Annibale D'Ercole

Formation and dynamical evolution of multiple stellar generations in globular clusters

We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.

Production and circulation of iron in elliptical galaxies and clusters of galaxies

We introduce the concept of cluster iron mass-to-light ratio (IMLR) and estimate its value for both the intracluster medium and the stars, finding the amount of iron to be nearly the same in the two cluster components. We discuss under which conditions the past supernova activity can account for the observed IMLR and provide evidence that either the past average rate of Type Ia supernovae was at least a factor of ∼10 higher than the present rate in ellipticals, or massive stars in clusters formed with a very flat initial mass function (IMF)

Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta

A large number of spectroscopic studies have provided evidence of the presence of multiple populations in globular clusters by revealing patterns in the stellar chemical abundances. This paper is aimed at studying the origin of these abundance patterns. We explore a model in which second generation (SG) stars form out of a mix of pristine gas and ejecta of the first generation of asymptotic giant branch stars. We first study the constraints imposed by the spectroscopic data of SG stars in globular clusters on the chemical properties of the asymptotic and super asymptotic giant branch ejecta. With a simple one-zone chemical model, we then explore the formation of the SG population abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations and on the helium distribution function. We carry out a survey of models and explore the dependence of the final SG chemical properties on the key parameters affecting the gas dynamics and the SG formation process. Finally, we use our chemical evolution framework to build specific models for NGC 2808 and M4, two Galactic globular clusters which show different patterns in the Na–O and Mg–Al anticorrelation and have different helium distributions. We find that the amount of pristine gas involved in the formation of SG stars is a key parameter to fit the observed O–Na and Mg–Al patterns. The helium distribution function for these models is in general good agreement with the observed one. Our models, by shedding light on the role of different parameters and their interplay in determining the final SG chemical properties, illustrate the basic ingredients, constraints and problems encountered in this self-enrichment scenario which must be addressed by more sophisticated chemical and hydrodynamic simulations.

Three-dimensional simulations of the interstellar medium in dwarf galaxies - I. Ram pressure stripping

We present 3D hydrodynamic simulations of ram pressure stripping in dwarf galaxies. Analogous studies on this subject usually deal with much higher ram pressures, typical of galaxy clusters, or mild ram pressure due to the gas halo of the massive galactic companions. We extend over previous investigations by considering flattened, rotating dwarf galaxies subject to ram pressures typical of poor galaxy groups. We study the ram pressure effects as a function of several parameters such as galactic mass and velocity, ambient gas density, and angle between the galactic plane and the direction of motion. It turns out that this latter parameter plays a role only when the gas pressure in the galactic centre is comparable to the ram pressure. Despite the low values of the ram pressure, some dwarf galaxies can be completely stripped after 1-2 hundred of million years. This pose an interesting question on the aspect of the descents and, more in general, on the morphological evolution of dwarf galaxies. In cases in which the gas is not completely stripped, the propagation of possible galactic wind may be influenced by the disturbed distribution of the interstellar matter. We also consider the modification of the ISM surface density induced by the ram pessure and find that the resulting compression may trigger star formation over long time spans.

Dynamical evolution and spatial mixing of multiple population globular clusters

In this paper we study the long-term dynamical evolution of multiple-population clusters, focusing on the evolution of the spatial distributions of the first- (FG) and second-generation (SG) stars.In previous studies we have suggested that SG stars formed from the ejecta of FG AGB stars are expected initially to be concentrated in the cluster inner regions. Here, by means of N-body simulations, we explore the time scales and the dynamics of the spatial mixing of the FG and the SG populations and their dependence on the SG initial concentration.Our simulations show that, as the evolution proceeds, the radial profile of the SG/FG number ratio, NSG/NFG, is characterized by three regions: 1) a flat inner part; 2) a declining part in which FG stars are increasingly dominant; and 3) an outer region where the NSG/NFG profile flattens again (the NSG/NFG profile may rise slightly again in the outermost cluster regions). The radial variation of NSG/NFG implies that the fraction of SG stars determined by observations covering a limited range of radial distances is not, in general, equal to the SG global fraction, (NSG/NFG)glob. The distance at which NSG/NFG equals (NSG/NFG)glob is approximately between 1 and 2 cluster half-mass radii. The results of our simulations suggest that in many Galactic globular clusters the SG should still be more spatially concentrated than the FG.[abridged]

Star formation feedback and metal enrichment by Types Ia and II supernovae in dwarf spheroidal galaxies: the case of Draco

We present 3D hydrodynamic simulations aimed at studying the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies. This evolution is driven by the explosions of Type II supernovae (SNe II) and Type Ia supernovae (SNe Ia), whose different contribution is explicitly taken into account in our models. We compare our results with detailed observations of the Draco galaxy. We assume star formation histories consisting . of a number of instantaneous bursts separated by quiescent periods. Diverse histories differ by the number of bursts, but all have the same total duration and give rise to the same amount of stars. Because of the large effectiveness of the radiative losses and the extended dark matter halo, no galactic wind develops, despite the total energy released by the supernovae is much larger than the binding energy of the gas. This explains why the galaxy is able to form stars for a long period (>3 Gyr), consistently with observations. In this picture, the end of the star formation and gas removal must result from external mechanisms, such as ram pressure and/or tidal interaction with the Galaxy. The stellar [Fe/H] distributions found in our models match very well the observed ones. We find a mean value ([Fe/H]) = -1.65 with a spread of ∼1.5 dex. The chemical properties of the stars derive by the different temporal evolution between SNe la and SNe II rate, and by the different mixing of the metals produced by the two types of supernovae. We reproduce successfully the observed [O/Fe]-[Fe/H] diagram. However, our interpretation of this diagram differs from that generally adopted by previous chemical models. In fact, we find that the break observed in the diagram is not connected with the onset of a galactic wind or with a characteristic time-scale for the sudden switchover of the SNe Ia, as sometimes claimed. Instead, we find that the chemical properties of the stars derive, besides the different temporal evolution of the SNe II and SNe Ia rates, from the spatial inhomogeneous chemical enrichment due to the different dynamical behaviour between the remnants of the two types of supernovae.

Multiple starbursts in Blue Compact Galaxies

In this paper we present some results concerning the eects of two instantaneous starbursts, separated by a quiescent period, on the dynamical and chemical evolution of Blue Compact Dwarf galaxies. In particular, we compare the model results to the galaxy IZw18, which is a very metal-poor, gas-rich dwarf galaxy, possibly experiencing its rst or second burst of star formation. We follow the evolution of a rst weak burst of star formation followed by a second more intense one occurring after several hundred million years. We nd that a galactic wind develops only during the second burst and that metals produced in the burst are preferentially lost, relative to the hydrogen gas. We predict the evolution of several chemical abundances (H, He, C, N, O, -elements, Fe) in the gas inside and outside the galaxy, by taking into account, in detail, the chemical and energetical contributions from type II and Ia supernovae. We nd that the abundances predicted for the star forming region are in good agreement with the H ii region abundances derived for IZw18. We also predict the abundances of C, N and O expected for the H i gas to be compared with future FUSE abundance determinations. We conclude that IZw18 must have experienced two bursts of star formation, one occurred300 Myr ago and a recent one with an age between 4{7 Myr. However, by taking into account also other independent estimates, such as the color-magnitude diagram and the spectral energy distribution of stars in IZw18, and the fact that real starbursts are not instantaneous, we suggest that it is more likely that the burst age is between 4 and 15 Myr.

Abundance gradient slopes versus mass in spheroids: predictions by monolithic models

We investigate whether it is possible to explain the wide range of observed gradients in early-type galaxies in the framework of monolithic models. To do so, we extend the set of hydrodynamical simulations by Pipino et al. by including low-mass ellipticals and spiral (true) bulges. These models satisfy the mass-metallicity and the mass-[α/Fe] relations. The typical metallicity gradients predicted by our models have a slope of -0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. However, we also find a few quite massive galaxies in which this slope is -0.5 dex per decade, in agreement with some recent data. In particular, we find a mild dependence from the mass tracers when we transform the stellar abundance gradients into radial variations of the Mg 2 line-strength index, but not in the Mg b . We conclude that, rather than a mass-slope relation, is more appropriate to speak of an increase in the scatter of the gradient slope with the galactic mass. We can explain such a behaviour with different efficiencies of star formation in the framework of the revised monolithic formation scenario, hence the scatter in the observed gradients should not be used as an evidence of the need of mergers. Indeed, model galaxies that exhibit the steepest gradient slopes are preferentially those with the highest star formation efficiency at that given mass.

Formation of multiple populations in globular clusters: constraints on the dilution by pristine gas

The star-to-star differences in the abundance of light elements observed in the globular clusters (GCs) can be explained assuming that a second generation (SG) of stars form in the gas ejected by the asymptotic giant branch (AGB) stars belonging to a first stellar generation. However, while Na and O appear to be anticorrelated in the cluster stars, from the stellar models they turn out to be correlated in the AGB ejecta. In order to reconcile the stellar theory with the observational findings, all the GC models invoke an early dilution of AGB ejecta with pristine gas occurring during the SG formation. Despite a vast consensus about the occurrence of such a dilution, the physical process behind it is still unknown. In the present paper we set some general constraints on the pristine gas dynamics and on the possible amount of pristine gas involved in the SG formation, making use of a one-zone chemical model. We find that such a dilution is a necessary ingredient in the SG star formation to explain the observed abundance patterns. We confirm the conclusion of our previous works showing that clusters must have been initially much more massive. We also show that models assuming that clusters had an initial mass similar to their current one, and adopting a large fraction of pristine gas to form SG stars, fail to reproduce the observed Na–O anticorrelation and are not viable. We finally show that the dilution event should be restricted in time, rather than extended for the full duration of SG formation.

Formation of (α/Fe) radial gradients in the stars of elliptical galaxies

The scope of this paper is two-fold: i) to test and improve our previous models of an outside-in formation for the majority of ellipticals in the context of the SN-driven wind scenario, by means of a careful study of gas inflows/outflows; ii) to explain the observed slopes, either positive or negative, in the radial gradient of the mean stellar [alpha/Fe], and their apparent lack of any correlation with all the other observables. In order to pursue these goals we present a new class of hydrodynamical simulations for the formation of single elliptical galaxies in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe. We find that all the models which predict chemical properties (such as the central mass-weighted abundance ratios, the colours as well as the [ ] gradient) within the observed ranges for a typical elliptical, also exhibit a variety of gradients in the [ ] ratio, in agreement with the observations (namely positive, null or negative). All these models undergo an outside-in formation, in the sense that star formation stops earlier in the outermost than in the innermost regions, owing to the onset of a galactic wind. The typical [ ] gradients predicted by our models have a slope of -0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. We can safely conclude that the history of star formation is fundamental for the creation of abundance gradients in ellipticals but that radial flows with different velocity in conjunction with the duration and efficiency of star formation in different galactic regions are responsible for the gradients in the [ ] ratios.