arXiv subjects

Annibale D'Ercole

Publications and source records attributed to Annibale D'Ercole.

At least 19 recordsLinked to original sources

arXiv2020

Hydrodynamic simulations of an isolated star-forming gas cloud in the Virgo cluster

We present a suite of three-dimensional, high-resolution hydrodynamic simulations that follow the evolution of a massive (10^7 M_sun) pressure confined, star-forming neutral gas cloud moving through a hot intra-cluster medium (ICM). The main goal of the analysis is to get theoretical insight into the lifetimes and evolution of stellar systems like the recently discovered star-forming cloud SECCO~1 in the Virgo cluster of galaxies, but it may be of general interest for the study of the star-forming gas clumps that are observed in the tails of ram pressure stripped galaxies. Building upon a previous, simple simulation, we explored the effect of different relative velocity of the cloud and larger temperature of the ICM, as well as the effect of the cloud self-gravity. Moreover, we performed a simulation including star-formation and stellar feedback, allowing for a first time a direct comparison with the observed properties of the stars in the system. The survivability of the cold gas in the simulated clouds is granted on timescales of the order of 1 Gyr, with final cold gas fractions generally $>0.75$. In all cases, the simulated systems end up, after 1 Gyr of evolution, as symmetric clouds in pressure equilibrium with the external hot gas. We also confirm that gravity played a negligible role at the largest scales on the evolution of the clouds. In our simulation with star formation, star formation begins immediately, it peaks at the earliest times and decreases monotonically with time. Inhomogeneous supernova explosions are the cause of an asymmetric shape of the gas cloud, facilitating the development of instabilities and the decrease of the cold gas fraction.

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arXiv2019

High-resolution three-dimensional simulations of gas removal from ultrafaint dwarf galaxies. I. Stellar feedback

The faintest Local Group galaxies found lurking in and around the Milky Way halo provide a unique test bed for theories of structure formation and evolution on small scales. Deep Subaru and Hubble Space Telescope photometry demonstrates that their stellar populations are old, and that the star formation activity did not last longer than 2 Gyr in these systems. A few mechanisms that may lead to such a rapid quenching have been investigated by means of hydrodynamic simulations, without providing any final assessment so far. This is the first in a series of papers aimed at analysing the roles of stellar feedback, ram pressure stripping, host-satellite tidal interactions and reionization in cleaning the lowest-mass Milky Way companions of their cold gas, by using high-resolution, three-dimensional hydrodynamic simulations. We simulate an isolated ultrafaint dwarf galaxy loosely modeled after Bootes I, and examine whether or not stellar feedback alone could drive a substantial fraction of the ambient gas out from the shallow potential well. In contrast to simple analytical estimates, but in agreement with previous hydrodynamical studies, we find that most of the cold gas reservoir is retained. Conversely, a significant fraction of the metal-enriched stellar ejecta crosses the boundaries of the computational box with velocities exceeding the local escape velocity and is, thus, likely lost from the system. Although the total energy output from multiple supernova explosions exceeds the binding energy of the gas, no galactic-scale outflow develops in our simulations and as such, most of the ambient medium remains trapped within the weak potential well of the model galaxy. It seems thus unavoidable that, in order to explain the dearth of gas in ultrafaint dwarf galaxies, we will have to resort to environmental effects. This will be the subject of a forthcoming paper.

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arXiv2018

Evolution of the Stellar Mass Function in Multiple-Population Globular Clusters

We present the results of a survey of N-body simulations aimed at studying the effects of the long-term dynamical evolution on the stellar mass function (MF) of multiple stellar populations in globular clusters. Our simulations show that if first-(1G) and second-generation (2G) stars have the same initial MF (IMF), the global MFs of the two populations are affected similarly by dynamical evolution and no significant differences between the 1G and the 2G MFs arise during the cluster's evolution. If the two populations have different IMFs, dynamical effects do not completely erase memory of the initial differences. Should observations find differences between the global 1G and 2G MF, these would reveal the fingerprints of differences in their IMFs. Irrespective of whether the 1G and 2G populations have the same global IMF or not, dynamical effects can produce differences between the local (measured at various distances from the cluster centre) 1G and 2G MFs; these differences are a manifestation of the process of mass segregation in populations with different initial structural properties. In dynamically old and spatially mixed clusters, however, differences between the local 1G and 2G MFs can reveal differences between the 1G and 2G global MFs. In general, for clusters with any dynamical age, large differences between the local 1G and 2G MFs are more likely to be associated with differences in the global MF. Our study also reveals a dependence of the spatial mixing rate on the stellar mass, another dynamical consequence of the multiscale nature of multiple-population clusters.

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arXiv2016

Evolution of Binary Stars in Multiple-Population Globular Clusters - II. Compact Binaries

We present the results of a survey of N-body simulations aimed at exploring the evolution of compact binaries in multiple-population globular clusters.We show that as a consequence of the initial differences in the structural properties of the first-generation (FG) and the second-generation (SG) populations and the effects of dynamical processes on binary stars, the SG binary fraction decreases more rapidly than that of the FG population. The difference between the FG and SG binary fraction is qualitatively similar to but quantitatively smaller than that found for wider binaries in our previous investigations.The evolution of the radial variation of the binary fraction is driven by the interplay between binary segregation, ionization and ejection. Ionization and ejection counteract in part the effects of mass segregation but for compact binaries the effects of segregation dominate and the inner binary fraction increases during the cluster evolution. We explore the variation of the difference between the FG and the SG binary fraction with the distance from the cluster centre and its dependence on the binary binding energy and cluster structural parameters. The difference between the binary fraction in the FG and the SG populations found in our simulations is consistent with the results of observational studies finding a smaller binary fraction in the SG population.

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arXiv2015

Rapidly rotating second-generation progenitors for the blue hook stars of ω Cen

Horizontal Branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in these clusters, that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the blue hook stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich "second generation" of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of ω Cen contains \sim 30% of horizontal branch stars 10 , or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to \sim 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose premain sequence accretion disc suffered an early disruption in the dense environment of the cluster's central regions where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.

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arXiv2015

Evolution of Binary Stars in Multiple-Population Globular Clusters

The discovery of multiple stellar populations in globular clusters has implications for all the aspects of the study of these stellar systems. In this paper, by means of N-body simulations, we study the evolution of binary stars in multiple-population clusters and explore the implications of the initial differences in the spatial distribution of different stellar populations for the evolution and survival of their binary stars. Our simulations show that initial differences between the spatial distribution of first-generation (FG) and second-generation (SG) stars can leave a fingerprint in the current properties of the binary population. SG binaries are disrupted more efficiently than those of the FG population resulting in a global SG binary fraction smaller than that of the FG. As for surviving binaries, dynamical evolution produces a difference between the SG and the FG binary binding energy distribution with the SG population characterized by a larger fraction of high binding energy (more bound) binaries. We have also studied the dependence of the binary properties on the distance from the cluster centre. Although the global binary fraction decreases more rapidly for the SG population, the local binary fraction measured in the cluster inner regions may still be dominated by SG binaries. The extent of the differences between the surviving FG and SG binary binding energy distribution also varies radially within the cluster and is larger in the cluster inner regions.

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arXiv2014

Pre-main sequence accretion and the formation of multiple populations in Globular Clusters

We investigate the viability of a model in which the chemical anomalies among Globular Cluster stars are due to accretion of gas onto the protostellar discs of low mass stars. This model has been suggested as a way to reduce the large initial cluster masses required by other models for the formation of multiple stellar generations. We numerically follow the evolution of the accreting stars, and we show that the structure of the seed star does not remain fully convective for the whole duration of the accretion phase. Stellar populations showing discrete abundances of helium in the core, that seem to be present in some clusters, might be formed with this mechanism only if accretion occurs before the core of the stars become radiative (within 2-3Myr) or if a thermohaline instability is triggered, to achieve full mixing after the accretion phase ends. We also show that the lithium abundances in accreted structures may vary by orders of magnitude in equal masses obtained by accreting different masses. In addition, the same thermohaline mixing which could provide a homogeneous helium distribution down to the stellar center, would destroy any lithium surviving in the envelope, so that both helium homogeneity and lithium survival require that the accretion phase be limited to the first couple of million years of the cluster evolution. Such a short accretion phase strongly reduces the amount of processed matter available, and reintroduces the requirement of an extremely large initial mass for the protocluster.

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arXiv2012

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]

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arXiv2012

Super AGB and AGB evolution and the chemical inventory in NGC 2419

We follow the scenario of formation of second generation stars in globular clusters by matter processed by hot bottom burning (HBB) in massive asymptotic giant branch (AGB) stars and Super AGB stars (SAGB). In the cluster NGC 2419 we assume the presence of an extreme population directly formed from the AGB and SAGB ejecta, so we can directly compare the yields for a metallicity Z=0.0003 with the chemical inventory of the cluster NGC 2419. At such a low metallicity, the HBB temperatures (well above 108K) allow a very advanced nucleosynthesis. Masses of about 6Mo deplete Mg and synthesize Si, going beyond Al, so this latter element results only moderately enhanced; sodium can not be enhanced. The models are consistent with the observations, although the predicted Mg depletion is not as strong as in the observed stars. We predict that the oxygen abundance must be depleted by a huge factor in the Mg poor stars. The HBB temperatures are close to the region where other p capture reactions on heavier nuclei become possible. We show that high potassium abundance found in Mg poor stars can be achieved during HBB, by p captures on the argon nuclei, if the relevant cross section(s) are larger than listed in the literature or if the HBB temperature is higher. Finally, we speculate that some calcium production is occurring owing to proton capture on potassium. We emphasize the importance of a strong effort to measure a larger sample of abundances in this cluster.

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arXiv2012

The role of super-asymptotic giant branch ejecta in the abundance patterns of multiple populations in globular clusters

In order to account for the chemical composition of a stellar second generation (SG), Globular Clusters (GCs) evolution models based on the asymptotic giant branch (AGB) scenario so far included only the yields available for the massive AGB stars, while the possible role of super-AGB ejecta was either extrapolated or not considered. In this work, we explore the role of super-AGB ejecta using yields recently calculated by Ventura and D'Antona. Models of clusters showing extended Na-O anticorrelations, like NGC 2808, indicate that a SG formation history similar to that outlined in our previous work is required: formation of an Extreme population with very large helium content from the pure ejecta of super-AGB stars, followed by formation of an Intermediate population by dilution of stellar ejecta with pristine gas. The very O-poor Na-rich Extreme stars can be accounted for once deep-mixing is assumed in SG giants forming in a gas with helium abundance Y> 0.34, which significantly reduces the atmospheric oxygen content, while preserving the sodium abundance. On the other hand, for clusters showing a mild O-Na anticorrelation, like M 4, the use of the new yields broadens the range of SG formation routes leading to abundance patterns consistent with observations. It is shown that models in which SG stars form only from super-AGB ejecta promptly diluted with pristine gas can reproduce the observations. We discuss the variety of small helium variations occurring in this model and its relevance for the horizontal branch morphology. In some of these models the duration of the SG formation episode can be as short as \sim10 Myr; the formation time of the SG is thus compatible with the survival of a cooling flow in the GC, previous to the explosion of the SG core collapse supernovae. We also explore models with formation of multiple populations in individual bursts, each lasting no longer than \sim10 Myr.

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arXiv2011

Binary Star Disruption in Globular Clusters with Multiple Stellar Populations

The discovery of multiple stellar populations in globular clusters raises fundamental questions concerning the formation and dynamical history of these systems. In a previous study aimed at exploring the formation of second-generation (SG) stars from the ejecta of first-generation (FG) AGB stars, and the subsequent dynamical evolution of the cluster, we showed that SG stars are expected to form in a dense subsystem concentrated in the inner regions of the FG cluster. In this paper we explore the implications of the structural properties of multiple-population clusters, and in particular the presence of the inner SG subsystem, for the disruption of binary stars. We quantify the enhancement of the binary disruption rate due to the presence of the central SG subsystem for a number of different initial conditions. Our calculations show that SG binaries, which are assumed to be more concentrated in the cluster inner regions, are disrupted at a substantially larger rate than FG binaries. Assuming a similar initial fraction of FG and SG binaries, our dynamical study indicates that the SG population is now expected to contain a significantly smaller binary fraction than the FG population.

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arXiv2011

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 into 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 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 all duration of the SG formation.

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arXiv2010

The fraction of globular cluster second-generation stars in the Galactic halo

Many observational studies have revealed the presence of multiple stellar generations in Galactic globular clusters. These studies suggest that second-generation stars make up a significant fraction of the current mass of globular clusters, with the second-generation mass fraction ranging from ~50 to 80 per cent in individual clusters. In this Letter we carry out hydrodynamical simulations to explore the dependence of the mass of second-generation stars on the initial mass and structural parameters and stellar initial mass function (IMF) of the parent cluster. We then use the results of these simulations to estimate the fraction, f_{SG,H}, of the mass of the Galactic stellar halo composed of second-generation stars that originated in globular clusters. We study the dependence of f_{SG,H} on the parameters of the initial mass function of the Galactic globular cluster system. For a broad range of initial conditions, we find that the fraction of mass of the Galactic stellar halo in second-generation stars is always small, f_{SG,H}<4-6 per cent for a Kroupa-1993 IMF and f_{SG,H}<7-9 per cent for a Kroupa-2001 IMF.

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arXiv2010

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.

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arXiv2010

Intermediate-Mass Black Holes in Early Globular Clusters

Spectroscopic and photometric observations show that many globular clusters host multiple stellar populations, challenging the common paradigm that globular clusters are "simple stellar populations" composed of stars of uniform age and chemical composition. The chemical abundances of second-generation (SG) stars constrain the sources of gas out of which these stars must have formed, indicating that the gas must contain matter processed through the high-temperature CNO cycle. First-generation massive Asymptotic Giant Branch (AGB) stars have been proposed as the source of this gas. In a previous study, by means of hydrodynamical and N-body simulations, we have shown that the AGB ejecta collect in a cooling flow in the cluster core, where the gas reaches high densities, ultimately forming a centrally concentrated subsystem of SG stars. In this Letter we show that the high gas density can also lead to significant accretion onto a pre-existing seed black hole. We show that gas accretion can increase the black hole mass by up to a factor of 100. The details of the gas dynamics are important in determining the actual black hole growth. Assuming a near-universal seed black hole mass and small cluster-to-cluster variations in the duration of the SG formation phase, the outcome of our scenario is one in which the present intermediate-mass black hole (IMBH) mass may have only a weak dependence on the current cluster properties. The scenario presented provides a natural mechanism for the formation of an IMBH at the cluster center during the SG star-formation phase.

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arXiv2009

Hydrodynamical simulations of Galactic fountains II: evolution of multiple fountains

Using 3D hydrodynamical simulations, we studied in detail the fountain flow and its dependence with several factors, such as the Galactic rotation, the distance to the Galactic center, and the presence of a hot gaseous halo. We have considered the observed size-frequency distribution of young stellar clusters within the Galaxy in order to appropriately fuel the multiple fountains in our simulations. The present work confirms the localized nature of the fountain flows: the freshly ejected metals tend to fall back close to the same Galactocentric region where they are delivered. Therefore, the fountains do not change significantly the radial profile of the disk chemical abundance. The multiple fountains simulations also allowed to consistently calculate the feedback of the star formation on the halo gas. Finally, we have also considered the possibility of mass infall from the intergalactic medium and its interaction with the clouds that are formed by the fountains. Though our simulations are not suitable to reproduce the slow rotational pattern that is typically observed in the halos around the disk galaxies, they indicate that the presence of an external gas infall may help to slow down the rotation of the gas in the clouds and thus the amount of angular momentum that they transfer to the coronal gas, as previously suggested in the literature.

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arXiv2008

Formation and Dynamical Evolution of Multiple Stellar Generations in Globular Clusters

Some globular clusters are observed to 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, assuming that the SG is formed by the gas ejected by AGB stars. This gas collects in a cooling flow into the cluster core, where it forms a SG star subsystem strongly concentrated in the cluster innermost regions with structural properties 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 study the two-population cluster dynamical evolution. 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 is largely unscathed by this early mass loss, and this early evolution leads to values of the SG to FG number ratio consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. Until mixing of the two populations is complete, the radial profile of the SG to FG number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions. (abridged)

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arXiv2008

About the Chemical Evolution of dSphs (and the peculiar Globular Cluster Omega Cen)

We present three dimensional hydrodynamical simulations aimed at studying the dynamical and chemical evolution of the interstellar medium (ISM) in isolated dwarf spheroidal galaxies (dSphs). This evolution is driven by the explosion of Type II and Type Ia supernovae, whose different contribution on both the dynamics and chemical enrichment is taken into account. Radiative losses are effective in radiating away the huge amount of energy released by SNe explosions, and the dSph is able to retain most of the gas allowing a long period (>2-3 Gyr) of star formation, as usually observed in this kind of galaxies. We are able to reproduce the stellar metallicity distribution function (MDF) as well as the peculiar chemical properties of strongly O-depleted stars observed in several dSphs. The model also naturally predicts two different stellar populations, with an anti-correlation between [Fe/H] and velocity dispersion, similarly to what observed in the Sculptor and Fornax dSphs. These results derive from the inhomogeneous pollution of the SNe Ia, a distinctive characteristic of our model. We also applied the model to the peculiar globular cluster (GC) Omega Cen in the hypothesis that it is the remnant of a formerly larger stellar system, possibly a dSph.

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