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Context: Understanding the origin of the oldest stars in the universe is crucial to shed light on its early chemical evolution and on the formation history of the Milky Way. The oldest stars were either the direct heirs of the Big-Bang nucleosynthesis (and thus mainly composed of H and He at their birth), or were born in clouds of primordial (zero metallicity) material polluted with the ejecta from one or a few stars. Thus their observational counterparts must be characterised by very low amounts of iron in their composition. Currently, there is a debate on whether the first stellar generations (and thus the first polluters of the interstellar medium) were composed only of massive stars, or were a combination of intermediate and massive objects. Goal: We aim to identify the nature of the progenitors of the most metal-poor stars observed ([Fe/H] ¿ ¿2 or Z ¿ 10¿4). Specifically, we intend to know whether these stars show the nucleosynthetic signatures of early supernova explosions of different types, or of a combination of the ejecta from massive and intermediate-mass stars. Methods: We compile theoretical nucleosynthetic yields existing in the literature for intermediate and massive stars. We propose a scenario in which the theoretical yields of either an intermediate mass star or a massive star, are mixed within a gas cloud of primordial matter. Our main assumption is that some of the currently observed most metal-poor stars were born in this polluted clouds and thus, their abundances should show the nucleosynthetic signatures of their parent stars. We intend to develop a code able to automatically derive abundances from theoretical nucleosynthetic yields at different degrees of dilution with primordial material, and subsequently identify statistically sound matches between the former theoretical abundances and the surface abundances of metal poor stars of [Fe/H] ¿ ¿2. Automatising this process is crucial, given the increasing number of new theoretical models and ob

2022/2023, 2n quadrimestre

2022/2023, 2n quadrimestre

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly notices of the Royal Astronomical Society following peer review. The version of record Althaus, L.G., et al. Carbon-oxygen ultra-massive white dwarfs in general relativity. Monthly notices of the Royal Astronomical Society, August 2023, vol. 523, n. 3, p. 4492-4503 is available online at: https://doi.org/10.1093/mnras/stad1720, We employ the La Plata stellar evolution code, lpcode, to compute the first set of constant rest-mass carbon–oxygen ultra-massive white dwarf evolutionary sequences for masses higher than 1.29 M¿ that fully take into account the effects of general relativity on their structural and evolutionary properties. In addition, we employ the lp-pul pulsation code to compute adiabatic g-mode Newtonian pulsations on our fully relativistic equilibrium white dwarf models. We find that carbon–oxygen white dwarfs more massive than 1.382 M¿ become gravitationally unstable with respect to general relativity effects, being this limit higher than the 1.369 M¿ we found for oxygen–neon white dwarfs. As the stellar mass approaches the limiting mass value, the stellar radius becomes substantially smaller compared with the Newtonian models. Also, the thermo-mechanical and evolutionary properties of the most massive white dwarfs are strongly affected by general relativity effects. We also provide magnitudes for our cooling sequences in different passbands. Finally, we explore for the first time the pulsational properties of relativistic ultra-massive white dwarfs and find that the period spacings and oscillation kinetic energies are strongly affected in the case of most massive white dwarfs. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural, evolutionary, and pulsational properties of white dwarfs with masses above ~1.30 M¿., We thank Domingo García-Senz for valuable comments about the possible impact of our models on Type Ia Supernovae and Detlev Koester for providing atmosphere models to the high surface gravities that characterise our relativistic ultra-massive white dwarf models. Part of this work was supported by PICT-2017-0884 from ANPCyT, PIP 112-200801-00940 grant from CONICET, and by the Spanish project PID 2019-109363GB-100. MEC acknowledges Grant RYC2021-032721-I funded by MCIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR. ST, RR and ARM acknowledge support from MINECO under the PID2020-117252GB-I00 grant and from the AGAUR/Generalitat de Catalunya grant SGR-386/2021. RR acknowledges support from Grant RYC2021-030837-I funded by MCIN/AEI/ 10.13039/501100011033 and by “European Union NextGenerationEU/PRTR”. This research has made use of NASA Astrophysics Data System. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement., Postprint (author's final draft)

We employ the La Plata stellar evolution code, LPCODE, to compute the first set of constant rest-mass carbon–oxygen ultra-massive white dwarf evolutionary sequences for masses higher than 1.29 M⊙ that fully take into account the effects of general relativity on their structural and evolutionary properties. In addition, we employ the LP-PUL pulsation code to compute adiabatic g-mode Newtonian pulsations on our fully relativistic equilibrium white dwarf models. We find that carbon–oxygen white dwarfs more massive than 1.382 M⊙ become gravitationally unstable with respect to general relativity effects, being this limit higher than the 1.369 M⊙ we found for oxygen–neon white dwarfs. As the stellar mass approaches the limiting mass value, the stellar radius becomes substantially smaller compared with the Newtonian models. Also, the thermo-mechanical and evolutionary properties of the most massive white dwarfs are strongly affected by general relativity effects. We also provide magnitudes for our cooling sequences in different passbands. Finally, we explore for the first time the pulsational properties of relativistic ultra-massive white dwarfs and find that the period spacings and oscillation kinetic energies are strongly affected in the case of most massive white dwarfs. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural, evolutionary, and pulsational properties of white dwarfs with masses above ∼1.30 M⊙.

Primordial black holes in the asteroid-mass window, which might constitute all the dark matter, can be captured by stars when they traverse them at low enough velocity. After being placed on a bound orbit during star formation, they can repeatedly cross the star if the orbit happens to be highly eccentric, slow down by dynamical friction and end up in the stellar core. The rate of these captures is highest in halos of high dark matter density and low velocity dispersion, when the first stars form at redshift ¿ 20. We compute this capture rate for low-metallicity stars of 0¿3 to 1 M , and find that a high fraction of these stars formed in the first dwarf galaxies would capture a primordial black hole, which would then grow by accretion up to a mass that may be close to the total star mass. We show the capture rate of primordial black holes does not depend on their mass over this asteroid-mass window, and should not be much affected by external tidal perturbations. These low-mass stellar black holes could be discovered today in low-metallicity, old binary systems in the Milky Way containing a surviving low-mass main- sequence star or a white dwarf, or via gravitational waves emitted in a merger with another compact object. No mechanisms in standard stellar evolution theory are known to form black holes below the Chandrasekhar mass, so detecting a low-mass black hole would fundamentally impact our understanding of stellar evolution, dark matter and the early Universe., We would like to acknowledge helpful discussions and advice fromN. Bellomo, J. L. Bernal, A. Escrivà, C. Germani, and J. Sal-vadó. This work was supported in part by Spanish grants CEX-2019-000918-M funded by MCIN/AEI/10.13039/501100011033,AYA2015-71091-P, and PID2019-108122GB-C32., Peer Reviewed, Postprint (author's final draft)

2022/2023, 1r quadrimestre

Faculty members, undergraduate and graduate students of the School of Communication and Aerospace Engineering (Polytechnical University of Catalonia) are participating in a series of studies to determine the thermospheric density. These studies involve planning a space mission, designing and constructing small satellites, and performing related data analysis. This article presents a method for determining the thermospheric density and summarises the academic context in which we develop our work. Several studies have reported the existence of a downtrend in thermospheric density, with relative values ranging from –2% to –7% per decade. Although it is well known that solar and geomagnetic activity are the main drivers of the variations of the thermospheric density, this downtrend was reported to be caused by the rise of greenhouse gases. We present an update of this progression, considering the last solar cycle (2009-2021) and using Two-Line Elements sets (TLE) of 1U CubeSats and the spherical satellites ANDE-2. TLEs were used to propagate the orbits numerically using SGP4 (Simplified General Perturbations), and then compute the average density between two consecutive TLEs by integrating the appropriate differential equation. Then, using the NRLMSISE-00 (Picone 2002) and JB2008 (Bowman 2008) atmospheric models, we calculated an average density deviation per year. We built a comprehensive time series of the thermospheric density values, ranging from 1967 to the present. We merged Emmert (2015) thermospheric density data and our results computed both with NRLMSISE-00 and with JB2008. A linear regression on the combined dataset yields a decreasing trend of –5.1% per decade. We also studied the geomagnetic and solar activity to isolate the possible greenhouse gasses effect during the considered period. Our results show a strong correlation between geomagnetic activity and density deviation near the solar minima, and we propose that the cause of the previously reported lon

2021/2022, 2n quadrimestre

2021/2022, 1r quadrimestre

2021/2022, 2n quadrimestre

Stellar models and nucleosynthetic yields of primordial to extremely metal-poor (EMP) stars are crucial to interpret the surface abundances of the most metal-poor stars observed and, ultimately, to better understand the earliest stellar populations. In addition, they are key ingredients of Galactic chemical evolution models. Aims. We aim to better characterise the evolution and fates, and determine updated nucleosynthetic yields of intermediate-mass stars between primordial and EMP metallicity (Z = 10-10, 10-8, 10-7, 10-6, and 10-5). We also probed uncertainties in the nucleosynthesis of the oldest intermediate-mass stars, namely those related to the treatment of convection and convective boundaries and those related to wind prescriptions during the asymptotic giant branch (AGB) phase. Methods. We analyse the evolution of models from their main sequence, through the thermally pulsing AGB (TP-AGB), to the latest stages of their evolution, using the Monash-Mount Stromlo stellar evolution code MONSTAR. The results were post-processed with the code MONSOON, which allowed for the determination of the nucleosynthetic yields of 77 species up to 62Ni. By comparing them to similar calculations existing in the literature, we inspected the effects of input physics on the nucleosynthesis of EMP models. Results. From the evolutionary point of view, as reported in former works, we identified proton ingestion episodes (PIEs) in our lowest-mass lowest-metallicity models. Models of Z = 10-10 and Z = 10-8 in a narrow initial mass range around 5 M experience the cessation of thermal pulses, and their final fates as type-I1/2 supernovae cannot be discarded. However, the initial mass range of models eventually leading to the formation of type-I1/2 and electron-capture supernovae is considerably reduced compared to former works. All the models of initial mass & 6-7 M experience a corrosive second dredge-up and, analogously to those experiencing PIEs, undergo significant metal enrichment, We thank the anonymous referee for their useful comments and clarifications. This work was supported by the Spanish project PID 2019-109363GB-100, and by the German Deutsche Forschungsgemeinschaft, DFG project number Ts 17/2–1. S.W.C. acknowledges federal funding from the Australian Research Council through a Future Fellowship (FT160100046) and Discovery Project (DP190102431). This research was supported by use of the Nectar Research Cloud, a collaborative Australian research platform supported by the National Collaborative Research Infrastructure Strategy (NCRIS)., We thank the anonymous referee for their useful comments and clarifications. This work was supported by the Spanish project PID 2019-109363GB-100, and by the German Deutsche Forschungsgemeinschaft, DFG project number Ts 17/2–1. S.W.C. acknowledges federal funding from the Australian Research Council through a Future Fellowship (FT160100046) and Discovery Project (DP190102431). This research was supported by use of the Nectar Research Cloud, a collaborative Australian research platform supported by the National Collaborative Research Infrastructure Strategy (NCRIS)., Peer Reviewed, Postprint (updated version)

Ultra-massive white dwarfs (MWD¿1.05M¿) are considered powerful tools to study type Ia supernovae explosions, merger events, the occurrence of physical processes in the Super Asymptotic Giant Branch (SAGB) phase, and the existence of high magnetic fields. Traditionally, ultra-massive white dwarfs are expected to harbour oxygen-neon (ONe) cores. However, new observations and recent theoretical studies suggest that the progenitors of some ultra-massive white dwarfs can avoid carbon burning, leading to the formation of ultra-massive white dwarfs harbouring carbon-oxygen (CO) cores. Here we present a set of ultra-massive white dwarf evolutionary sequences with CO cores for a wide range of metallicity and masses. We take into account the energy released by latent heat and phase separation during the crystallization process and by 22Ne sedimentation. Realistic chemical profiles resulting from the full computation of progenitor evolution are considered. We compare our CO ultra-massive white dwarf models with ONe models. We conclude that CO ultra-massive white dwarfs evolve significantly slower than their ONe counterparts mainly for three reasons: their larger thermal content, the effect of crystallization, and the effect of 22Ne sedimentation. We also provide colors in several photometric bands on the basis of new model atmospheres. These CO ultra-massive white dwarf models, together with the ONe ultra-massive white dwarf models, provide an appropriate theoretical framework to study the ultra-massive white dwarf population in our Galaxy., We thank the comments of our expert referee, P.-E. Tremblay that helped us to improve the original version of this work. This work was partially supported by the NASA grants 80NSSC17K0008 and 80NSSC20K0193 and the University of Colorado Boulder. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://ww w.cosmos.esa.int/web/gaia/dpac/consortium). Part of this work was supported by AGENCIA through the Programa de Modernizaci ón Tecnol ógica BID 1728/OC-AR, by the PIP 112200801-00940 grant from CONICET, by grant G149 from National University of La Plata, and by the Spanish project PID 2019-109363GB-100. ST acknowledges the support by the MINECO grant AYA-2017-86274- P and by the AGAUR (SGR-661/2017)., Peer Reviewed, Postprint (published version)

[Context] Stellar models and nucleosynthetic yields of primordial to extremely metal-poor (EMP) stars are crucial to interpret the surface abundances of the most metal-poor stars observed and, ultimately, to better understand the earliest stellar populations. In addition, they are key ingredients of Galactic chemical evolution models., [Aims] We aim to better characterise the evolution and fates, and determine updated nucleosynthetic yields of intermediate-mass stars between primordial and EMP metallicity (Z = 10−10, 10−8, 10−7, 10−6, and 10−5). We also probed uncertainties in the nucleosynthesis of the oldest intermediate-mass stars, namely those related to the treatment of convection and convective boundaries and those related to wind prescriptions during the asymptotic giant branch (AGB) phase., [Methods] We analyse the evolution of models from their main sequence, through the thermally pulsing AGB (TP-AGB), to the latest stages of their evolution, using the Monash-Mount Stromlo stellar evolution code MONSTAR. The results were post-processed with the code MONSOON, which allowed for the determination of the nucleosynthetic yields of 77 species up to 62Ni. By comparing them to similar calculations existing in the literature, we inspected the effects of input physics on the nucleosynthesis of EMP models., [Results] From the evolutionary point of view, as reported in former works, we identified proton ingestion episodes (PIEs) in our lowest-mass lowest-metallicity models. Models of Z = 10−10 and Z = 10−8 in a narrow initial mass range around 5 M⊙ experience the cessation of thermal pulses, and their final fates as type-I1/2 supernovae cannot be discarded. However, the initial mass range of models eventually leading to the formation of type-I1/2 and electron-capture supernovae is considerably reduced compared to former works. All the models of initial mass ≳6–7 M⊙ experience a corrosive second dredge-up and, analogously to those experiencing PIEs, undergo significant metal enrichment in their envelopes. The associated increase in their opacities allows them to develop a solar-like TP-AGB or TP-super-AGB, ultimately becoming white dwarfs. Except for those undergoing the cessation of thermal pulses, all of our models show the nucleosynthetic signatures of both efficient third dredge-up and hot-bottom burning, with the activation of the NeNa cycle and the MgAlSi chains. This leads to the creation of vast amounts of CNO, with typical [N/Fe] > 4), and the characteristic abundance signature [N/Fe] > [C/Fe] > [O/Fe]. Our nucleosynthetic yields present dramatic differences with respect to recent results existing in the literature for intermediate-mass models of similar metallicities. The reason for these discrepancies lay in the poorly known input physics related to stellar winds and, above all, the treatment of convection and convective boundaries.

Ultra-massive white dwarfs ( M WD1 . 05 M ) are considered powerful tools to study Type Ia supernovae explosions, merger events, the occurrence of physical processes in the superasymptotic giant branch phase, and the existence of high magnetic fields. Traditionally, ultra-massive white dwarfs are expected to harbour oxygen-neon (ONe) cores. However, new observations and recent theoretical studies suggest that the progenitors of some ultra-massive white dwarfs can avoid carbon burning, leading to the formation of ultra-massive white dwarfs harbouring carbon-oxygen (CO) cores. Here, we present a set of ultra-massive white dwarf evolutionary sequences with CO cores for a wide range of metallicity and masses. We take into account the energy released by latent heat and phase separation during the crystallization process and by 22Ne sedimentation. Realistic chemical profiles resulting from the full computation of progenitor evolution are considered. We compare our CO ultra-massive white dwarf models with ONe models. We conclude that CO ultra-massive white dwarfs evolve significantly slower than their ONe counterparts mainly for three reasons: their larger thermal content, the effect of crystallization, and the effect of 22Ne sedimentation. We also provide colours in several photometric bands on the basis of new model atmospheres. These CO ultra-massive white dwarf models, together with the ONe ultra-massive white dwarf models, provide an appropriate theoretical framework to study the ultra-massive white dwarf population in our Galaxy.

The existence of ultra-massive white dwarf stars, MWD & 1.05M , has been reported in several studies. These white dwarfs are relevant for the role they play in type Ia Supernova explosions, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Aims. We explore the formation of ultra-massive, carbon-oxygen core white dwarfs resulting from single stellar evolution. We also study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. The aim is to provide a theoretical basis that can eventually help to discern the core composition of ultra-massive white dwarfs and the scenario of their formation. Methods. We consider two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs that involve rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We find that reducing standard mass-loss rates by a factor larger than 5 - 20 yields the formation of carbon-oxygen cores more massive than 1.05 M as a result of the slow growth of carbon-oxygen core mass during the thermal pulses. We also performed a series of evolutionary tests of solar-metallicity models with initial masses between 4 and 9.5 M , and with different core rotation rates. We find that ultra-massive carbon-oxygen core white dwarfs are formed even for the lowest rotation rates we analyzed, and that the range of initial masses leading to these white dwarfs widens as the rotation rate of the core increases, whereas the initial mass range for the formation of oxygen-neon core white dwarfs decreases significantly. Finally, we compare our findings with the predictions from ultra-massive white dwarfs resulting from the mer, Part of this work was supported by PIP 112-200801-00940 grant from CONICET, by MINECO grants AYA2014-59084-P, AYA2017-86274-P, and PID2019-109363GB-I00 / AEI / 10.13039/501100011033, by grant G149 from University of La Plata, and by the AGAUR grant SGR-661/201. ARM acknowledges support from the MINECO under the Ramón y Cajal programme (RYC-2016-20254). This research has made use of NASA Astrophysics Data System., Peer Reviewed, Postprint (published version)

2021/2022, 1r quadrimestre

Part of this work was supported by PIP 112-200801-00940 Grant from CONICET, by MINECO grants AYA2014-59084-P, and AYA2017-86274-P, by grant G149 from University of La Plata, and by the AGAUR grant SGR-661/201. This research has made use of NASA Astrophysics Data System, Ultra-massive white dwarfs are relevant for many reasons: their role as type Ia supernova progenitors, the occurrence of physical processes in the asymptotic giant branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Some hydrogen-rich ultra-massive white dwarfs are pulsating stars and, as such, they offer the possibility of studying their interiors through asteroseismology., Postprint (published version)

This work deals with the design of a femto-satellite for scientific studies. The goal of the femto-satellite is to model the density of the thermosphere by measuring the orbital decay caused by atmospheric drag. The proposed femto-satellite, having a mass of maximum 100 g by definition, is embedded in a PocketQube structure and equipped with solar panels in order to guarantee operability during the entire expected lifetime of approximately three years. In order to ensure a constant drag coefficient regardless of the attitude, the femto-satellite is to be covered by a spherical structure, which moreover allows for solar illumination of the solar panels.

2020/2021, 1r quadrimestre

2020/2021, 2n quadrimestre

[Context] Observed abundances of extremely metal-poor stars in the Galactic halo hold clues for understanding the ancient universe. Interpreting these clues requires theoretical stellar models in a wide range of masses in the low-metallicity regime. The existing literature is relatively rich with extremely metal-poor massive and low-mass stellar models. However, relatively little information is available on the evolution of intermediate-mass stars of Z ≲ 10-5, and the impact of the uncertain input physics on the evolution and nucleosynthesis has not yet been systematically analysed. [Aims] We aim to provide the nucleosynthetic yields of intermediate-mass Z = 10-5 stars between 3 and 7.5 M⊙, and quantify the effects of the uncertain wind rates. We expect these yields could eventually be used to assess the contribution to the chemical inventory of the early universe, and to help interpret abundances of selected C-enhanced extremely metal-poor (CEMP) stars. [Methods] We compute and analyse the evolution of surface abundances and nucleosynthetic yields of Z = 10-5 intermediate-mass stars from their main sequence up to the late stages of their thermally pulsing (Super) AGB phase, with different prescriptions for stellar winds. We use the postprocessing code MONSOON to compute the nucleosynthesis based on the evolution structure obtained with the Monash-Mount Stromlo stellar evolution code MONSTAR. By comparing our models and others from the literature, we explore evolutionary and nucleosynthetic trends with wind prescriptions and with initial metallicity (in the very low-Z regime). We also compare our nucleosynthetic yields to observations of CEMP-s stars belonging to the Galactic halo. [Results] The yields of intermediate-mass extremely metal-poor stars reflect the effects of very deep or corrosive second dredge-up (for the most massive models), superimposed with the combined signatures of hot-bottom burning and third dredge-up. Specifically, we confirm the reported tre

[Context] Ultra-massive white dwarfs are relevant for many reasons: Their role as type Ia supernova progenitors, the occurrence of physical processes in the asymptotic giant branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Some hydrogen-rich ultra-massive white dwarfs are pulsating stars and, as such, they offer the possibility of studying their interiors through asteroseismology. On the other hand, pulsating helium-rich ultra-massive white dwarfs could be even more attractive objects for asteroseismology if they were found, as they should be hotter and less crystallized than pulsating hydrogen-rich white dwarfs, something that would pave the way for probing their deep interiors. [Aims] We explore the pulsational properties of ultra-massive helium-rich white dwarfs with carbon-oxygen and oxygen-neon cores resulting from single stellar evolution. Our goal is to provide a theoretical basis that could eventually help to discern the core composition of ultra-massive white dwarfs and the scenario of their formation through asteroseismology, anticipating the possible future detection of pulsations in helium-rich ultra-massive white dwarfs. [Methods] We focus on three scenarios for the formation of helium-rich ultra-massive white dwarfs. First, we consider stellar models coming from two recently proposed single-star evolution scenarios for the formation of ultra-massive white dwarfs with carbon-oxygen cores that involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant branch stars. Finally, we contemplate ultra-massive oxygen-neon core white-dwarf models resulting from standard single-star evolution. We compute the adiabatic pulsation gravity-mode periods for models in a range of effective temperatures, embracing the instability strip of average-mass pulsating helium-rich white dwarfs, and we compare the characteristics of the mode-Trapping

Context. The existence of ultra-massive white dwarf stars, M & 1.05 M, has been reported in several studies. These white dwarfs are relevant for the role they play in type Ia supernova explosions, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double-white-dwarf mergers. Aims. We aim to explore the formation of ultra-massive, carbon-oxygen core white dwarfs resulting from single stellar evolution. We also intend to study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. The aim is to provide a theoretical basis that can eventually help to discern the core composition of ultra-massive white dwarfs and the circumstances of their formation. Methods. We considered two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs, which involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We find that reducing standard mass-loss rates by a factor larger than 5−20 yields the formation of carbon-oxygen cores more massive than 1.05 M as a result of the slow growth of carbon-oxygen core mass during the thermal pulses. We also performed a series of evolutionary tests of solar-metallicity models with initial masses between 4 and 9.5 M and with different core rotation rates. We find that ultra-massive carbon-oxygen core white dwarfs are formed even for the lowest rotation rates we analyzed, and that the range of initial masses leading to these white dwarfs widens as the rotation rate of the core increases, whereas the initial mass range for the formation of oxygen-neon core white dwarfs decreases significantly. Finally, we compared our findings with the predictions from ultra-massive w

2020/2021, 1r quadrimestre

Contexto: Los CubeSats constituyen el estándar de satélites pequeños más extendido, prácticamente desde que fueron presentados a finales de la década de 1990. Los CubeSats revolucionaron la industria espacial. Gracias a su pequeño tamaño y por el hecho implementar componentes comercializados, su coste es muy bajo, y con ello abren posibilidades de exploración espacial a instituciones educativas, al sector privado y a países en vías de desarrollo. Objetivos: Tal y como ocurre con satélites de mayor tamaño, el control de actitud puede resultar crítico para el éxito de su misión, y dicho control de actitud requiere un buen conocimiento del tensor de inercia del satélite. En este trabajo, partimos del diseño preliminar de un péndulo de torsión destinado a obtener tensores de inercia de CubeSats. Nuestro objetivo es desarrollar un código que utilice como datos de entrada las configuraciones específicas que le demos al CubeSats situado sobre el péndulo, así como las correspondientes medidas de periodo de oscilación. Como información de salida ofrece la posición del centro de masas del péndulo, sus momentos de inercia y, finalmente, el tensor de inercia completo para el sistema de referencia deseado y la precisión de los datos de salida. El programa desarrollado se puede utilizar para mejorar el diseño del péndulo original. Métodos: Aplicaremos conceptos y leyes básicos de mecánica de rotación (e.g. centro de masas, tensor de inercia, teorema de ejes paralelos, matrices de rotación, ecuaciones de Euler para rotación). Dicha información y el diseño del péndulo de torsión propuesto en un trabajo anterior serán utilizados para implementar un código en Matlab. Resultados: El principal resultado de nuestro trabajo es el programa en Matlab, flexible en cuanto opciones de uso y a la vez sencillo para el usuario. El programa cumple todos los requisitos inicialmente propuestos, e incluso otros adicionales relacionados, sobre todo, con la posibilidad de simular experimentos y con el

2019/2020, 2n quadrimestre

Context: The use of unmanned aerial vehicles (more popularly known as drones) has experienced a fast growth in the last two decades, especially over urban areas. This led to important safety concerns and thus to the development of a body of astringent regulations aimed to minimize eventual hazards. For this reason, the drone industry must develop suitable strategies to take full advantage of the use of drones, while simultaneously meeting safety and legal requirements. Goals: In line with the former concerns and opportunities, this work focuses on the development of an efficient risk-aware path planning for Unmanned Aerial Systems (UAS) in urban environments. Methods: We use a well-known open-source robotics middleware Robot Operating System (ROS), together with The Open Motion Planning Library, which allow to solve a wide range of complex motion planning problems. Specifically, our proposed strategy is based on the Rapidly-Exploring Random Tree ¿Star¿ algorithm, which allows rapid and efficient exploration of the different paths and, ultimately, convergence to an optimal solution. Our work is actually a 3-dimensional extension of 2-dimension risk-based path planner developed at the Politecnico di Torino. This extension, and some additional optimization goals implied structural changes in the original code which had to be carefully implemented and tested. Results: Given a start and a target position, as well as a 2.5-dimensional risk-based map, the path planner is able to find the optimal path while minimizing the effective risk. The 2.5-dimensional risk-based map quantifies the risks of operating UAS eventually affecting the population on the ground (computed at discrete flight altitudes). In addition, it defines no-flight zones and obstacles at the considered flight altitude. Moreover, the originally proposed approach was extended to consider the Quality of Service of the mobile network used to communicate with the aircraft. In this scenario, the path planner perf, Outgoing

Abridged: Observed abundances of extremely metal-poor (EMP) stars in the Halo hold clues for the understanding of the ancient universe. Interpreting these clues requires theoretical stellar models at the low-Z regime. We provide the nucleosynthetic yields of intermediate-mass Z= 10-5 stars between 3 and 7.5 Msun , and quantify the effects of the uncertain wind rates. We expect these yields can be eventually used to assess the contribution to the chemical inventory of the early universe, and to help interpret abundances of selected C-enhanced EMP stars. By comparing our models and other existing in the literature, we explore evolutionary and nucleosynthetic trends with wind prescriptions and with initial metallicity. We compare our results to observations of CEMP-s stars belonging to the Halo. The yields of intermediate-mass EMP stars reflect the effects of very deep second dredge-up (for the most massive models), superimposed with the combined signatures of hot-bottom burning and third dredge-up. We confirm the reported trend that models with initial metallicity Z ini <= 0.001 give positive yields of 12C,15N,16O , and 26Mg . The 20Ne,21Ne , and 24Mg yields, which were reported to be negative at Z ini = 0.0001, become positive for Z= 10-5 . The results using two different prescriptions for mass-loss rates differ widely in terms of the duration of the thermally-pulsing (Super) AGB phase, overall efficiency of the third dredge-up episode, and nucleosynthetic yields. The most efficient of the standard wind rates frequently used in the literature seems to favour agreement between our yield results and observational data. Regardless of the wind prescription, all our models become N-enhanced EMP stars., Postprint (published version)

The European Agency is currently developing a 3U CubeSat, aimed to transmit to Earth well-defined reference signals which will be used to calibrate ground-based Cosmic microwave background (CMB) polarimeters. In order to establish reliable communication with the ground based stations, this CubeSat's orientation must be highly accurate. In order to achieve the required accuracy, the development of an Attitude Orbit Control System (AOCS) based on reaction wheels was proposed. Once in operation, this system will orientate and control the nanosatellite into the desired position. The goal of this Bachelor's Thesis is to design and test the AOCS system in 1 axis with reaction wheels. The detailed design involves the definition of components, as well as the design of a proportional integral and derivative (PID) controller to direct the system and test its performance. Once the complete design and successful tests are performed, the system will be on board of the 3U ESA CubeSat., Outgoing

Nowadays, the study of the dynamics of the upper atmosphere has become a must in order to study satellite trajectories at low orbit with accuracy. The region we are interested in this project is the lower part of the thermosphere (between 100 and 300 km) where the atmosphere density begins to be considerably important. There are several atmosphere models which forecast the physical conditions at that altitudes. However, these models have a considerable error when they compute the density (usually between 15-20%). Due to that, a group of researchers at UPC aims to reduce these errors between the forecast and real values. Within this research, this project aims to study the average density along-orbit that satellites may encounter at ionospheric altitudes using two different atmospheric models (NRLMSISE-00 and JB2008) and check if we can obtain the density with relative accuracy in case of a problem with the satellites. To achieve that, we have developed a code in MATLAB based on the Simplified General Perturbation 4 (SGP4) orbit propagator in which we compute the drag perturbation on our own using an atmospheric model. This, nevertheless, introduces an important factor to consider and study, which is the determination of the ballistic coefficient. Once we developed and test the code, we chose two different satellites to simulate their trajectories and make the study: a 1U CubeSat (ArduSat-X) and a spherical satellite (STARSHINE-2). Both satellites were in different situations in terms of solar activity which allowed us to study the problem in different conditions. Apart from that, as STARSHINE-2 is spherical, it allowed us to determine how important is to consider the ballistic coefficient with accuracy. The results have been against what we expected to get and were a bit surprising for us. The main conclusion we got is that the errors from the atmospheric models are bigger than what we read in any bibliography. We get values around 40-50% of error with STARSHINE-2

Article redactat en col.laboració amb l'àrea de Relacions Externes-CBL, Un programa de la Politécnica quiere acercar a los niños cuestiones científicas y tecnológicas para dar respuesta a sus curiosidades. Los escolares envían preguntas a profesores que, mediante vídeos amenos, las responden mediante ejemplos comprensibles, Objectius de Desenvolupament Sostenible::4 - Educació de Qualitat, Postprint (published version)

Context: Society tends towards a territorial imbalance which concentrates population in urban areas. Infrastructure and quality transport are essential for economy and regional growth, as economic boost also leads to unbalanced urban development. In addition, sustainability, defined as the ability to simultaneously meet the needs of current society whilst preserving the life conditions of future generations, is becoming increasingly important. As a consequence, large cities rely on Collective Public Transport (CPT) as a solution to reduce the problems associated to the use of private vehicles. CPT allows citizens’ mobility, optimizes space, limits toxic emissions and noise, and promotes social equity by making transportation accessible to almost the entire population. Thus, its social benefits vastly overcome the associated economic expenses. Actually, maintaining high levels of service due to social policies of public administrations generates operating costs which make transport companies unprofitable. The largest expenses of these companies are due to staff costs, thus, finding means of reducing these expenses is one of the premises of public transport operators, which increasingly rely on specialized software for this purpose. Aim: In this project, we present the methodology and database design process of a system aimed to optimize tasks associated to the logistics of a CPT company. Tools and methods: The database design presented here is a Transmodel based database. Transmodel (Reference Data Model For Public Transport) is a European standard, which consists of a conceptual model, and sets the basis for the design of an arbitrary system in the public transport sector. Transmodel is a very complex and comprehensive model which is not intended to be implemented as a whole. Actually, the designer's task is to establish which entities or parts of the model are necessary for a specific function of the system. We use the Transmodel standard, to efficiently desi, Contexto: La sociedad tiende hacia un desequilibrio territorial que concentra la población en zonas urbanas. Las infraestructuras y el transporte de calidad son esenciales para el desarrollo económico y el crecimiento de las regiones, aunque este impulso económico también provoca un desarrollo urbano desequilibrado. Además, el concepto de sostenibilidad, definido como la capacidad de cubrir con las necesidades de las generaciones actuales con métodos que perduren y no causen perjuicio a las generaciones venideras es cada vez más importante. En este sentido, la apuesta de las grandes urbes continúa siendo el Transporte Público Colectivo (TPC), ya que es la solución para solventar las problemáticas que el uso del vehículo privado provoca, permitiendo la movilidad de las personas, optimizando el espacio, las emisiones y el ruido y fomentando la equidad social, haciendo que el transporte sea accesible a prácticamente toda la población. De esta manera, aunque su rentabilidad no sea económica al menos siempre lo es de tipo social. Mantener los altos niveles de servicio, basado en políticas sociales de las administraciones públicas, genera costes operativos que hacen que las empresas operadoras de TPC sean deficitarias. El mayor gasto de estas empresas proviene de los costes de personal. Encontrar métodos de disminuir estos gastos es una de las premisas de las operadoras de transporte público que cada vez más se apoyan en software especializado para éste fin. Objetivo: En este proyecto, se presenta la metodología y el proceso de diseño de la base de datos para un sistema que tiene como objetivo optimizar las tareas asociadas a los procesos operativos de una compañía de TPC. Herramientas y metodología: El diseño de la base de datos que aquí se presenta se basa en el Transmodel. El Transmodel (Reference Data Model For Public Transport) es un estándar europeo, que presenta un modelo conceptual y que establece las bases de cómo debe ser diseñado cualquier sistema en el sector

Context: Attitude control of a satellite relies upon a precise determination of its inertia tensor. However, the determination of the inertia tensor of a complex system, as is the case of satellites, completely escapes analytic methods. Even though the inertia tensor can be obtained by means of CAD models, this method is subject to a degree of uncertainties due to required approximations in the materials definition and the geometry of some components. Furthermore, cabling is often difficult to integrate into CAD designs, which may introduce an additional source of error. Aims: In this Degree Thesis, we propose the design of a torsion pendulum which will allow us to determine all the components of the inertia tensor of a CubeSat-type object. We intend to achieve more efficiency and better accuracy than performing standard CAD simulations. Methodology: Our starting point is a general idea of a pendulum, Our sketch involves a set of fixed parts, mostly with structural purposes, and a set of mobile parts. The latter must be able to rotate about the axis of a torsion fiber. Besides, it must connect to a platform on top of which the CubeSat whose tensor of inertia we intend to determine will be placed. By measuring the period of rotation of the mobile parts (which include the platform and CubeSat), it is easy to derive the inertia moment of rotating set about the axis given by the torsion fiber. The detailed design of the pendulum was executed with the software SolidWorks. Foundations of Classical Mechanics and rotation matrix Algebra, as well as the software MatLab were required to develop the operation analysis of the proposed pendulum. Results: We designed, obtained detailed maps, and developed stress analysis calculations of the pendulum. Besides, we completed the mathematical analysis which allows to build the inertia tensor of a CubeSat, by using individual inertia moments, derived from a set of period measurements obtained with different orientations of the CubeSat, Contexto: El control de actitud de un satelite se basa en una determinaci ´ on precisa de ´ su tensor de inercia, sin embargo, la determinacion del tensor de inercia de un sistema ´ complejo, como es el caso de los satelites, escapa por completo a los m ´ etodos anal ´ ´ıticos. Aunque el tensor de inercia se puede obtener por medio de modelos CAD, este metodo ´ esta sujeto a un grado de incertidumbre debido a las aproximaciones requeridas en la ´ definicion de materiales y en la geometr ´ ´ıa de algunos de los dispositivos. Ademas, el ´ cableado a menudo es dif´ıcil de integrar en los disenos ˜ CAD, e introduce una fuente de error adicional. Objetivos: En este trabajo de final de grado, proponemos realizar el diseno y el an ˜ alisis ´ de las bases del funcionamiento de un pendulo de torsi ´ on que permite determinar todos ´ los componentes del tensor de inercia de un objeto tipo CubeSat, con mayor eficiencia y precision que la que nos permitir ´ ´ıa un analisis de ´ CAD. Metodolog´ıa: Partimos de una idea general de un pendulo, que incluye un conjunto de par- ´ tes fijas con funcion principalmente estructural, y de partes que pueden rotar alrededor de ´ una fibra de torsion, unida a una plataforma sobre la cual se puede situar el ´ CubeSat cuyo tensor de inercia deseamos conocer. A partir de las medidas del periodo de rotacion de ´ la plataforma podemos determinar de manera sencilla el momento de inercia del conjunto de partes giratorias alrededor del eje de la fibra de torsion. Para realizar el dise ´ no deta- ˜ llado del dispositivo utilizamos el software SOLIDWORKS. Para analizar las bases de su funcionamiento utilizamos conocimientos de Mecanica Cl ´ asica y ´ Algebra de matrices de ´ rotacion, as ´ ´ı como el software MATLAB. Resultados: Hemos disenado, presentado los planos, y realizado un an ˜ alisis num ´ erico de ´ estres sobre el p ´ endulo propuesto. Adem ´ as hemos completado el desarrollo matem ´ atico ´ que permite construir el tensor de inercia del Cu

Getting a better understanding of the evolution and nucleosynthetic yields of the most metal-poor stars ( Z ¿ 10 ¿5 ) is critical because they are part of the big picture of the history of the primitive universe. Yet many of the remaining unknowns of stellar evolution lie in the birth, life, and death of these objects. We review stellar evolution of intermediate-mass Z = 10 ¿5 models existing in the literature, with a particular focus on the problem of their final fates. We emphasise the importance of the mixing episodes between the stellar envelope and the nuclearly processed core, which occur after stars exhaust their central He (second dredge-up and dredge-out episodes). The depth and efficiency of these episodes are critical to determine the mass limits for the formation of electron-capture SNe. Our knowledge of these phenomena is not complete because they are strongly affected by the choice of input physics. These uncertainties affect stars in all mass and metallicity ranges. However, difficulties in calibration pose additional challenges in the case of the most metal-poor stars. We also consider the alternative SN I1/2 channel to form SNe out of the most metal-poor intermediate-mass objects. In this case, it is critical to understand the thermally pulsing Asymptotic Giant Branch evolution until the late stages. Efficient second dredge-up and, later, third dredge-up episodes could be able to pollute stellar envelopes enough for the stars to undergo thermal pulses in a way very similar to that of higher initial Z objects. Inefficient second and/or third dredge-up may leave an almost pristine envelope, unable to sustain strong stellar winds. This may allow the H-exhausted core to grow to the Chandrasekhar mass before the envelope is completely lost, and thus let the star explode as an SN I1/2. After reviewing the information available on these two possible channels for the formation of SNe, we discuss existing nucleosynthetic yields of stars of metallicity Z = 10, Peer Reviewed, Postprint (author's final draft)

Twenty years ago, the first low-cost airlines in Spain appeared. The introduction of these companies called low cost in the Spanish territory have benefited us exponentially in the traffic of passengers. More specifically, we can say that in Barcelona at the El Prat de Llobregat airport last year there was a great increase in circulation by low cost companies. One of the national companies par excellence is Vueling Airlines, which is located its main operating base in El Prat del Llobregat. It is the largest airline within Spanish territory in terms of destinations and by size of fleet and the second by number of passengers transported within Spanish territory, only surpassed by Irish Ryanair. The relationship between the EETAC (School of Telecommunications and Aerospace Engineering of Castelldefels) with the offices of Vueling Airlines has always collaborated so that students can develop their engineering skills from practices and collaborations. Work with the company from final research projects. Based on these collaborations, we have been able to develop research projects like this, which begin with the active participation in the Department of Vueling Operations Engineering, specifically in the field of Weight and Balance. Vueling Airlines is one of the pioneer airlines in the aeronautical sector in Spain. In order for a large company like this to remain the country's first, it is necessary that there is constant optimization and supervision of its operation. The Weight and Balance department is in charge of this aspect but focusing only on the weight and balance of the aircraft. This document attempts to analyze the program in charge of the distribution of passengers within the company's own aircraft. First of all, we analyze the current distribution program for passengers of the Vueling Airlines company, finding their weak points and possible points to improve. A fieldwork has been carried out with all the data of distribution of previous flights of Vueling. O

The main objective of this project is to design an aerial vehicle which can contribute in planetary research in a more optima l way than the current missions. It has been decided that an aerobot, which consists in a balloon and scientific instrumentation (also known as payload) hanging from this balloon, would be a suitable option due t o its better resolution than orbiters and it s better mobilit y than rovers. This balloon is filled with a buoyant gas that must be lighter than the atmospheric air in order to make possible the balloon flight. In this project there is a study of the different aerobot and buoyant gas options that coul d be employed for this mission. In more specific terms, the chosen aerobot is a helium Super Pressure Balloon made of LLDPE, which has a positive pressure differential pressure between the internal and atmospheric pressure, so its float altitude would kee p constant along its flight. The balloon size and mass , internal properties , vertical speed during ascension and lifetime of the balloon are some of the aerobot characteristics that have been computed in this project. This balloon endurance is determined b y the film leakage features and helium diffusion . Moreover, a study of Mars has also been performed in order to apply the design of the aerobot to the singular Martian conditions. An atmosphere model for Mars has been settled , the Viking - Pathfinder , from which the balloon characteristics have been determined. Furthermore, there have been exposed different extra systems that could improve the performance of the aerobot, increasing its exploration capabilities or its endurance . Finally, it has been conclud ed that a planetary aerobot is a system that would improve to a large extent the current exploration of solar system bodies with a substantial atmosphere due to it is proved that its design is possible . However, it is also needed a great improvement and fu ture work at the balloon envelope materials and buoyancy technology o

This project focuses on the study previous to the development of an Attitude Determination and Control System (ADCS) of a small space telescope, the Humble Space Telescope (hST). The ADCS is essential for the telescope operation as it must guarantee a good observation of the space regions of interest. The proposed system is based on the use of quaternions to lighten the computational weight. The QUEST (Quaternion Estimator) algorithm will be used for the attitude determination from the vector observations made with the sensors, mainly star trackers. The control part will be done by means of a PD (Proportional Derivative) controller using a control law based on the error quaternion between the actual and the desired attitude. The stability of the proposed control law will be verified by a Lyapunov study. An empirical study of the effect of the propotional and derivative gains on the system's response will be carried out. The main hardware for the control part will be reaction wheels, combined with the use of magnetorquers for the desaturation of the reaction wheels. The numerical simulations that are going to be conducted will verify the viability of the proposed system.

This project aims to study the drag coefficient (CD) for satellites in a free molecular flow. The study is mainly based on Schaaf & Chambre and Schamberg models. These are old models (from 1958), but still are the most widely used nowadays. The two models are analyzed for flat and spherical geometries, considering both specular and diffuse reflection. In order to set the scenario of our study, we describe the thermosphere and justify the interest of studying this region of the terrestrial atmosphere. Then, we describe the specific type of flux considered in this work. The parameters which divide the behaviour of the gases into different regimes, as well as the basic characteristics of the free molecular flow and the interaction between it and the surface of the satellite, are explained. The first model studied is the one by Schaaf and Chambre, considering the two types of reflection for each geometry. We observe that, in the case of flat plate geometry, the angle of attack is the most important parameter for the determination of CD. For the spherical case, we see that CD adopts almost constant values, since the angle of attack is irrelevant due to the geometry, and the temperature does not cause great variations. With respect to Schamberg model we obtain large variations of CD due to variations in the angle of attack, as in the previous model. The case of the sphere also gives a constant CD in the specular case. There is also a clear dependency on the wall temperature for the diffuse and intermediate cases. The values obtained for CD under the diffuse reflection show large discrepancies between models. In fact, Schamberg's model gives values that are at odds with empirical values under certain conditions. For this reason, we also analyze a third model: Sentman's model for diffuse reflection. This model gives results similar to those of Schaaf & Chambre. Additionally, the bibliography points to serious shortcomings of Schamberg's model in the diffuse reflection case