Dissertations / Theses on the topic 'Astronomy and Astrophysics'

In this work, the long-term evolution of globulettes, low-mass globules found in H II regions, is studied through numerical hydrodynamic simulations. It has been proposed by Gahm et al. (2007) that these clouds may form free-floating planetary mass objects due to shock compression, caused by heating from the intense UV radiation of the central OB star cluster. To address this possibility, lifetimes are calculated for three different 3D simulated cases, similar to globulettes found in the Rosette Nebula. A plane-parallel approximation of the radiation field is used, as well as an inhomogeneous initial density distribution. The ionizing radiation will cause the globulettes to photo-evaporate, creating a rocket acceleration effect from the mass ejected on the heated side of the cloud. For a typical globulette with an initial mass of 29.5 Jupiter masses a lifetime of 50 000 yrs is estimated. This estimate is compared to the analytical models of Mellema et al. (1998) and Bertoldi and McKee (1990) which suggest longer lifetimes; the discrepancy is attributed to fragmentation of the clouds in the numerical simulation, which is not adequately described by the models. Synthesized H-alpha images and lightcurves are presented, indicating that the bright rims of small clouds are only likely to be visible in dim parts of the Rosette Nebula. The morphology of simulated clouds generally agrees with observations. While the code does not include self-gravity, the gravitational stability of the clouds is studied indirectly. It is concluded that clouds in the planetary mass range are stable against gravitational collapse, from supporting thermal pressure alone, when in pressure equilibrium with the heated ionization front. However, gravity may play a significant role during the initial shock compression.

This thesis is concerned with the detection of pulsed TeV γ-rays from millisecond pulsars. These stars appear to include some very efficient producers of high energy particles, but the mechanisms by which they produce TeV γ-rays are still a matter of debate. After an introductory section, there is a brief description of the principles used in the atmospheric Cerenkov technique. The design and operation of the University of Durham atmospheric Cerenkov telescopes are reviewed. The main analysis techniques used to search for periodic signals are then described. The effects on periodic signals of binary motion of a source are discussed. These are a particularly important consideration for observations of millisecond pulsars, where high timing accuracy is required. One of the problems of detecting TeV sources is the cosmic ray background. A means of rejecting background events in TeV γ -ray telescopes is considered in chapter 5. The technique is developed for the Durham Mark III telescope. Substantial rejection of the cosmic ray background is achieved, with minimal loss of source events. The evolutionary scenarios which lead to the formation of millisecond pulsars are outlined. Two models for 7-ray emission are discussed briefly and applied to six known millisecond pulsars. Empirical results on these and two other pulsars are also presented. In particular, a detection of PSR 1855+09 is reported, and an upper limit to the flux from PSR 1957+20 is derived. All the empirical fluxes are compatible with the emission models, but the 'polar gap' model may be favoured. The final chapter summarises the results obtained and suggests some directions for future work on the 7-ray emission from millisecond pulsars.

One of the most important scientific discoveries to be had this century is the spectroscopic characterization of Earth-like exoplanets to determine the occurrence rate of worlds capable of supporting life and to potentially answer: are we alone in the universe? To accomplish these lofty goals requires an advancement in the technology to separate the overwhelming starlight from that of the exoplanet. I believe starshades are the key technology that will enable these discoveries within our lifetime. This dissertation work is a contribution to the advancement of starshade technology to put us on the path towards discovery.

In this dissertation I present a number of suborbital methods developed for testing small-scale starshades, which include a Vertical Takeoff Vertical Landing rocket, the surface of a dry lake bed, and the heliostat of a solar telescope. The results from our high contrast observations are used to validate the optical model I developed to conduct tolerance analyses that will drive future starshade designs. The results from testing a formation flying sensor on the VTVL rocket demonstrate the rocket’s potential for conducting starshade experiments in the stratosphere.

This dissertation (along with [Novicki, et al. (2016)]) presents the first astronomical observations with a starshade that provide photometric measurements of stars, previously unobserved in the visible spectrum, in the proximity of Vega. These observations led to the development of a visual feedback system for the heliostat that allows us to push farther in separation and inner working angle. These high contrast observations were made using a starshade in the most flight-like configuration (in terms of Fresnel number, inner working angle, and resolution) to date.

The results of this dissertation have helped demonstrate the effectiveness and practicality of starshades for starlight suppression and have outlined a path forward to further advance starshade technology through optical testing and high contrast astronomy.

The work presented in this thesis covers two topics: a study of Intermediate Polars (non- synchronously rotating magnetic cataclysmic variables) and a survey of the Galactic plane, both carried out with the EXOSAT observatory. The first part of the thesis describes a study of all the Intermediate Polars observed by EXOSAT. The X-ray light curves and spectra of each source are considered, with particular emphasis on the spin modulation. By treating the Intermediate Polars as an homogenous set of objects, a model for the accretion flow and emission region is developed in which the modulation is due to a combination of photo-electric absorption and occultation effects. The modulation seen can be explained in terms of a single, large emission area in the vicinity of the magnetic pole of the white dwarf. Further analysis is presented concerning the differences in the observed properties of GK Per, between outburst and quiescence. The results of these observations are interpretted in terms of a decreasing mass accretion rate and are shown to be in accord with the overall model presented. Finally the concept of the system equilibrium is discussed and used to determine the magnetic moments of the white dwarfs in each system from their spin periods and X-ray luminosities. The values found are shown to be significantly model dependent. The second part of the thesis concerns a survey of the Galactic plane. Scanning observations are used to produce a map of the medium energy X-ray emission and from this a catalogue of 70 sources is compiled. The source list includes many which were previously unidentified. Comments are made on the nature of the sources detected and on the two diffuse components (the Galactic Ridge and the Galactic Bulge) which were observed. Pointed observations were also made at seven of the previously unidentified sources which were detected in the map. These observations are analysed in an attempt to determine the type of object which is producing the emission, and also to gain some insight into possible origins of the two diffuse emission components.

We discuss various topics concerning the propagation, generation, and detec-tionof high-frequency (HF) radio waves in the Earth's ionosphere. With re-gardsto propagation, we derive a full wave Hamiltonian and a polarization evo-lutionequation for electromagnetic waves in a cold, stratified magnetoplasma.With regards to generation, we will be concerned with three experiments con-ducted at the ionosphere- radio wave interaction research facilities at Sura, Rus-siaand Tromsø, Norway. These facilities operate high power HF transmittersthat can inject large amplitude electromagnetic waves into the ionosphere andexcite numerous nonlinear processes. In an experiment conducted at the Surafacility, we were able to measure the full state of polarization of stimulatedelectromagnetic emissions for the first time. It is expected that by using thetechnique developed in this experiment it will be possible to study nonlinearpolarization effects on powerful HF pump waves in magnetoplasmas in the fu-ture.In another experiment conducted at the Sura facility, the pump frequencywas swept automatically allowing rapid, high-resolution measurements of SEEdependence on pump frequency with minimal variations in ionospheric condi-tions.At the Tromsø facility we discovered by chance a highly variable, pumpinduced, HF emission that most probably emanated from pump excited spo-radicE. Regarding detection, we have proposed a set of Stokes parametersgeneralized to three dimension space; and we have used these parameters in aninvention to detect the incoming direction of electromagnetic waves of multiplefrequencies from a single point measurement.

Stars with masses roughly less than half a solar mass, M stars, constitute 75% of all stars in the Galaxy. However, due to their enigmatic nature, it remains difficult to measure these stars' physical properties. This project performs such characterization through the investigation of eclipsing binary star systems. By utilizing precise photometric data from NASA's Kepler Mission, we present this study on all M star eclipsing binaries in the Kepler dataset. We modeled our selected targets' light curves using existing code and vetted our sample for prime candidates for radial velocity follow up. Using available time on various telescopes, we obtained radial velocity measurements and are able to determine the effective temperatures of our target sample; a proposal has been submitted for further study with these and other observatories. With this information an absolute mass scale for the eclipsing systems allows us to fully characterize the individual stars in terms of their mass, radius, and temperature, leading us to derived luminosities. Ultimately, we hope to fill the current knowledge gap that concerns this dominant class of planet formation in the galaxy.

This thesis focuses on gravitational wave (GW) astrophysics with Pulsar Timing Arrays (PTAs). Firstly it is shown that anisotropy in the GW background may be present, and that its characterization at different angular scales carries important information. The standard analysis for isotropic backgrounds is then generalized by decomposing the angular distribution of the GW energy density into multipole moments. Generalized overlap reduction functions (ORFs) are computed for a generic level of anisotropy and PTA configuration. A rigorous analysis is then done of the assumptions made when calculating ORFs. It is shown that correlated phase changes introduce previously unmodeled effects for pulsars pairs separated by less than a radiation wavelength. The research then turns to the study of continuous GW sources from supermassive black hole binaries (SMBHBs). Here it shown that the detection of GWs from SMBHB systems can yield direct information about the masses and spins of the black holes, provided that the GW-induced timing fluctuations both at the pulsar and at Earth are detected. This in turn provides a map of the nonlinear dynamics of the gravitational field and a new avenue to tackle open problems in astrophysics connected to the formation and evolution of SMBHs.

With the development of more and more elegant and sensitive interferometric gravitational wave detectors, we are expecting the first direct detection of gravitational waves in a short time. This triggers huge interest to develop more powerful tools to perform data analysis on these signals, and to develop a good understanding of the analysis so that confident conclu- sions can be made. A further step would be to view into the future, as the first detections will boost the scientific demands for more powerful future generation detectors, which identifies the task of optimising the site of such detectors. Bayesian Inference plays a vital role in data analysis, and one excellent example that demon- strates its usefulness is its ability to resolve the tension between multiple models using the methodology of Bayesian Model Selection. In this thesis we apply this methodology to the timing data of pulses from the pulsar 1E 2259+586. With a set of different choices for the prior range, a fair and quantitative comparison can be made between two competing models: that of so-called successive anti-glitches and an anti-/normal glitch pair. Our analysis of the data shows a consistent support for the successive anti-glitches model, with a Bayes Factor of ∼ 45, where the uncertainty has been estimated from nested sampling and from multiple runs that are slightly different, but still within a factor of two, showing a general consistency. Simplifying the timing model will only make the Bayes Factor even bigger, while the two event model is overwhelmingly supported over the one event model. In gravitational wave data analysis, posteriors are generally complicated structures contain- ing multiple modes. A novel algorithm to achieve efficient sampling for multi-modal pos- teriors, known as mixed MCMC, is proposed in this thesis. This enables communication between multiple regions within the parameter space by adopting a novel jump proposal. We present the mixed MCMC algorithm and first apply it to a toy model problem, where the likelihood may be determined theoretically. By comparing the theoretical and empiri- cally sampled values of 2 log(L) for credible regions that correspond to 68.27%, 95.45% and 99.73%, we conclude that for our illustrative model the sampling result of mixed MCMC is consistent with the theoretical prediction with small uncertainty. Since it does not re- quire multiple chains with different temperatures, mixed MCMC can boost the efficiency of sampling by design, compared with (for example) parallel tempering MCMC. The sampling strategy of mixed MCMC can be helpful for not only Bayesian Inference, but also more general problems like the global optimisation of future generations of Gravita- tional Wave Detectors. As we expect such problem to be intrinsically high dimensional and multi-modal, mixed MCMC is a suitable sampling method, and we develop and apply it in this thesis. Based on our analysis it is concluded that for both a 3-detector-network and a 5-detector-network, Australia hosts the “best” site, in the sense that such site is most flex- ible, i.e. it can be involved in the largest number of detector networks, involving different component sites, that have a high ‘Figure of Merit’. The work of gravitational wave data analysis leads to the ultimate goal of making a direct detection of gravitational waves, which in turn requires the ability of distinguish astronomi- cal signals from a noisy background, and assess the significance of each gravitational wave ‘trigger’ (i.e. candidate event) appropriately. There are two types of method for estimating significance and these differ by the key distinction of either removing the foreground events from the background estimation or keeping them in the analysis. This thesis presents the results of a Mock Data Challenge (MDC), carried out within the LIGO Scientific Collabo- ration using different data analysis pipelines, designed to investigate these two methods for estimating significance. It contains a variety of background complexity ranging from simple, realistic to complex, and foreground event rate ranging from zero, low, medium and high. Analysis of the MDC results illustrated that generally all methods for determining the sig- nificance agree well with each other, irrespective of the background complexity. However, a discrepancy became apparent between the results for removal or non-removal of foreground events, for events below a significance level of < 10−3. Our results demonstrated that the removal method is an unbiased estimator for the mean of the significance. However, as the most scientifically interesting events are likely to have a very small numerical value for their significance, such method would overestimate that significance for most of the realisations.

Recent developments in the application of the atmospheric Cerenkov technique to 7-ray astronomy are reviewed here. These include new methods of signal to noise enhancement and the increasing diversity of stellar systems positively identified as Very High Energy 7-ray sources. Four Cataclysmic Variable systems were observed using the University of Durham Atmospheric Cerenkov telescopes during the course of 1990 and 1991. The statistical analysis performed in the search for a 7-ray signal, above a threshold energy of approximately 0-4 TeV, from three of these objects, H0253+193, EF Eridani and VW Hydri, is described here. The results of this brief survey are discussed in the context of current ideas as to the mechanisms by which Very High Energy 7-rays may be emitted from accreting binary star systems of this type. The analysis techniques applied to Cataclysmic Variable data were extended to an x-ray binary system, Sgr X-7. For comparison, the analysis of data recorded on two radio pulsars, PSR 1855+09 and PSR 1509-58, having more accurately known pulse signatures than the accreting systems is also described here, together with that of the globular cluster 47 Tucanae, which may emit a steady 7-ray flux; an upper limit is placed upon the level of Very High Energy 7-ray emission from this object. Extension of the Very High Energy 7-ray source catalogue will require a further improvement beyond the current signal to noise ratios of atmospheric Cerenkov telescopes. Some features characteristic of the atmospheric Cerenkov emission triggered by Very High Energy 7-rays as opposed to other cosmic ray particles, which could be exploited in an attempt to reduce interference from the latter, are reviewed. The first attempt to obtain directional information from the relative time of arrival of a Cerenkov flash at the telescopes at the University of Durham Southern Hemisphere site, and thus isolate an anisotropic 7-ray flux is reported here.

I buchi neri (BHs) sono oggetti variegati ed affascinanti in Natura. Il loro regno si estende dai buchi neri stellari con masse $sim 10-10^2 msun$ fino a BH supermassivi di $10^{9-10} msun$ al centro delle galassie. Mentre i primi sono il lascito dell'evoluzione stellare, i secondi sono il risultato di multipli merger di aloni di dark matter nella cosmologia $Lambda$CDM. Quando due BHs sono abbastanza vicini, formano una binaria (BHBs) ed emettono onde gravitazionali (GWs) che possiamo rilevare con i nostri interferometri. Come i BHs, anche le BHBs possono dividersi in diverse sottopopolazioni, ognuna con le proprie proprietà e caratteristiche: BHBs stellari (SBHBs) si formano dalla co-evoluzione di una binaria di stelle o in ambienti densi, mentre BHBs massicce (MBHBs) sono il risultato di merger di galassie. Le sfide nell'astronomia delle GW sono ancora numerose e richiedono varie conoscenze ed abilità per essere risolte. Per questa ragione, incomincio questa Tesi presentando dei risultati per SBHBs nei primi capitoli e virando verso MBHBs sul finale. Ogni capitolo ha la sua breve introduzione e discussione dei risultati. Nel capitolo 1 introduco alcuni concetti base della Relatività Generale (GR) legati all'emissione delle GWs. Riassumo lo stato corrente dell'astronomia delle GW. Spiego inoltre come GWs da BHBs possono essere modellizzate sotto alcune assunzioni ragionevoli e riporto alcune formule utili a capire i concetti dei paragrafi successivi. Nel capitolo 2 studio il minimo ordine Post-Newtonian (PN) necessario per seguire il segnale di SBHBs in LISA e realizzare una stima dei parametri corretta. Infatti gli SBHBs compiono numerosi cicli in LISA e quindi una descrizione accurata del segnale d'onda è necessaria per evitare bias nella stima dei parametri. Mostro come il fattore che determina il minimo ordine PN è il tempo alla coalescenza e che sistemi più vicini al merger richiedono più contributi PN. Applico questo risultato ad una popolazione di SBHBs in LISA per ottenere delle stime più realistiche, trovando che la maggior parte delle srogenti può essere descritta con correzzioni fino al 2PN, mentre i sistemi che mergono durante il tempo di missione richiedono contributi PN fino al 2.5PN o 3PN. L'argomento del capitolo 3 è un modello per descrivere BHs al di sopra del pair-instability mass gap, cioè BHs con massa $> 120 msun$. Presento un semplice modello e, sotto l'assuzione che la formazione della binaria non cambi oltre il gap, stimo il numero di eventi per i detector attuali, ET e LISA. Inoltre, suggerisco che le binarie non risolte possano formare un background casuale in LISA. Nel capitolo 4, mi muovo verso i MBHBs, osservabili solo dallo spazio con LISA. Introduco i concetti legati alla formazione ed evoluzione di MBHBs e le possibilità dell'astronomia multimessaggera. Descrivo anche come possiamo stimare i parametri della sorgente con il formalismo della Fisher matrix. Nle capitolo 5 presento un lavoro a cui ho contribuito sulla possibilità di osservare un modulazione Doppler in banda X durante l'inspiral di MBHBs. Prima del merger, emissione in banda X può essere prodotto dal gas che accresce sui BHs e il moto orbitale della binaria può imprimere una modulazione al segnale elettromagnetico in fase con il segnale di GWs. Osservare questa modulazione permetterebbe di determinare l'esatta posizione della sorgente nell'area di cielo stimata da LISA. Dalla nostra analisi, stimiamo di poter osservare qualche modulazione durante tutta la missione. Nel capitolo 6 riporto la stime dei parametri per MBHBs in funzione del tempo alla coalescenza. In particolare, mi concentro su posizione in cielo, distanza di luminosità, chirp mass e mass ratio e come i loro errori si riduciono mentre il sistema si avvicina al merger. A beneficio della community, rilascio i dati e delle formule analitiche per descrivere l'evoluzione di questi parametri. Discuto infine le prospettive multimessangere.
Black holes (BHs) are variegated and fascinating objects in Nature. Their realm extends from the stellar BHs with mass $sim 10-10^2 msun$ to the supermassive BHs of $10^{9-10} msun$ that reside in the center of galaxies. While the former are the expected outcome of stellar evolution, the latter are the results of multiple dark matter halo mergers in the standard $Lambda$CDM scenario. When two BHs are close enough, they form a binary BHs (BHBs) and emit gravitational waves (GWs) that we can detect with our inteferometers. Similarly to BHs, also BHBs can be divided into different sub-populations, each with its unique features and characteristics: stellar BHBs (SBHBs) form from the co-evolution of binary stars or in dense region, while massive BHBs (MBHBs) are the result of galaxy mergers. The challenges of GW astronomy are still numerous and require different knowledge and expertise to be solved. For this reason, I start this Thesis presenting result for SBHBs in the initial chapters and moving to MBHBs in the end. Each chapter has its own brief introduction and discussion of the main results and conclusions. In Chapter 1 I introduce some basic General Relativity (GR) concepts related to the emission of GWs. I summarise the current status of GW astronomy. I explain how GWs from BHBs can be easily modeled under some reasonable assumptions and report some formulas useful to understand the concepts of the following chapters. In Chapter 2 I study the minimum Post-Newtonian (PN) order necessary to accurately track SBHBs in LISA and perform an unbiased parameter estimation. SBHBs are expected to spend a large number of cycles in band, therefore an accurate waveform is necessary to avoid biases in the binary parameters. I show that the main factor affecting the PN accuracy is the time to coalescence with systems closer to merger requiring higher PN contributions. I apply the previous result to a realistic population of SBHBs in LISA in order to draw more realistic estimates: I find that most of the sources can be modeled with just 2PN corrections while systems merging during LISA time mission require up to 2.5PN and 3PN contributions. The topic of Chapter 3 is a model to describe SBHBs above the pair-instability mass gap, i.e. BHs with mass $> 120 msun$. I build a simple approach and, under the assumption that the binary formation does not change beyond the mass gap, I estimate the detected rate for current detectors, ET and LISA. Finally I also suggest the possibility that the undetected sources form a new source of stochastic background in LISA. In Chapter 4 I move to MBHBs, detectable only from space by LISA. I provide an introduction on MBHBs formation and evolution and the multimessenger possibilities. I also explain how we estimate source information with the so-called Fisher matrix formalism. In Chapter 5 I present a work I contributed where we explore the possibility to detect a Doppler modulated X-ray emission during the inspiral of MBHBs. In the last stage of merger, X-ray emission is expected as the result of gas accretion on each BHs and the orbital motion of the binary might imprint a Doppler modulation on the electromagnetic (EM) emission in phase with the GW signal. The detection of this modulation would allow to pinpoint the exact source location in the relatively large error area provided by LISA. From our analysis, we estimate few modulation detections over LISA time mission. Finally in Chapter 6 I report the results for the parameter estimation of MBHBs on the fly, i.e. as function of time before coalescence. In particular I focus on sky position, luminosity distance, chirp mass and mass ratio and how their errors decrease as the system approaches merger. For the benefit of the community, I release also the complete set of data and analytical fits to describe the time evolution in the aforementioned parameters. Finally I discuss the multimessenger prospects.

An analysis of the detection properties of an actively shielded 86cm (^3) germanium detector is given. Results are presented of observations of the Crab Nebula, its associated pulsar PSR0531+21, the Seyfert galaxy NGC1275, and the COS B source 2CG195+5. Evidence is presented for gamma-ray line emission from the Crab at energies of 404.7 and 1049.8 keV with intensities at the top of the atmosphere of (7.2 ± 2.1) x 10(^-3) and (2.03 ± 0.46) X 10(^-2) s (^-1) respectively. A line at 78.9 keV has also been detected at the 4.1o level which shows time variability. The peak flux at the top of the atmosphere was measured to be (1.1 ± 0.26) x 10(^-2) photons cm"(^-2)s(^-1) .We report the detection of redshifted annihilation radiation from the type II Seyfert galaxy, NGC 1275.The measured flux at the top of the atmosphere is (7.1 ± 2.2) x 10(^-3)photons cm(^-2)s(^-1),

Chapter One introduces the theory of galactic microlensing and develops the necessary formulae needed to discuss extended source events in the subsequent Chapters. Some of the complications encountered by groups observing such events are discussed, as are a few of the more notable events themselves. In Chapter Two an extended source model for microlensing is presented and applied to different atmosphere models, with different surface brightness profiles including simple one and two parameter limb darkening models and the more sophisticated and recently developed "Next Generation" stellar atmosphere models. It is shown that microlensing can distinguish between these different surfaces brightness profiles and thus, the underlying stellar atmosphere models, for realistic observational strategies. In Chapter Three a second stellar atmosphere models is introduced. This model includes the effects of a non-radial surface brightness profile, i.e. starspots. Such effects are interesting for several reasons. Firstly, the existence or otherwise of starspots is an important indicator of stellar surface activity and would provide valuable information for the testing and development of more sophisticated stellar atmosphere models. Additionally, there has been concern that starspots could mimic planetary microlensing lightcurves making it important to consider how their observational signatures could be distinguished from those of planets. The microlensing signatures of starspots are considered for point mass lens in Chapter Three and for fold caustic crossings in Chapter Four. In Chapter Five the extended source model used previously is applied to a source model with a small level of radial and temperature variability, to allow examination of how such events, if observed, would compare to standard microlensing events. In Chapter Six an investigation is made of the spectroscopic signatures of microlensing from circumstellar envelopes and the opportunities of using microlensing to diagnose bulk motion in these envelopes during caustic crossing events is examined.

Gamma-ray bursts (GRBs) are still puzzling scientists even 40 years after their discovery. Questions concerning the nature of the progenitors, the connection with supernovae and the origin of the high-energy emission are still lacking clear answers. Today, it is known that there are two populations of GRBs: short and long. It is also known that long GRBs are connected to supernovae (SNe). The emission observed from GRBs can be divided into two phases: the prompt emission and the afterglow. This thesis presents spectral analysis of the early X-ray afterglow of GRBs observed by the {\it Swift} satellite. For the majority of GRBs the early X-ray afterglows are well described by an absorbed power-law model. However, there exists a number of cases where this power-law component fails in fully describing the observed spectra and an additional blackbody component is needed. In the paper at the end of this thesis, a time-resolved spectral analysis of 74 GRBs observed by the X-ray telescope on board {\it Swift} is presented. Each spectrum is fitted with a power-law and a power-law plus blackbody model. The significance of the added thermal component is then assessed using Monte Carlo simulations. Six new cases of GRBs with thermal components in their spectra are presented, alongside three previously reported cases. The results show that a cocoon surrounding the jet is the most likely explanation for the thermal emission observed in the majority of GRBs. In addition, the observed narrow span in radii points to these GRBs being produced in similar environments.

QC 20171031

This thesis is concerned with non pulsating Low Mass X-Ray Binaries as Very High Energy γ-ray sources, in particular Scorpius X-1; the brightest of these objects in X-rays, and the most likely to be detectable at energies above 250 GeV. After a first introductory chapter, experimental techniques presently used in Very High Energy γ-ray Astronomy are reviewed. In the third chapter statistical techniques used to quantify count rate excesses and orbital modulations are described and applied to data from Scorpius X-1. Data taken in 1988 and 1989 showing a 3cr count excess, reported previously elsewhere, are found to show orbital modulation at the 3% statistical level. The analysis of data taken on 1990 shows no signal. Periodicity tests, in particular the Rayleigh test, are also described. The principles of a segmented fast algorithm for period searches in Cerenkov data using memory limited, but relatively fast, micro-computers are shown in the fourth chapter. Various of these machines can be used simultaneously in order to achieve a large efficiency. A method to perform various trials per independent frequency is also presented. The results of period searches in data from three selected objects (Scorpius X-1, GX 5-1 and Supernova 1987A) are presented. No periodicity is found at significant level in these data. The complete power spectrum of four segments of data from Cygnus X-3 showing a signal near to 12.59 ms are also shown. The final chapter considers theoretical models developed previously for more massive systems accounting for the different physical scenario of these low mass systems. The process of pair production between high energy photons and the radiation field of the accretion disc appears as the tightest constraint on how close to the neutron star high energy photons can be produced.

Researchers have been categorizing asteroids by color for decades in an attempt to better understand asteroid composition and potential links to the meteorite population. However, only recently through large data collection surveys like the Sloan Digital Sky Survey (SDSS) has the asteroid population as a whole been studied. This research will look at a subset of asteroids with the highest reflectivity differences as reported by Carvano et al. (2010) in order to answer the question: Can visible wavelength ambiguous taxonomic asteroid types be an indicator of a non-homogeneous surface?

This research studied asteroid 2453 (Wabash) in great detailed with visible spectrophotometry and near-infrared spectra. The results show that although a minor non-homogeneous surface was identified the non-homogenous surface is the not the primary source of the SDSS detected taxonomic variation.

The computational advances of the past several decades have allowed theoretical astrophysics to proceed at a dramatic pace. Numerical simulations can now simulate the formation of individual molecules all the way up to the evolution of the entire universe. Observational astrophysics is producing data at a prodigious rate, and sophisticated analysis techniques of large data sets continue to be developed. It is now possible for terabytes of data to be effectively turned into stunning astrophysical results. This is especially true for the field of star and planet formation. Theorists are now simulating the formation of individual planets and stars, and observing facilities are finally capturing snapshots of these processes within the Milky Way galaxy and other galaxies. While a coherent theory remains incomplete, great strides have been made toward this goal.

This dissertation discusses several projects that develop models of star and planet forma- tion. This work spans large spatial and temporal scales: from the AU-scale of protoplanetary disks all the way up to the parsec-scale of star-forming clouds, and taking place in both contemporary environments like the Milky Way galaxy and primordial environments at redshifts of z ~ 20.

Particularly, I show that planet formation need not proceed in incremental stages, where planets grow from millimeter-sized dust grains all the way up to planets, but instead can proceed directly from small dust grains to large kilometer-sized boulders. The requirements for this model to operate effectively are supported by observations. Additionally, I draw suspicion toward one model for how you form high mass stars (stars with masses exceeding ~ 8 M_sun), which postulates that high-mass stars are built up from the gradual accretion of mass from the cloud onto low-mass stars. I show that magnetic fields in star forming clouds thwart this transfer of mass, and instead it is likely that high mass stars are created from the gravitational collapse of large clouds. This work also provides a sub-grid model for computational codes that employ sink particles accreting from magnetized gas. Finally, I analyze the role that radiation plays in determining the final masses of the first stars to ever form in the universe. These stars formed in starkly different environments than stars form in today, and the role of the direct radiation from these stars turns out to be a crucial component of primordial star formation theory.

These projects use a variety of computational tools, including the use of spectral hydrodynamics codes, magneto-hydrodynamics grid codes that employ adaptive mesh refinement techniques, and long characteristic ray tracing methods. I develop and describe a long characteristic ray tracing method for modeling hydrogen-ionizing radiation from stars. Additionally, I have developed Monte Carlo routines that convert hydrodynamic data used in smoothed particle hydrodynamics codes for use in grid-based codes. Both of these advances will find use beyond simulations of star and planet formation and benefit the astronomical community at large.

The stars and galaxies are not where they seem. In the process of imaging and measurement, the light from distant objects is distorted, blurred, and skewed by several physical effects on scales from megaparsecs to microns. Charge-coupled devices (CCDs) provide sensitive detection of this light, but introduce their own problems in the form of systematic biases. Images of these stars and galaxies are formed in CCDs when incoming light generates photoelectrons which are then collected in a pixel’s potential well and measured as signal. However, these signal electrons can be diverted from purely parallel paths toward the pixel wells by transverse fields sourced by structural elements of the CCD, accidental imperfections in fabrication, or dynamic electric fields induced by other collected charges. These charge transport anomalies lead to measurable systematic errors in the images which bias cosmological inferences based on them. The physics of imaging therefore deserves thorough investigation, which is performed in the laboratory using a unique optical beam simulator and in computer simulations of charge transport.

On top of detector systematics, there are often biases in the mathematical analysis of pixelized images; in particular, the location, shape, and orientation of stars and galaxies. Using elliptical Gaussians as a toy model for galaxies, it is demonstrated how small biases in the computed image moments lead to observable orientation patterns in modern survey data. Also presented are examples of the reduction of data and fitting of optical aberrations of images in the lab and on the sky which are modeled by physically or mathematically-motivated methods.

Finally, end-to-end analysis of the weak gravitational lensing signal is presented using deep sky data as well as in N-body simulations. It is demonstrated how measured weak lens shear can be transformed by signal matched filters which aid in the detection of mass overdensities and separate signal from noise. A commonly-used decomposition of shear into two components, E- and B-modes, is thoroughly tested and both modes are shown to be useful in the detection of large scale structure. We find several astrophysical sources of B-mode and explain their apparent origin. The methods presented therefore offer an optimal way to filter weak gravitational shear into maps of large scale structure through the process of cosmic mass cartography.

In recent years, general relativistic magnetohydrodynamic (GRMHD) simulations have produced more realistic models for black hole disks and jets. However the complexity of the simulations has created a disconnect between simulations and theory: it is often unclear whether the simulated physics is correctly described by standard, semi-analytic disk and jet models. In this thesis, we describe new GRMHD simulations of black hole disks and jets. We compare the simulations to standard disk and jet models. We show that GRMHD thin disks are well described by the Novikov-Thorne model, and GRMHD jets are well described by the Blandford-Znajek model. Then, guided by the simulations, we develop two extensions of the standard thin disk model: a radially varying \(\alpha(r)\) viscosity prescription and an analytical disk solution with nonzero stress at the innermost stable circular orbit.
Astronomy

Studying dwarf galaxies can shed light on the original building blocks of galaxy formation. Most large galaxies are thought to be built up over billions of years through the collisions and mergers of smaller galaxies. The dwarf galaxies we see today are the evolved remnants of those building blocks, and by understanding their nature and evolution, we can study the raw ingredients of galaxy formation.

Blue Compact Dwarf galaxies (BCDs) and Almost Dark galaxies are at opposite extremes of today's population of dwarf galaxies. BCDs are exceptionally compact and host very intense starbursts, while Almost Dark galaxies are much more diffuse and have weak stellar populations.

This work studies the evolutionary context of BCDs by using deep, high-resolution images to study the detailed structure of their components, and by fitting our multi-wavelength observations with models to describe the properties of their stars, gas, and dust. BCDs appear to have exceptionally compact old stellar populations and unusually large star formation rates, when compared to typical dwarf galaxies.

By contrast, the optically faint, gas-dominated Almost Dark galaxies have extremely low star formation rates and weak stellar populations. In particular, one of the Almost Darks studied in this work has very unusual properties and is in disagreement with widely-studied scaling relations for large samples of galaxies. It appears to have too little stellar mass, a distribution of HI that is too extended to be supported by its modest rotation, and the highest well-measured gas mass-to-light ratio ever observed.

These two extreme classes may represent evolutionary stages that all galaxies pass through, and appear to be extreme ends of the broad continuum of dwarf galaxy properties. In order to use today's dwarf galaxies as windows into the building blocks of early galaxy formation, these unusual states and evolutionary pathways must be understood.

In this thesis, I explore two topics in exoplanet science. The first is the prevalence of Earth-size planets in the Milky Way Galaxy. To determine the occurrence of planets having different sizes, orbital periods, and other properties, I conducted a survey of extrasolar planets using data collected by NASA’s Kepler Space Telescope. This project involved writing new algorithms to analyze Kepler data, finding planets, and conducting follow-up work using ground-based telescopes. I found that most stars have at least one planet at or within Earth’s orbit and that 26% of Sun-like stars have an Earth-size planet with an orbital period of 100 days or less.

The second topic is the connection between the properties of planets and their host stars. The precise characterization of exoplanet hosts helps to bring planet properties like mass, size, and equilibrium temperature into sharper focus and probes the physical processes that form planets. I studied the abundance of carbon and oxygen in over 1000 nearby stars using optical spectra taken by the California Planet Search. I found a large range in the relative abundance of carbon and oxygen in this sample, including a handful of carbon-rich stars. I also developed a new technique called SpecMatch for extracting fundamental stellar parameters from optical spectra. SpecMatch is particularly applicable to the relatively faint planet-hosting stars discovered by Kepler.

Red giants with solar-like oscillations are astrophysical laboratories for probing the Milky Way. The Kepler Space Telescope revolutionized asteroseismology by consistently monitoring thousands of targets, including several red giants in eclipsing binaries. Binarity allows us to directly measure stellar properties independently of asteroseismology. In this dissertation, we study a subset of eight red giant eclipsing binaries observed by Kepler with a range of orbital periods, oscillation behavior, and stellar activity. Two of the systems do not show solar-like oscillations at all. We use a suite of modeling tools to combine photometry and spectroscopy into a comprehensive picture of each star's life. One noteworthy case is a double red giant binary. The two stars are nearly twins, but have one main set of solar-like oscillations with unusually low-amplitude, wide modes, likely due to stellar activity and modest tidal forces acting over the 171 day eccentric orbit. Mixed modes indicate the main oscillating star is on the secondary red clump (a core-He-burning star), and stellar evolution modeling supports this with a coeval history for a pair of red clump stars. The other seven systems are all red giant branch stars (shell-H-burning) with main sequence companions. The two non-oscillators have the strongest magnetic signatures and some of the strongest lifetime tidal forces with nearly-circular 20-34 day orbits. One system defies this trend with oscillations and a 19 day orbit. The four long-period systems (> 100 days) have oscillations, more eccentric orbits, and less stellar activity. They are all detached binaries consistent with coevolution. We find the asteroseismic scaling laws are approximately correct, but fail the most for stars that are least like the Sun by systematically overestimating both mass and radius. Strong magnetic activity and tidal effects often occur in tandem and act to suppress solar-like oscillations. These red giant binaries offer an unprecedented opportunity to test stellar physics and are important benchmarks for ensemble asteroseismology. Future asteroseismic studies should know they are excluding magnetically active stars and close binaries and be aware that asteroseismic masses and radii are both overestimated.

A high resolution measurement of the distribution of star formation within galaxies is key to understanding the emergence of galactic structure. The aim of this thesis is to understand how the structure of galaxies is built by developing a new method to spatially resolve their star formation. Using Ha maps for 2676 galaxies, this thesis shows where star formation is distributed in galaxies during the epoch 0.7 < z < 1.5 when a third of the total star formation in the history of the universe occurred. Across the star formation rate - stellar mass plane (the "main sequence"), star formation is `spatially coherent': in galaxies with higher than average star formation rates, Ha is enhanced throughout the disk; similarly, in galaxies with low star formation rates Ha is depressed throughout the disk. This places constraints both on the mechanisms for enhancing and quenching star formation as well as on how the structure of galaxies is built. The disk scale length of star formation in galaxies is larger than that of the stars, a direct demonstration that the disks of galaxies grow inside-out. While most star formation in most galaxies occurs in disks, not all of it does. With the first spatially resolved measurement of the Balmer decrement at z > 1, it can be seen that galaxies with M_* > 10¹⁰M[special characters omitted] have significant dust attenuation toward their centers. This means that we are witnessing the build-up of the dense stellar cores of massive galaxies through dust-obscured in-situ star formation. The most massive galaxies are thought to have formed their dense stellar cores at even earlier cosmic epochs. This thesis presents the first confirmed example of a massive galaxy core in the process of formation at z = 2.3. It has one of the highest velocity dispersions ever measured for a normal star forming galaxy and also appears to be building through very dense, dust-enshrouded star formation.

Feedback from active galactic nuclei (AGN) is thought to regulate the growth of supermassive black holes (SMBHs) and galaxies. The most direct evidence of AGN feedback is probably galactic outflows. This thesis addresses the link between SMBHs and their host galaxies from four different observational perspectives. First, I study the local correlation between black hole mass and the galactic halo potential (the MBH – V_c relation) based on Very Large Array (VLA) HI observations of galaxy rotation curves. Although there is a correlation, it is no tighter than the well-studied M_BH – σ* relation between the black hole mass and the potential of the galactic bulge, indicating that physical processes, such as feedback, could link the evolution of the black hole to the baryons in the bulge. In what follows, I thus search for galactic outflows as direct evidence of AGN feedback. Second, I use the Atacama Large Millimeter Array (ALMA) to observe a luminous obscured AGN that hosts an ionized galactic outflow and find a compact but massive molecular outflow that can potentially quench the star formation in 10

6 years.The third study extends the sample of known ionized outflows with new Magellan long-slit observations of 12 luminous obscured AGN. I find that most luminous obscured AGN (L_bol > 10⁴⁶ ergs s^–1) host ionized outflows on 10 kpc scales, and the size of the outflow correlates strongly with the luminosity of the AGN. Lastly, to capitalize on the power of modern photometric surveys, I experiment with a new broadband imaging technique to study the morphology of AGN emission line regions and outflows. With images from the Sloan Digital Sky Survey (SDSS), this method successfully constructs images of the [OIII]λ5007 emission line and reveals hundreds of extended emission-line systems. When applied to current and future surveys, such as the Large Synoptic Survey Telescope (LSST), this technique could open a new parameter space for the study of AGN outflows. In summary, through multi-phase and multi-scale galactic outflows, AGN feedback can link the growth of SMBHs with the evolution of galaxies.

We study the problem of separating stars and galaxies in the Hyper Suprime-Cam (HSC) multi-band imaging data at high galactic latitudes. We show that the current separation technique implemented in the HSC pipeline is unable to produce samples of stars with i 24 without a significant contamination from galaxies (> 50%). We study various methods for measuring extendedness in HSC with simulated and real data and find that there are a number of available techniques that give nearly optimal results; the extendedness measure HSC is currently using is among these. We develop a star/galaxy separation method for HSC based on the Extreme Deconvolution (XD) algorithm that uses colors and extendedness simultaneously, and show that with it we can generate samples of faint stars keeping contamination from galaxies under control to i ≤ 25. We apply our star/galaxy separation method to carry out a preliminary study of the structure of the Milky Way (MW) with main sequence (MS) stars using photometric parallax relations derived for the HSC photometric system. We show that it will be possible to generate a tomography of the MW stellar halo to galactocentric radii ∼ 100 kpc with ∼ 10⁶ MS stars in the HSC Wide layer once the survey has been completed. We report two potential detections of the Sagittarius tidal stream with MS stars in the XMM and GAMA15 fields at ≈ 20 kpc and ≈ 40 kpc respectively.

This thesis reports on the characterization of the thermal and chemical distribution of Jupiter’s polar regions. The quantities are derived from mid-infrared images covering all longitudes at unprecedented spatial resolution using the COMICS instrument at the Subaru Telescope on the nights of January 24 and 25, 2016. Because of Jupiter’s slight axial tilt of 3° and low angular resolution and incomplete longitudinal coverage of previous mid-infrared observations, the physical and chemical properties of Jupiter’s polar regions have been poorly characterized. In advance of the exploration of the structure of Jupiter’s polar regions by the Juno spacecraft, this study focuses on mapping the 3-dimensional structure of Jupiter’s polar regions, specifically to characterize the polar vortices and compact regions of auroral influence. Using mid-infrared images taken in the 7.8 μm - 24.2 μm range, the 3-dimensional temperature field, para-H2 fraction, aerosol opacity, and the constraint on the distribution of gaseous-NH3 are determined on a range from 400 mbar to 100 mbar. Retrievals of these atmospheric parameters were performed using NEMESIS, a radiative transfer forward model and retrieval code. Results indicate that there are vortices at both poles, each with very distinct boundaries approximately 70° latitude in the north and -75° latitude in the south. The boundaries can be defined by sharp thermal gradients extending at least from the upper troposphere (500 mbar of atmospheric pressure) and into the stratosphere (0.1 mbar of atmospheric pressure). These polar regions are characterized by lower temperatures and lower para-hydrogen concentration, compared with the regions immediately outside the vortex boundaries.

Studies of scaling relations in groups and clusters of galaxies have shown that the X-ray properties of groups deviate the most from the self-similar prediction. This is because groups are more affected by non-gravitational processes due to their shallower potential well, a behaviour which makes groups an ideal class of systems for the study of the impact of feedback. From the observational point of view, the study of the X-ray properties of groups, especially at high redshifts is hindered by their lower surface brightness compared to their more massive counterparts. We present the result from the Chandra Deep Group Survey, a survey dedicated to find high redshift groups in the deepest observations available in the Chandra archive. We found 26 groups and 36 clusters with available redshifts, with largest redshift being 1.3. We have used this sample to investigate the evolution of cool cores in these two classes of systems using cooling time divided by the age of the cluster as a parameter to describe the cooling state. We have found that groups and clusters have similar evolution in their cool core properties. Both classes of systems have a wide spread in the cool core parameter at low redshifts, which then narrows at high redshifts showing a lack of strong cool core systems.

Stellar halos give insight into the initial conditions that existed when a host galaxy first formed and provide details on disrupted satellites via their different stellar populations. An algorithm that is computationally inexpensive compared to hydrodynamic simulations is necessary in order to theoretically study the structure and formation of galactic stellar halos in sufficient detail to probe substructure. CoSANG (Coupling Semi-Analytic/N-body Galaxies) is a new computational method that we are developing which couples pure dark matter N-body simulations with a semi-analytic galaxy formation model. At each timestep, results from the N-body simulation feed into the semi-analytic code, whose results feed back into the N-body code making the evolution of the dark matter and baryonic matter dependent on one another. CoSANG will enable a variety of galaxy formation science, including analysis of stellar populations, halo merging, satellite accretion, supermassive black holes, and indirect and direct dark matter detection.

In this dissertation, I will describe the new simulation code CoSANG. The results from the extensive testing phase on CoSANG will be presented which indicate CoSANG is properly simulating feedback from galaxies within a dark matter halo. I used this validated code to analyze a CoSANG zoom simulation of a 10¹²M solar masses dark matter halo. Results showed a flatter inner halo near the disk and a more spherical outer halo which is expected when a galaxy exists at the center of a dark matter halo. A comparison is made with a simulation run with the same initial conditions, but with the baryonic component simulated using a hydrodynamic algorithm. The semi-analytic model predicted galaxy types better than the hydrodynamic simulation leading to the conclusion that the CoSANG halo is more accurate. I also present a dark matter direct detection analysis on the CoSANG zoom halo to measure the dark matter velocity distributions and modulation amplitudes. The CoSANG results show that the dark matter velocity distribution does not fit well to a Maxwell Boltzmann distribution and the modulation amplitudes derived indicate an anisotropic dark matter velocity distribution. Future work will include tagging dark matter particles with stellar properties to build and evolve a stellar halo.

The injection of energy and momentum into the interstellar medium by young massive stars’ intense radiation fields and their fast, radiatively driven winds can have a profound influence on their formation and environment. Massive star forming regions are rare and highly obscured, making the early moments of their formation difficult to observe. Instead, we must turn to theory to elucidate the physics involved in the formation of massive stars and massive star clusters (MSCs), which can host thousands of massive stars. In my thesis, I developed analytical and numerical techniques to study the formation of massive stars and how stellar wind feedback affects the dynamics of gas that surrounds MSCs. To estimate the initial rotation rates of massive stars at birth, I developed a protostellar angular momentum evolution model for accreting protostars to determine if magnetic torques can spin down massive stars during their formation. I found that magnetic torques are insufficient to spin down massive stars due to their short formation times and high accretion rates. Radiation pressure is likely the dominate feedback mechanism regulating massive star formation. Therefore detailed simulation of the formation of massive stars requires an accurate treatment of radiation. For this purpose, I developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new tool, I performed a suite of three-dimensional adaptive mesh refinement radiation-hydrodynamic simulations of the formation of massive stars from collapsing massive pre-stellar cores. I found that mass is channeled to the massive star via dense infalling filaments that are uninhibited by radiation pressure and gravitational and Rayleigh-Taylor instabilities. To determine the importance of stellar wind feedback in young MSCs, I used observations to constrain a range of kinetic energy loss channels for the hot gas produced by the shock-heating of stellar winds to explain the low X-ray luminosities observed in Hii regions. I demonstrated that the energy injected by stellar winds is not a significant contributor to stellar feedback in young MSCs.