Dissertations / Theses on the topic 'Star fields'

A star graph consists of a vertex to which a set of edges are connected. Such an object can be used to, among other things, model the electromagnetic properties of quantum wires. A scalar field theory is constructed on the star graph and its properties are investigated. It turns out that there exist Kirchoff's rules for the conserved charges in the system leading to restrictions of the possible type of boundary conditions at the vertex. Scale invariant boundary conditions are investigated in detail.
En stjärngraf består av en nod på vilken vilken ett antal kanter är anslutna. Ett sådant objekt kan bland annat användas till att modellera de elektromagnetiska egenskaperna hos kvanttrådar. En skalärfältsteori konstrueras på stjärngrafen och dess egenskaper undersöks. Det visar sig att det exisisterar en typ av Kirchoffs lagar för de konserverade laddningarna i systemet. Detta leder till restriktioner på vilka randvillkor som är möjliga vid noden. Skalinvarianta randvillkor undersöks i detalj.

A star graph consists of an arbitrary number of segments that are joined at a point which is called the vertex. In this work it is investigated from a pure theoretical point of view, in the framework of quantum field theory. As a concrete physical application, the electric conductance tensor is obtained. In particular it is shown that this conductance behaves differently according to whether the scattering matrix associated with the vertex of the graph has bound-state poles or not.

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Astronomy. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Despite much improvement over the past several decades, women continue to be underrepresented across many STEM fields. In this study, I draw upon past research to theorize that (1) there exist a substantial gender performance gap among STEM researchers and that (2) the gap is disproportionately larger among star performers, i.e., individuals who produce output many times greater than others holding the same job (Aguinis & O’Boyle, 2014). I then discuss how a gender performance gap specifically among star performers can be more harmful to the underrepresented group than an equivalent gap among average performers. To investigate the possible existence of such gender performance gaps, I assess the research productivity of all researchers in the fields of mathematics, materials sciences, and genetics who have published in the past decade at least one article in the most influential journals in their fields. Using the process of distribution pitting (Joo, Aguinis & Bradley, 2017), I identify the best-fitting theoretical distributions and associated dominant generative mechanisms that shape individual performance across the three STEM fields. Assessment of the shapes of the performance distributions confirms the existence of considerable gender performance gaps in favor of men, although the gap was substantially lower in the field of genetics compared to in the others. In addition, the findings suggest that (1) individual STEM researchers vary in performance predominantly due to differences in their accumulation rates (i.e., average output produced per time period), and (2) women’s research output accumulation rates are lower (on average) and also less variable compared to men’s. Implications for theory and practice based on these findings are discussed.

This dissertation is a study of the physical conditions in star-forming regions, and combines observational data and theoretical calculations. We studied the physical conditions of Orions Veil, which is an absorbing screen that lies along the line of sight to the Orion H II region. We computed photoionization models of the Veil. We combined calculations with UV, radio, and optical spectra that resolve the Veil into two velocity components. We derive many physical parameters for each component seen in 21 cm absorption. We find the magnetic field energy dominates turbulent and thermal energies in one component while the other component is close to equipartition between turbulent and magnetic energies. We observe H2 absorption for highly excited levels. We find that the low ratio of H2/H0 in the Veil is due to the high UV flux incident upon the Veil. We detect blueshifted S+2 and P+2 ions which must arise from ionized gas between the neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We determine the ionized and neutral layers of the Veil will collide in less than 85,000 years. The second part of this dissertation involved self-consistently calculating the thermal and chemical structure of an H II region and photodissociation region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of initial conditions. We describe improvements made to the spectral synthesis code Cloudy which made these calculations possible. These include the addition of a molecular network with ~1000 reactions involving 68 molecules and improved treatment of the grain physics. Archival data are used to derive important physical characteristics of observed H II regions and PDRs. These include stellar temperatures, electron densities, ionization parameters, UV flux, and PDR density. The contribution of the H II region to PDR emission line diagnostics is also calculated. Finally, these calculations are used to derive emission line ratios than can tell us the equation of state in star-forming regions.

The object of this dissertation is to provide an observational study of the effects of interstellar magnetic fields on star-formation regions. This is part of a long-standing research project that uses the techniques of radio astronomy to measure magnetic field strengths in the interstellar medium of our galaxy. Interstellar magnetic fields are believed to play a crucial role in the star-formation process therefore a comprehensive study of magnetic fields is necessary in understanding the origins of stars. These projects use observational data obtained from the Very Large Array (VLA) in Socorro, NM. The data reveal interstellar magnetic field strengths via the Zeeman effect in radio frequency spectral lines. This information provides an estimate of the magnetic energy in star-forming interstellar clouds in the Galaxy, and comparisons can be made with these energies and the energies of self-gravitation and internal motions. From these comparisons, a better understanding of the role of magnetic fields in the origins of stars will emerge. The regions observed include the giant molecular clouds and star-forming regions of Cygnus X and NGC 6334. NGC 6334 A is a compact HII region at the center of what is believed to be a large, rotating molecular torus (based on studies by Kramer et al. (1997)). This is a continuing study based on initial measurements of the HI and OH Zeeman effect (Sarma et al. (2000)). The current study includes OH observations performed by the VLA at a higher spatial resolution than previously published data, and allows for a better analysis of the spatial variations of the magnetic field. A new model of the region is also developed based on OH opacity studies, dust continuum maps, radio spectral lines, and infrared (IR) maps. The VLA has been used to study the Zeeman effect in the 21cm HI line seen in absorption against radio sources in the Cygnus-X region. These sources are mostly galactic nebulae or HII regions, and are bright and compact in this region of the spectrum. HI absorption lines are strong against these regions and the VLA is capable of detecting the weak Zeeman effect within them.

We carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and their relevant physical properties available in the catalogues, among them were considered the density, variable stars known, colour-magnitude diagrams, and luminosity. The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time images. The camera was configured to take 10 frames per second. Due to this, the brighter stars observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly, it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that also enabled us to produce images with better angular resolution than previously achieved with conventional CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that obtained with space telescopes. The detection of the stars in each of the globular clusters and the photometry was performed via difference image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated to produce high-precision time-series photometry of very crowded stellar regions. We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the variable star population in the globular clusters. In NGC 6715, we found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type. In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2 SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables (16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study. Ephemerides and photometric measurements for the variable stars are available in electronic form through the Strasbourg Astronomical Data Centre.

This document describes study of an active star forming region in our galaxy (the Milky Way) known as W22. Physical conditions in these regions can help us in understanding star formation processes in the universe and hence the structure and evolution of the universe. Zeeman effect measurements in 18 cm OH absorption line were carried out in order to estimate the line of sight magnetic field strength in the molecular cloud associated with this star forming region. Other physical parameters such as hydrogen column density, optical depth, critical magnetic field were also determined from these measurements. The region was mapped at 18 cm and the distribution of molecular gas within this star forming complex was described.

Dans les galaxies en interaction, de colossales forces de marée perturbent la morphologie des progéniteurs pour engendrer les longs bras d'étoiles, gaz et poussières que l'on observe parfois. En plus de leur effet destructeur, les forces de marée peuvent, dans certain cas, se placer dans une configuration protectrice appelée mode compressif. De tels modes protègent alors la matière en leur sein, en augmentant son énergie de liaison. Cette thèse se concentre sur l'étude de ce régime peu connu en quantifiant ses propriétés grâce à des outils numériques et analytiques appliqués à un spectaculaire système de galaxies en fusion, communément appelé les Antennes. Des simulations N-corps de cette paire de galaxies montrent la présence de modes compressifs dans les régions où les observations révèlent un sursaut de formation stellaire. De plus, les temps et énergies caractéristiques de ces modes correspondent à ceux de la formation de sous-structures autogravitantes telles que des amas stellaires et des naines de marée. Des comparaisons avec les taux de formation stellaire dérivés de simulations hydrodynamiques confirment la corrélation entre les positions des modes compressifs et les sites où la formation des étoiles est certainement amplifiée. Mis bout-à-bout, ces résultats suggèrent que les modes compressifs des champs de marée jouent un role important dans la formation et l'évolution des jeunes amas, au moins d'un point de vue statistique, sur une échelle de temps de l'ordre de dix millions d'années. Des résultats préliminaires de simulations d'associations stellaires soulignent l'importance de plonger les amas dans leur environnement galactique en évolution, pour tenir compte précisément de leur morphologie et évolution interne. Ces conclusions ont été étendues à de nombreuses configurations d'interaction et restent robustes aux variations des principaux paramètres caractérisant les paires de galaxies. Nous notons cependant une nette anti-corrélation entre l'importance du mode compressif et la distance entre ces galaxies. De nouvelles études incluant les aspects hydrodynamiques sont maintenant en cours et aideront à préciser le rôle exact du mode compressif dans la formation et la survie des amas d'étoiles. Les premières comparaisons avec de telles simulations suggèrent que les modes compressifs agissent en tant que catalyseurs ou amorces de la formation stellaire.

This document describes studies of two independent regions of the interstellar medium (ISM). These studies have the common element that both pertain to regions in our Galaxy that are known to be associated with present-day star formation. These studies aim to help us understand the ISM, star formation, and ultimately where we came from, since, after all, our star, the Sun, is itself the product of star formation 4.5 billion years ago. The first project measured the Zeeman Effect on the 21 cm H I absorption line in order to create a map of the line of sight magnetic fields near a star forming region called W3. From the map of the field, it was possible to create a three dimensional model of the magnetic field morphology. Also, calculating the various energies associated with W3 revealed that it is most likely in virial equilibrium, not expanding or contracting. The second project used an instrument on the Hubble Space Telescope (HST) to measure the abundance of iron in a neutral region near the Orion Nebula called Orion’s Veil. One of the goals of this project is to investigate whether solid dust grains can be destroyed by ionizing radiation by comparing the amount of solid iron in Orion’s Veil to the amount in the nearby ionized regions. By measuring the depletion of iron in the neutral Veil and comparing it to the depletion of iron the H+ regions of the Orion Nebula, it was possible to conclude that iron was not being released into the gas phase by ultraviolet photons. In addition, oscillator strengths for two Fe II transitions were measured.

In astronomy, the Sun has an important role. It keeps the solar-system together and is thesource for life, heat, light and energy to Earth. As any other star or planet, the Sun has amagnetic field. The magnetic field of the Sun has a great impact on the Sun itself as well asits surrounding. The magnetic field shapes solar wind, causes flares and drives coronal massejections radiating towards the Earth (and other planets). The Sun's magnetic field is still notfully understood, and therefore it is useful to study other stars with properties similar to theSun. So by studying young solar-type stars, the evolution of the Sun can be more easilyunderstood. The aim of this project is to study the surface magnetic field in a youngsolar-type star, π1 UMa to see how the magnetic field is distributed and if there are anypatterns like polarity reversals. Magnetic field generates polarisation and with Stokes vector Iand V, polarisation can be described. Earlier measurements from two time-epochs (2014 and2015) of Stokes I and V have been obtained from the spectropolarimeter NARVAL. To getthe desired mean polarisation profiles of the star, a technique called least squaredeconvolution was applied which increases the signal-to-noise level. To reconstruct themagnetic topology the Zeeman-Doppler imaging technique was used. Then we obtained thesurface magnetic field maps of both measurements. No change of the polarity of magneticfield at the visible stellar pole was found. Most of the magnetic field energy was contained inthe spherical harmonic modes with angular degrees l=1-3. The star shows dominance in thetoroidal component so the study seem to agree with the previously established trend thatyounger and faster rotating stars have predominantly toroidal magnetic fields and older starswith slower rotation rate, like the Sun, have predominantly poloidal field. Looking at themagnetic field plots, the star show dominance in the azimuthal field component, and themean magnetic field strength is similar to one found in the previous study. The results of thesurface magnetic field in our study thus agrees with previous study of the same star. With thiswe can conclude that the Sun's magnetic field probably been different when it was younger, and possibly similar to the star analyzed in this study.

We present a complete census of all Herschel-detected sources within the six massive lensing clusters of the HST Frontier Fields (HFF). We provide a robust legacy catalogue of 263 sources with Herschel fluxes, primarily based on imaging from the Herschel Lensing Survey and PEP/HerMES Key Programmes. We optimally combine Herschel, Spitzer and WISE infrared (IR) photometry with data from HST, VLA and ground-based observatories, identifying counterparts to gain source redshifts. For each Herschel-detected source we also present magnification factor (mu), intrinsic IR luminosity and characteristic dust temperature, providing a comprehensive view of dust-obscured star formation within the HFF. We demonstrate the utility of our catalogues through an exploratory overview of the magnified population, including more than 20 background sub-LIRGs unreachable by Herschel without the assistance gravitational lensing.

Black hole-neutron star (BH-NS) mergers are promising candidates for the progenitors of short-duration gamma-ray bursts (GRBs). With the right initial conditions, the neutron star becomes tidally disrupted, eventually forming a dense, accreting disk around the black hole. The thermal energy of this black hole-disk system can be extracted via neutrino processes, while the spin energy of the black hole can be extracted via magnetic processes. Either (or even a combination of these) processes could feasibly power a relativistic jet with energy ≥~ 10 49 erg and duration ≤~ 2 s, hence producing a short-duration GRB. In this thesis, we investigate BH-NS mergers with three-dimensional, pseudo-Newtonian simulations. We use the simulation code Charybdis, which uses a dimensionally-split, reconstruct-solve-average scheme (i.e. using Riemann solvers) to solve the Euler equations of hydrodynamics. Although the code is based on a Newtonian framework, it includes pseudo- Newtonian approximations of local gravitational wave effects and the innermost stable circular orbit of the BH, which are both general relativistic phenomena. The code also includes the effects of global neutrino emission, shear viscosity and self-gravity. This thesis comprises two main projects. The first project is a parameter study of the equation of state, which encapsulates the relationship between the pressure of a fluid and its other thermodynamic properties. Although the EOS is well understood at low densities, it is yet to be constrained at supranuclear densities, and so must be treated as a parameter in numerical studies of BH-NS mergers. We present simulations using three existing EOSs, in order to investigate their effect on the merger dynamics. We find that the EOS strongly influences the fate of the NS, the properties of the accretion disk, and the neutrino emission. In the second project, we begin upgrading Charybdis to include magnetic field effects, in order to investigate the magnetic processes described above. We implement existing reconstruction and Riemann solver algorithms for the equations of magnetohydrodynamics, and present 1D tests to compare them. When modelling magnetic fields in more than one dimension, we must also deal with the divergence-free condition, ∇. B = 0. We develop a new constrained transport algorithm to ensure our code maintains this condition, and present 2D tests to confirm its accuracy. This algorithm has many advantages over existing ones, including easier implementation, greater computational efficiency and better parallelisation. Finally, we present preliminary tests that use these algorithms in simulations of BH-NS mergers.

The two SCUBA HAlf-Degree Extragalactic Survey (SHADES) fields are amongst the richest places in the sky in terms of multi-wavelength coverage. They comprise an eastern section of the Lockman Hole (LH) and the central portion of the Subaru- XMM/Newton Deep Field (SXDF). In this thesis, I have obtained extremely deep, multi-frequency radio imaging of the SHADES fields using the GiantMetre-wave Radio Telescope (GMRT) and the Very Large Array (VLA), at 610MHz and 1.4GHz, respectively. These data are used to analyse the nature of the sub-milliJansky (sub-mJy) radio population, which has been hotly debated in the last few years: are they powered by star-forming or nuclear activity? To tackle the problem, I employ different approaches making use of the large variety of multi-wavelength data in the SHADES fields. I begin by analysing the spectral index, α610MHz 1.4GHz , of radio sources detected in the LH, to explore the dominant emission mechanism. Based on a robust 10 σ detection criterion, I find a constantmedian spectral index of α610MHz 1.4GHz ≈ −0.6 to −0.7 for sources between S1.4GHz ≈ 200 μJy and 10mJy. This result suggests that the galaxy population in the sub-mJy regime is powered by optically-thin synchrotron emission – starforming galaxies or lobe-dominated active galactic nuclei (AGN). Making use of X-ray observations in the LH, I show that the fraction of radio sources detected in the hard X-ray band (between 2 and 10 keV) decreases from 50 to 15 per cent between S1.4GHz ≈ 1mJy and . 100 μJy, which strongly suggests a transition from AGN to star-forming galaxies. Based on the deep, multi-wavelength coverage of the SXDF, I explore the behaviour of the far-infrared (FIR)/radio correlation as a function of redshift. I combine the q24 factor – the logarithmic flux density ratio between Spitzer 24-μm and VLA 1.4- GHz flux densities – with available photometric redshifts and find strong evidence that the correlation holds out to z ≈ 3.5. Based on M82-like k-corrections and using a high-significance (S1.4GHz > 300 μJy) radio sub-sample, I find a mean and scatter of q24 = 0.71 ± 0.47. Monte-Carlo simulations based on these findings show that fewer sources deviate from the correlation at fainter flux densities (i.e. fewer radioloud AGN). I predict that the radio-loud fraction drops from 50 per cent at ∼ 1mJy to zero at . 100 μJy. The validity of the FIR/radio correlation out to very high redshifts adds credibility to identifications of sub-millimetre (submm) galaxies (SMGs) made at radio wavelengths. Based on a sample of 45 radio-identified SMGs in the LH, I find a median radio spectral index of α610MHz 1.4GHz = −0.72 ± 0.07, which suggests that optically-thin synchrotron is the dominant radio emission mechanism. Finally, as anAppendix I include a theoretical treatment that constrains the average geometry of the dusty, torus-like structures believed to obscure a large fraction of the AGN population. I use the distribution of column densities (NH) obtained from deep ∼ 1Msec X-ray observations in the Chandra Deep Field South. I find that to reproduce the wide observed range of NH, the best torus model is given by a classical “donut”- shaped distribution with an exponential angular dependency of the density profile.

L'étude du champ électromagnétique autour des étoiles à neutrons est l'une des méthodes vitales pour comprendre la physique des pulsars. Alors que la plupart des publications utilisent l'hypothèse d'un champ électromagnétique dipolaire centré standard, des études récentes se sont concentrées sur l'inclusion de composantes de champ multipolaire plus élevées et ont présenté une image plus générale pour les pulsars dans lesquels le moment du dipôle magnétique est décalé du centre de l'étoile. Ce travail discute des conséquences d'un dipôle magnétique rotatif excentré dans le vide en montrant diverses caractéristiques des lignes de champ magnétique et de l'émission de pulsar. Une étude à large bande du spectre du rayonnement pulsar est également présentée par la création de cartes des différentes régions d'émission des pulsars distinguées sur la base de leur fréquence dans le but principal de rechercher l'évolution du profil d'impulsion avec la fréquence. La thèse présente tous les résultats ci-dessus accompagnés des discussions nécessaires pour comprendre les modèles théoriques utilisés et les détails des méthodes numériques appliquées
Studying the electromagnetic field around neutron stars is one of the vital methods to understand the physics of the pulsars. While major literature uses assumption of a standard centred dipolar electromagnetic field, recent studies have focused on including higher multipolar field components and have presented a more generalized picture for pulsars in which the magnetic dipole moment is shifted off from the centre of the star. This work discusses the consequences of an off centred rotating magnetic dipole in vacuum by showing various characteristic features of magnetic field lines and pulsar emission. A broadband spectrum study of pulsar radiation is also laid out by creating maps of different emission regions of pulsars distinguished on the basis of their frequency with the main aim of looking for the evolution of the pulse profile with frequency. The thesis presents all the above results accompanied by the necessary discussions to understand the theoretical models used and the details of the numerical methods applied

In this thesis we investigated two major issues in astrophysical flows: the transport of magnetic fields in highly conducting fluids in the presence of turbulence, and the turbulence evolution and turbulent dynamo amplification of magnetic fields in collisionless plasmas. The first topic was explored in the context of star-formation, where two intriguing problems are highly debated: the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (the so called \"magnetic flux problem\") and the formation of rotationally sustained protostellar discs in the presence of the magnetic fields which tend to remove all the angular momentum (the so called \"magnetic braking catastrophe\"). Both problems challenge the ideal MHD description, usually expected to be a good approximation in these environments. The ambipolar diffusion, which is the mechanism commonly invoked to solve these problems, has been lately questioned both by observations and numerical simulation results. We have here investigated a new paradigm, an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD). We tested the TRD through fully 3D MHD numerical simulations, injecting turbulence into molecular clouds with initial cylindrical geometry, uniform longitudinal magnetic field and periodic boundary conditions. We have demonstrated the efficiency of the TRD in decorrelating the magnetic flux from the gas, allowing the infall of gas into the gravitational well while the field lines migrate to the outer regions of the cloud. This mechanism works for clouds starting either in magnetohydrostatic equilibrium or initially out-of-equilibrium in free-fall. We estimated the rates at which the TRD operate and found that they are faster when the central gravitational potential is higher. Also we found that the larger the initial value of the thermal to magnetic pressure ratio (beta) the larger the diffusion process. Besides, we have found that these rates are consistent with the predictions of the theory, particularly when turbulence is trans- or super-Alfvénic. We have also explored by means of 3D MHD simulations the role of the TRD in protostellar disks formation. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low mass stars. Previous studies showed that an enhanced microscopic diffusivity of about three orders of magnitude larger than the Ohmic diffusivity would be necessary to enable the formation of a rotationally supported disk. However, the nature of this enhanced diffusivity was not explained. Our numerical simulations of disk formation in the presence of turbulence demonstrated the efficiency of the TRD in providing the diffusion of the magnetic flux to the envelope of the protostar during the gravitational collapse, thus enabling the formation of rotationally supported disks of radius ~ 100 AU, in agreement with the observations. The second topic of this thesis has been investigated in the framework of the plasmas of the intracluster medium (ICM). The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action which is known to amplify the magnetic energy until close equipartition with the kinetic energy. This is generally derived employing a collisional MHD model. However, this is poorly justified a priori since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless-MHD description. We have studied here the turbulence statistics and the turbulent dynamo amplification of seed magnetic fields in the ICM using a single-fluid collisionless-MHD model. This introduces an anisotropic thermal pressure with respect to the direction of the local magnetic field and this anisotropy modifies the MHD linear waves and creates kinetic instabilities. Our collisionless-MHD model includes a relaxation term of the pressure anisotropy due to the feedback of the mirror and firehose instabilities. We performed 3D numerical simulations of forced transonic turbulence in a periodic box mimicking the turbulent ICM, assuming different initial values of the magnetic field intensity and different relaxation rates of the pressure anisotropy. We showed that in the high beta plasma regime of the ICM where these kinetic instabilities are stronger, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model in the description of the statistical properties of the turbulence. Also, the amplification of the magnetic energy due to the turbulent dynamo action when considering an initial seed magnetic field is similar to the collisional-MHD model, particularly when considering an instantaneous anisotropy relaxation. The models without any pressure anisotropy relaxation deviate significantly from the collisional-MHD results, showing more power in small-scale fluctuations of the density and velocity field, in agreement with a significant presence of the kinetic instabilities; however, the fluctuations in the magnetic field are mostly suppressed. In this case, the turbulent dynamo fails in amplifying seed magnetic fields and the magnetic energy saturates at values several orders of magnitude below the kinetic energy. It was suggested by previous studies of the collisionless plasma of the solar wind that the pressure anisotropy relaxation rate is of the order of a few percent of the ion gyrofrequency. The present study has shown that if this is also the case for the ICM, then the models which best represent the ICM are those with instantaneous anisotropy relaxation rate, i.e., the models which revealed a behavior very similar to the collisional-MHD description.
Nesta tese, investigamos dois problemas chave relacionados a fluidos astrofísicos: o transporte de campos magnéticos em plasmas altamente condutores na presença de turbulência, e a evolução da turbulência e amplificação de campos magnéticos pelo dínamo turbulento em plasmas não-colisionais. O primeiro tópico foi explorado no contexto de formação estelar, onde duas questões intrigantes são intensamente debatidas na literatura: a necessidade da difusão de fluxo magnético durante o colapso gravitacional de nuvens moleculares, a fim de explicar as intensidades dos campos magnéticos observadas em proto-estrelas (o denominado \"problema do fluxo magnético\"), e a formação de discos proto-estelares sustentados pela rotação em presença de campos magnéticos, os quais tendem a remover o seu momento angular (a chamada \"catástrofe do freamento magnético\"). Estes dois problemas desafiam a descrição MHD ideal, normalmente empregada para descrever esses sistemas. A difusão ambipolar, o mecanismo normalmente invocado para resolver estes problemas, vem sendo questionada ultimamente tanto por observações quanto por resultados de simulações numéricas. Investigamos aqui um novo paradigma, um mecanismo de difusão alternativo baseado em reconexão magnética rápida induzida pela turbulência, que denominamos reconexão turbulenta (TRD, do inglês turbulent reconnection diffusion). Nós testamos a TRD através de simulações numéricas tridimensionais MHD, injetando turbulência em nuvens moleculares com geometria inicialmente cilíndrica, permeadas por um campo magnético longitudinal e fronteiras periódicas. Demonstramos a eficiência da TRD em desacoplar o fluxo magnético do gás, permitindo a queda do gás no poço de potencial gravitacional, enquanto as linhas de campo migram para as regiões externas da nuvem. Este mecanismo funciona tanto para nuvens inicialmente em equilíbrio magneto-hidrostático, quanto para aquelas inicialmente fora de equilíbrio, em queda livre. Nós estimamos as taxas em que a TRD opera e descobrimos que são mais rápidas quando o potencial gravitacional é maior. Também verificamos que quanto maior o valor inicial da razão entre a pressão térmica e magnética (beta), mais eficiente é o processo de difusão. Além disto, também verificamos que estas taxas são consistentes com as previsões da teoria, particularmente quando a turbulência é trans- ou super-Alfvénica. Também exploramos por meio de simulações MHD 3D a influência da TRD na formação de discos proto-estelares. Sob condições MHD ideais, a remoção do momento angular do disco progenitor pelo campo magnético da nuvem pode evitar a formação de discos sustentados por rotação durante a fase principal de acreção proto-estelar de estrelas de baixa massa. Estudos anteriores mostraram que uma super difusividade microscópica aproximadamente três ordens de magnitude maior do que a difusividade ôhmica seria necessária para levar à formação de um disco sustentado pela rotação. No entanto, a natureza desta super difusividade não foi explicada. Nossas simulações numéricas da formação do disco em presença de turbulência demonstraram a eficiência da TRD em prover a diffusão do fluxo magnético para o envelope da proto-estrela durante o colapso gravitacional, permitindo assim a formação de discos sutentados pela rotação com raios ~ 100 UA, em concordância com as observações. O segundo tópico desta tese foi abordado no contexto dos plasmas do meio intra-aglomerado de galáxias (MIA). A amplificação e manutenção dos campos magnéticos observados no MIA são normalmente atribuidas à ação do dínamo turbulento, que é conhecidamente capaz de amplificar a energia magnética até valores próximos da equipartição com a energia cinética. Este resultado é geralmente derivado empregando-se um modelo MHD colisional. No entanto, isto é pobremente justificado a priori, pois no MIA o caminho livre médio de colisões íon-íon é da ordem das escalas dinâmicas, requerendo então uma descrição MHD não-colisional. Estudamos aqui a estatística da turbulência e a amplificação por dínamo turbulento de campos magnéticos sementes no MIA, usando um modelo MHD não-colisional de um único fluido. Isto indroduz uma pressão térmica anisotrópica com respeito à direção do campo magnético local. Esta anisotropia modifica as ondas MHD lineares e cria instabilidades cinéticas. Nosso modelo MHD não-colisional inclui um termo de relaxação da anisotropia devido aos efeitos das instabilidades mirror e firehose. Realizamos simulações numéricas 3D de turbulência trans-sônica forçada em um domínio periódico, mimetizando o MIA turbulento e considerando diferentes valores iniciais para a intensidade do campo magnético, bem como diferentes taxas de relaxação da anisotropia na pressão. Mostramos que no regime de plasma com altos valores de beta no MIA, onde estas instabilidades cinéticas são mais fortes, uma rápida taxa de relaxação da anisotropia produz resultados similares ao modelo MHD colisional na descrição das propriedades estatísticas da turbulência. Além disso, a amplificação da energia mangética pela ação do dínamo turbulento quando consideramos um campo magnético semente, é similar ao modelo MHD colisional, particularmente quando consideramos uma relaxação instantânea da anisotropia. Os modelos sem qualquer relaxação da anisotropia de pressão mostraram resultados que se desviam significativamente daqueles do MHD colisional, mostrando mais potências nas flutuações de pequena escala da densidade e velocidade, em concordância com a presença significativa das instabilidades cinéticas nessas escalas; no entanto, as flutuações do campo magnético são, em geral, suprimidas. Neste caso, o dínamo turbulento também falha em amplificar campos magnéticos sementes e a energia magnética satura em valores bem abaixo da energia cinética. Estudos anteriores do plasma não-colisional do vento solar sugeriram que a taxa de relaxação da anisotropia na pressão é da ordem de uma pequena porcentagem da giro-frequência dos íons. O presente estudo mostrou que, se este também é o caso para o MIA, então os modelos que melhor representam o MIA são aqueles com taxas de relaxação instantâneas, ou seja, os modelos que revelaram um comportamento muito similar à descrição MHD colisional.

Polarized deep inelastic scattering experiments play a vital role in the exploration of the spin structure of the proton. The polarized proton-proton collider at RHIC provides direct access to the gluon spin distribution through longitudinal double spin asymmetry measurements of inclusive jets, pions, and dijets. This thesis presents the measurement of the dijet double differential cross-section in proton-proton collisions at center of mass energies of √s = 500 GeV. The data represent an integrated luminosity of 8.7 pb-1 recorded by the STAR detector during the 2009 RHIC run. A comprehensive jet analysis was performed to determine the ideal jet algorithm and jet parameters used in √s = 500 GeV collisions at the STAR detector. The cross-section is measured as a function of the dijet invariant mass (30 ≤ Mij ≤ 152 GeV) in the mid rapidity region with a maximum rapidity range of |ymax| ≤ 0.8. This result shows agreement with theoretical next-to-leading order pQCD calculations, motivating the use of dijet asymmetries at STAR to further constrain the shape of Δg(x).

Kampung is a pervasive concept in Malay Culture and considered counter urban in contemporary discourse. Rural to urban migration of the Malays from kampung to cities occur at an accelerated pace in urbanizing Malaysia. Rural migrants are said to remain attached to their rural kampung lifestyles and find the socio-spatial character of urban environment difficult to adapt to. Previous studies on rural kampung by anthropologists and social scientists have unpacked the socio-economic and cultural aspects of kampung Malays in rural area. My study of migrants in Alor Star and Kuala Lumpur is focused on the landscape meanings of kampung and explores how these ideas have been brought across to a city environment. I investigated the meanings and symbolic values that kampung holds to the rural-urban migrants through a ‘landscape lens’. I recorded the experiences of the rural-urban migrants in adapting to an urban landscape, identified kampung elements to which people have strong attachment with and highlighted the kampung characteristics that could be maintained or replicated in order to address the maladaptation of the migrants and enhance their urban living experience. Study participants were rural-urban migrant respondents from rural kampung in Yan, Kedah who have either moved to Kuala Lumpur or Alor Star. The case studies in the two cities were carried out using qualitative methods including photo elicitation, in-depth interviews, model mapping techniques and participant observation. Respondents provided narratives of their journey from kampung, moving to the city, and their process of adapting and settling in cities. Challenges in adaptation to city living spaces included spatial use, privacy, social relationships, safety and surveillance. My findings demonstrated that the memory of kampung plays a significant part in guiding the life of respondents in the city, and that the image of kampung is pervasive in the daily social and spatial practice of rural-urban migrants, guiding respondents’ level of adaptation and place-making in the city landscape. The use of landscape as lens was helpful in interpreting the complex and multivalent kampung meanings. Addressing a dynamic kampung idea through a landscape framework highlights the strong parallels between kampung and the early landscape concepts. The process of unweaving the meanings of kampung have illustrated that kampung ideas have the potential to inspire a landscape design language that could mitigate the harsh contrast between rural and urban Malaysia.

We directly detect dust emission in an optically detected, multiply imaged galaxy lensed by the Frontier Fields cluster MACSJ0717.5+3745. We detect two images of the same galaxy at 1.1 mm with the AzTEC camera on the Large Millimeter Telescope leaving no ambiguity in the counterpart identification. This galaxy, MACS0717_Az9, is at z > 4 and the strong lensing model (mu = 7.5) allows us to calculate an intrinsic IR luminosity of 9.7 x 10(10) L-circle dot and an obscured star formation rate of 14.6 +/- 4.5 M-circle dot yr(-1). The unobscured star formation rate from the UV is only 4.1 +/- 0.3 M-circle dot yr(-1), which means the total star formation rate (18.7 +/- 4.5 M-circle dot yr(-1)) is dominated (75%-80%) by the obscured component. With an intrinsic stellar mass of only 6.9 x 10(9) M circle dot, MACS0717_Az9 is one of only a handful of z. >. 4 galaxies at these lower masses that is detected in dust emission. This galaxy lies close to the estimated star formation sequence at this epoch. However, it does not lie on the dust obscuration relation (IRX-beta) for local starburst galaxies and is instead consistent with the Small Magellanic Cloud attenuation law. This remarkable lower mass galaxy, showing signs of both low metallicity and high dust content, may challenge our picture of dust production in the early universe.

Neste trabalho investigamos os efeitos de choques (induzidos por supernovas) e de turbulência magneto-hidrodinâmica no processo de formação estelar. Primeiramente, considerando o impacto de um remanescente de supernova (RSN) com uma nuvem neutra magnetizada derivamos analiticamente um conjunto de condições através das quais estas interações podem levar à formação de estruturas densas capazes de tornarem-se gravitacionalmente instáveis e formar estrelas. Usando estas condições, construímos diagramas do raio do RSN, $R_$, versus a densidade inicial da nuvem, $n_c$, os quais delimitam um domínio no espaço paramétrico onde a formação estelar é permitida. Estes diagramas foram testados através de simulações numéricas magneto-hidrodinâmicas tridimensionais (3D MHD) onde seguimos a evolução espaço-temporal da interação de um RSN com uma nuvem auto-gravitante. Verificamos que a análise numérica está de acordo com os resultados previstos pelos diagramas. Observamos ainda que a presença de um campo magnético fraco, $\\sim 1 \\; \\mu$G, inicialmente homogêneo e perpendicular à velocidade de impacto do RSN, resulta em uma pequena diminuição da região permitida para formação estelar nos diagramas quando comparado a diagramas para nuvens não magnetizadas. Já um campo magnético mais intenso ($\\sim 10\\;\\mu$G) causa um encolhimento significativo nestas, como esperado. Embora derivados de considerações analíticas simples estes diagramas fornecem uma ferramenta útil para identificar locais onde a formação estelar pode ter sido induzida pelo impacto de uma onda de choque de SN. Aplicações a algumas regiões de nossa Galáxia (como a Grande Concha de CO na direção de Escorpião e a Nuvem Periférica 2 na direção da constelação de Cassiopeia) mostram que a formação estelar nestes locais pode ter sido induzida por uma onda de choque de um RSN em passado recente, quando se consideram valores específicos para as condições iniciais das nuvens impactadas.%, para valores específicos de raio do RSN e uma faixa de densidades iniciais possíveis para estas nuvens. Avaliamos também a eficiência de formação estelar efetiva para estas interações e encontramos que esta é geralmente menor do que os valores observados para a nossa Galáxia (sfe $\\sim$ 0.01$-$0.3). Este resultado é consistente com outros trabalhos da literatura e também sugere que este mecanismo, embora poderoso para induzir a formação de estruturas, turbulência supersônica e eventualmente formação estelar local, não parece ser suficiente para induzir a formação estelar global em galáxia normais, nem mesmo quando o campo magnético é desprezado. Além do estudo acima, exploramos ainda a formação estelar considerando a injeção prévia de turbulência (por um mecanismo físico arbitrário) em nuvens magnetizadas. Para uma nuvem ou glóbulo de nuvem molecular formar estrelas deve haver transporte de fluxo magnético das regiões internas mais densas para as regiões externas menos densas da nuvem, de outra forma o colapso poderá ser impedido pela força magnética. Consideramos aqui um novo mecanismo. Reconexão magnética rápida, a qual ocorre em presença de turbulência, pode induzir um processo de difusão eficiente dos campos magnéticos. Neste trabalho investigamos esse processo por meio de simulações numéricas 3D MHD e suas implicações para a formação estelar, estendendo um estudo prévio realizado para nuvens de simetria cilíndrica e sem auto-gravidade (Santos-Lima et al. 2010). Aqui consideramos nuvens mais realistas com potenciais gravitacionais esféricos (devido a estrelas embebidas) e também levando em conta os efeitos da auto-gravidade do gás. Determinamos, pela primeira vez, quais as condições em que o transporte do campo magnético devido à difusão por reconexão turbulenta leva uma nuvem inicialmente subcrítica a tornar-se super-crítica e capaz de colapsar para formar estrelas. Nossos resultados indicam que a formação de um núcleo supercrítico é resultado de uma complexa interação entre gravidade, auto-gravidade, intensidade do campo magnético e turbulência aproximadamente trans-sônica e trans-Alfvénica. Em particular, a auto-gravidade favorece a difusão do campo magnético por reconexão turbulenta e, como resultado, seu desacoplamento do gás colapsante torna-se mais eficiente do que quando apenas um campo gravitacional externo está presente. Demonstramos que a difusão por reconexão turbulenta é capaz de remover fluxo magnético da maior parte das nuvens investigadas, porém somente uma minoria desenvolve núcleos aproximadamente críticos ou super-críticos, o que é consistente com as observações. A formação destes é restrita ao seguinte intervalo de condições iniciais para as nuvens: razão pressão térmica-pressão magnética, $\\beta \\sim 1$ a $3$, razões entre a energia turbulenta e a energia magnética $E_/E_\\sim 1.62$ a $2.96$, e densidades $50 < n < 140$ cm$^$, quando consideramos massas estelares M$_{\\star}\\sim 25$M$_{\\odot}$, implicando uma massa total da nuvem (gás + estrelas) M$_\\lesssim 120$M$_{\\odot}$.
In this work, we have investigated the effects of shocks (induced by supernovae) and magnetohydrodynamical turbulence in the process of star formation. Considering first, the impact of a supernova remnant (SNR) with a neutral magnetized cloud we derived analytically a set of conditions through which these interactions can lead to the formation of dense structures able to become gravitationally unstable and form stars. Using these conditions, we have built diagrams of the SNR radius, $R_{SNR}$, versus the initial cloud density, $n_c$, that constrain a domain in the parameter space where star formation is allowed. These diagrams have been also tested by means of three-dimensional magneto-hydrodynamical (3D MHD) numerical simulations where the space-time evolution of a SNR interacting with a self-gravitating cloud is followed. We find that the numerical analysis is in agreement with the results predicted by the diagrams. We have also found that the effects of a weak homogeneous magnetic field ($\\sim 1 \\; \\mu$G) approximately perpendicular to the impact velocity of the SNR results only a small decrease of the allowed zone for star formation in the diagrams when compared with the diagrams with non-magnetized clouds. A larger magnetic field ($\\sim 10\\;\\mu$G) on the other hand, causes a significant shrinking of the star formation zone, as one should expect. Although derived from simple analytical considerations, these diagrams provide a useful tool for identifying sites where star formation could be triggered by the impact of a SN blast wave. Applications of them to a few regions of our own Galaxy (e.g., the large CO shell in the direction of Scorpious, and the Edge Cloud 2 in the direction of the Cassiopeia constellation) have revealed that star formation in those sites could have been triggered by shock waves from SNRs in a recent past, when considering specific values of the SNR radius and the initial conditions in the neutral clouds. We have also evaluated the effective star formation efficiency for this sort of interaction and found that it is generally smaller than the observed values in our Galaxy (sfe $\\sim$ 0.01$-$0.3). This result is consistent with previous work in the literature and also suggests that the mechanism presently investigated, though very powerful to drive structure formation, supersonic turbulence and eventually, local star formation, does not seem to be sufficient to drive $global$ star formation in normal star forming galaxies, not even when the magnetic field is neglected. Besides the study above, we have also explored star formation considering a priori injection of turbulence (by an arbitrary physical mechanism) in magnetized clouds. For a molecular cloud clump to form stars some transport of magnetic flux may be required from the denser, inner regions to the outer regions of the cloud, otherwise this can prevent the gravitational collapse. We have considered here a new mechanism. Fast magnetic reconnection which takes place in the presence of turbulence can induce a process of reconnection diffusion of the magnetic field. In this work, we have investigated this process by means of 3D MHD numerical simulations considering its implications on star formation. We have extended a previous study which considered clouds with cylindrical geometry and no self-gravity (Santos-Lima et al. 2010). Here, we considered more realistic clouds with spherical gravitational potentials (from embedded stars) and also accounted for the effects of the gas self-gravity. We demonstrated that reconnection diffusion takes place. We have also, for the first time, determined the conditions under which reconnection diffusion is efficient enough to make an initially subcritical cloud clump to become supercritical and collapse. Our results indicate that the formation of a supercritical core is regulated by a complex interplay between gravity, self-gravity, magnetic field strength and nearly transonic and trans-Alfvénic turbulence. In particular, self-gravity helps reconnection diffusion and, as a result, the magnetic field decoupling from the collapsing gas becomes more efficient than in the case when only an external gravitational field is present. We have demonstrated that reconnection diffusion is able to remove magnetic flux from most of the collapsing clumps analysed, but only a few of them develop nearly critical or supercritical cores, which is consistent with the observations. Their formation is restricted to a range of initial conditions for the clouds as follows: thermal to magnetic pressure ratios $\\beta \\sim$ 1 to 3, turbulent to magnetic energy ratios $E_{turb}/E_{mag}\\sim 1.62$ to $2.96$, and densities $50 < n < 140$ cm$^{-3}$, when considering stellar masses M$_{\\star}\\sim 25$M$_{\\odot}$, implying total (gas+stellar) masses M$_{tot} \\lesssim 120$M$_{\\odot}$.

Observational evidence that star formation proceeds in a clustered manner raises a question on the mass function of star clusters and their evolution. However, we have a limited scope of these processes in the Milky Way galaxy. The M31 galaxy is the nearest stellar system similar to our Galaxy, therefore, it is the most suitable one to provide clues for understanding the star cluster population and the evolution of galactic structures. However, detailed study of stellar populations and star clusters is a challenging task for ground-based observations due to crowded stellar fields. Using Subaru telescope Suprime-Cam wide-field images, a survey of clusters was carried out in the disk region of the M31 galaxy southwest field, which is a close analogue to that of the Solar neighborhood in terms of chemical composition, stellar density, and quiescent star formation. Data analysis methods and programs were developed, tested, and applied for crowded wide-field image reduction and evolutionary parameter determination of semi-resolved star clusters. The main results are: (1) enhanced star cluster formation activity in M31 occurred ~70 Myr ago; (2) approximately 10% of stars born in star clusters remain there at 100 Myr age; (3) the characteristic lifetime of a cluster of ~10^4 solar masses mass is ~300 Myr; (4) the mass function of star clusters in M31 is similar to that in other low star formation activity galaxies and it is best described by the Schechter's function with a... [to full text]
Daugėjant įrodymų, kad žvaigždės formuojasi spiečiais, jų masių funkcijos ir evoliucijos savybes kol kas galima nustatyti empiriniais stebėjimais. Tačiau tirti žvaigždėdaros procesą mūsų Galaktikoje galima ribotai. Andromedos galaktika (M31) yra artimiausia žvaigždžių sistema, panaši į Galaktiką. Todėl ji tinkamiausia žvaigždžių spiečių populiacijai tirti, tačiau ilgą laiką buvo tirta ribotai dėl tankių žvaigždžių laukų stebėjimo sudėtingumo. Naudojant „Subaru“ teleskopo Suprime-Cam plataus lauko nuotraukas, buvo ištirti žvaigždžių spiečiai M31 galaktikos disko srityje, kuri pagal cheminę sudėtį, žvaigždinį tankį ir mažą žvaigždėdaros spartą yra analogiška Saulės aplinkai mūsų Galaktikoje. Buvo sukurti, išbandyti ir pritaikyti duomenų analizės metodai ir programos tankiems plataus lauko vaizdams apdoroti ir pusiau išskiriamų žvaigždžių spiečių evoliuciniams parametrams nustatyti. Pagrindiniai rezultatai: (1) žvaigždžių spiečių formavimosi spartos sustiprėjimas M31 galaktikoje įvyko prieš ~70 mln. m.; (2) maždaug 10% žvaigždžių spiečių gyvena ilgiau nei 100 mln. m.; (3) būdingoji ~10^4 Saulės masių spiečiaus gyvavimo trukmė yra ~300 mln. m.; (4) žvaigždžių spiečių masių pasiskirstymas panašus į kitų mažos žvaigždėdaros spartos galaktikų žvaigždžių spiečių pasiskirstymą ir jį gerai nusako Schechter'io funkcija, kurios charakteringoji masė ~2x10^5 Saulės masių. Tai rodo, kad M31 galaktikoje yra gausi vidutinės masės žvaigždžių spiečių populiacija, kuri mūsų Galaktikoje yra... [toliau žr. visą tekstą]

Silicon monoxide maser emission has been detected in many evolved stars in circumstellar envelopes in different vibrationally-excited rotational transitions. It is considered a good tracer to study the dynamics in a region close to the photosphere of the star. We present multi-epoch, total intensity, high-resolution images of 43 GHz, v=1, J=1-0 SiO maser emission toward the Mira variable R Cas. In total we have 23 epochs of data for R Cas at approximate monthly intervals over an optical pulsation phase range of φ = 0.158 to φ = 1.782. These maps show a ring-like distribution of the maser features in a shell, which is assumed to be centred on the star at average radius of 1.6 → 2.3 times the radius of star, R⋆. It is clear from these images that the maser emission is significantly extended around the star. At some epochs a faint outer arc can be seen at about 4 R⋆. The intensity of the emission waxes and wanes during the stellar phase. Some maser features are seen infalling as well as outflowing. We have made initial comparisons of our data with models by Gray et al. (2009). We have investigated the polarization morphology by mapping the linear and circular polarization of SiO masers in the v=1, J=1-0 transition. We found that some of the polarization vectors are either tangential or radial, which indicate a bimodal structure of the linear polarization morphology. Other angles can be seen as well. This is consistent with a radial, stellar-centred magnetic field in the SiO maser shell. We found in some isolated features the fractional linear polarization exceeds 100%. In other features, the polarization angle abruptly flips by 90◦. We found that our data are in the regime that the Zeeman splitting rate g is much greater than the stimulated emission rate R which in turn is greater than the decay rate , which indicates that the solution of Goldreich et al. (1973) can be applied.

Nos encontramos em uma localização vantajosa para o estudo da formação e evolução de galáxias espirais. Situados no disco da Via-Láctea, somos capazes de fazer observações detalhadas sobre as estrelas individuais que a compõem. Com o avanço tecnológico das últimas décadas, foi possível coletar um grande conjunto de informações, (e.g. composição química, cinemática, astrometria e parâmetros atmosféricos), com alta resolução para uma vasta parte do volume Galáctico. Com o objetivo de compreender melhor a nossa Galáxia, desenvolvemos uma ferramenta, chamada StarHorse, que estima parâmetros como distâncias, idades, massas e avermelhamento utilizando informação disponível em levantamentos espectroscópicos, fotométricos e astrométricos. O código StarHorse estima os parâmetros através de um método Bayesiano, que constrói uma distribuição de probabilidade a partir do cálculo de verossimilhança entre observações e modelos de evolução teórica e a partir de priores Galácticos bem conhecidos. Os parâmetros que o StarHorse estima são cruciais para estudos de arqueologia Galáctica. Com eles é possível investigar a estrutura, o histórico de formação estelar, a função de massa inicial, o mapa tridimensional da nossa Galáxia e também adicionar vínculos a modelos quemodinâmicos da Via Láctea Neste trabalho, focaremos na descrição e validação do método, testando sua aplicabilidade em levantamentos recentes de espectroscópica, astrometria e fotometria. Também disponibilizamos catálogos com distâncias e extinção para comunidade astronômica. As nossas distâncias e extinções se tornaram referência dentro da colaboração APOGEE e foram liberadas junto com o seu Data Release 14. Ademais, catálogos foram liberados para os surveys RAVE, Gaia-ESO e GALAH. Neste trabalho, exploramos os resultados do StarHorse, especialmente os resultados APOGEE, em um contexto amplo de arqueologia Galáctica.
We are in an advantageous position to study the formation and evolution of disk galaxies. By being inside the Milky Way, we are able to make detailed observations about the individual stars that compose it. With the technological revolution of the latest years, it has been possible to collect a massive set of information, (e.g. chemical composition, kinematics, astrometry, and atmospheric parameters), with high resolution for a large portion of the Galactic volume. With the goal to understand better our Galaxy, we developed a tool, called StarHorse, that can estimate distances, ages, masses, and extinction from the available spectroscopic, astrometric, and photometric information. StarHorse makes these estimates through a Bayesian method, that builds a probability distribution over the models by calculating a likelihood function between observation and stellar evolution models, and by using common knowledge about our Galaxy as priors. The parameters that StarHorse estimates are crucial to Galactic archaeology studies. With them, we can investigate the structure, the star formation history, the initial mass function, the three-dimensional dust map of our Galaxy, and provide constraints to chemodynamical models of the Milky Way. In this work, we focus on the description and validation of the method, testing its applicability in recent spectroscopic and astrometric surveys. We also make available catalogs with distances and extinctions to the astronomy community. Our distances and extinctions became a reference inside the APOGEE-team and were released as part of the SDSS Data Release 14. Moreover, we made available catalogs also to other spectroscopic surveys such as Gaia-ESO, RAVE, and GALAH. In this work, we also explore these results, especially for APOGEE, in a broad Galactic archaeology context.

This paper is an introduction to the subject of magnetic fields on stars, with a focus on hotter stars. Basic astrophysical concepts are explained, including: spectroscopy, stellar classification, general structure and evolution of stars. The Zeeman effect and how absorption line splitting  is used to detect and measure magnetic fields is explained. The properties of a prominent type of magnetic massive star, Ap-stars, are delved into. These stars have very stable, global, roughly dipolar magnetic fields theorized to have been captured during star formation and maintained  throughout their lifetime. Different approaches to making models of the magnetic field of massive stars are explained, in particular the oblique rotator model. Finally, the use of the oblique rotator model is illustrated by finding possible field structures that would reproduce magnetic field observations of the star HD 37776.This paper is an introduction to the subject of magnetic fields on stars, with a focus on hotter stars. Basic astrophysical concepts are explained, including: spectroscopy, stellar classification, general structure and evolution of stars. The Zeeman effect and how absorption line splitting  is used to detect and measure magnetic fields is explained. The properties of a prominent type of magnetic massive star, Ap-stars, are delved into. These stars have very stable, global, roughly dipolar magnetic fields theorized to have been captured during star formation and maintained  throughout their lifetime. Different approaches to making models of the magnetic field of massive stars are explained, in particular the oblique rotator model. Finally, the use of the oblique rotator model is illustrated by finding possible field structures that would reproduce magnetic field observations of the star HD 37776.

Magnetic fields are present throughout the universe and are very important for many astrophysical processes. Magnetic field influences a star throughout its life and affects nearby objects such as planets. Stellar magnetic field can be detected by measuring the Zeeman splitting of spectral lines in the intensity spectra (Stokes I) if the field is strong, or by analyzing polarization spectra if the field is weak. Magnetic fields in stars similar to the Sun are ubiquitous but, in general, relatively weak. Until recently these fields were detected through circular polarization (Stokes V) only since linear polarization (Stokes QU) is significantly weaker. The information embedded in different Stokes spectra is used for reconstruction of the surface magnetic field topology with Zeeman Doppler imaging (ZDI) technique. However, cool stars often have complex field geometries and this, combined with a low field strength, partial Stokes parameter observations and the presence of cool spots, makes accurate magnetic mapping difficult. We have performed numerical tests of ZDI to investigate some of the problems of magnetic inversions and ways to overcome them. The most reliable results were found when magnetic field and temperature inhomogeneities were modelled simultaneously and all four Stokes parameters were included in the reconstruction process. We carried out observations of active cool stars in all four Stokes parameters trying to find an object with linear polarization signatures suitable for ZDI. The RS CVn star II Peg was identified as a promising target, showing exceptionally strong linear polarization signatures. We reconstructed the magnetic field in II Peg using full Stokes vector observations for the first time in a cool star. Compared to the magnetic maps recovered from the Stokes IV spectra, the four Stokes parameter results reveal a significantly stronger and more complex surface magnetic field and a more compact stellar magnetosphere. Spectropolarimetric observations and magnetic inversions can also be used to investigate magnetic activity of the young Sun and its implications for the solar system past. To this end, we studied a sample of six stars with parameters very similar to the present Sun, but with ages of only 100-650 Myr. Magnetic field maps of these young solar analogues suggest a significant decrease of the field strength in the age interval 100-250 Myr and a possible change in the magnetic field topology for stars older than about 600 Myr.

The majority of stars in the Milky Way are M dwarfs whichmake up 75 % of stars in the vicinity of the sun. As the magneticfield of stars can significantly affect the interiors and theatmospheres of the exoplanets they host within the stellar system,characterising this magnetic field is of major importance.In this work, we selected a sample of 23 exoplanet hostM dwarfs and analysed the observations made by the highresolutionspectropolarimeters ESPaDOnS and NARVAL availablein PolarBase for these stars. In particular, we used theLSD Stokes I and V spectra to measure the projection alongthe line of sight of the average magnetic field over the stellarsurface. 60% of stars had a likely magnetic signatures with adetection greater than 2. By taking the maximum value foreach star over the different observations, the median magneticfield strength measured was 55 G and only a few stars exceeded100 G in strength.

Magnetic fields are prevalent in a wide variety of low mass stellar systems and play an important role in their evolution. Yet the process through which these fields are generated is not well understood. To understand how such systems can generate strong field structures characterization of these fields is required. Radio emission traces the fields directly and the properties of this emission can be modeled leading to constraints on the field geometry and magnetic parameters. The new Karl Jansky Very Large Array (VLA) provides highly sensitive radio observations. My thesis involves combining VLA observations with the development of magnetospheric emission models in order to characterize the magnetic fields in two fully convective cool star systems: (1) Young Stellar Objects (YSOs); (2) Ultracool dwarf stars. I conducted multi epoch observations of DG Tau, a YSO with a highly active, collimated outflow. The radio emission observed from this source was found to be optically thick thermal emission with no indication of the magnetic activity observed in X-rays. I determined that the outflow is highly collimated very close to the central source, in agreement with jet launching models. Additionally, I constrained the mass loss of the ionized component of the jet and found that close to the central source the majority of mass is lost through this component. Using lower angular resolution observations, I detected shock formations in the extended jet of DG Tau and modeled their evolution with time. Taking full advantage of the upgraded bandwidth on the VLA, I made wideband observations of two UCDs, TVLM513-46 and 2M 0746+20. Combining these observations with previously published and archival VLA observations I was able to fully characterize the spectral and temporal properties of the radio emission. I found that the emission is dominated by a mildly polarized, non-thermal quiescent component with periodic strongly polarized flare emission. The spectral energy distribution and polarization of the quiescent emission is well modeled using gyrosynchrotron emission with a mean field B~100 G, mildly relativistic power-law electrons with a density ne~105-6 cm-3, and source size of R~2R*. We were able to model the pulsed emission by coherent electron cyclotron radiation from a small number of isolated loops of high magnetic field (2-3 kG) with scale heights~1.2-2.7 stellar radii. The loops are well-separated in magnetic longitude, and are not part of a single dipolar magnetosphere. The overall magnetic configuration of both stars appears to confirm recent suggestions that radio over-luminous UCD's have `weak field' non-axisymmetric topologies, but with isolated regions of high magnetic field.

This paper describes the thesis work of Schnerr (2007) entitled ”Magnetic fields and mass loss in massive stars“, which aimed at a better understanding of the impact of magnetic fields on the winds of massive stars.

Mercury-manganese (HgMn) stars belong to the class of chemically peculiar (CP) stars. It was recently discovered that some HgMn stars have spots of chemical elements on their surfaces. According to conventional picture of CP stars, magnetic field facilitates the formation and long term stability of chemical spots by controlling stratification of elements in stellar atmosphere. However, previous attempts to find magnetic field in HgMn stars set an upper limit on its strength at the level of about 20-100 Gauss. Observational evidence suggested that even weaker magnetic fields can be responsible for the formation of chemical spots. The main goal of our work was to verify this possibility. The search for weak magnetic fields requires the use of least-squares deconvolution (LSD) technique.  This method combines information from many spectral lines providing a mean line profile with increased signal-to-noise ratio. Up to now there was no extensive comparison of the LSD profile with real spectral lines. We showed that the LSD profile of the intensity spectrum does not behave like a real spectral line as a function of chemical composition. However, for circular polarization, LSD profile resembles the profile of a spectral line with mean atomic parameters. We performed a comprehensive search for magnetic field in 47 HgMn stars and their companions, based on high-quality spectropolarimetric data obtained with the HARPSpol polarimeter at the ESO 3.6-m telescope. With the help of LSD technique, an upper limit on the mean longitudinal magnetic field was brought down to 2-10 G for most stars. We concluded that magnetic field is not responsible for the spot formation in HgMn stars. We obtained full rotational phase coverage for the HgMn stars φ Phe and 66 Eri. This enabled us to investigate line profile variability, reconstruct surface maps of chemical elements, and perform a search for magnetic field with very high sensitivity. For φ Phe we derived surface maps of Y, Sr, Ti, Cr, and obtained an upper limit of 4 G on the field strength. We also found marginal indication of vertical stratification of Y and Ti. No magnetic field was detected in both components of 66 Eri, with an upper limit of 10-24 G. We discovered chemical spots of Y, Sr, Ba, and Ti, in the primary star. We demonstrated a relation between the binary orbit and the morphology of these spots.