The age-metallicity relation in the thin disk of the galaxy

Abstract

HST trigonometric distances, photometric metallicities, isochronic ages from the second revised version of the Geneva-Copenhagen survey, and uniform spectroscopic F e and M g abundances from our master catalog are used to construct and analyze the age-metallicity and age-relative M g abundance relations for stars of the thin disk. The influences of selection effects are discussed in detail. It is demonstrated that the radial migration of stars does not lead to appreciable distortions in the age dependence of the metallicity. During the first several billion years of the formation of the thin disk, the interstellar material in this disk was, on average, fairly rich in heavy elements ( [F e/H] ≈ −0.2) and poorly mixed. However, the metallicity dispersion continuously decreased with age, from σ [F e/H] ≈ 0.22 to ≈ 0.13. All this time, the mean relative abundance of M g was somewhat higher than the solar value ( [M g/F e] ≈ 0.1). Roughly four to five billion years ago, the mean metallicity began to systematically increase, while retaining the same dispersion; the mean relative M g abundance began to decrease immediately following this. The number of stars in this subsystem increased sharply at the same time. These properties suggest that the star-formation rate was low in the initial stage of formation of the thin disk, but abruptly increased about four to five billion years ago.

Peculiarities of α-element abundances in Galactic open clusters

“…The mean absolute uncertainties for the cluster ages are presented in the figure, but the uncertainties in the ages are reflected more accurately by their relative uncertainties, which are ≈ 30 %, on average [3]. Figure 3a shows that the behavior of the field red giants is in good agreement with the behavior of the field dwarfs in [34] described above-the systematic decrease in the metallicities before five billion years is obvious, and the dependence becomes flat with further increase in the age. (The one point near 1.7 bilion years that disrupts the smooth behavior of the age dependence of the metallicity for the field giants has a small mean-square error, but represents an average over only five stars, and is unlikely to be reliable.)…”

“…Selection effects, systematic effects associated with random errors in the age determinations, and the effects of radial migration of stars were analyzed. It was shown in [34] that, during the first several billion years of the formation of the thin disk, the interstellar material incident on the disk possessed approximately the same heavy-element abundance ( [Fe/H] ≈ −0.2) and a low degree of homogeneity, but the dispersion in the metallicity σ[Fe/H] decreased smoothly from ≈ 0.22 to ≈ 0.13 with age. However, approximately four to five billion years ago, the mean metallicity began to systematically increase, while preserving the same dispersion.…”

“…According to modern views, α-elements are primarily synthesized in the cores of nassive stars (M > 10M ⊙ ) in late stages of their evolution, and are ejected into the interstellar medium when they explode as Type II supernovae (see, e. g., [34,37]). Type II supernovae also give rise to some iron-peak elements, but iron-peak elements are produced primarily in Type Ia supernovae, which correspond to the final stage in the evolution of close binary stars with masses < 8M ⊙ (see, e. g., [38]).…”

“…A similar behavior for the age dependence of the metallicity is also displayed by thin-disk stars with uniform spectroscopic iron abundances from the compiled catalog [35], which contains about 900 F-G dwarfs and ages estimated from the chromospheric activity of the stars. It is proposed in [34] that the low star-formation rate in the initial stages of formation of the Galactic thin disk suddenly increased about four to five billion years ago. This means that the metallicity in the thin disk over the past ∼ 5 billion years can be considered to be related to age, so that the observed differences in the relative abundances of elements in the open clusters compared to the field stars can be interpreted in terms of the chemical evolution of the interstellar matter.…”

“…A complete sample of 14 000 F-G dwarfs of the thin disk located within 70 pc of the Sun was compiled in [34], based on the catalog [27]. Selection effects, systematic effects associated with random errors in the age determinations, and the effects of radial migration of stars were analyzed.…”