Abundance gradients in the galactic disk: space and time variations

Source: arXiv

ABSTRACT Recent work on abundance gradients have focussed not only on their magnitudes, but also on their spatial and temporal variations. In this work, we analyze the behaviour of radial abundance gradients in the galactic disk giving special emphasis on these variations. The data used includes planetary nebulae and objects in different age brackets, namely open clusters, HII regions, cepheid variables and stars in OB associations. We find evidences for a space variation of the radial gradients as measured for element ratios such as O/H, S/H, Ne/H, Ar/H and [Fe/H], in the sense that the gradients tend to flatten out at large galactocentric distances. Moreover, near the bulge-disk interface a steep decrease in the abundances is observed. The time evolution of the gradients is also evaluated on the basis of approximate ages attributed to the central stars of planetary nebulae and open cluster stars. It is concluded that the available data is consistent with a time flattening of the gradients during the last 6 to 8 Gyr, a time interval in which the age determinations are probably more accurate.

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    ABSTRACT: We summarize radial velocity studies of selected stars in the old, distant clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31. Cluster members are identified using optical and infrared color-magnitude diagrams, as well as radial velocities derived from high-resolution echelle spectra. Three members of M67 were observed similarly, and those velocities compare extremely well with prior measures. Mean cluster radial velocities are determined. We also employ the highest quality spectra to analyze the chemical compositions of all six clusters for [Fe/H], as well as abundances of alpha-elements, iron-peak elements, and those synthesized in either the s-process or the r-process. In Be 21 our observed star is found to be rotating rapidly and overabundant in lithium, the second Li-rich star found in this sparse cluster. We confirm the lack of correlation between abundance and age. For the outer disk, the abundance gradient for [Fe/H] deviates from the trend defined near the solar neighborhood. Rather than declining with increasing galactocentric distance, [Fe/H] appears to reach a ``basement'' at [Fe/H]~-0.5 beyond RGC~10-12 kpc. Our radial abundance distribution for [Fe/H] is not inconsistent with the radial abundance discontinuity exhibited by Cepheids. We find enhanced [O/Fe], [alpha/Fe], and [Eu/Fe] in the outer disk, revealing a rapid star formation history. The outer disk also exhibits enhancements for s-process elements. We compare the open cluster compositions with those of the thin disk, thick disk, halo, bulge, and dwarf spheroidal galaxies. None of these stellar populations perfectly matches the abundance ratios of the outer disk open clusters. Several key points arise from these comparisons: (1) [O/Fe] and [alpha/Fe] resemble those of the thick disk. (2) [Na/Fe] and [Al/Fe] are enhanced relative to those of the thin disk. (3) [Ni/Fe] and [Mn/Fe] are in accord with those of the thin disk, while [Co/Fe] may be slightly enhanced. (4) The neutron-capture elements indicate different ratios of s-process to r-process material, with no cluster showing a pure r-process distribution. (5) An unusual pattern exists among the alpha-elements, with [/Fe] enhanced while [/Fe] is normal. Similar abundance ratios have been reported for Galactic bulge giants and indicate a common but not necessarily shared nucleosynthetic history between the bulge and the outer disk. Enhanced ratios of [Al/Fe] and [Co/Fe] offer another possible similarity between the bulge and the outer disk. An intriguing but tentative conclusion is that the outer disk open cluster abundance ratios are consistent with the outer disk being formed via a merger event or series of merger events. The basement in [Fe/H] and enhanced [alpha/Fe] suggest that the outer disk formed from a reservoir of gas with a star formation history distinct from the solar neighborhood. That the open clusters may be associated with an accreted dwarf galaxy or galaxies is appealing, since the clusters are young and have [alpha/Fe] ratios indicating a rapid star formation history. However, the high [alpha/Fe] ratios are unlike those seen in any current dwarf galaxies at the same [Fe/H]. Therefore, the open clusters may have formed as a result of star formation triggered by a merger event or series of mergers in the outer disk. The ages of the outer disk open clusters would then be a measure of when the merger(s) occurred. However, Be 29 is a candidate merger member, while Be 31 is not. One problem with the merger scenario is that open clusters with presumably very different origins have similar and unusual compositions. This paper makes use of observations obtained at the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under contract from the National Science Foundation. We also employ data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
    The Astronomical Journal 01/2005; 130:597-625. · 4.97 Impact Factor
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    ABSTRACT: We model the abundance gradients in the disk of the Milky Way for several chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba, La and Eu), and compare our results with the most recent and homogeneous observational data. We adopt a chemical evolution model able to well reproduce the main properties of the solar vicinity. We compute, for the first time, the abundance gradients for all the above mentioned elements in the galactocentric distance range 4 - 22 kpc. The comparison with the observed data on Cepheids in the galactocentric distance range 5-17 kpc gives a very good agreement for many of the studied elements. In addition, we fit very well the data for the evolution of Lanthanum in the solar vicinity for which we present results here for the first time. We explore, also for the first time, the behaviour of the abundance gradients at large galactocentric distances by comparing our results with data relative to distant open clusters and red giants and select the best chemical evolution model model on the basis of that. We find a very good fit to the observed abundance gradients, as traced by Cepheids, for most of the elements, thus confirming the validity of the inside-out scenario for the formation of the Milky Way disk as well as the adopted nucleosynthesis prescriptions. Comment: 11 pages, 9 figures, accepted for publication in A&A
    Astronomy and Astrophysics 09/2006; · 5.08 Impact Factor
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    ABSTRACT: Radial abundance gradients are a common feature of spiral galaxies, and in the case of the Galaxy both the magnitude of the gradients and their variations are among the most important constraints of chemical evolution models. Planetary nebulae (PN) are particularly interesting objects to study the gradients and their variations. Owing to their bright emission spectra, they can be observed even at large galactocentric distances, and the derived abundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex, particularly for the elements that are not synthesized in their progenitor stars. On the other hand, as the offspring of intermediate mass stars, with main sequence masses in the interval of 1 to 8 solar masses, they are representative of objects with a reasonable age span. In this paper, we present an estimate of the time variation of the O/H radial gradient in a sample containing over 200 nebulae with accurate abundances. Our results are consistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to -0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpc during the last 5 Gyr. Comment: 9 pages, 7 encapsulated postscript figures, LaTeX, uses Astronomy and Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and Astrophysics (2002), Also available at:
    Astronomy and Astrophysics 10/2002; · 5.08 Impact Factor

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