1,272 research outputs found
An inhomogeneous model for the Galactic halo: a possible explanation for the spread observed in s- and r-process elements
No full textAims. We propose an explanation for the considerable scatter
of the abundances of neutron capture elements observed in low-metallicity stars
in the solar vicinity, compared to the small star-to-star scatter
observed for the α-elements.
Methods. We have developed a stochastic
chemical evolution model in which the main assumption is a random formation
of new stars subject to the condition that the cumulative mass distribution
follows a given initial mass function.
Results. With our model, we are able to reproduce the different spreads of neutron
capture elements and α-elements in low-metallicity stars.
Conclusions. The reason for different observed spreads in neutron
capture elements and α-elements resides in the random birth of stars, coupled
with different stellar mass ranges, from which α-elements and neutron capture
elements originate. In particular, the site of production of α-elements is the
whole range of massive stars, from 10 to 80
whereas the mass range of production for neutron capture
elements lies between 12 and 30
The neutron-capture and alpha-elements abundance ratios scatter in old stellar populations. Cosmological simulations of the stellar halo
No full textWe investigate the origin of the abundance ratios and scatter of the
neutron-capture elements Sr, Ba and Eu in the stellar halo of a Milky Way-mass
galaxy formed in a hydrodynamical cosmological simulation, and compare them
with those of -elements. For this, we implement a novel treatment for
chemical enrichment of Type II supernovae which considers the effects of the
rotation of massive stars on the chemical yields and differential enrichment
according to the life-times of progenitor stars. We find that differential
enrichment has a significant impact on the early enrichment of the interstellar
medium which is translated into broader element ratio distributions,
particularly in the case of the oldest, most metal-poor stars. We find that the
[element/Fe] ratios of the elements O, Mg and Si have systematically
lower scatter compared to the neutron-capture elements ratios Sr, Ba and Eu at
[Fe/H], which is dex for the former and between
and dex for the latter. The different scatter levels found for the
neutron-capture and -elements is consistent with observations of old
stars in the Milky Way. Our model also predicts a high scatter for the [Sr/Ba]
ratio, which results from the treatment of the fast-rotating stars and the
dependence of the chemical yields on the metallicity, mass and rotational
velocities. Such chemical patterns appear naturally if the different ejection
times associated to stars of different mass are properly described, without the
need to invoke for additional mixing mechanisms or a distinct treatment of the
and neutron-capture elements.Comment: MNRAS accepte
The oldest stars of the bulge: new information on the ancient Galaxy
No full textRecently the search for the oldest stars have started to focus on the Bulge region. The Galactic bulge hosts extremely old stars, with ages compatible with the ages of the oldest halo stars. The data coming from these recent observations present new chemical signatures and therefore provide complementary constraints to those already found in the halo. So, the study of the oldest bulge stars can improve dramatically the constraints on the nature of first stars and how they polluted the pristine ISM of our Galaxy. We present our first results regarding the light elements (CNO) and the neutron capture elements. Our findings in the oldest bulge stars support the scenario where the first stellar generations have been fast rotators
Manganese evolution in Omega Centauri: a clue to the cluster formation mechanisms?
No full textWe model the evolution of manganese relative to iron in the progenitor system of the globular cluster Omega Centauri by means of a self-consistent chemical evolution model. We use stellar yields that already reproduce the measurements of [Mn/Fe] versus [Fe/H] in Galactic field disc and halo stars, in Galactic bulge stars and in the Sagittarius dwarf spheroidal galaxy. We compare our model predictions to the Mn abundances measured in a sample of 10 red giant members and six subgiant members of ω Cen. The low values of [Mn/Fe] observed in a few, metal-rich stars of the sample cannot be explained in the framework of our standard, homogeneous chemical evolution model. Introducing cooling flows that selectively bring to the cluster core only the ejecta from specific categories of stars does not help to heal the disagreement with the observations. The capture of field stars does not offer a viable explanation either. The observed spread in the data and the lowest [Mn/Fe] values could, in principle, be understood if the system experienced inhomogeneous chemical evolution. Such an eventuality is qualitatively discussed in this paper. However, more measurements of Mn in ω Cen stars are needed to settle the issue of Mn evolution in this cluster
Chemical evolution of neutron capture elements in our Galaxy and in the dwarf spheroidal galaxies of the Local Group
No full text2005/2006In questa tesi si è modellata l'evoluzione delle abbondanze di diversi elementi a cattura neutronica (Ba, Eu, La, Sr, Y e Zr) nella via Lattea ed inoltre si sono estese le nostre predizioni ad alcune galassie nane sferoidali del Gruppo Locale. Due meccanismi principali di cattura neutronica sono generalmente considerati: il processo lento (s-process) ed il processo rapido (r-process), ove lento e veloce è definito in base al tempo scala del decadimento β. I calcoli di nucleosintesi per l' r-process sono ancora pochi, a causa della difficoltà nel modellare la fisica del processo stesso e della scarsita di conoscenza sui siti di produzione di questi elementi. Per l' s-process invece alcuni calcoli sono disponibili ma i siti di produzioni sono anch'essi non del tutto ben determinati. Grazie all'adozione di un modello di evoluzione chimica per la via Lattea che già riproduce l'evoluzione di molti altri elementi (H, He, C, N, O, elementi α e elementi del picco del ferro), siamo in grado di comparare i nostri risultati con nuovi e accurati dati stellari di elementi a cattura neutronica e siamo in grado di porre forti vincoli sulla nucleosintesi degli elementi studiati. Possiamo quindi suggerire il sito stellare di produzione per ogni elemento. In particolare, l'eventuale componente derivatante dall'r-process é prodotta in un intervallo di massa da 10 a 30 Mʘ, mentre la componente derivante dall's-process viene prodotta dal stelle in un intervallo di massa da l a 3 Mʘ. Usando lo stesso modello di evoluzione chimica, esteso a differenti distanze dal centro della Galassia, abbiamo ottenuto risultati sui gradienti radiali nella via Lattea. Abbiamo confrontato i risultati del modello non solo per gli elementi a cattura neutronica, ma anche per gli elementi α e gli elementi del picco del ferro, con nuovi dati di stelle cefeidi. Per la prima volta con questi dati é possibile verificare le predizioni riguardanti i gradienti di elementi molto pesanti. Abbiamo concluso che il modello, con uno scenario inside-out per la costruzione del disco e una distribuzione costante di densita del gas durante la fase di alone, può essere considerato molto soddisfacente; in effetti, per quasi tutti gli elementi considerati con le nostre prescrizioni di nucleosintesi, il modello riproduce bene i gradienti di abbondanza osservati. Abbiamo dato una possibile spiegazione. alla notevole dispersione nelle abbondanze degli elementi a cattura neutronica osservati nelle stelle a bassa metallicita nelle vicinanze solari, paragonata alla piccola dispersione fra stella e stella per quel che riguarda gli elementi α. Abbiamo infatti sviluppato un modello di evoluzione chimica stocastico, nel quale l'assunzione principale una formazione casuale di nuove stelle, soggetta però alla condizione che la distribuzione totale di massa segua la funzione iniziale di massa. Col nostro modello siamo in grado di riprodurre le diverse caratteristiche degli elementi a cattura neutronica e degli elementi α. La ragione di questo, si basa nella nascita casuale di stelle accoppiata ai differenti intervalli di massa delle stelle che producono gli elementi a cattura neutronica e gli elementi α. In particolare, il sito di produzione degli elementi α e composto da tutte le stelle massicce, mentre l'intervallo di produzione dei elementi a cattura neutronica ha un limite superiore a 30 Mʘ . Abbiamo infine testata le prescrizioni a cattura neutronica anche per le galassie nane sferoidali del Gruppo Locale. Abbiamo usato un modello di evoluzione chimica che gia in grado di riprodurre le abbondanze per gli elementi α in questi sistemi. Abbiamo concluso che le stesse prescrizioni usate nella via Lattea riproducono le caratteritiche principali degli elementi a cattura neutronica anche nelle galassie nane sferoidali per cui abbiamo dati osservativi. Per quelle in cui non abbiamo dati ossevativi abbiamo dato soltanto delle predizioni. I risultati del nostro modello mostrano inoltre che l'evoluzione chimica di questi elementi nelle galassie nane sferoidali differente dall'evoluzione nelle vicinanze solari. Questo causato dalle loro differenti storie di formazione stellare rispetto a quella della nostra Galassia e indicano che le galassie nane sferoidali (almeno quelle che vediamo ai giorni nostri) non possono essere i costituenti elementari da cui si formata la nostra Galassia.We model the evolution of the abundances of several neutron capture elements (Ba, Eu, La, Sr, Y and Zr) in the Milky Way and then we extend our predictions to some dwarf spheroidal galaxies of the Local Group. Two major neutron capture mechanisms on iron seeds are generally invoked: the slow process (s-process) and the rapid process (r-process), where the slow and the rapid are defined relative to the timescale of the β-decay. Nucleosynthesis calculations for r-process are very few, owing to the difficulties in modelling the physics the r-process and the lack of knowledge about the sites of productions of these elements. For s-process elements instead some calculations are available but the sites of production are also uncertain. By adopting a chemical evolution model for the Milky Way already reproducing the evolution of several chemical elements (H, He, C, N, O, α-elements and iron peak elements), we compare our theoretical results with accurate and new stellar data of neutron capture elements and we are able to impose strong constraints on the nucleosynthesis of the studied elements. We can suggest the stellar sites .of production for each element. In particular, the r-process component of each element (if any) is produced in the mass range from 10 to 30 Mʘ, whereas the s-process component arises from stars in the range from l to 3 Mʘ. Using the same chemical evolution model, extended to different galactocentric distances, we obtain results on the radial gradients of the Milky Way. We compare the results of the model not only for the neutron capture elements but also for α-elements and iron peak elements with new data of Cepheids stars. For the first time with these data, it is possible to verify the predictions for the gradients of very heavy elements. We conclude that the model, with an inside-out scenario for the building up of the disc and a constant density distribution of the gas for the halo phase, can be considered successful; in fact, for almost all the considered elements with our nucleosynthesis prescriptions, the model well reproduces the observed abundance gradients. We give a possible explanation to the considerable scatter of neutron capture elements observed in low metallicity stars in the solar vicinity, compared to the small star tostar scatter observed for the α-elements. In fact, we have developed a stochastic chemical evolution model, in which the main assumption is a random formation of new stars, subject to the condition that the cumulative mass distribution follows a given initial mass function. With our model we are able to reproduce the different features of neutron capture elements and α-elements. The reason for this resides in the random birth of stars coupled with different stellar mass ranges from where α-elements and neutron capture elements originate. In particular, the site of production of α-elements is the whole range of the massive stars, whereas the mass range of production for neutron capture elements has an upper limit of 30 Mʘ . Finally, we test the prescriptions for neutron capture elements also for the dwarf spheroidal galaxies of the Local Group. We use a chemical evolution model already able to reproduce the abundances for α-elements in these systems. We conclude that the same prescriptions used for the Milky Way well reproduce the main features of neutron capture elements also in the dwarf spheroidal galaxies for which we have observational data. In dwarf spheroidal galaxies for which we do not have observational data we only give predictions. We predict that the chemical evolution of these elements in dwarf spheroidal galaxies is different from the evolution in the solar vicinity. This is due to their different histories of star formation relative to our Galaxy and indicates that dwarf spheroidal galaxies (we see nowadays) cannot be the building blocks of our Galaxy.XIX Ciclo1977Versione digitalizzata della tesi di dottorato cartacea
About twin primes and distribution of primes
No full textThis paper give us a demonstration of twin primes conjecture using approximation of function �(iupsilon) that we introduce in section 6. Section 1-5 give us introduction to terminology and a clarification on (iupsilon) terms. In particular section
5 is really important because of its Lemma. Section 7 reassume foregoing explanations and it give us two theorems and one corollary;the theorem 7.2 give us exact approximation of twin primes counting function
Silicon depletion in damped Ly α systems. The S/Zn method
No full textilicates are an important component of interstellar dust that has been poorly investigated in high redshift galaxies. As a preliminary step to studying silicates at high redshift, we survey silicon depletions in damped Ly α (DLA) systems. Silicon depletion is mild in the Galactic interstellar medium (ISM) and is expected to be weaker in most DLA systems, so we introduce a method for improving the accuracy of DLA depletion measurements. We compare abundance ratios measured in the gas with calculations of total abundance ratios of gas and dust predicted by models of galactic chemical evolution tailored for DLA systems. To tune the model parameters, we use the dust-free observational diagram S/Zn versus Zn/H, and we also compare the look back time estimated from the absorption redshift with the evolutionary time predicted by the model. By applying our method to a large set of DLA column densities, we succeeded in measuring the depletion of silicon in 74 systems. For comparison, we also measure iron and magnesium depletions (105 and 10 systems, respectively) with the same method. The mean depletion of silicon that we derive, ⟨ δSi ⟩ ≃ -0.27 ± 0.16 dex, is surprisingly close to that of iron, ⟨ δFe ⟩ ≃ -0.42 ± 0.28 dex, despite iron being much more depleted than silicon in the Galactic ISM. Silicon depletion in DLA systems does not correlate with metallicity, at variance with iron depletion, for which we confirm a rise with [Fe/H] found in previous work. Magnesium depletion seems to behave more in accordance with silicon than with iron. The different behaviors of the silicon and iron depletions suggests a complex history of dust production at the early stages of galactic chemical evolution
Established and Outsiders at the Same Time - Self-Images and We-Images of Palestinians in the West Bank and in Israel
No full textPalestinians frequently present a harmonizing and homogenizing we-image of their own national we-group, as a way of counteracting Israeli attempts to sow divisions among them, whether through Israeli politics or through the dominant public discourse in Israel. However, a closer look reveals the fragility of this homogenizing we-image which masks a variety of internal tensions and conflicts. By applying methods and concepts from biographical research and figurational sociology, the articles in this volume offer an analysis of the Middle East conflict that goes beyond the polar opposition between “Israelis” and “Palestinians”. On the basis of case studies from five urban regions in Palestine and Israel (Bethlehem, Ramallah, East Jerusalem, Haifa and Jaffa), the authors explore the importance of belonging, collective self-images and different forms of social differentiation within Palestinian communities. For each region this is bound up with an analysis of the relevant social and socio-political contexts, and family and life histories. The analysis of (locally) different figurations means focusing on the perspective of Palestinians as members of different religious, socio-economic, political or generational groupings and local group constellations – for instance between Christians and Muslims or between long-time residents and refugees. The following scholars have contributed to this volume: Ahmed Albaba, Johannes Becker, Hendrik Hinrichsen, Gabriele Rosenthal, Nicole Witte, Arne Worm and Rixta Wundrak. Gabriele Rosenthal is a sociologist and professor of Qualitative Methodology at the Center of Methods in Social Sciences, University of Göttingen. Her major research focus is the intergenerational impact of collective and familial history on biographical structures and actional patterns of individuals and family systems. Her current research deals with ethnicity, ethno-political conflicts and the social construction of borders. She is the author and editor of numerous books, including The Holocaust in Three Generations (2009), Interpretative Sozialforschung (2011) and, together with Artur Bogner, Ethnicity, Belonging and Biography (2009)
- …
