Predictions Of Chemical Evolution Models Research Articles

Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang

Abundances of chemical elements in the interstellar and circumgalactic media of high-redshift galaxies offer important constraints on the nucleosynthesis by early generations of stars. Damped Lyα absorbers (DLAs) in spectra of high-redshift background quasars are excellent sites for obtaining robust measurements of element abundances in distant galaxies. Past studies of DLAs at redshifts z > 4 have measured abundances of ≲0.01 solar. Here we report the discovery of a DLA at z = 4.7372 with an exceptionally high degree of chemical enrichment. We estimate the H i column density in this absorber to be log (N H I/cm−2) = 20.48 ± 0.15. Our analysis shows unusually high abundances of carbon and oxygen ([C/H] = 0.88 ± 0.17, [O/H] = 0.71 ± 0.16). Such a high level of enrichment a mere 1.2 Gyr after the Big Bang is surprising because of insufficient time for the required amount of star formation. To our knowledge, this is the first supersolar absorber found at z > 4.5. We find the abundances of Si and Mg to be [Si/H] = and [Mg/H] = , confirming the metal-rich nature of this absorber. By contrast, Fe shows a much lower abundance ([Fe/H] = ). We discuss implications of our results for galactic chemical evolution models. The metallicity of this absorber is higher than that of any other known DLA and is >2 orders of magnitude above predictions of chemical evolution models and the N H I-weighted mean metallicity from previous studies at z > 4.5. The relative abundances (e.g., [O/Fe] = 2.29 ± 0.05, [C/Fe] = 2.46 ± 0.08) are also highly unusual compared to predictions for enrichment by early stars.

The Gaia-ESO Survey: Galactic evolution of lithium from iDR6

Context. After more than 50 years, astronomical research still struggles to reconstruct the history of lithium enrichment in the Galaxy and to establish the relative importance of the various 7Li sources in enriching the interstellar medium (ISM) with this fragile element. Aims. To better trace the evolution of lithium in the Milky Way discs, we exploit the unique characteristics of a sample of open clusters (OCs) and field stars for which high-precision 7Li abundances and stellar parameters are homogeneously derived by the Gaia-ESO Survey (GES). Methods. We derive possibly un-depleted 7Li abundances for 26 OCs and star forming regions with ages from young (∼3 Myr) to old (∼4.5 Gyr), spanning a large range of galactocentric distances, 5 < RGC/kpc < 15, which allows us to reconstruct the local late Galactic evolution of lithium as well as its current abundance gradient along the disc. Field stars are added to look further back in time and to constrain 7Li evolution in other Galactic components. The data are then compared to theoretical tracks from chemical evolution models that implement different 7Li forges. Results. Thanks to the homogeneity of the GES analysis, we can combine the maximum average 7Li abundances derived for the clusters with 7Li measurements in field stars. We find that the upper envelope of the 7Li abundances measured in field stars of nearly solar metallicities (−0.3 < [Fe/H]/dex < +0.3) traces very well the level of lithium enrichment attained by the ISM as inferred from observations of cluster stars in the same metallicity range. We confirm previous findings that the abundance of 7Li in the solar neighbourhood does not decrease at super-solar metallicity. The comparison of the data with the chemical evolution model predictions favours a scenario in which the majority of the 7Li abundance in meteorites comes from novae. Current data also seem to suggest that the nova rate flattens out at later times. This requirement might have implications for the masses of the white dwarf nova progenitors and deserves further investigation. Neutrino-induced reactions taking place in core-collapse supernovae also produce some fresh lithium. This likely makes a negligible contribution to the meteoritic abundance, but could be responsible for a mild increase in the 7Li abundance in the ISM of low-metallicity systems that would counterbalance the astration processes.

The Gaia-ESO Survey: a new approach to chemically characterising young open clusters

Context. Young open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to ~0.7 dex. Regarding the behaviour of the other s-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages. Aims. In this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu I, Sr I, Sr II, Y II, Zr II, Ba II, La II, and Ce II. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time. Methods. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the Gaia-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun. Results. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm that [Ba/Fe] is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with [Y/Fe] ratios covering values between 0 and +0.3 dex. For the other s-process elements we find that [X/Fe] ratios are solar at all ages. Conclusions. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of the s-process at young ages, and we strongly encourage further critical observations.

Helium abundances and its radial gradient from the spectra of H ii regions and ring nebulae of the Milky Way

ABSTRACT We determine the radial abundance gradient of helium in the disc of the Galaxy from published spectra of 19 H ii regions and ring nebulae surrounding massive O-type stars. We revise the Galactocentric distances of the objects considering Gaia DR2 parallaxes (Gaia Collaboration 2018) and determine the physical conditions and the ionic abundance of He+ in a homogeneous way, using between 3 and 10 He i recombination lines in each object. We estimate the total He abundance of the nebulae and its radial abundance gradient using four different ionization correction factor (ICF; He) schemes. The slope of the gradient is always negative and weakly dependent on the ICF(He) scheme, especially when only the objects with log(η) < 0.9 are considered. The slope values go from −0.0078 to −0.0044 dex kpc−1, consistent with the predictions of chemical evolution models of the Milky Way and chemodynamical simulations of disc galaxies. Finally, we estimate the abundance deviations of He, O, and N in a sample of ring nebulae around Galactic Wolf–Rayet stars, finding a quite similar He overabundance of about +0.24 ± 0.11 dex in three stellar ejecta ring nebulae.

A Formation Timescale of the Galactic Halo from Mg Isotopes in Dwarf Stars*

Abstract We determine magnesium isotopic abundances of metal-poor dwarf stars from the galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star nucleosynthesis in the galactic halo and constrain the timescale of its formation. We observed a sample of eight new halo K dwarfs in a metallicity range of −1.9 < [Fe/H] < −0.9 and 4200 < T eff(K) < 4950, using the HIRES spectrograph at the Keck Observatory (R ≈ 105 and 200 ≤ S/N ≤ 300). We obtain magnesium isotopic abundances by spectral synthesis on three MgH features and compare our results with galactic chemical evolution models. With the current sample, we almost double the number of metal-poor stars with Mg isotopes determined from the literature. The new data allow us to determine the metallicity when the 26Mg abundances start to become important, [Fe/H] ∼ −1.4 ± 0.1. The data with [Fe/H] > −1.4 are somewhat higher (1–3σ) than previous chemical evolution model predictions, indicating perhaps higher yields of the neutron-rich isotopes. Our results using only AGB star enrichment suggest a timescale for formation for the galactic halo of about 0.3 Gyr, but considering also supernova enrichment, the upper limit for the timescale formation is about 1.5 Gyr.

How can star formation be sustained?

AbstractThere is overwhelming evidence that the Milky Way has formed its stars at a relatively constant rate throughout the Hubble time. This implies that its stock of cold gas was not in place since the beginning but it has been acquired slowly through gas accretion. The gas accretion must have been at low metallicity in order to reconcile the metallicities observed in the disc with the predictions of chemical evolution models. But how does this gas accretion take place? I review the current evidence of gas accretion into the Milky Way and similar galaxies through the infall of cold gas clouds and satellites. The conclusion from these studies is that the infalling gas at high column densities observed in HI emission is a least one order of magnitude below the value required to sustain star formation, thus accretion must come from a different channel. The likely reservoir for gas accretion is the cosmological corona of virial-temperature gas in which every galaxy must be embedded. At the interface between the disc and the corona the cold high-metallicity disc gas and the hot coronal medium must mix efficiently and this mixing causes the cooling and accretion of the lower corona. I show how this mechanism reproduces the kinematics of the neutral extraplanar gas in the Milky Way and other nearby galaxies and the ionised high-velocity clouds observed in HST spectra. I conclude with the speculation that the loss in efficiency of the disc-corona interaction is the ultimate cause for the evolution of disc galaxies towards the red sequence.

Quantifying the uncertainties of chemical evolution studies

This is the second paper of a series which aims at quantifying the uncertainties in chemical evolution model predictions related to the underlying model assumptions. Specifically, it deals with the uncertainties due to the choice of the stellar yields. We adopt a widely used model for the chemical evolution of the Galaxy and test the effects of changing the stellar nucleosynthesis prescriptions on the predicted evolution of several chemical species. We find that, except for a handful of elements whose nucleosynthesis in stars is well understood by now, large uncertainties still affect the model predictions. This is especially true for the majority of the iron-peak elements, but also for much more abundant species such as carbon and nitrogen. The main causes of the mismatch we find among the outputs of different models assuming different stellar yields and among model predictions and observations are: (i) the adopted location of the mass cut in models of type II supernova explosions; (ii) the adopted strength and extent of hot bottom burning in models of asymptotic giant branch stars; (iii) the neglection of the effects of rotation on the chemical composition of the stellar surfaces; (iv) the adopted rates of mass loss and of (v) nuclear reactions, and (vi) the different treatments of convection. Our results suggest that it is mandatory to include processes such as hot bottom burning in intermediate-mass stars and rotation in stars of all masses in accurate studies of stellar evolution and nucleosynthesis. In spite of their importance, both these processes still have to be better understood and characterized. As for massive stars, presupernova models computed with mass loss and rotation are available in the literature, but they still wait for a self-consistent coupling with the results of explosive nucleosynthesis computations.

Stellar metallicity distributions in local dwarf spheroidal galaxies: a comparison between model and observations

AbstractWe analyze the main populations of 6 local Dwarf Spheroidal Galaxies by comparing the observed stellar metallicity distributions with the predictions of chemical evolution models. The predicted metallicity distributions of stars generally exhibit a low range of metallicities, a peak below the one of the MW disc in the solar neighborhood (but similar to the Halo), and a sharp decrease at higher metallicities in agreement with observations. The position of the peak is related to the low star formation rates adopted whereas the sharp decrease is a consequence of the occurrence of strong galactic winds. In the low metallicity tail of the distributions we predict a low number of stars in agreement with observations.

Stellar mass loss, rotation and the chemical enrichment of early-type galaxies

We present a comparison between the [Ca,C,N/Fe]-mass relations observed in local spheroids and the results of a chemical evolution model which already successfully reproduces the [Mg/Fe]-mass and the [Fe/H]-mass relations in these systems. We find that the [Ca/Fe]-mass relation is naturally explained by such a model without any additional assumption. In particular, the observed under-abundance of Ca with respect to Mg can be attributed to the different contributions from supernovae Type Ia and supernovae Type II to the nucleosynthesis of these two elements. For C and N, we consider new stellar yields that take into account stellar mass loss and rotation. These yields have been shown to successfully reproduce the C and N abundances in Milky Way metal-poor stars. The use of these new stellar yields produces a good agreement between the chemical evolution model predictions and the integrated stellar population observations for C. In the case of N, the inclusion of fast rotators and stellar mass-loss nucleosynthesis prescriptions improves our predictions for the slope of the [N/Fe] vs. sigma relation, but a zero point discrepancy of 0.3 dex remains. This work demonstrates that current stellar yields are unable to simultaneously reproduce the large mean stellar [<N/Fe>] ratios inferred from integrated spectra of elliptical galaxies and the low N abundance measured in the gas of high redshift spheroids from absorption lines. However, it seems reasonable to suggest that there may be uncertainties in either the inferred stellar or gas-phase N abundances at the level of 0.3 dex. (abriged)

The Effects of Stellar Yields with Rotation on Chemical Evolution Models

We summarise the results of recent work investigating the effects of stellar yields which take into account rotation on the predictions of chemical evolution models for early populations of galaxies.

Stellar yields with rotation and their effect on chemical evolution models

We compute the evolution of different abundance ratios in the Milky Way (MW) for two different sets of stellar yields. In one of them stellar rotation is taken into account and we investigate its effects on the chemical evolution model predictions. Moreover, we show that some abundance ratios offer an important tool to investigate the halo-disk discontinuity. For the first time it is shown that the effect of a halt in the star formation between the halo/thick disk and thin disk phases, already suggested from studies based both on Fe/O vs. O/H and Fe/Mg vs. Mg/H, should also be seen in a C/O versus O/H plot if C is produced mainly by low- and intermediate-mass stars (LIMS). The idea that C originates mainly from LIMS is suggested by the flat behavior of the [C/Fe] ratio as a function of metallicity, from [Fe/H]=-2.2 to solar, and by the fact that very recent C/O measurements for stars in the MW halo and disk seem to show a discontinuity around log(O/H)+12=8.4. Finally, a more gentle increase of N abundance with metallicity (or time), relative to models adopting the yields of van den Hoek and Groenewegen (1997), is predicted by using the stellar yields of Meynet and Maeder (2002 - which include stellar rotation but not hot-bottom burning) for intermediate mass stars. This fact has some implications for the timescales of N enrichment and thus for the interpretation of the nature of Damped Lyman Alpha Systems.

Chemical evolution of dwarf spheroidal and blue compact galaxies

We studied the star formation and chemical evolution in a sample of eight dwarf spheroidal (dSph) galaxies of the Local Group and in blue compact galaxies (BCGs) by means of comparison between the predictions of chemical evolution models and several observed abundance ratios. Detailed models with up-to-date nucleosynthesis taking into account the role played by supernovae of different types (II, Ia) were developed for both types of galaxies, allowing us to follow the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca and Fe). The models are specified by the prescriptions of the star formation and galactic wind efficiencies chosen to reproduce the main features of these galaxies. The BCGs are characterized by a star formation proceeding in several short bursts separated by long quiescent periods and by a low wind efficiency, whereas one or two long bursts and a very efficient wind well describe the dSph galaxies. We also investigated a possible connection in the evolution of dSph and BCGs and compared the predictions of the models to the abundance ratios observed in damped Lyman α systems (DLAs). The main conclusions are: (i) the observed distribution of [α/Fe] versus [Fe/H] in dSph galaxies is mainly a result of the star formation rate coupled with the wind efficiency; (ii) a low star formation efficiency (ν= 0.01–1 Gyr−1) and a high wind efficiency (wi∼ 5–15) are required to reproduce the observational data for dSph galaxies; (iii) the low gas content of these galaxies is the result of the combined action of gas consumption by star formation and gas removal by galactic winds; (iv) the abundance ratios of the BCGs are reproduced by models with two to seven bursts of star formation and an efficiency in the range ν= 0.1–0.9 Gyr−1; (v) the low values of N/O observed in BCGs are the natural result of a bursting star formation; (vi) a connection between dSph and BCGs in a unified evolutionary scenario is unlikely; (vii) the models for both the dSph galaxies and BCGs reproduce the abundance ratios observed in DLAs, but imply different formation scenarios for these objects; (viii) a suitable amount of primary N produced in massive stars can be perhaps an explanation for the low plateau in the [N/α] distribution observed in DLAs, if real.

A revision of the solar neighbourhood metallicity distribution

We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centred on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood has a metallicity higher than [Fe/H]=0. The percentage of mid-metal-poor stars ([Fe/H]$<$-0.5) is approximately 4 per cent, in agreement with present estimates of the thick disc. In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections due to the increasing thickness of the stellar disc with age, we show that the Simple Closed-Box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age-metallicity relation and abundance ratios suggest that the Simple Closed-Box model may be a viable model of the chemical evolution of the Galaxy at solar radius.

The significance of the interstellar Li-7/Li-6 ratio

view Abstract Citations (20) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Significance of the Interstellar 7Li/ 6Li Ratio Steigman, Gary Abstract The present abundances of Li-6 and Li-7, when compared with the solar system values, can provide valuable information on stellar and cosmic-ray nucleosynthesis, stellar structure and evolution, and Galactic chemical evolution in the solar vicinity in the last 4.6 Gyr. Unfortunately, the interpretation of stellar data from observations of main-sequence stars in young clusters and of pre-main-sequence stars are clouded by large and uncertain corrections for burning of lithium and for non-LTE effects, and the interpretation of observations of interstellar Li are rendered uncertain by large and unknowable corrections for ionization and depletion onto grains and/or molecules. In this Letter it is shown that the singular origin for Population I Li-6 offers the potential for exploiting bounds to interstellar beryllium and boron to infer an upper bound to the present interstellar Li-6 abundance. Then the recent observations (Lemoine et al., 1992; Meyer et al., 1993) of the interstellar Li-7/Li-6 ratio may be utilized to bound the present abundance of Li-7. This approach is independent of any model for the evolution of Li-7. The current data suggest that the Li abundance has barely increased, and may well have decreased, in the 4.6 Gyr since the formation of the solar system. This result is inconsistent with the predictions of chemical evolution models and raises the question of the reliability of the sun as a probe of the abundances in the local ISM 4.6 Gyr ago. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/186962 Bibcode: 1993ApJ...413L..73S Keywords: Interstellar Matter; Lithium Isotopes; Metallicity; Milky Way Galaxy; Abundance; Solar System; Astrophysics; GALAXY: ABUNDANCES; ISM: ABUNDANCES; SUN: ABUNDANCES full text sources ADS |