TEXTBOOK:
Pagel 1997 (1st edition) 2009 (2nd edition) Cambridge University Press
NUCLEOSYNTHESIS AND CHEMICAL EVOLUTION OF GALAXIES

REVIEWS:


Tinsley (1980) Fundamentals of Cosmic Physics, 5, 287
EVOLUTION OF THE STARS AND GAS IN GALAXIES

McWilliam 1997 ARAA, 35, 503
ABUNDANCE RATIOS AND GALACTIC CHEMICAL EVOLUTION
The metallicity of stars in the Galaxy ranges from [Fe/H]= -4 to +0.5 dex, and the solar iron abundance is ɛ(Fe) = 7.51 +/- 0.01 dex. The average values of [Fe/H] in the solar neighborhood, the halo, and Galactic bulge are -0.2, -1.6, and -0.2 dex respectively. Detailed abundance analysis reveals that the Galactic disk, halo, and bulge exhibit unique abundance patterns of O, Mg, Si, Ca, and Ti and neutron-capture elements. These signatures show that environment plays an important role in chemical evolution and that supernovae come in many flavors with a range of element yields. The 300-fold dispersion in heavy element abundances of the most metal-poor stars suggests incomplete mixing of ejecta from individual supernova, with vastly different yields, in clouds of ~10^6 Msun. The composition of Orion association stars indicates that star-forming regions are significantly self-enriched on time scales of 80 million years. The rapid self-enrichment and inhomogeneous chemical evolution models are required to match observed abundance trends and the dispersion in the age-metallicity relation.

Freeman & Bland-Hawthorn 2002 ARAA, 40, 487
THE NEW GALAXY: SIGNATURES OF ITS FORMATION
The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. Within the broad context of hierachical structure formation, we have only a crude picture of how galaxies like our own came into existence. A detailed physical picture where individual stellar populations can be associated with (tagged to) elements of the protocloud is far beyond our current understanding. Important clues have begun to emerge from both the Galaxy (near-field cosmology) and the high redshift universe (far-field cosmology). Here we focus on the fossil evidence provided by the Galaxy. Detailed studies of the Galaxy lie at the core of understanding the complex processes involved in baryon dissipation. This is a necessary first step toward achieving a successful theory of galaxy formation.

Tolstoy, Hill & Tosi 2009 ARAA, 47, 371
STAR-FORMATION HISTORIES, ABUNDANCES, AND KINEMATICS OF DWARF GALAXIES IN THE LOCAL GROUP
Within the Local Universe galaxies can be studied in great detail star by star, and here we review the results of quantitative studies in nearby dwarf galaxies. The color-magnitude diagram synthesis method is well established as the most accurate way to determine star-formation histories of galaxies back to the earliest times. This approach received a large boost from the exceptional data sets that wide-field CCD imagers on the ground and the Hubble Space Telescope could provide. Spectroscopic studies using large ground-based telescopes such as VLT, Magellan, Keck, and HET have allowed the determination of abundances and kinematics for significant samples of stars in nearby dwarf galaxies. These studies have shown how the properties of stellar populations can vary spatially and temporally. This leads to important constraints to theories of galaxy formation and evolution. The combination of spectroscopy and imaging and what they have taught us about dwarf galaxy formation and evolution is the aim of this review.

Nomoto, Kobayashi & Tominaga 2013 ARAA, 51, 457
NUCLEOSYNTHESIS IN STARS AND THE CHEMICAL ENRICHMENT OF GALAXIES
After the Big Bang, production of heavy elements in the early Universe takes place starting from the formation of the first stars, their evolution, and explosion. The first supernova explosions have strong dynamical, thermal, and chemical feedback on the formation of subsequent stars and evolution of galaxies. However, the nature of the Universe's first stars and supernova explosions has not been well clarified. The signature of the nucleosynthesis yields of the first stars can be seen in the elemental abundance patterns observed in extremely metal-poor stars. Interestingly, those patterns show some peculiarities relative to the solar abundance pattern, which should provide important clues to understanding the nature of early generations of stars. We thus review the recent results of the nucleosynthesis yields of mainly massive stars for a wide range of stellar masses, metallicities, and explosion energies. We also provide yields tables and examine how those yields are affected by some hydrodynamical effects during supernova explosions, namely, explosion energies from those of hypernovae to faint supernovae, mixing and fallback of processed materials, asphericity, etc. Those parameters in the supernova nucleosynthesis models are constrained from observational data of supernovae and metal-poor stars. Nucleosynthesis yields are then applied to the chemical evolution model of our Galaxy and other types of galaxies to discuss how the chemical enrichment process occurred during evolution.



CASE STUDIES:

Heger & Woosley 2002 ApJ, 567, 532
THE NUCLEOSYNTHETIC SIGNATURE OF POPULATION III
Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 Msolar). Could these stars have left a distinct nucleosynthetic signature? We explore the nucleosynthesis of helium cores in the mass range MHe=64-133 Msolar, corresponding to main-sequence star masses of approximately 140-260 Msolar. Above MHe=133 Msolar, without rotation and using current reaction rates, a black hole is formed, and no nucleosynthesis is ejected. For lighter helium core masses, ~40-63 Msolar, violent pulsations occur, induced by the pair instability and accompanied by supernova-like mass ejection, but the star eventually produces a large iron core in hydrostatic equilibrium. It is likely that this core, too, collapses to a black hole, thus cleanly separating the heavy-element nucleosynthesis of pair instability supernovae from those of other masses, both above and below. Indeed, black hole formation is a likely outcome for all Population III stars with main-sequence masses between about 25 and 140 Msolar (MHe=9-63 Msolar) as well as those above 260 Msolar. Integrating over a distribution of masses, we find that pair instability supernovae produce a roughly solar distribution of nuclei having even nuclear charge (Si, S, Ar, etc.) but are remarkably deficient in producing elements with odd nuclear charge-Na, Al, P, V, Mn, etc. This is because there is no stage of stable post-helium burning to set the neutron excess. Also, essentially no elements heavier than zinc are produced owing to a lack of s- and r-processes. The Fe/Si ratio is quite sensitive to whether the upper bound on the initial mass function is over 260 Msolar or somewhere between 140 and 260 Msolar. When the yields of pair instability supernovae are combined with reasonable estimates of the nucleosynthesis of Population III stars from 12 to 40 Msolar, this distinctive pattern of deficient production of odd-Z elements persists. Some possible strategies for testing our predictions are discussed.

Cayrel et al. 2004 A&A, 416, 1117
FIRST STARS V - ABUNDANCE PATTERNS FROM C TO ZN AND SUPERNOVA YIELDS IN THE EARLY GALAXY
Very high-quality spectra of very metal-poor dwarfs and giants were obtained with the ESO VLT and UVES spectrograph. These stars are likely to have descended from the first generation(s) of stars formed after the Big Bang, and their detailed composition provides constraints on issues such as the nature of the first supernovae, the efficiency of mixing processes in the early Galaxy, the formation and evolution of the halo of the Galaxy, and the possible sources of reionization of the Universe. This paper presents the abundance analysis of an homogeneous sample of 35 giants of extremely low metallicity: 30 of our 35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have [Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power of R˜45 000 and with signal-to-noise ratios of 100-200 per pixel over the wavelength range 330-1000 nm, are greatly superior to those of the classic studies of McWilliam et al. (\cite{MPS95}) and Ryan et al. (\cite{RNB96}).
Abundances of 17 elements from C to Zn have been measured in all stars, including K and Zn, which have not previously been detected in stars with [Fe/H] < -3.0. Among the key results, we discuss the oxygen abundance (from the forbidden [OI] line), the different and sometimes complex trends of the abundance ratios with metallicity, the very tight relationship between the abundances of certain elements (e.g., Fe and Cr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within the error bars, the trends of the abundance ratios with metallicity are consistent with those found in earlier literature, but in many cases the scatter around the average trends is much smaller than found in earlier studies, which were limited to lower-quality spectra. We find that the cosmic scatter in several element ratios may be as low as 0.05 dex.
The evolution of the abundance trends and scatter with declining metallicity provides strong constraints on the yields of the first supernovae and their mixing into the early ISM. The abundance ratios found in our sample do not match the predicted yields from pair-instability hypernovae, but are consistent with element production by supernovae with progenitor masses up to 100 M
. Moreover, the composition of the ejecta that have enriched the matter

Karlsson, Johnson & Bromm 2008 ApJ, 679, 6
UNCOVERING THE CHEMICAL SIGNATURE OF THE FIRST STARS IN THE UNIVERSE
The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allow us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to ‘‘overshoot,’’ reaching enrichment levels of [Ca/H] ~-2.5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be <0.07, or <40% by mass, assuming that metal-free stars have masses in excess of 10 Msun. We further predict, based on theoretical estimates for the relative number of PISNe, that the expected fraction of second-generation stars below [Ca / H]=-2 with a dominant (i.e., >90%) contribution from PISNe is merely ~10-4 to 5 x 10-4. The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of ~4 smaller. With the advent of next-generation telescopes and new, deeper surveys, we should be able to test these predictions.

Becker et al. 2012 ApJ, 744, 91
IRON AND α-ELEMENT PRODUCTION IN THE FIRST ONE BILLION YEARS AFTER THE BIG BANG
We present measurements of carbon, oxygen, silicon, and iron in quasar absorption systems existing when the universe was roughly one billion years old. We measure column densities in nine low-ionization systems at 4.7 < z < 6.3 using Keck, Magellan, and Very Large Telescope optical and near-infrared spectra with moderate to high resolution. The column density ratios among C II, O I, Si II, and Fe II are nearly identical to sub-damped Lyα systems (sub-DLAs) and metal-poor ([M/H] <= -1) DLAs at lower redshifts, with no significant evolution over 2 <~ z <~ 6. The estimated intrinsic scatter in the ratio of any two elements is also small, with a typical rms deviation of <~ 0.1 dex. These facts suggest that dust depletion and ionization effects are minimal in our z > 4.7 systems, as in the lower-redshift DLAs, and that the column density ratios are close to the intrinsic relative element abundances. The abundances in our z > 4.7 systems are therefore likely to represent the typical integrated yields from stellar populations within the first gigayear of cosmic history. Due to the time limit imposed by the age of the universe at these redshifts, our measurements thus place direct constraints on the metal production of massive stars, including iron yields of prompt supernovae. The lack of redshift evolution further suggests that the metal inventories of most metal-poor absorption systems at z >~ 2 are also dominated by massive stars, with minimal contributions from delayed Type Ia supernovae or winds from asymptotic giant branch stars. The relative abundances in our systems broadly agree with those in very metal-poor, non-carbon-enhanced Galactic halo stars. This is consistent with the picture in which present-day metal-poor stars were potentially formed as early as one billion years after the big bang.


Salvadori, Schneider & Ferrara 2007 MNRAS, 381, 647
COSMIC STELLAR RELICS IN THE GALACTIC HALO
We study the stellar population history and chemical evolution of the Milky Way (MW) in a hierarchical ΛCDM model for structure formation. Using a Monte Carlo method based on the semi-analytical extended Press & Schechter formalism, we develop a new code GALAXY MERGER TREE AND EVOLUTION (GAMETE) to reconstruct the merger tree of the Galaxy and follow the evolution of gas and stars along the hierarchical tree. Our approach allows us to compare the observational properties of the MW with model results, exploring different properties of primordial stars, such as their initial mass function and the critical metallicity for low-mass star formation, Zcr. In particular, by matching our predictions to the metallicity distribution function (MDF) of metal-poor stars in the Galactic halo we find that: (i) a strong supernova (SN) feedback is required to reproduce the observed properties of the MW; (ii) stars with [Fe/H] < -2.5 form in haloes accreting Galactic medium (GM) enriched by earlier SN explosions; (iii) the fiducial model (Zcr = 10-4Zsolar, mPopIII = 200 Msolar) provides an overall good fit to the MDF, but cannot account for the two hyper-metal-poor (HMP) stars with [Fe/H] < -5 the latter can be accommodated if Zcr <= 10-6 Zsolar but such model overpopulates the `metallicity desert', that is, the range -5.3 < [Fe/H] < -4 in which no stars have been detected; (iv) the current non-detection of metal-free stars robustly constrains either Zcr > 0 or the masses of the first stars mPopIII > 0.9 Msolar (v) the statistical impact of truly second-generation stars, that is, stars forming out of gas polluted only by metal-free stars, is negligible in current samples; and (vi) independent of Zcr, 60 per cent of metals in the GM are ejected through winds by haloes with masses M < 6 × 109 Msolar, thus showing that low-mass haloes are the dominant population contributing to cosmic metal enrichment. We discuss the limitations of our study and comparison with previous work.

Lecureur et al. 2007 A&A, 465, 799
OXYGEN, SODIUM, MAGNESIUM, AND ALUMINIUM AS TRACERS OF THE GALACTIC BULGE FORMATION
This paper investigates the peculiar behaviour of the light even (alpha-elements) and odd atomic number elements in red giants in the galactic bulge, both in terms of the chemical evolution of the bulge, and in terms of possible deep-mixing mechanisms in these evolved stars.
Abundances of the four light elements O, Na, Mg, and Al are measured in 13 core He-burning giant stars (red clump stars) and 40 red giant branch stars.
We show that the abundance patterns point towards a chemical enrichment dominated by massive stars at all metallicities. Oxygen, magnesium, and aluminium ratios with respect to iron are overabundant with respect to both galactic disks (thin and thick) for [Fe/H] > -0.5. A formation timescale for the galactic bulge shorter than for both the thin and thick disks is therefore inferred. To isolate the massive-star contribution to the abundances of O, Mg, Al, and Na, we use Mg as a proxy for metallicity (instead of Fe), and further show that: (i) the bulge stars [O/Mg] ratio follows and extends the decreasing trend of [O/Mg] found in the galactic disks to higher metallicities. This is a challenge for predictions of O and Mg yields in massive stars, which so far predicted no metallicity dependence in this ratio; (ii) the [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disk, the thick disk, and the bulge. The bulge trend is well represented by the predicted metallicity-dependent yields of massive stars, whereas the galactic disks have Na/Mg ratios that are too high at low metallicities, pointing to an additional source of Na from AGB stars; (iii) contrary to the case of the [Na/Mg] ratio, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disk stars, and the theoretical yields by massive stars agree with the observed ratios, leaving no space for AGB contribution to Al.

Caffau et al. 2012 A&A, 542, 51
A PRIMORDIAL STAR IN THE HEART OF THE LION
The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang.
We report the study of a primordial star situated in the centre of the constellation Leo (SDSS J102915+172927).
The star, selected from the low-resolution spectrum of the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters were derived from the photometry. The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections.
An iron abundance of [Fe/H ] = -4.89 makes SDSS J102915+172927 one of the lowest [Fe/H] stars known. However, the absence of measurable C and N enhancements indicates that it has the lowest metallicity, Z ≤ 7.40 × 10
-7 (metal-mass fraction), ever detected. No oxygen measurement was possible.
Conclusions: The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed. Even lower metallicity stars may yet be discovered, with a chemical composition closer to the composition of the primordial gas and of the first supernovae.


Cooke et al. 2011, MNRAS, 417, 1534
THE MOST METAL-POOR DAMPED LYΑ SYSTEMS: INSIGHTS INTO CHEMICAL EVOLUTION IN THE VERY METAL-POOR REGIME
We present a high spectral resolution survey of the most metal-poor damped Lyα absorption systems (DLAs) aimed at probing the nature and nucleosynthesis of the earliest generations of stars. Our survey comprises 22 systems with iron abundance less than 1/100 solar; observations of seven of these are reported here for the first time. Together with recent measures of the abundances of C and O in Galactic metal-poor stars, we reinvestigate the trend of C/O in the very metal-poor (VMP) regime and we compare, for the first time, the O/Fe ratios in the most metal-poor DLAs and in halo stars. We confirm the near-solar values of C/O in DLAs at the lowest metallicities probed, and find that their distribution is in agreement with that seen in Galactic halo stars. We find that the O/Fe ratio in VMP DLAs is essentially constant, and shows very little dispersion, with a mean []=+0.39 ± 0.12, in good agreement with the values measured in Galactic halo stars when the oxygen abundance is measured from the [O I] λ6300 line. We speculate that such good agreement in the observed abundance trends points to a universal origin for these metals. In view of this agreement, we construct the abundance pattern for a typical VMP DLA and compare it to model calculations of Population II and Population III nucleosynthesis to determine the origin of the metals in VMP DLAs. Our results suggest that the most metal-poor DLAs may have been enriched by a generation of metal-free stars; however, given that abundance measurements are currently available for only a few elements, we cannot yet rule out an additional contribution from Population II stars.


Venn et al. 2004 AJ, 128, 1177
STELLAR CHEMICAL SIGNATURES AND HIERARCHICAL GALAXY FORMATION
To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [α/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [α/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V<-420 km s-1), but this is not supported by other element ratios. Other element ratios compared in this paper include r- and s-process abundances, where we find a significant offset in the [Y/Fe] ratios, which results in a large overabundance in [Ba/Y] in most dSph stars compared with Galactic stars. Thus, the chemical signatures of most of the dSph stars are distinct from the stars in each of the kinematic components of the Galaxy. This result rules out continuous merging of low-mass galaxies similar to these dSph satellites during the formation of the Galaxy.

Travaglio et al. 1999 ApJ, 521, 691
GALACTIC CHEMICAL EVOLUTION OF HEAVY ELEMENTS: FROM BARIUM TO EUROPIUM
We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable for reproducing a large set of Galactic (local and nonlocal) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process com-ponents. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(a, n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reac- tion 22Ne(a, n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribu- tion is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, there is a sharp production of the Ba-peak elements around 1/4solar. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution, given by the Galactic chemical evolution model at the epoch of the formation of the solar system. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars), showing that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.



Kobayashi et al. 2006, ApJ, 653, 1145
GALACTIC CHEMICAL EVOLUTION: CARBON THROUGH ZINC
We calculate the evolution of heavy-element abundances from C to Zn in the solar neighborhood, adopting our new nucleosynthesis yields. Our yields are calculated for wide ranges of metallicity (Z=0-Zsolar) and the explosion energy (normal supernovae and hypernovae), based on the light-curve and spectra fitting of individual supernovae. The elemental abundance ratios are in good agreement with observations. Among the α-elements, O, Mg, Si, S, and Ca show a plateau at [Fe/H]<~-1, while Ti is underabundant overall. The observed abundance of Zn ([Zn/Fe]~0) can be explained only by the high-energy explosion models, as it requires a large contribution of hypernovae. The observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements (Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5<~[Fe/H]<~-1, and the observed trend at the lower metallicity can be explained by the energy effect. We also show the abundance ratios and the metallicity distribution functions of the Galactic bulge, halo, and thick disk. Our results suggest that the formation timescale of the thick disk is ~1-3 Gyr.



Schönrich
& Binney (2009) MNRAS, 396, 203
CHEMICAL EVOLUTION WITH RADIAL MIXING
Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and α-elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva-Copenhagen survey (GCS) of the solar neighbourhood and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age-metallicity plane, although this plane was not used in the fitting process. Although this model's star formation rate is monotonically declining, its disc naturally splits into an α-enhanced thick disc and a normal thin disc. In particular, the model's distribution of stars in the ([O/Fe], [Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration, and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high σz from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of σz with age than predicted by theory. The metallicity gradient in the interstellar medium is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cut-off in the star formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.


Norris et al. 2010, ApJ, 723, 1632
CHEMICAL ENRICHMENT IN THE FAINTEST GALAXIES: THE CARBON AND IRON ABUNDANCE SPREADS IN THE BOOTES I DWARF SPHEROIDAL GALAXY AND THE SEGUE 1 SYSTEM
We present an AAOmega spectroscopic study of red giants in the ultra-faint dwarf galaxy Boo ̈tes I (MV~-6) and the Segue 1 system (MV −1.5), either an extremely low luminosity dwarf galaxy or an unusually extended globular cluster. Both Boo ̈tes I and Segue 1 have significant abundance dispersions in iron and carbon. Boo ̈tes I has a mean abundance of [Fe/H] = −2.55 ± 0.11 with an [Fe/H] dispersion of σ = 0.37 ± 0.08, and abundance spreads of Δ[Fe/H] = 1.7 and Δ[C/H] = 1.5. Segue 1 has a mean of [Fe/H] = −2.7 ± 0.4 with [Fe/H] dispersion of σ = 0.7±0.3, and abundances spreads of Δ[Fe/H] = 1.6 and Δ[C/H] = 1.2. Moreover, Segue 1 has a radial-velocity member at four half-light radii that is extremely metal-poor and carbon-rich, with [Fe/H] = −3.5, and [C/Fe] = +2.3. Modulo an unlikely non-member contamination, the [Fe/H] abundance dispersion confirms Segue 1 as the least-luminous ultra-faint dwarf galaxy known. For [Fe/H] < −3.0, stars in the Milky Way’s dwarf galaxy satellites exhibit a dependence of [C/Fe] on [Fe/H] similar to that in Galactic field halo stars. Thus, chemical evolution proceeded similarly in the formation sites of the Galaxy’s extremely metal-poor halo stars and in the ultra-faint dwarf galaxies. We confirm the correlation between (decreasing) luminosity and both (decreasing) mean metallicity and (increasing) abundance dispersion in the Milky Way dwarf galaxies at least as faint as MV = −5. The very low mean iron abundances and the high carbon and iron abundance dispersions in Segue 1 and Boo ̈tes I are consistent with highly inhomogeneous chemical evolution starting in near zero-abundance gas. These ultra-faint dwarf galaxies are apparently surviving examples of the very first bound systems.



OLDER WORKS:

Hoyle (1954) ApJS, 1, 121
ON NUCLEAR REACTIONS OCCURING IN VERY HOT STARS.I. THE SYNTHESIS OF ELEMENTS FROM CARBON TO NICKEL.

Burbidge, Burbidge, Fowler & Hoyle (1957) Reviews of Modern Physics, 29, 548
SYNTHESIS OF THE ELEMENTS IN STARS

Cameron (1957) PASP, 69, 201

NUCLEAR REACTIONS IN STARS AND NUCLEOGENESIS

Schmidt (1959) ApJ, 129, 243
THE RATE OF STAR FORMATION

Schmidt (1963) ApJ, 137, 758
THE RATE OF STAR FORMATION II: THE RATE OF FORMATION OF STARS OF DIFFERENT MASS.

Larson (1976) MNRAS, 176, 31
MODELS FOR THE FORMATION OF DISC GALAXIES
Previously calculated models of the collapse of protogalaxies with rotational and axial symmetry are extended to galaxies characterized by both a spheroidal component and a disk component containing a substantial fraction of the total mass. The predicted disk-to-bulge ratio is found to depend mainly on the assumed rate of star formation and its time variation. It is shown that formation of the spheroidal component requires an early stage of rapid star formation, while formation of the disk requires a later stage of much slower star formation to allow the residual gas to settle into the disk before forming stars. The calculations suggest that the slow phase of star formation may involve relatively diffuse gas, the rapid phase may involve strongly clumped gas, and the degree of clumping may depend on the intensity of the turbulence or collisions within the gas. The models also predict stellar metallicity distributions and kinematic properties that are in qualitative agreement with those of the stellar populations in the Milky Way and other spirals, as well as a long time scale for the formation of the outer regions of the disk.

Pagel & Patchett (1975) MNRAS, 172, 13
METAL ABUNDANCES IN NEARBY STARS AND THE CHEMICAL HISTORY OF THE SOLAR NEIGHBORHOOD
A simplified picture of chemical evolution in the solar neighborhood based on the instantaneous recycling approximation and incorporating the effects of recent theoretical assumptions is developed to indicate how these assumptions are related to observational constraints imposed by the dependence of stellar chemical composition on formation time as well as by the statistical distribution of metal abundances in long-lived stars such as G dwarfs. Observational data relating to these questions are reviewed, and the cumulative distribution of metal abundances in G dwarfs is derived from photometric data on nearby stars. The observed distribution is found to be approximately lognormal. Modifications of the simple model are described in detail, including prompt initial enrichment, the early version of metal-enhanced star formation, a more sophisticated version of this model, and inhomogeneous collapse and infall. It is shown that one of the two-component disk-halo models of Ostriker and Thuan (1975) is a particularly successful model.

Larson & Tinsley (1978) ApJ, 221, 554
CHEMICAL EVOLUTION AND THE FORMATION OF GALACTIC DISKS
The chemical evolution of two collapse models for the formation of disk galaxies is calculated in detail, and the results are compared with properties of our own and other spiral galaxies. The models show at least qualitative agreement with empirical stellar and interstellar abundance gradients and with color gradients in spiral galaxies. The outer parts of the model disks are also in general agreement with the metallicity and age distributions of stars in the solar neighborhood, and with correlations between metallicity and kinematics for nearby stars. It is concluded that the process of disk formation by gradual accumulation of gas into a plane may account in a natural way for many properties of disk galaxies. In general, gas flows within or from outside galaxies are expected to have very important effects on chemical evolution.

Tinsley (1979) ApJ, 229, 1046
STELLAR LIFETIMES AND ABUNDANCE RATIOS IN CHEMICAL EVOLUTION
The frequency of Type I supernovae (SN I) in galaxies with unusual star-formation activity, as well as the large envelopes of SN I, is regarded as almost conclusive evidence that the precursors of SN I belong to a young stellar population. It is assumed that white dwarfs, SN I, and SN II represent the deaths of stars in the mutually exclusive mass ranges below 4 solar masses, from 4 to 6.5 solar masses, and above 6.5 solar masses, respectively. Consequences of tentative assumptions based partly on empirical evidence for iron production by SN I and carbon production by low-mass stars are tested against published stellar abundances. From a variety of models for chemical abundances, it is found that the relative abundances of primary elements are essentially functions of time, determined by progenitor stellar lifetimes, although the absolute abundance levels depend mainly on the model parameters.

Gilmore & Wyse (1991) ApJL, 367, L55
CHEMICAL EVOLUTION WITH BURSTS OF STAR FORMATION - ELEMENT RATIOS IN DWARF GALAXIES
It is demonstrated that galaxies in which star formation proceeds in a small number of bursts evolve their chemical elements in ratios which are very different from those in galaxies with continuing star formation. Systematic changes of element ratios with overall chemical abundance are determined to a large extent by the onset and duration of the star formation bursts and can be very different from those seen in the solar neighborhood. In particular, it is shown that an underabundance of oxygen relative to iron, such as is observed in the Large Magellanic Cloud, occurs naturally when star formation proceeds in a small number of widely separated bursts, as is inferred from the age distribution of LMC field stars and clusters. There is no need to invoke either variations in the stellar initial mass function or metal-enhanced winds.