Dynamical and chemical properties of magnetised star-forming regions

Zweifellos spielen die Sterne die führende Rolle am Nachthimmel, und ihre Beobachtungen gehen auf den Ursprung der Menschheit zurück, wenn auch anfangs aus einer Perspektive des Staunens und der Mythologie. Doch erst im XX. Jahrhundert begannen die Wissenschaftler, das frühe Leben der Sterne systematisch zu erforschen. Die Sternentstehung ist zu einem zentralen Thema der modernen Astrophysik geworden, und wo, wann und wie Sterne geboren werden, sind Fragen, die noch gründlich beantwortet werden müssen. Noch in jüngster Zeit haben sich die Astrochemie und die Astrobiologie auf diesem Gebiet immer mehr durchgesetzt. Tatsächlich ist die Sternentstehung auch eine Geschichte von zunehmender chemischer Komplexität, die um mindestens einen eigentümlichen Stern herum zur Entstehung von Leben führte. Diese Doktorarbeit konzentriert sich auf mehrere Aspekte der Entstehung massearmer Sterne, hauptsächlich aus astrochemischer Sicht. Prästellare Kerne —kalte und dichte Fragmente von Molekülwolken am Rande des Gravitationskollapses— stellen den Geburtsort sonnenähnlicher Sterne dar. In den ersten beiden Teilen dieser Arbeit konzentriere ich mich auf diese Art von Objekten, indem sie die Isotopenzusammensetzung der reichlich vorhandenen Moleküle im kalten interstellaren Medium untersuche. Tatsächlich werden Fraktionierungsprozesse, die den molekularen Isotopengehalt verändern, als ein guter diagnostischer Tracer für die verschiedenen Sternentstehungsphasen angesehen. Insbesondere analysiere ich zunächst das Stickstoff-Isotopenverhältnis im Diazenylium (N2H+) in einer kleinen Stichprobe von prästellaren Kernen. In den letzten Jahrzehnten hat die Fraktionierung von Stickstoff in der Astrochemie großes Interesse erlangt, da sie es uns ermöglichen könnte, die verschiedenen Materialien, die das eigentliche Planetensystem bilden, mit der ursprünglichen interstellaren Materie zu verbinden. Sein Isotopenverhältnis kann uns daher helfen, grundlegende Fragen zu beantworten, wie zum Beispiel, wie und in welchem Ausmaß unser Planet seine Zusammensetzung vom ursprünglichen Sonnennebel geerbt hat. Um dieses Ziel zu erreichen, ist jedoch ein umfassendes Verständnis der Stickstoffchemie erforderlich, und weitere Beobachtungsdaten, wie sie in dieser Arbeit vorgestellt werden, können weitere Einschränkungen für die chemischen Modelle liefern. Der zweite Teil handelt vom prästellaren Kern L1544. Insbesondere konzentriere ich mich auf den Deuterierungsgrad —d.h. die Fraktionierung von Wasserstoff— dieses Objekts und analysiere mehrere Rotationsübergänge von zwei üppig vorhandenen Ionen: Diazenylium und protoniertes Kohlenmonoxid (HCO+). Diese aktuellen, qualitativ hochwertigen Daten erlauben es, zuverlässige Informationen über die räumliche Verteilung des Deuteriumanteils abzuleiten. Die Kombination eines fortschrittlichen Modells des Strahlungstransportes mit einem hochmodernen chemischen Netzwerk wird genutzt um auch die Chemie dieser Moleküle in L1544 zu untersuchen. Auf die prästellare Phase folgt die protostellare, in dem sich als Folge des Gravitationskollapses ein zentrales Objekt —ein Protostern— bildet. IRAS 15398-3359, das zu dieser Kategorie von Objekten gehört, wird im letzten Teil dieser Arbeit untersucht. Ich konzentriere mich auf die Untersuchung von Magnetfeldern, von denen bekannt ist, dass sie bei der Sternentstehung eine wichtige Rolle spielen. Tatsächlich können Magnetfelder die Gasströme regulieren, und bieten eine zusätzliche Druckquelle, die die Gravitationskraft ausgleicht. Ich benutze polarimetrische Beobachtungen der thermischen Staubemission aus dem protostellaren Kern, um die magnetische Stärke und Morphologie abzuleiten. Insbesondere bei letzterem stelle ich fest, dass die Feldlinien eine charakteristische Sanduhrform aufweisen, die auf einen magnetisch bedingten Kollaps hinweist. Zukünftige Beobachtungen mit höherer Empfindlichkeit und besserer Winkelauflösung, möglicherweise mit den neuesten interferometrischen Einrichtungen, werden uns helfen, all diese miteinander verbundenen Aspekte der Sternentstehungstheorie noch mehr zu erhellen und Schritt für Schritt zu einem vollständigen Verständnis der Theorie zu gelangen.Without a doubt, stars play the leading role in the night sky, and their observations trace back to the origin of humankind, even though at the beginning from a perspective of wonder and mythology. However, it was only in the XX century that scientists began systematically to study the early life of stars. Stellar formation has become a central subject in modern astrophysics, and where, when, and how stars are born are questions that still need thorough answers. Even more recently, astrochemistry and astrobiology became more and more prominent in this field. In fact, star formation is also a story of increasing chemical complexity, which around at least one peculiar star culminated in the emergence of life. This thesis focuses on several aspects of low-mass star formation, mainly from an astrochemical point of view. Prestellar cores —cold and dense fragments of molecular clouds on the verge of gravitational collapse— represent the birth place of Sun-like stars. In the first two parts of this work I concentrate on this kind of objects, studying the isotopic composition of abundant molecules in the cold interstellar medium. In fact, fractionation processes, which alter the molecular isotopic content, are considered a good diagnostic tracer of the different star-forming phases. In particular, I first analyse the nitrogen isotopic ratio in diazenylium (N2H+) in a small sample of prestellar cores. In the last few decades, nitrogen fractionation has become of key interest in astrochemistry, since it could allow us to link the various materials that constitute the actual planetary system to the pristine interstellar matter. Its isotopic ratio can hence help us answering fundamental questions, such as how and to which extent our planet inherited its composition from the primordial Solar Nebula. To achieve this goal, however, a comprehensive understanding of nitrogen chemistry is needed, and more observational data, such as the ones presented in this work, can provide further constraints for the chemical models. The second part aims its attention to the prestellar core L1544. In particular, I focus on the deuteration level —i.e. hydrogen fractionation— of this object, analysing several rotational transitions of two abundant ions: diazenylium and protonated carbon monoxide (HCO+). These recent, high-quality data allow to derive reliable information on the spatial distribution of the deuterium fraction. The combination of an advanced radiative transfer model with a state-of-the-art chemical network is used to investigate also the chemistry of these molecules in L1544. The protostellar phase follows the prestellar one when, as a consequence of the gravitational collapse, a central object —a protostar— is formed. IRAS 15398-3359, which belongs to this category of objects, is studied in the last part of this thesis. I focus on the study of magnetic fields, which are known to play an important role in star formation. In fact, they can regulate gas flows, and they provide an extra source of pressure balancing the gravitational pull. I use polarimetric observations of the dust thermal emission arising from the protostellar core to derive the magnetic strength and morphology. Concerning the latter, in particular, I find that the field lines present a characteristic hourglass shape, which is indicative of a magnetically-driven collapse. Future observations with higher sensitivity and better angular resolution, possibly with the most recent interferometric facilities, will help us enlighten even more all these interconnected aspects of the star formation theory, moving step by step towards a complete understanding of it

Interstellar ammonia emission: unveiling the dynamics of a star-forming core

Questo lavoro di Tesi si occupa dello studio della regione di recente formazione stellare nota come Barnard 59. L’analisi si basa principalmente sullo studio dell'emissione molecolare di ammoniaca, che consente di determinare numerose proprietà della sorgente, quali temperatura, densità molecolare e struttura cinematica. L'approccio matematico consiste in un fit non lineare degli spettri a disposizione. Accanto a questi dati sono stati utilizzati i dati del telescopio Herschel per caratterizzare l'emissione della polvere presente nel gas, al fine di determinare le mappe di temperatura della sorgente e la densità di colonna di idrogeno molecolare. I risultati ottenuti mostrano che Barnard 59 presenta un nucleo denso e freddo, cinematicamente coerente e contraddistinto da valori nella dispersione di velocità inferiori alla velocità sonica del mezzo interstellare. In prossimità di alcuni YSOs, però, ed in particolare del più giovane tra di essi (denominato B11), le temperature sono più elevate e la dispersione di velocità aumenta. L'analisi delle mappe di queste due variabili ha consentito di investigare gli effetti di feedback protostellare. I risultati ottenuti indicano che la parte più densa e fredda di Barnard 59 è caratterizzata da bassi livelli di turbolenza e da moti esclusivamente subsonici, in accordo con l'opinione generale secondo cui i dense cores si formano per dissipazione della turbolenza supersonica che contraddistingue invece le nubi molecolari. Attorno a B11, invece, l'entità della turbolenza aumenta, come probabile conseguenza degli outflows di cui tale oggetto è sorgente. Barnard 59 appare interessato, in conclusione, da attività di formazione stellare che iniettando turbolenza ed energia disturba il gas nel mezzo interstellare. D'altra parte, lo studio dello stato dinamico della sorgente rivela che il dense core è comunque gravitazionalmente legato: gli effetti di feedback sono più che altro locali, e non ne alterano lo stato globale

Cathodic protection with localised galvanic anodes in slender carbonated concrete elements

A combined experimental and numerical investigation was carried out with the aim of determining whether few localised galvanic anodes per unit length could protect the reinforcement of slender carbonated concrete elements, exposed to atmospheric conditions, which could not be repaired with traditional methods. Initially, the cathodic behaviour of steel under galvanostatic polarisation was determined on small-size specimens obtained from a real element. A correlation of potential versus applied current was obtained. The current distribution in slender elements was then determined through finite elements simulations, considering various scenarios of carbonation and humidity. Results showed that, in spite of the high electrical resistivity of carbonated concrete, anodes with spacing of 0.45 m are enough to protect corroding reinforcement in most exposure conditions, even in thin parts of element. Estimated anode durations were of the order of several years or even decades; however, it was shown that also reinforcement in dry (carbonated or alkaline) concrete, which does not need to be protected, contributes to anode consumption. Although other aspects play a role on the performance of a cathodic protection system (such as the effectiveness of anode-encasing material and of electrical connection to reinforcement), the results obtained are supportive of a repair strategy based on the use of localised galvanic anodes and can be generalised to slender elements exposed to atmospheric conditions suffering carbonation induced corrosion

Investigation on the effect of supplementary cementitious materials on the critical chloride threshold of steel in concrete

open3noThe critical chloride threshold is a key parameter in the service life design of reinforced concrete structures exposed to chloride-bearing environments. This paper investigates the role of concrete composition, and particularly the effect of supplementary cementing materials, on the chloride threshold. To simulate real exposure conditions, ponding tests were carried out on reinforced concrete specimens with bars in free corrosion conditions and corrosion initiation was detected through corrosion potential and corrosion rate measurements. After two and a half years, the ponding was followed by an ageing period and the initiation of corrosion was further detected with anodic potentiostatic polarisation tests. Results of the tests showed several limitations of the approach based on chloride penetration and monitoring of free corrosion parameters to investigate the chloride threshold. In spite of this, a possible role of natural pozzolan and coal fly ash additions in leading to higher values of the chloride threshold and ground limestone in promoting lower values of the chloride threshold could be observed.Lollini, Federica; Redaelli, Elena; Bertolini, LucaLollini, Federica; Redaelli, Elena; Bertolini, Luc

Nitrogen fractionation in ammonia and its insights on nitrogen chemistry

Context. Observations of $\rm ^{14}N/^{15}N$ in the interstellar medium are becoming more frequent thanks to the increased telescope capabilities. However, interpreting these data is still puzzling. In particular, measurements of $\rm ^{14}N/^{15}N$ in diazenylium revealed high levels of anti-fractionation in cold cores. Aims. Furuya & Aikawa (2018), using astrophysical simulations coupled with a gas-grain chemical code, concluded that the $^{15}$N-depletion in prestellar cores could be inherited from the initial stages, when $\rm ^{14}N^{15}N$ is selectively photodissociated and 15N atoms deplete onto the dust grain, forming ammonia ices. We aim to test this hypothesis. Methods. We targeted three sources (the prestellar core L1544, the protostellar envelope IRAS4A, and the shocked region L1157-B1) with distinct degrees of desorption or sputtering of the ammonia ices. We observed the NH3 isotopologues with the GBT, and we inferred the $\rm ^{14}N/^{15}N$ via a spectral fitting of the observed inversion transitions. Results. $^{15}$NH3(1,1) is detected in L1544 and IRAS4A, whilst only upper limits are deduced in L1157-B1. The NH3 isotopic ratio is significantly lower towards the protostar than at the centre of L1544, where it is consistent with the elemental value. We also present the first spatially resolved map of NH3 nitrogen isotopic ratio towards L1544. Conclusions. Our results are in agreement with the hypothesis that ammonia ices are enriched in $^{15}$N, leading to a decrease of the $\rm ^{14}N/^{15}N$ ratio when the ices are sublimated into the gas phase for instance due to the temperature rise in protostellar envelopes. The ammonia $\rm ^{14}N/^{15}N$ value at the centre of L1544 is a factor of 2 lower than that of N2H+, suggesting that the dominant formation pathway is hydrogenation of N atoms on dust grains, followed by non-thermal desorption.Comment: Accepted for publication in A&A on 29/05/2

Effects of nanosilica on early age stages of cement hydration

Effects of nanosilica on cement hydration have been broadly investigated in the literature and early age cement hydration, as awhole, has been mainly considered, disregarding the substages of the hydration. The hydration of cement is characterized by different substages and nanosilica effect on the hydration could be a result of diverse, even contradictory, behavior of nanosilica in individual stages of the hydration. In this study, effects of nanosilica on different substages of cement hydration are investigated. Isothermal calorimetry results show that at early ages (initial 72 hours) the effects of nanosilica depend on the phenomenon by which the hydration is governed:when the hydration is chemically controlled, that is, during initial reaction, dormant period, and acceleratory period, the hydration rate is accelerated by adding nanosilica; when the hydration is governed by diffusion process, that is, during postacceleratory period, the hydration rate is decelerated by adding nanosilica. The Thermal Gravimetric Analysis on the samples at the hardened state (after 28 days of curing) reveals that, after adding nanosilica, the hydration degree slightly increased compared to the plain paste

The challenge of the performance-based approach for the design of reinforced concrete structures in chloride bearing environment

The performance-based approach, published by the International Federation for Structural Concrete (fib), was applied for the design of a RC element in a marine environment, with corrosion resistant reinforcement, to analyse the potentiality of the model as well as the possible reasons which limit its use. Results showed that the fib model allows to compare different solutions and to consider the benefits connected with the use of preventative measures. However the definition of reliable values for some input parameters, as the critical chloride threshold for corrosion resistant reinforcement, is demanded to the designer and this aspect clearly limits a widespread use

Corrosion of Steel in Concrete and Its Prevention in Aggressive Chloride-Bearing Environments

This keynote paper deals with the durability of reinforced concrete (RC) structures exposed to aggressive environments characterized by high concentration of chloride ions, namely, marine environments or the use of de-icing salts. The mechanism of chloride-induced corrosion of steel in concrete is introduced, and its influence on the service life of RC structures is analyzed. Factors affecting the time to corrosion initiation are described with regard to both concrete properties and environmental exposure conditions. Design approaches available for achieving durability targets associated with the design service life are analyzed, focusing on studies carried out by the authors in recent years at the mCD Concrete Durability lab of Politecnico di Milano, which were aimed at improving the protection provided to the steel bars by the concrete cover, investigating the advantages of using corrosion-resistant stainless steel bars and developing the electrochemical technique of cathodic prevention