Tracing Chemical Evolution Over the Extent of the Milky Way's Disk with Apogee Red Clump Stars (original) (raw)
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I discuss three different topics concerning the chemical evolution of the Milky Way (MW). 1) The metallicity distribution of the MW halo; it is shown that this distribution can be analytically derived in the framework of the hierarchical merging scenario for galaxy formation, assuming that the component sub-haloes had chemical properties similar to those of the progenitors of satellite galaxies of the MW. 2) The age-metallicity relationship (AMR) in the solar neighborhood; I argue for caution in deriving from data with important uncertainties (such as the age uncertainties in the Geneva-Copenhagen Survey) a relationship between average metallicity and age: derived relationships are shown to be systematically flatter than the true ones and should not be directly compared to models. 3) The radial mixing of stars in the disk, which may have important effects on various observables (scatter in AMR, extension of the tails of the metallicity distribution, flatenning of disk abundance prof...
Chemical enrichment and star formation in the Milky Way disk
Astronomy and Astrophysics, 2004
In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a sample composed by 424 late-type dwarfs. We show that the velocity dispersion of a stellar group correlates with the age of this group, according to a law proportional to t 0.26 , where t is the age of the stellar group. The temporal evolution of the vertex deviation is considered in detail. It is shown that the vertex deviation does not seem to depend strongly on the age of the stellar group. Previous studies in the literature seem to not have found it due to the use of statistical ages for stellar groups, rather than individual ages. The possibility to use the orbital parameters of a star to derive information about its birthplace is investigated, and we show that the mean galactocentric radius is likely to be the most reliable stellar birthplace indicator. However, this information cannot be presently used to derive radial evolutionary constraints, due to an intrinsic bias present in all samples constructed from nearby stars. An extensive discussion of the secular and stochastic heating mechanisms commonly invoked to explain the age-velocity dispersion relation is presented. We suggest that the age-velocity dispersion relation could reflect the gradual decrease in the turbulent velocity dispersion from which disk stars form, a suggestion originally made by and supported by several more recent disk evolution calculations. A test to distinguish between the two types of models using high-redshift galaxies is proposed.
Monthly Notices of the Royal Astronomical Society, 2015
The radially averaged metallicity distribution of the ISM and the young stellar population of a sample of 20 disk galaxies is investigated by means of an analytical chemical evolution model which assumes constant ratios of galactic wind mass loss and accretion mass gain to star formation rate. Based on this model the observed metallicities and their gradients can be described surprisingly well by the radially averaged distribution of the ratio of stellar mass to ISM gas mass. The comparison between observed and model predicted metallicity is used to constrain the rate of mass loss through galactic wind and accretion gain in units of the star formation rate. Three groups of galaxies are found: galaxies with either mostly winds and only weak accretion, or mostly accretion and only weak winds, and galaxies where winds are roughly balanced by accretion. The three groups are distinct in the properties of their gas disks. Galaxies with approximately equal rates of mass-loss and accretion gain have low metallicity, atomic hydrogen dominated gas disks with a flat spatial profile. The other two groups have gas disks dominated by molecular hydrogen out to 0.5 to 0.7 isophotal radii and show a radial exponential decline, which is on average steeper for the galaxies with small accretion rates. The rates of accretion ( 1.0 × SFR) and outflow ( 2.4 × SFR) are relatively low. The latter depend on the calibration of the zero point of the metallicity determination from the use of HII region strong emission lines.
Inhomogeneous chemical evolution of the Galactic disk: evidence for sequential stellar enrichment?
Journal of Hypertension - J HYPERTENSION, 1997
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 {DELTA}[M/H] ~0.6dex 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 ...
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Galaxy Evolution: Connecting the Distant Universe with the Local Fossil Record, 1999
The supernova yields of several heavy elements including α-, iron-group, and r-process elements are obtained as a function of the mass of their progenitor main-sequence stars M ms from the abundance patterns of extremely metal-poor stars with a procedure recently proposed by . The ejected masses of α-and iron-group elements increase with M ms , whereas more Eu is ejected from supernovae with lower M ms . For these several heavy elements, it is shown that the average abundance ratios weighted by the Salpeter initial mass function coincide with the ratios observed in stars with −2 < [Fe/H] < −1 within 0.1 dex. It follows that the correlations of stellar abundance ratios with the metallicity are twofold. One is the abundance ratios for [Fe/H] < −2.5 imprinted by the nucleosynthesis in individual supernovae on the timescale ∼ 10 7 yr and the other for [Fe/H] > −2 results from the mixing of the products from a whole site of the nucleosynthesis, taking place on the timescale longer than 10 9 yr.
Galaxy Formation and Chemical Evolution
The manner the galaxy accretes matter, along with the star formation rates at different epochs, influences the evolution of the stable isotopic inventories of the galaxy. A detailed analysis is presented here to study the dependence of the galactic chemical evolution on the accretion scenario of the galaxy along with the star formation rate during the early accretionary phase of the galactic thick disk and thin disk. Our results indicate that a rapid early accretion of the galaxy during the formation of the galactic thick disk along with an enhanced star formation rate in the early stages of the galaxy accretion could explain the majority of the galactic chemical evolution trends of the major elements. Further, we corroborate the recent suggestions regarding the formation of a massive galactic thick disk rather than the earlier assumed low mass thick disk.
The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years
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We calculate the stellar mass-metallicity relation at five epochs ranging to z ∼ 2.3. We quantify evolution in the shape of the mass-metallicity relation as a function of redshift; the mass-metallicity relation flattens at late times. There is an empirical upper limit to the gas-phase oxygen abundance in star-forming galaxies that is independent of redshift. From examination of the mass-metallicity relation and its observed scatter we show that the flattening at late times is a consequence of evolution in the stellar mass where galaxies enrich to this empirical upper metallicity limit; there is also evolution in the fraction of galaxies at a fixed stellar mass that enrich to this limit. The stellar mass where metallicities begin to saturate is ∼ 0.7 dex smaller in the local universe than it is at z ∼ 0.8.
Astronomy and Astrophysics, 2008
Aims. We study the nucleosynthesis of several neutron capture elements (barium, europium, lanthanum, and yttrium) in local group dwarf spheroidal (dSph) galaxies and in the Milky Way by comparing the predictions of detailed chemical evolution models with the observed data. Methods. We compare the evolution of [Ba/Fe], [Eu/Fe], [La/Fe], [Y/Fe], [Ba/Y], [Ba/Eu], [Y/Eu], and [La/Eu] observed in dSph galaxies and in our Galaxy with predictions of detailed chemical evolution models. The models for all dSph galaxies and for the Milky Way are able to reproduce several observational features of these galaxies, such as a series of abundance ratios and the stellar metallicities distributions. The Milky Way model adopts the two-infall scenario, whereas the most important features of the models for the dSph galaxies are the low star-formation rate and the occurrence of intense galactic winds. Results. We predict that the [s-r/Fe] ratios in dSphs are generally different than the corresponding ratios in the Milky Way, at the same [Fe/H] values. This is interpreted as a consequence of the time-delay model coupled with different star formation histories. In particular, the starformation is less efficient in dSphs than in our Galaxy and it is influenced by strong galactic winds. Our predictions are in very good agreement with the available observational data. Conclusions. The time-delay model for the galactic chemical enrichment coupled with different histories of star formation in different galaxies allow us to succesfully interpret the observed differences in the abundance ratios of s-and r-process elements, as well as of α-elements in dSphs and in the Milky Way. These differences strongly suggest that the main stellar populations of these galaxies could not have had a common origin and, consequently, that the progenitors of local dSphs might not be the same objects as the building blocks of our Galaxy.
Chemical enrichment and star formation in the Milky Way disk. II. Star formation history
Astronomy and Astrophysics, 2000
A chromospheric age distribution of 552 late-type dwarfs is transformed into a star formation history by the ap- plication of scale height corrections, stellar evolutionary cor- rections and volume corrections. We show that the disk of our Galaxy has experienced enhanced episodes of star formation at 0-1 Gyr, 2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still uncertain. The star sample birthsites are dis- tributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate. These results are compared with the metal-enrichment rate, given by the age-metallicity relation, with the expected epochs of close encounters between our Galaxy and the Magellanic Clouds, and with previous determinations of the star forma- tion history. Simulations are used to examine the age-dependent smearing of the star formation history due to age uncertainties, and the broadening of the recovered features, as well as to...