in the ratio [α/Fe] in the Galaxy occurs in those lyrids belonging to a thick disk; this is indicative of a long formation period for these subsystems. An inconsistency is found between the chemical composition of some lyrids and their kinematics. It is assumed that they are all most likely of extragalactic origin.
KeywordsRR Lyrae variables α-element abundances galactic subsystems
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