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Astron. Astrophys. 318, 231-251 (1997)


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Inhomogeneous chemical evolution of the Galactic disk: evidence for sequential stellar enrichment?

L.B.  van den Hoek 1 and T.  de Jong 1, 2

1 Astronomical Institute `Anton Pannekoek', Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Space Research Organisation of the Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received 13 November 1995 / Accepted 19 June 1996

Abstract

We investigate the origin of the abundance variations observed among similarly aged F and G dwarfs in the local Galactic disk. We argue that orbital diffusion of stars in combination with radial abundance gradients is probably insufficient to explain these variations.

We show that episodic and local infall of metal-deficient gas can provide an adequate explanation for iron and oxygen variations as large as [FORMULA] [M/H] [FORMULA] 0.6 dex among stars formed at a given age in the solar neighbourhood (SNBH). However, such models appear inconsistent with the observations because they: 1) result in current disk ISM abundances that are too high compared to the observations, 2) predict stellar abundance variations to increase with the lifetime of the disk, and 3) do not show substantial scatter in the [Fe/H] vs. [O/H] relation. Notwithstanding, our results do suggest that metal-deficient gas infall plays an important role in regulating the chemical evolution of the Galactic disk.

We demonstrate that sequential enrichment by successive stellar generations within individual gas clouds can account for substantial abundance variations as well. However, such models are inconsistent with the observations because they: 1) are unable to account for the full magnitude of the observed variations, in particular for [Fe/H], 2) predict stellar abundance variations to decrease with the lifetime of the disk, and 3) result in current abundances far below the typical abundances observed in the local disk ISM.

We present arguments in support of combined infall of metal-deficient gas and sequential enrichment by successive stellar generations in the local Galactic disk ISM. We show that galactic chemical evolution models which take into account these processes simultaneously are consistent with both the observed abundance variations among similarly aged F and G dwarfs in the SNBH and the abundances observed in the local disk ISM. For reasonable choices of parameters, these models can reproduce [FORMULA] [M/H] for individual elements M = C, O, Fe, Mg, Al, and Si as well as the scatter observed in abundance-abundance relations like [O/Fe]. For the same models, the contribution by sequential stellar enrichment to the magnitude of the observed abundance variations can be as large as [FORMULA] 50%.

We discuss the impact of sequential stellar enrichment and episodic infall of metal-deficient gas on the inhomogeneous chemical evolution of the Galactic disk *.

Key words: Galaxy: evolution; abundances; solar neighbourhood – ISM: abundances – galaxies: ISM

Send offprint requests to: L.B. van den Hoek (bobby@astro.uva.nl)

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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