Principali reazioni termonucleari nelle stelle

In questo elaborato vengono messe in evidenza le principali reazioni termonucleari che avvengono al centro delle stelle, e i meccanismi che ne governano il funzionamento. Nel primo capitolo vengono fornite le nozioni introduttive utili a comprendere i capitoli successivi; viene data una descrizione generale riguardo alla natura e alla struttura delle stelle e riguardo alla produzione di energia termonucleare, principale fonte di sostentamento della vita di ogni stella. Nel secondo capitolo vengono descritte ampiamente le principali catene di reazioni termonucleari che avvengono all'interno delle stelle, ponendo particolare attenzione alla combustione di idrogeno attraverso le catene protone-protone e il ciclo CNO, di cui si mette a disposizione un confronto, e alla successiva combustione di elio. Infine vengono brevemente descritte le reazioni termonucleari successive alla combustione dell'elio, comprendenti il bruciamento di carbonio e la produzione dei cosiddetti elementi alfa. Si conclude con la descrizione della produzione degli elementi chimici nei primi tre minuti della vita del nostro universo, anche detta nucleosintesi primordiale, in accordo con la teoria cosmologica del Big Bang

Exploring the cosmological synergy between galaxy cluster and cosmic void number counts

Galaxy clusters and cosmic voids are the most extreme objects of our Universe in terms of mass and size, tracing two opposite sides of the large-scale matter density field. By studying their abundance as a function of their mass and radius, respectively, i.e. the halo mass function (HMF) and void size function (VSF), it is possible to achieve fundamental constraints on the cosmological model. While the HMF has already been extensively exploited providing robust constraints on the main cosmological model parameters (e.g. $\Omega_{\rm m}$, $\sigma_8$ and $S_8$), the VSF is still emerging as a viable and effective cosmological probe. Given the expected complementarity of these statistics, in this work we aim at estimating the costraining power deriving from their combination. To achieve this goal, we exploit realistic mock samples of galaxy clusters and voids extracted from state-of-the-art large hydrodynamical simulations, in the redshift range $0.2 \leq z \leq 1$. We perform an accurate calibration of the free parameters of the HMF and VSF models, needed to take into account the differences between the types of mass tracers used in this work and those considered in previous literature analyses. Then, we obtain constraints on $\Omega_{\rm m}$ and $\sigma_8$ by performing a Bayesian Markov Chain Monte Carlo analysis. We find that cluster and void counts represent powerful independent and complementary probes to test the cosmological framework. In particular, we found that the constraining power of the HMF on $\Omega_{\rm m}$ and $\sigma_8$ improves drastically with the VSF contribution, increasing the $S_8$ constraint precision by a factor of about $60\%$.Comment: 12 pages, 7 figures, submitted to MNRA

The Northern Cross Fast Radio Burst project -- III. The FRB-magnetar connection in a sample of nearby galaxies

Fast radio bursts (FRBs) are millisecond radio transients observed at cosmological distances. The nature of their progenitors is still a matter of debate, although magnetars are invoked by most models. The proposed FRB-magnetar connection was strengthened by the discovery of an FRB-like event from the Galactic magnetar SGR J1935+2154. In this work, we aim to investigate how prevalent magnetars such as SGR J1935+2154 are within FRB progenitors. We carried out an FRB search in a sample of seven nearby (< 12 Mpc) galaxies with the Northern Cross radio telescope for a total of 692 h. We detected one 1.8 ms burst in the direction of M101 with a fluence of $58 \pm 5$ Jy ms. Its dispersion measure of 303 pc cm$^{-3}$ places it most-likely beyond M101. Considering that no significant detection comes indisputably from the selected galaxies, we place a 38 yr$^{-1}$ upper limit on the total burst rate (i.e. including the whole sample) at the 95\% confidence level. This upper limit constrains the event rate per magnetar $\lambda_{\rm mag} < 0.42$ magnetar$^{-1}$ yr$^{-1}$ or, if combined with literature observations of a similar sample of nearby galaxies, it yields a joint constraint of $\lambda_{\rm mag} < 0.25$ magnetar$^{-1}$ yr$^{-1}$. We also provide the first constraints on the expected rate of FRBs hypothetically originating from ultraluminous X-ray (ULX) sources, since some of the galaxies observed during our observational campaign host confirmed ULXs. We obtain $< 13$ yr$^{-1}$ per ULX for the total sample of galaxies observed. Our results indicate that bursts with energies $E>10^{34}$ erg from magnetars like SGR J1935+2154 appear more rarely compared to previous observations and further disfavour them as unique progenitors for the cosmological FRB population, leaving more space open to the contribution from a population of more exotic magnetars, not born via core-collapsed supernovae.Comment: 9 pages, 4 figures, published in A&

Optical design and performance simulations for the 1.49 keV beamline of the BEaTriX X-ray facility

The BEaTriX (Beam Expander Testing X-ray) facility, now operational at INAF-Brera Astronomical Observatory, will represent a cornerstone in the acceptance roadmap of Silicon Pore Optics (SPO) mirror modules, and will so contribute to the final angular resolution of the ATHENA X-ray telescope. By expansion and collimation of a microfocus X-ray source via a paraboloidal mirror, a monochromation stage, and an asymmetric crystal, BEaTriX enables the full-aperture illumination of an SPO mirror module with a parallel, monochromatic, and broad (140 mm × 60 mm) X-ray beam. The beam then propagates in a 12 m vacuum range to image the point spread function of the mirror module, directly on a focal plane camera. Currently the 4.51 keV beamline, based on silicon crystals, is operational in BEaTriX. A second beamline at 1.49 keV, which requires a separate paraboloidal mirror and organic crystals (ADP) for beam expansion, is being realized. As for monochromators, the current design is based on asymmetric quartz crystals. In this paper, we show the current optical design of the 1.49 keV beamline and the optical simulations carried out to predict the achievable performances in terms of beam collimation, intensity, and uniformity. In the next future, the simulation activity will allow us to determine manufacturing and alignment tolerances for the optical components

One ligand, two regulators and three binding sites: How KDPG controls primary carbon metabolism in Pseudomonas

Effective regulation of primary carbon metabolism is critically important for bacteria to successfully adapt to different environments. We have identified an uncharacterised transcriptional regulator; RccR, that controls this process in response to carbon source availability. Disruption of rccR in the plant-associated microbe Pseudomonas fluorescens inhibits growth in defined media, and compromises its ability to colonise the wheat rhizosphere. Structurally, RccR is almost identical to the Entner-Doudoroff (ED) pathway regulator HexR, and both proteins are controlled by the same ED-intermediate; 2-keto-3-deoxy-6-phosphogluconate (KDPG). Despite these similarities, HexR and RccR control entirely different aspects of primary metabolism, with RccR regulating pyruvate metabolism (aceEF), the glyoxylate shunt (aceA, glcB, pntAA) and gluconeogenesis (pckA, gap). RccR displays complex and unusual regulatory behaviour; switching repression between the pyruvate metabolism and glyoxylate shunt/gluconeogenesis loci depending on the available carbon source. This regulatory complexity is enabled by two distinct pseudo-palindromic binding sites, differing only in the length of their linker regions, with KDPG binding increasing affinity for the 28 bp aceA binding site but decreasing affinity for the 15 bp aceE site. Thus, RccR is able to simultaneously suppress and activate gene expression in response to carbon source availability. Together, the RccR and HexR regulators enable the rapid coordination of multiple aspects of primary carbon metabolism, in response to levels of a single key intermediate

X-ray tests of the ATHENA mirror modules in BEaTriX: from design to reality

The BEaTriX (Beam Expander Testing X-ray) facility is now operative at the INAF-Osservatorio Astronomico Brera (Merate, Italy). This facility has been specifically designed and built for the X-ray acceptance tests (PSF and Effective Area) of the ATHENA Silicon Pore Optics (SPO) Mirror Modules (MM). The unique setup creates a parallel, monochromatic, large X-ray beam, that fully illuminates the aperture of the MMs, generating an image at the ATHENA focal length of 12 m. This is made possible by a microfocus X-ray source followed by a chain of optical components (a paraboloidal mirror, 2 channel cut monochromators, and an asymmetric silicon crystal) able to expand the X-ray beam to a 6 cm × 17 cm size with a residual divergence of 1.5 arcsec (vertical) × 2.5 arcsec (horizontal). This paper reports the commissioning of the 4.5 keV beam line, and the first light obtained with a Mirror Module

Cosmology from probe combination: synergies between the number counts of galaxy clusters and cosmic voids

This Thesis work concerns the complementary study of the abundance of galaxy clusters and cosmic voids identified in cosmological simulations, at different redshifts. In particular, we focus our analyses on the combination of the cosmological constraints derived from these probes, which can be considered statistically independent, given the different aspects of Universe density field they map. Indeed, we aim at showing the orthogonality of the derived cosmological constraints and the resulting impressive power of the combination of these probes. To perform this combination we apply three newly implemented algorithms that allow us to combine independent probes. These algorithms represent a flexible and user-friendly tool to perform different techniques for probe combination and are implemented within the environment provided by the large set of free software C++/Python CosmoBolognaLib. All the new implemented codes provide simple and flexible tools that will be soon applied to the data coming from currently available and next-generation wide-field surveys to perform powerful combined cosmological analyses

Definition of the Binding Architecture to a Target Promoter of HP1043, the Essential Master Regulator of Helicobacter pylori

HP1043 is an essential orphan response regulator of Helicobacter pylori orchestrating multiple crucial cellular processes. Classified as a member of the OmpR/PhoB family of two-component systems, HP1043 exhibits a highly degenerate receiver domain and evolved to function independently of phosphorylation. Here, we investigated the HP1043 binding mode to a target sequence in the hp1227 promoter (Php1227). Scanning mutagenesis of HP1043 DNA-binding domain and consensus sequence led to the identification of residues relevant for the interaction of the protein with a target DNA. These determinants were used as restraints to guide a data-driven protein-DNA docking. Results suggested that, differently from most other response regulators of the same family, HP1043 binds in a head-to-head conformation to the Php1227 target promoter. HP1043 interacts with DNA largely through charged residues and contacts with both major and minor grooves of the DNA are required for a stable binding. Computational alanine scanning on molecular dynamics trajectory was performed to corroborate our findings. Additionally, in vitro transcription assays confirmed that HP1043 positively stimulates the activity of RNA polymerase

Parameter identification of degrading and pinched hysteretic systems using a modified Bouc–Wen model

The Bouc–Wen (BW) model is a successful differential equations model used to describe a wide range of nonlinear hysteretic systems. However, it is unable to describe force degradation, stiffness degradation and pinching effects. Therefore, Baber and Noori proposed a generalisation, developing the Bouc–Wen–Baber–Noori (BWBN) model. Nevertheless, it is composed of many parameters and complex pinching and degrading functions. Thus, it is necessary to develop a simpler and reliable model to be used for practical applications. In this paper, a modified BW model is proposed. It involves a more direct physical meaning of each parameter and allows achieving a substantial reduction of computational effort and numerical deficiencies. This is obtained through simpler pinching and degrading functions that entail a decrease of the number of parameters. The result is a straightforward model, capable of predicting the behaviour of degrading and pinched hysteretic systems. An application of the proposed scheme to a real case is also presented, in which reinforced concrete bridge piers that were physically tested in the laboratory are considered. The force–displacement data are used to perform the identification process of the model parameters via a Genetic Algorithm. The numerical results are accurate since they coincide with the experimental ones

Insight into the essential role of the Helicobacter pylori HP1043 orphan response regulator: Genome-wide identification and characterization of the DNA-binding sites

Many bacterial regulatory genes appear to be dispensable, as they can be deleted from the genome without loss of bacterial functionalities. In Helicobacter pylori, the hp1043 gene, also known as hsrA, is one of the transcriptional regulator that is essential for cell viability. This gene could not be deleted, nor the amount of protein modulated, supporting the hypothesis that HP1043 could be involved in the regulation of crucial cellular processes. Even though detailed structural data are available for the HP1043 protein, its targets are still ill-defined. Using Chromatin Immunoprecipitation-sequencing (ChIP-seq), one of the most powerful approaches to characterize protein-DNA interactions in vivo, we were able to identify genome-wide several new HP1043 binding sites. Moreover, in vitro DNA binding assays enabled precise mapping of the HP1043 binding sites on the new targets, revealing the presence of a conserved nucleotide sequence motif. Intriguingly, a significant fraction of the newly identified binding sites overlaps promoter regions controlling the expression of genes involved in translation. Accordingly, when protein translation was blocked, a significant induction of almost all HP1043 target genes was detected. These observations prompted us to propose HP1043 as a key regulator in H. pylori, likely involved in sensing and in coordinating the response to environmental conditions that provoke an arrest of protein synthesis. The essential role of HP1043 in coordinating central cellular processes is discussed