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The quality and quantity of polarimetric data being collected for stellar sources creates new opportunities for studying stellar properties and evolution, and also leads to new challenges for modeling and interpreting such data. Inspired by fresh prospects for detecting the Hanle effect to study photospheric magnetic fields, we have focused attention on purely geometrical aspects for polarimetric variability in the example of oblique magnetic rotators. In the case of axisymmetric fields, we highlight two key facts: (a) polarimetric lightcurves necessarily exhibit a certain time symmetry with rotation phase, and (b) variations in the polarization position angle can be modeled based on geometrical projection effects, independent of the photospheric magnetic field. These conclusions also have general applicability, such as to Thomson scattering and the transverse Zeeman effect. The authors gratefully acknowledge that funding for this work was provided by the National Science Foundation, grant AST-0807664.

Clumped structures in wind flows have substantially altered our interpretations of multiwavelength data for understanding mass loss from massive stars. Embedded wind shocks have long been the favored explanation for the hot plasma production and X-ray generation in massive star winds. This contribution reports on line profile shapes fromthe clump bowshock model and summarizes the temperature and emission measure distributions throughout the wind for this model with a focus on results that can be tested against observations.The authors acknowledge funding support for this work from a NASA grant(NNH09CF39C

The action of X-rays is commonly invoked to explain the wind properties of low-luminosity O-type stars. These stars have significantly smaller mass loss rates than predicted radiation-driven wind theories. In this respect they may resemble the first generation of supermassive stars in the early universe which presumably had weak winds due to their low metallicity. We present the high-resolution X-ray spectrum of a weak-wind star, mu Col, and discuss the potential for X-ray emission line strengths and profiles to discriminate among proposed mechanisms for the generation of X-rays in stellar winds, and in resolving the weak-wind problem.

Classical Cepheids and eclipsing binary systems are powerful probes for measuring stellar fundamental parameters and constraining stellar astrophysics. A Cepheid in an eclipsing binary system is even more powerful, constraining stellar physics, the distance scale and the Cepheid mass discrepancy. However, these systems are rare, only three have been discovered. One of these, OGLE-LMC-CEP1812, presents a new mystery: where the Cepheid component appears to be younger than its red giant companion. In this work, we present stellar evolution models and show that the Cepheid is actually product of a stellar merger during main sequence evolution that causes the Cepheid to be a rejuvenated star. This result raises new questions into the evolution of Cepheids and their connections to smaller-mass anomalous Cepheids.

Massive star winds are understood to be structured. Structures can come in the form of co-rotating interaction regions, which are globally organized flow streams that thread the winds. Structures can also be stochastic in nature, generically referred to as "clumps". The theory for interpreting the radio emissions from randomly distributed microclumps in single star winds is established. Results are presented here for macroclumping, in which the radiative transfer is sensitive to the clump geometry. Two cases are compared: spherical clumps and pancake-like fragments. The geometry of macroclumps can influence the power-law slope of the long wavelength spectral energy distribution.

<p> Core-collapse supernovae are among the most consequential astronomical events. They impact galaxy evolution, chemical enrichment of the Universe, and the creation of exotic objects (e.g., black holes and neutron stars). However, aspects of supernovae such as explosion asymmetry and progenitor mass loss are not well understood. Young, nearby supernova remnants are excellent laboratories to uniquely constrain some these fundamental properties. In this thesis, I investigate two nearby oxygen-rich supernova remnants and measure the proper motion of their ejecta to estimate their center of expansions and explosion ages. These properties are important for determining central compact object ‘kick’ velocities, guiding searches for surviving companions, and creating 3D remnant reconstructions. </p> <p><br></p> <p>I estimate the center of expansion and age of two supernova remnants, 1E0102.2-7219 (E0102) and N132D utilizing two epochs of Hubble Space Telescope imaging to measure the proper motion of their ejecta. For E0102, the proper motions show evidence for a nonhomologous expansion, which combined with spectral observations, support the idea that this remnant is expanding into an asymmetric circumstellar environment. Using the new proper-motion derived age and center of expansion, I provide a new ‘kick’ velocity estimate for E0102’s candidate neutron star. For N132D, I measure the proper motion of the ejecta both visually and using a novel computer vision procedure which identifies and measures the proper motions of the knots. I find that N132D’s ejecta are still ballistic, along with evidence of explosion asymmetry. My results represent the first proper-motion derived center of expansion and age of N132D. </p> <p><br></p> <p>Finally, I investigate diffuse interstellar bands observed towards progenitor candidates of core-collapse supernovae to test whether time variability can be a possible probe of the mass loss and surrounding environments of these systems. I find evidence of time variability in diffuse interstellar band carriers located in two of these environments. This is especially unusual as diffuse interstellar bands are normally attributed to the interstellar medium. These findings imply that the sources of these bands are closer to the stellar objects than previously thought and can provide insight into the currently unknown sources of diffuse interstellar bands. </p>

The Polstar mission will provide for a space-borne 60cm telescope operating at UV wavelengths with spectropolarimetric capability capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter massive stars' evolution, and finding the consequences for the stellar remnant population and the stirring and enrichment of the interstellar medium, by addressing four key science objectives. In addition, Polstar will determine drivers for the alignment of the smallest interstellar grains, and probe the dust, magnetic fields, and environments in the hot diffuse interstellar medium, including for the first time a direct measurement of the polarized and energized properties of intergalactic dust. Polstar will also characterize processes that lead to the assembly of exoplanetary systems and that affect exoplanetary atmospheres and habitability. Science driven design requirements include: access to ultraviolet bands: where hot massive stars are brightest and circumstellar opacity is highest; high spectral resolution: accessing diagnostics of circumstellar gas flows and stellar composition in the far-UV at 122-200nm, including the NV, SiIV, and CIV resonance doublets and other transitions such as NIV, AlIII, HeII, and CIII; polarimetry: accessing diagnostics of circumstellar magnetic field shape and strength when combined with high FUV spectral resolution and diagnostics of stellar rotation and distribution of circumstellar gas when combined with low near-UV spectral resolution; sufficient signal-to-noise ratios: ~103 for spectropolarimetric precisions of 0.1% per exposure; ~102 for detailed spectroscopic studies; ~10 for exploring dimmer sources; and cadence: ranging from 1-10 minutes for most wind variability studies, to hours for sampling rotational phase, to days or weeks for sampling orbital phase. The ISM and exoplanet science program will be enabled by these capabilities driven by the massive star science.

Dans le milieu interstellaire, parmi les 200 molécules observées, les hydrocarbures sont présents en abondance. La formation des hydrocarbures CHy(+) se fait entre un C+ et un atome d'hydrogène ou une molécule. Par la suite, ces hydrocarbures réagissent entre eux pour former des hydrocarbures plus gros. Ils sont donc les précurseurs de tous les hydrocarbures présents dans le milieu interstellaire ; il est par conséquent nécessaire de les étudier en détail. Ces hydrocarbures CHy(+) sont soumis à divers processus physiques dans le milieu interstellaire notamment la collision avec un électron, l'absorption d'un photon ultraviolet ou d'un rayon cosmique. Ils vont être excités par ces processus et gagner un excès d'énergie qu'ils vont libérer par fragmentation, ce qui a pour conséquence de redistribuer les espèces. Des simulations précises dans la chimie du milieu interstellaire nécessitent donc une connaissance précise des taux de réaction et des rapports de branchement de fragmentation. Afin de documenter tous ces rapports de branchement, peu importe le processus physique ou chimique mis en jeu, nous avons construit expérimentalement des « breakdown curves » semi-empiriques qui sont les rapports de branchement des voies de fragmentation en fonction de l'énergie interne déposée lors du processus.L'expérience a été réalisée en utilisant le multi détecteur silicium AGAT et l'accélérateur ANDROMEDE. Les molécules CHy(+) produites à haute vitesse (3 u.a) collisionnent avec des atomes d’Hélium au repos dans le référentiel du laboratoire. Le dispositif expérimental permet de détecter tous les fragments, même les neutres, et de résoudre toutes les voies de fragmentation. Nous avons mesuré les rapports de branchement de fragmentation des CHyq+ (y=2-4, q=0-3) et les distributions d'énergie cinétique des fragments neutres.A partir des rapports de branchement, des distributions d'énergie cinétique et des énergies de dissociation théorique, nous avons construit les breakdown curves qui se sont révélés en bon accord avec des rapports de branchement expérimentaux déjà existant dans la littérature pour la photodissociation, la recombinaison dissociative et les collisions électroniques. Enfin, un modèle a été développé pour prédire les rapports de branchement de réactions chimiques ainsi que leur évolution avec la température. / In the interstellar medium, among the 200 molecules observed, the hydrocarbons are in abundance. The formation of hydrocarbons CHy(+) is done between a C+ and a hydrocarbon atom or molecule. Thereafter, these hydrocarbons are reacting between them to form bigger hydrocarbons. Therefore, they are the precursor of all the hydrocarbons present in the interstellar medium, so it is necessary to study them in details. These hydrocarbons CHy(+) are under a lot of different physical processes in the interstellar medium including the collision with an electron, the absorption of an ultra violet photon or a cosmic ray. They will be excited by these processes and gain excess energy they will liberate by fragmentation which leads to a redistribution of species. Therefore, a precise knowledge of the rate of reaction and of the branching ratios of the fragmentation is needed to do specific simulations in the chemistry of the interstellar medium. In order to document all these branching ratios, no matter the physical or chemical process at stake, we experimentally built semi-empirical breakdown curves which are the branching ratios of the paths of fragmentation as a function of the internal energy of the molecule.The experiment was done using the AGAT silicon multi-detector and the ANDROMEDE accelerator. CHy(+) molecules produced at high velocity (3 u.a.) are collided with He atom at rest in the lab. Thanks to the experimental developments, all fragments, neutral or charged, are separately identified, allowing to resolve all fragmentation channels. Therefore, we have been able to measure fragmentation branching ratios for CHyq+ (y=2-4, q=0-3) and the kinetic energy distributions of the neutral fragments.From the branching ratios, the kinetic energy distributions and the theoretical dissociation energies we built BDCs that revealed to be in accordance with the experimental branching ratios which already exists in the literature concerning the photo dissociation, the dissociative recombination and the electronic collisions. Finally, a model has been developed to predict the chemical reactions of the branching ratios as well as to predict their evolution with the temperature.

Space travelers in science fiction can drop out of hyperspace and make a pinpoint landing on any strange new world without stopping to get their bearings, but real-life space navigation is an art characterized by limited information and complex mathematics that yield no easy answers. This study investigates, for the first time ever, what position and velocity estimation errors can be expected by a starship arriving at a distant star - specifically, a miniature probe like those proposed by the Breakthrough Starshot initiative arriving at Proxima Centauri. Such a probe consists of nothing but a small optical camera and a small microprocessor, and must therefore rely on relatively simple methods to determine its position and velocity, such as observing the angles between its destination and certain guide stars and processing them in an algorithm known as an extended Kalman filter. However, this algorithm is designed for scenarios in which the position and velocity are already known to high accuracy. This study shows that the extended Kalman filter can reliably estimate the position and velocity of the Starshot probe at speeds characteristic of current space probes, but does not attempt to model the filter’s performance at speeds characteristic of Starshot-style proposals. The gravity of the target star is also estimated using the same methods.