To investigate if magnetic fields are present in Wolf–Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf–Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3σ (〈Bz〉 = 258 ± 78 G). Among the other targets, the highest value for the longitudinal magnetic field, 〈Bz〉 = 327 ± 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the 〈Bz〉 variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.

In this paper we investigate the multiwavelengths properties of the magnetic early B-type star HR7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM X-ray telescope. Modeling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR7355 magnetosphere. A comparison between HR7355 and a similar analysis for the Ap star CUVir, allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR7355 and is likely relevant for magnetospheres of other magnetic early type stars.

We report the detection of the auroral radio emission from the early-type magnetic star HD 142301. New VLA observations of HD 142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD 142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD 142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense (and vice versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD 142301 magnetosphere.

Context. High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths. Aims. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R = f∕i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability. Methods. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. We focus on wind integration for total line luminosities, taking account of radiative pumping and stellar occultation. While the case of a variable stellar radiation field was explored in the first paper, the effects of wind variability are emphasized in this work. Results. We formulate an expression for the ratio of line luminosities f∕i that closely resembles the classic expression for the on-the-spot result. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example for wind integration, particularly since this produces no variability for the on-the-spot case. The f∕i ratio can be significantly modulated owing to evolving wind properties. The extent of the variation depends on how the timescale for the wind flow compares to the timescale over which the line emissivities change. Conclusions. While a variety of factors can ellicit variable line ratios, a time-varying mass-loss rate serves to demonstrate the range of amplitude and phased-dependent behavior in f∕i line ratios. Importantly, we evaluate how variable mass loss might bias measures of f∕i. For observational exposures that are less than the timescale of variable mass loss, biased measures (relative to the time-averaged wind) can result; if exposures are long, the f∕i ratio is reflective of the time-averaged spherical wind.

Polycyclic aromatic hydrocarbons (PAHs) are the most abundant large organic molecules in space. They are thought to be the main contributor to the unidentified infrared (UIR) emission bands from the interstellar medium (ISM) for several reasons: UIR intensities correspond to carbon abundance, indicating the presence of a carbon-based molecule; UIRs are found in extremely harsh environments which means the source must be a stable molecule. The most important evidence is if the bands in mid-infrared (MIR) or 'fingerprint' region match those of PAHs. Through the infrared spectroscopy of matrix-isolated polycyclic aromatic hydrocarbons a compound's unique neutral and ionized vibrational modes can be identified. In this work, the PAH anthracoronene (AntCor, C36H18) is suspended in a matrix of water-ice, irradiated with ultraviolet (UV) light, and then analyzed using Fourier Transform Infrared (FTIR) spectroscopy. AntCor has not been studied in water ice before, and therefore the vibrational transition data collected (i.e. band positions and intensities) has been compared to coronene and anthracene, the parent molecules, and with theoretical predictions made using density functional theory. The data from this work will be incorporated into the NASA Ames PAH IR Database, where it will be applied to astronomical observations of the unidentified infrared emissions of the ISM, as well as observations of infrared absorption features in dense molecular clouds.

I use single-epoch, SDSS spectroscopy of RR Lyraes identified in the Catalina survey to separate the spectra into same-temperature groups. Then I draw temperature-phase diagrams of the groups. I find shocked stars, improperly phased stars, low amplitude stars, and a few that are more likely eclipsing binaries. The RR Lyraes are then given precise metallicities by measurements of the CaII K and H-β, H-γ, and H-δ lines. This leads to better distance measurements, which allow me to confirm a distinction between the inner and outer galactic halo.

It is some observational evidence that dense cores are the birthplace of low-mass stars. These regions, apparently quiescent, are capable of surviving several free-fall times and, potentially, collapse and form stars. Despite its importance, little is known about the very first evolutionary stages of these objects. The observational challenge that studying these diffuse and extended objects represent is preventing us to reveal their properties. It is easier, from an observational point of view, to study brighter sources as the more evolved Class-0 objects. It is possible then, through theoretical models, to trace the history back of these objects and find the initial conditions that, ideally, are those of the dense cores on which they formed. On the other hand, from the theoretical point of view, many studies have considered for decades the effect of the magnetic field in models. However, the lacks of instrumentation and observational techniques have prevented us from testing the predictions. Fortunately, several telescopes have developed polarimetric system during the last years allowing us to study for the first time the magnetic fields in a reliable manner. The complex interplay in dense cores among gravity, thermal pressure, turbulence, rotation, and magnetic field, in not well characterized due to all the previously described issues and, as a consequence, not well understood theoretically. The aim is, then, to improve our understanding on how the low-mass starless dense cores form, survive, and evolve. We faced this aim in a twofold approach. Firstly, we characterized observationally the physical, chemical, and magnetic properties of magnetized dense cores in their most initial stages, in order to understand the real initial conditions of the star-formation process. Secondly, we have compared interferometric observations of a Class-0 source to theoretical models of magnetized cloud collapse to derive the bettersuited initial conditions to form it, and the most relevant physical processes involved. To study the very young dense cores, we selected the Pipe nebula. This cloud presents very low star-formation efficiency (~0.06 %) and it is permeated by a uniform magnetic field. The nebula harbours more than a hundred very young dense cores mostly quiescent. We have mapped dense cores with densities below 10(5) cm(-3), lower than the typical values reported in literature. These cores present structures compatible with Bonnor-Ebert spheres, which suggests that they can be in a state close to hydrostatic equilibrium with the environment. Moreover, we discovered a very rich and varied chemistry, unexpected taking into account previous works in similar sources. Even in such young and diffuse objects, it is possible to distinguish differentiated chemical properties that allow us to propose an observational characterization and to suggest a possible evolutionary sequence. Some of the cores present chemical properties compatible with ages of 1 million years, but the lack of signposts of collapse suggests that active supporting non-thermal sources are acting. The lack of spherical symmetry also implies that some anisotropic force is present. The reported sub-Alfvénic turbulence points to magnetic field as this agent, which would cause the flattened shapes. NGC 1333 IRAS 4A is the ideal source to test magnetized low-mass cloud collapse models as it is a young Class-0 source with a collapsing envelope of gas and dust and a detected magnetic field with a clear hour-glass shape. We have confirmed that its properties can be satisfactorily explained with the standard model of star formation. The ideal-MHD models lead to better results, and the use of a temperature profile improves the agreement with the observational data. The initial conditions of the models, with sizes of ~0.1 pc and centrally peaked densities, agree with the results found toward the Pipe nebula dense cores. The intensity of the magnetic fields used in the models can be scaled to the values obtained for the diffuse gas with a power-law such as B-alpha-ro (1/2) typical of magnetized clouds evolving through ambipolar diffusion. From a technical point of view, the method used can establish the starting point in the way the ALMA data will be analyzed. The high quality of the data will make possible this kind of analysis, and foresees a huge improvement in our understandings of the star-formation process. / És una evidència observacional que els nuclis densos són llocs de naixement d’estrelles de baixa massa. Aquestes regions aparentment inactives són capaces de sobreviure diverses vegades l’escala temporal de caiguda lliure i, potencialment, col•lapsar per formar estrelles. Malgrat la seva importància se’n coneixen pocs detalls dels primers estadis evolutius d’aquests objectes. El desafiament observacional que representa estudiar objectes tan difusos i estesos ens dificulta revelar-ne les respostes. Resulta molt més senzill, des d’un punt de vista observacional, estudiar fonts més brillants com les més evolucionades Classe-0, de les quals se’n pot revertir la història en base a models teòrics i trobar-ne les condicions inicials que són, idealment, aquelles dels nuclis densos on s’han format. Per altra banda, des d’un punt de vista teòric, molts estudis han considerat l’efecte del camp magnètic en els seus models durant dècades. No obstant això, la mancança d’instrumentació i tècniques observacionals impedien contrastar-ne les prediccions. Per fortuna, diversos telescopis han desenvolupat sistemes polarimètrics durant els darrers anys permetent estudiar per primera vegada i de forma fiable el camp magnètic. Per tot això, la complexa interacció als nuclis densos entre gravitació, pressió tèrmica, turbulència, rotació i camp magnètic no està ben caracteritzada observacionalment i, com a conseqüència, tampoc ben entesa teòricament. L’objectiu és, aleshores, aprofundir en la comprensió de com es formen, sobreviuen i evolucionen els nuclis densos de baixa massa. Aquest objectiu l’hem enfrontat seguint dues vessants. En primer lloc, hem caracteritzat observacionalment les propietats físiques, químiques i magnètiques dels nuclis densos magnetitzats als seus estadis més primigenis, a fi d’entendre les vertaderes condicions inicials del procés de formació estel•lar. En segon lloc, hem comparat les observacions interferomètriques d’una font Classe-0 amb models teòrics de col•lapse de núvols magnetitzats, per derivar-ne les condicions inicials més adients per formar-la i els processos físics que n’han dominat l’evolució. Per a l’estudi dels nuclis densos primigenis hem seleccionat la nebulosa de la Pipa, que presenta una eficiència de formació estel•lar molt baixa (~0.06 %) i està penetrada per un camp magnètic uniforme. La nebulosa alberga més d’un centenar de nuclis densos molt joves majoritàriament inactius. Hem mapat nuclis densos amb densitat per sota de 10(5) cm(-3), molt per sota dels valors reportats a la literatura. Aquests nuclis presenten una estructura compatible amb esferes de Bonnor-Ebert, el que suggereix que es poden trobar en situació d’equilibri hidrostàtic amb el seu entorn. A més, hem descobert una química molt rica i variada, inesperada tenint en compte els treballs previs en fonts d’aquest tipus. Inclús en objectes tan joves i difusos, és possible distingir característiques químiques pròpies que permeten definir grups i establir una possible seqüència química evolutiva. Alguns dels objectes mostren propietats químiques típiques d’edats de 1 milió d’anys, però la mancança d’indicis de col•lapse gravitatori suggereix que hi ha actives fonts de suport no tèrmiques. La falta de simetria esfèrica també implica que alguna força anisotròpica està actuant. La turbulència sub-Alfvénica apunta a que el camp magnètic pot ser aquest agent, el que causaria les formes aplanades. NGC 1333 IRAS 4A és la font ideal per a testejar els model de col•lapse magnetitzat a baixa massa perquè es una Classe-0 jove amb un embolcall de gas i pols en fase de col•lapse on es detecta un camp magnètic amb clara morfologia de rellotge d’arena. Hem confirmat que les seves propietats poden ser explicades satisfactòriament amb el model estàndard de formació estel•lar. Els models de magnetohidrodinàmica idealitzada condueixen a millors resultats, i l’ús de un perfil de temperatura millora l’acord amb les dades. Les condicions inicials dels models, amb mides de ~0.1 pc i densitats creixents cap al centre, concorden amb els resultats als nuclis densos de la nebulosa de la Pipa. La intensitat del camp magnètic inicial usat pels models poden ser escalats als valors obtinguts per al gas difús de la Pipa amb una llei del tipus B-alfa-ro(1/2) típica de núvols magnetitzats. Des d’un punt de vista més tècnic, el mètode que hem emprat pot establir un punt de referència en la manera en que les futures dades d’ALMA seran analitzades. La alta qualitat de les dades farà possible aquest tipus d’anàlisis, i fa preveure una gran millora en l’enteniment del procés de formació estel•lar.

This thesis presents a starburst model for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, provide estimates for the age of the recent starburst activity. We also derive the total H$_2$ gas mass in the measured regions of the central 1 kpc starburst disk of M 82. In addition, we apply the model to represent various stages of starburst evolution in a well known sample of nearby luminous infrared galaxies (LIRGs). In this way, we interpret the relationship between the degree of molecular excitation and ratio of FIR to CO luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions. We conclude with an assessment of the strengths and weaknesses of this approach to dating starbursts, and suggest future work for improving the model.

This thesis presents a starburst model for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, provide estimates for the age of the recent starburst activity. We also derive the total H$_2$ gas mass in the measured regions of the central 1 kpc starburst disk of M 82. In addition, we apply the model to represent various stages of starburst evolution in a well known sample of nearby luminous infrared galaxies (LIRGs). In this way, we interpret the relationship between the degree of molecular excitation and ratio of FIR to CO luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions. We conclude with an assessment of the strengths and weaknesses of this approach to dating starbursts, and suggest future work for improving the model.

Giant molecular clouds (GMCs), out of which stars form, are complex, dynamic systems, which both influence and are shaped by the process of star formation. In this dissertation, I examine three different facets of the dynamical motions within GMCs. Collapse modes in different dimensional objects. Molecular clouds contain lower dimensional substructures, such as filaments and sheets. The collapse properties of finite filaments and sheets differ from those of spherical objects as well as infinite sheets and filaments. I examine the importance of local collapse modes of small central perturbations, relative to global collapse modes, in different dimensional objects to elucidate whether strong perturbations are required for molecular clouds to fragment to form stars. I also calculate the dependence of the global collapse timescale upon the aspect ratio of sheets and filaments. I find that lower dimensional objects are more readily fragmented, and that for a constant density, lower dimensional objects and clouds with larger aspect ratios collapse more slowly. An edge-driven collapse mode also exists in sheets and filaments and is most important in elongated filaments. The failure to consider the geometry of a gas cloud is shown to lead to an overestimation of the star formation rate by up to an order of magnitude. Molecular tracers of turbulent energy dissipation. Molecular clouds contain supersonic turbulence that simulations predict will decay rapidly via shocks. I use shock models to predict which species emit the majority of the turbulent energy dissipated in shocks and find that carbon monoxide, CO, is primarily responsible for radiating away this energy. By combining these shock models with estimates for the turbulent energy dissipation rate of molecular clouds, I predict the expected shock spectra of CO from molecular clouds. I compare the results of these shock models to predictions for the emission from the unshocked gas in GMCs and show that mid-to-high rotational transitions of CO (e.g., J = 8 to 7), should be dominated by shocked gas emission and should trace the turbulent energy being dissipated in molecular clouds. Orion-Eridanus superbubble. The nearby Orion star forming region has created a large bubble of hot plasma in the local interstellar medium referred to as the Orion-Eridanus superbubble. This bubble is unusual in that it is highly elongated, is believed to be oriented roughly parallel to the galactic plane, and contains bright filamentary features on the Eridanus side. I fit models for a wind driven bubble in an exponential atmosphere to the Orion-Eridanus superbubble and show that the elongation of the bubble cannot be explained by such a model in which the scale height of the galactic disk is the typical value of 150 pc. Either a much smaller scale height must be adopted or some additional physics must be added to the model. I also show that the Eridanus filaments cannot be equilibrium objects ionized by the Orion star forming region. / Graduate / 0606 / andyrpon@gmail.com