This review summarizes the properties of the stellar population in bulges observed in nearby and distant spiral galaxies. Particular emphasis is placed on comparison with elliptical galaxies, when possible. The sample-selection criteria and choices in data analysis are addressed when they may be involved in discrepancies among published results.

We argue that the discrepancies observed in H ii regions between abundances derived from optical recombination lines (ORLs) and collisionally excited lines (CELs) might well be the signature of a scenario of the enrichment of the interstellar medium (ISM) proposed by Tenorio-Tagle (1996). In this scenario, the fresh oxygen released during massive supernova explosions is confined within the hot superbubbles as long as supernovae continue to explode. Only after the last massive supernova explosion does the metal-rich gas start to cool down and fall onto the galaxy in the form of metal-rich droplets. Full mixing of these metal-rich droplets and the ISM occurs during photoionization by the next generations of massive stars. During this process, the metal-rich droplets give rise to strong recombination lines of the metals, leading to the observed ORL–CEL discrepancy.

Several spectroscopic studies have shown that stars with giant planets are particularly metal-rich compared with average field stars. In this paper we review the most recent results concerning the study of the chemical abundances of planet-host stars. Abundance distributions for several elements are presented or discussed, including those of iron-peak and alpha-elements, and the light elements lithium (both 7Li and 6Li) and beryllium. The impact of these results on the theories of planet formation and evolution is discussed.

From stellar-evolution models and from observations of Wolf–Rayet stars it is known that massive stars are releasing metal-enriched gas during their Wolf–Rayet phase by means of strong stellar winds. Although H ii-region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is not yet reliably known to what extent the diagnostic H ii gas is already contaminated by chemically processed stellar-wind matter. In a recent paper, we therefore analyzed our models of radiation-driven and wind-blown H ii bubbles around an isolated 85M⊙ star of originally Solar metallicity with respect to its chemical abundances. Although the hot stellar-wind bubble (SWB) is enriched with 14N during the WN phase and even more so with 12C and 16O during the WC phase of the star, we found that at the end of the stellar lifetime the mass ratios of the traced elements N and O in the warm ionized gas are insignificantly higher than Solar, whereas an enrichment of 22% above Solar is found for C. The transport of enriched elements from the hot SWB to the cool gas occurs mainly by means of mixing of hot gas with cooler at the back side of the SWB shell.

We present a spectrophotometric study of circumnuclear star-forming regions (CNSFRs) in the early-type spiral galaxies NGC 2903, NGC 3351 and NGC 3504, all of them of over Solar metallicity according to standard empirical calibrations. A detailed determination of their abundances is performed after careful subtraction of the very prominent underlying stellar absorption. It is found that most regions exhibit the highest abundances in H II-region-like objects. The relative N/O and S/O abundances are discussed. It is also shown that CNSFRs, as a class, segregate from the disk H II region family, clustering around smaller “softness parameter” – η′ – values, and therefore higher ionizing temperatures.

We present a detailed study on the kinematics of metal-rich stars with and without planets, and their relation with the Hyades, Sirius and Hercules dynamical streams in the Solar neighbourhood. We compare the kinematic behaviour of known planet-host stars with that of the remaining targets belonging to the CORALIE volume-limited sample, in particular its metal-rich population. The high average metallicity of the Hyades stream is confirmed. The planet-host targets exhibit a kinematic behaviour similar to that of the metal-rich comparison subsample, rather than to that of the comparison sample as a whole, thus supporting the hypothesis of a primordial origin for the metal excess observed in stars with known planetary companions. According to the scenarios proposed as an explanation for the dynamical streams, systems with giant planets could have formed more easily in metal-rich inner Galactic regions

We discuss theoretical predictions and observational findings obtained for radiatively driven winds of massive stars, with emphasis on their dependence on metallicity. If these winds are not strongly clumped or the clumping properties are independent of metallicity z, theory and observations agree very well, and mass-loss rates and terminal velocities scale as Ṁ ∝ z0.62±015 and υ∞ ∝ z0.13, respectively. This dependence could be validated only for winds with Solar and subsolar abundances, due to the lack of supersolar-metallicity test cases. The actual values for the mass-loss rates are uncertain, due to unknown clumping properties of the wind, and currently accepted numbers might be overestimated by factors in between ∼2 and 10.

Using the MIKE spectrograph, mounted on the 6.5-m Magellan/Clay telescope at the Las Campanas observatory in Chile, we have obtained highresolution spectra for 60 F and G dwarf stars, all likely members of a density enhancement in the local velocity distribution, referred to as the Hercules stream. By comparing with an existing sample of 102 thin- and thick-disk stars we have used space velocities, detailed elemental abundances, and stellar ages to trace the origin of the Hercules stream. We find that the Hercules-stream stars exhibit a wide spread in stellar ages, metallicities, and element abundances. However, the spreads are not random but separate the Hercules stream into the abundance and age trends outlined by either the thin disk or the thick disk. We hence claim that the major constituents of the Hercules stream actually are thin- and thick-disk stars. These diverse properties of the Hercules stream indicate a dynamical origin, probably caused by the Galactic bar. However, we can at the moment not entirely rule out the possibility that the Hercules stream could be the remnants of a relatively recent merger event.

We study the effect of assuming different formation timescales for the thick and thin disks on the variation of the abundance ratios of several elements with metallicity. We show that, if the thin disk was formed on a onger timescale (≃7 Gyr) than the thick disk (≃0.8 Gyr), the abundance ratio shifts between the thick and thin disk, as a function of the metallicity, can be well explained. Moreover, these observations offer a powerful constraint on stellar yields in general (massive stars, low- and intermediate-mass stars, and SN Ia) and their dependence on metallicity.

Results of several investigations of FGK stars in the Solar neighborhood have suggested that thin-disk stars with an iron abundance similar to that of the Sun appear to have higher abundances of other elements, such as silicon, titanium, and nickel. Offsets could arise if the samples contain stars with ages, mean Galactocentric distances, or kinematics that differ on average from the Solar values. They could also arise due to systematic errors in the abundance determinations, if the samples contain stars that are different from the Sun regarding their atmospheric parameters. We re-examine this issue by studying a sample of 80 nearby stars with Solarlike colors and luminosities. Among these Solar analogs, the objects with Solar iron abundances exhibit Solar abundances of carbon, silicon, calcium, titanium, and nickel.

Chemical abundances of metal-rich H II regions: why?

Ionized nebulae (H II regions) trace the sites of massive-star formation in spiral and irregular galaxies. The rapid evolution of these stars, ending in supernova explosions, and the subsequent recycling of nucleosynthesis products into the interstellar medium, make H II regions essential probes of the present-day chemical composition of star-forming galaxies across the Universe. The study of nebular abundances is therefore crucial for understanding the chemical evolution of galaxies. In the following pages I will provide an optical astronomer's perspective on some of the issues concerning the measurement of abundances in metal-rich H II regions, by focusing on the observational difficulties that are peculiar to the high-metallicity regime, discussing some of the most recent abundance determinations from H II regions in the metal-rich zones of spiral galaxies, and indicating some possibilities for further progress. Throughout this paper I will use the oxygen abundance as a proxy for the metallicity (oxygen makes up roughly half of the metal content of the interstellar medium), and assume the Solar value from Asplund et al. (2004), 12 + log(O/H)⊙ = 8.66. Elements besides oxygen will not be discussed in great detail.

Motivations

Why measure abundances of metal-rich H II regions? After all, as we will see in Section 2, metal-rich H II regions pose difficulties to the observer that are not present at lower metallicities, i.e. roughly below half the Solar O/H value. However, high abundances are encountered in a variety of astrophysical contexts, and the study of ionized nebulae often provides the only way to measure these abundances.

Although damped Lyman-alpha (DLA) systems are usually considered metal-poor, it has been suggested that this could be due to observational bias against metal-enriched absorbers. I review recent surveys to quantify the particular issue of dust obscuration bias and demonstrate that there is currently no compelling observational evidence to support the hypothesis of a widespread effect due to extinction. On the other hand, a small subset of DLAs may be metal-rich and I review some recent observations of these metal-rich absorbers and the detection of diffuse interstellar bands in one DLA at z ∼ 0.5.

I present a review of chemical-evolution models of the Solar neighborhood. I pay special attention to the ingredients necessary to reproduce the observed [Xi/Fe] ratios in nearby metal- and super-metal-rich stars, and to the chemical properties of the Solar vicinity, focusing on [Fe/H]≥–0.1. I suggest that the observed abundance trends are due to material synthesized and ejected by intermediate-mass stars with Solar metallicity in the AGB stage, and also by massive stars with (super)solar metallicity in the stellar wind and supernovae stages. The required tool to build chemicalevolution models that reach supersolar metallicities is the computation of stellar yields for stellar metallicities higher than the initial Solar value. With these models it might be possible to estimate the importance of merger events in the recent history of the Galactic disk as well as the relevance of radial stellar migration from the inner to the outer regions of the Galaxy. I also present a short review of the photospheric Solar abundances and their relation to the initial Solar abundances.

Even though metals constitute only a few per cent of the total mass fraction of stars, they have a huge impact on the way stars and galaxies evolve. In that respect, metallicity in the Universe is, like the salt in a dish, a small amount that can completely change its flavour!

The metal-rich stars have never attracted as much attention as the metal-poor halo stars, which tell us about the first supernovae and the early chemical evolution of our Galaxy. However, metal-rich stars are of interest in their own right and can shed new lights on very topical subjects. For instance, it is now well established that stars rich in metals are more likely to harbour giant planets. This awareness has elicited careful and detailed abundance studies of ever more metal-rich stars. As a byproduct, trends of the abundances of many elements at high metallicity are now available and await an interpretation in terms of stellar nucleosynthesis and chemical-evolution models. The extent to which these observed trends are in line with what is expected from the current stellar and chemical-evolution models largely remains to be checked and this is one of the main topics of these proceedings.

Putting the subject into a larger context, let us recall that the attainment of adequate models of the high-metallicity regime is of great interest for the study of the central regions of galaxies, which are thought to have higher-than-solar metallicity. Also, it appears that many quasar environments are metal-rich out to redshifts of at least 5.

We present long-slit spectra for 11 early-type galaxies observed with the Keck telescope. We measure rotation-velocity and velocity-dispersion profiles together with 20 Lick line-strength gradients. Gradients of indices are transformed into ages, metallicities and [α/Fe] using stellar-population models that take into account variations in chemical-abundance ratios. We find that the line-strength gradients are mainly due to radial variations of metallicity, although small gradients of [α/Fe] and age are also present. Contrary to what is expected in simple collapse models, galaxies in our sample have both positive and negative [α/Fe] profiles. This rules out a solely inside-out or outside-in formation mechanism for all early-type galaxies. Metallicity gradients correlate with the shape of the isophotes and the rotational velocity but do not correlate with the mass of the galaxies. Galaxies with younger populations in their centres have steeper metallicity gradients. Our results suggest a scenario whereby galaxies form through the merger of smaller structures and the degree of dissipation during those mergers increases when the masses of the progenitor galaxies decrease.