Galactic Evolution Models Research Articles

The Geneva-Copenhagen survey of the Solar neighbourhood II

Context.Ages, metallicities, space velocities, and Galactic orbits of stars in the Solar neighbourhood are fundamental observational constraints on models of galactic disk evolution. Understanding and minimising systematic errors and sample selection biases in the data is crucial for their interpretation.

Silicon and Carbon Isotopic Ratios in AGB Stars: SiC Grain Data, Models, and the Galactic Evolution of the Si Isotopes

PresolarSiCgrainsofthemainstream,Y,andZtypearebelievedtocomefromcarbonstars.WecomparedtheirCand Si isotopicratios withtheoretical modelsfor theenvelopecompositions of AGB stars.Two setsof models (FRANEC and Monash) use a range of stellar masses (1.5–5M� ) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of 12 C/ 13 C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22 Ne neutron source dominates Si nucleosynthesis, the effect of the 13 C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Yand Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The 12 C/ 13 C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains’ C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the 29 Si/ 28 Si ratio in the Galaxy as function of metallicity Z .A tZ <0:01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon

Planetary nebulae as probes for galactic chemical evolution

The role of planetary nebulae as probes for the galactic chemical evolution is reviewed. Their abundances throughout the Galaxy are discussed for key elements, in particular oxygen and other alpha elements. The abundance distribution derived from planetary nebulae leads to the establishment of radial abundance gradients in the galactic disk that are important constraints to model the chemical evolution of the Galaxy. The radial gradient, well determined for the solar neighborhood, is examined for distinct regions. For the galactic anticenter in particular, the observational data confirm results from galactic evolution models that point to a decreasing in the gradient slope at large galactocentric distances. The possible time evolution of the radial gradient is also examined comparing samples of planetary nebulae of different ages, and the results indicate that a flattening in the gradient occurred, which is confirmed by some galactic evolution models. The galactic bulge is another important region whose modeling can be constrained by observational results obtained from planetary nebulae. Results derived in the last few years indicate that bulge nebulae have an abundance distribution similar to that of disk objects, however with a larger dispersion.

Interstellar Helium Trapped with the COLLISA Experiment on theMiRSpace Station—Improved Isotope Analysis by In Vacuo Etching

We have redetermined the helium isotopic composition of the local interstellar cloud (LIC) in the framework of the Swiss-Russian Collection of Interstellar Atoms (COLLISA) collaboration. Based on in vacuo etching analyses of foils that have been exposed to the interstellar neutral particle flux on board the Mir space station, we obtain (3He/4He)LIC = (1.62 ± 0.29) × 10-4. This is the most precise determination of the He isotopic composition of the LIC, with errors being lower by more than a factor of 2 compared to an earlier experiment on similar foils and compared to the analysis of interstellar pick-up ions. Comparing our improved result with current models of Galactic chemical evolution reveals that close to 100% of all Galactic low-mass stars must have undergone extra mixing associated with cool bottom processing. The 3He abundance relative to 4He in the LIC is within the uncertainty identical to the value of (1.66 ± 0.06) × 10-4 inferred for the protosolar cloud (PSC), showing that no significant evolution of the 3He abundance took place since 4.6 Gyr ago at the solar distance (~8 kpc) from the Galactic center. Our value sets new rigorous constraints on Galactic evolution models. Since the Big Bang nucleosynthesis theory predicts a value of ≤(1.1 ± 0.2) × 10-4, a noticeable increase of the relative abundance of 3He has apparently occurred in the Galaxy before the formation of the solar system, but, contrary to earlier expectations, only a modest or negligible increase is registered after the formation of the solar system.

Early star formation in the Galaxy from beryllium and oxygen abundances

We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called ``accretion'' and ``dissipative'' populations) are neatly separated in the [O/Fe] vs. Log(Be/H) diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ~5-10) than the accretion component. The latter shows a large scatter in the [O/Fe] vs. Log(Be/H) diagram, probably resulting from the inhomogeneous enrichment in oxygen and iron of the protogalactic gas. Despite the limitation of the sample, the data suggest that the combined use of products of spallation reactions (like beryllium) and elemental ratios of stellar nucleosynthesis products (like [O/Fe]) can constrain theoretical models for the formation and early evolution of our Galaxy.

The early build-up of dust in galaxies: A study of damped Lyαsystems

We present a study of the early build-up of dust in high redshift galaxies. The study is based on the analysis of 38 Damped Lyα systems (DLAs) for which we derive the fraction of iron atoms in dust form, f Fe . The sample is representative of metal-poor galaxies in the redshift range 0.6 < z < 3.4 selected on the basis of their absorption HI column density (N(HI) ≥ 2 x 10 20 atoms cm -2 ). We find that the dust fraction increases with metallicity, from f Fe ≃ 0 at [Fe/H] ∼ -2 dex, up to f Fe ≃ 0.9 at solar metallicity; the increase is fast below [Fe/H] ≃ -1 dex and mild at higher metallicities. We also find some evidence for an increase of f Fe with cosmic time; a large fraction of the systems younger than 3 Gyr has f Fe ≤ 0.5. These results indicate that the dust-to-metal ratio increases in the course of chemical evolution, at variance with the hypothesis of an approximately constant dust-to-metal ratio, commonly adopted in models of galactic evolution. This hypothesis is consistent with local and high-redshift data only when the metallicity is relatively high ([Fe/H] ≥ -1 dex). The results of this work suggest that the main mechanisms of dust formation may be rather sensitive to the level of metallicity attained by a galaxy in the course of its chemical evolution. A metallicity-dependent dust production by SNe II seems to be the most promising mechanism to explain the rise of f Fe at [Fe/H] ≤ -1 dex.

The mutable galaxies

Soon after the discovery of the expansion of the universe, astronomers hoped to determine the parameters of the universe by comparing distant galaxies with nearby ones. Observing distant galaxies, however, meant observing them as they were long ago. A crucial assumption in these studies was that galaxies hardly change, in the time scale of the expansion of the universe. Beatrice Tinsley was one of the first astronomers to point out that galaxies do change. Here we discuss a simplified model of galactic evolution, and show that the comparison of its prediction with observations uncovers clues to how galaxies evolve with time

Hypernovae/Gamma-Ray Bursts in the Galactic Center as Possible Sources of Galactic Positrons

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV 26Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of 56Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from 56Co decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of 56Ni in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.

Big Bang Nucleosynthesis

Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation now that extragalactic deuterium is being measured. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman-α clouds, x-ray gas in clusters, the Sunyaev-Zeldovich effect, and the microwave anisotropy are made and shown to be consistent with the BBN value.

Lithium abundances for 185 main-sequence stars

We present a survey of lithium abundances in 185 main-sequence field stars with K and based on new measurements of the equivalent width of the Lii line in high-resolution spectra of 130 stars and a reanalysis of data for 55 stars from Lambert et al. ([CITE]). The survey takes advantage of improved photometric and spectroscopic determinations of effective temperature and metallicity as well as mass and age derived from Hipparcos absolute magnitudes, offering an opportunity to investigate the behaviour of Li as a function of these parameters. An interesting result from this study is the presence of a large gap in the plane, which distinguishes "Li-dip"stars like those first identified in the Hyades cluster by Boesgaard & Tripicco ([CITE]) from other stars with a much higher Li abundance. The Li-dip stars concentrate on a certain mass, which decreases with metallicity from about 1.4 at solar metallicity to 1.1 at . Excluding the Li-dip stars and a small group of lower mass stars with K and , the remaining stars, when divided into four metallicity groups, may show a correlation between Li abundance and stellar mass. The dispersion around the -mass relation is about 0.2 dex below and 0.3 dex above this metallicity, which cannot be explained by observational errors or differences in metallicity. Furthermore, there is no correlation between the residuals of the -mass relations and stellar age, which ranges from 1.5 Gyr to about 15 Gyr. This suggests that Li depletion occurs early in stellar life and that parameters other than stellar mass and metallicity affect the degree of depletion, e.g. initial rotation velocity and/or the rate of angular momentum loss. It cannot be excluded, however, that a cosmic scatter of the Li abundance in the Galaxy at a given metallicity contributes to the dispersion in Li abundance. These problems make it difficult to determine the Galactic evolution of Li from the data, but a comparison of the upper envelope of the distribution of stars in the plane with recent Galactic evolutionary models by Romano et al. ([CITE]) suggests that novae are a major source for the Li production in the Galactic disk; their occurrence seems to be the explanation for the steep increase of Li abundance at .

Chemo-spectrophotometric evolution of spiral galaxies--IV. Star formation efficiency and effective ages of spirals

We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilize detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low-mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low-mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low-mass ones. We also show that these galaxies (having rotational velocities in the 80–400 km s−1 range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.

Carbon Isotope Abundances in Comets

Rotational lines of 13C14N have been identified in high-resolution (λ/Δλ ~ 60,000) echelle spectra of the CN B2Σ+-X2Σ+ (0-0) band in three comets. The 12C/13C abundance ratios determined using a full fluorescence excitation model for comets Levy (C/1990 K1), Austin (C/1989 X1), and Okazaki-Levy-Rudenko (C/1989 XIX) are 90 ± 10, 85 ± 20, and 93 ± 20, respectively, consistent with the solar system ratio, 90. A lower limit for the nitrogen isotope ratio, 12C14N/12C15N 200, found for comet Levy is consistent with previous comet measurements and the solar system value, 272. The mean CN carbon isotope ratio in the five comets measured to date is 12C14N/12C15N = 90 ± 10, and the mean for the three molecular species C2, HCN, and CN measured in nine comets is 101 ± 15. Consistency of the cometary carbon isotope ratios with the bulk solar system value indicates (1) chemical homogeneity in the outer protosolar nebula, (2) minimal isotopic fractionation in the protosolar precursor molecular cloud, and (3) that comets formed coevally with the solar system. The 14% difference between the solar system (90) and the present solar neighborhood interstellar 12C/13C ratio (77 ± 7) may be indicative of significant Galactic 13C enrichment over the past 4.6 Gyr. However, even though models can match to within a factor of 2 the solar system abundances, including the carbon isotope ratio, other evidence suggests that simple homogeneous Galactic evolution models may not be adequate to explain detailed stellar and interstellar abundances in the Galaxy.

Odd-Shaped Pieces Confuse Puzzle of Galaxy Evolution

ASTRONOMYUsing new detectors that operate at infrared and submillimeter wavelengths, along with the Hubble Space Telescope's two Deep Field views of the early universe, astronomers had hoped that views of the most distant visible galaxies would clarify how galaxies grow and change. But the distant galaxies' properties are so uncertain that cosmologists have trouble fitting them into models of galactic evolution.

Carbon Stars and Nucleosynthesis in Galaxies

The role of carbon stars in the build-up of chemical elements in galaxies is discussed on the basis of stellar evolution calculations and estimated stellar yields, abundance analyses of AGB stars, galactic-evolution models and abundance trends among solar-type disk stars. We conclude that the AGB stars in general, and carbon stars in particular, probably are main contributors of s-elements, that their contributions of flourine and carbon are quite significant, and that possibly their contributions of lithium, 13C and 22Ne are of some importance. Also contributions of N, Na and Al are discussed. The major uncertainties that characterize almost any statement concerning these issues are underlined.

Cosmic deuterium

The knowledge of the primordial deuterium to hydrogen ratio provides one of the most reliable tests of the early Universe nucleosynthesis models and a direct estimate of the cosmic baryon density. Evaluations have been traditionally made using D/H estimations in the interstellar medium, extrapolated backwards in time with the use of galactic evolution models. Direct primordial D/H measurements have been carried out only recently in the direction of quasars. These measurements of deuterium abundances along with observations made in the solar system and in the interstellar medium are presented. New results that indicate spatial variations of the deuterium abundance in the interstellar medium at the level of ∼ 50% over scales possibly as small as ∼ 10 pc, may question our global vision of deuterium evolution until the causes for the origin of these variations are understood. With a conservative point of view, observations thus suggest that the primordial D/H value should be within the range 1. × 10 −5−3. × 10 −4, leading to a relatively low baryon content Universe. Since the actual evolution of deuterium from primordial nucleosynthesis to now is not known in details, more observations, hopefully to be made with the Hubble Space Telescope, FUSE the Far Ultraviolet Spectroscopic Explorer (launched in 1999). or from the ground with the largest telescopes (Keck, VLT,), should reveal the evolution of that key element, and better constrain its primordial abundance.

Modeling the Radial Abundance Distribution of the Transition Galaxy NGC 1313

NGC 1313 is the most massive disk galaxy showing a flat radial abundance distribution in its interstellar gas, a behavior generally observed in Magellanic and irregular galaxies. We have attempted to reproduce this flat abundance distribution using a multiphase chemical evolution model, which has been previously used successfully to depict other spiral galaxies along the Hubble morphological sequence. We found that it is not possible to reproduce the flat radial abundance distribution in NGC 1313 and at the same time be consistent with observed radial distributions of other key parameters, such as the surface gas density and star formation profiles. We conclude that a more complicated galactic evolution model, including radial flows and possibly mass loss due to supernova explosions and winds, is necessary to explain the apparent chemical uniformity of the disk of NGC 1313.

Oxygen in Unevolved Metal-Poor Stars from Keck Ultraviolet HIRES Spectra

The determination of the abundance of oxygen (O) is important in our understanding of mass¨ spectrum of previous generations of stars, the evolution of the Galaxy, stellar evolution, and the age- metallicity relation. We have measured O in 24 unevolved stars with Keck HIRES observations of the OH lines in the ultraviolet spectral region at a spectral resolution of D45,000. The spectra have high signal-to-noise ratios, typically 60¨110, and high dispersion, 0.022 per pixel. Very special care has been Ae taken in determining the stellar parameters in a consistent way and we have done this for two diUerent, plausible temperature scales. The O abundance from OH has been computed by spectrum synthesis tech- niques for all 24 stars plus the Sun for which we have a Keck spectrum of the daytime sky. In addition, we determined O abundances from the O I triplet with our stellar parameters and the published equiva- lent widths of the three O I lines from six sources. The comparison of data analyzed with the same, consistently determined, parameter sets show generally excellent agreement in the O abundances; diUer- ences in the origin of the models (not the parameters) may result in abundance diUerences of 0.07 to 0.11 dex. We show that the O abundances from OH and from O I are reliable and independent and average the two for the adopted O. This averaging has the great bene—t of neutralizing uncertainties in the parameters since OH and O I strengths depend on eUective temperature and gravity in opposite direc- tions. For these cool, unevolved stars we —nd that O is enhanced relative to Fe with a completely linear relation between (O/H) and (Fe/H) over 3 orders of magnitude with very little scatter; taking the errors into account in determining the —ts, we —nd (O/H) \) 0.66 (^0.02) (Fe/H) ) 0.05 (^0.04). The O abundances from 76 disk stars of Edvardsson et al. have a measured slope of 0.66 (identical to our halo dwarf stars) and —t this relationship smoothly. The relation between (O/Fe) and (Fe/H) is robustly linear and shows no sign of a break at metallicities between (1.0 and (2.0, as has been discussed by others. At low metallicities, (Fe/H) (3.0, (O/Fe) ( )1.0. The —t to this relationship (taking the errors into account) is (O/Fe) \( 0.35 (^0.03) (Fe/H) ) 0.03 (^0.05). The enrichment of O is prob- ably still from massive stars and Type II supernovae; however, the absence of a break in (O/Fe) versus (Fe/H) runs counter to traditional galactic evolution models, and the interplay of Type II and Type Ia supernovae in the production of O and Fe should be reexamined. It appears that either Fe or O can be used as a chronometer in studies of galactic evolution.

Primordial nucleosynthesis.

With the advent of the new extragalactic deuterium observations, Big Bang nucleosynthesis (BBN) is on the verge of undergoing a transformation. In the past, the emphasis has been on demonstrating the concordance of the BBN model with the abundances of the light isotopes extrapolated back to their primordial values by using stellar and galactic evolution theories. As a direct measure of primordial deuterium is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. This robustness remains even through major model variations such as an assumed first-order quark-hadron phase transition. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is nonbaryonic. Comparison of baryonic density arguments from Lyman-alpha clouds, x-ray gas in clusters, and the microwave anisotropy are made.

Big Bang Nucleosynthesis and the Density of Baryons in the Universe

Now that extragalactic deuterium observations are being made, Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman-α clouds, x-ray gas in clusters, and the microwave anisotropy are made and shown to be consistent with the BBN value.

Supernovae as thermal and kinetic energy input to their environment

We present some results of numerical studies of supernova remnant (SNR) evolution in the context of supernova effects on galactic and globular cluster evolution. The characteristics of a SNR depend on the environment in which the explosion occurs. In particular, we show that parameters such as the density and the metallicity of the environment significantly influence the evolution of the remnant, and thus change its effects on the global environment (e.g., globular clusters, galaxies) as a source of thermal and kinetic energy. We conducted our studies using a one-dimensional, explicit Lagrangian hydrodynamics code with a tensor form of the artificial viscosity for treating shocks. We implemented ionization effects in the equation of state and a metallicity dependent cooling function. Global time-dependent quantities such as the total kinetic and thermal energies and the radial extent are calculated for a grid of models. Our results can easily be incorporated as input physics in the numerical modeling of galactic or globular cluster evolution. We present here selected results to illustrate their significance.