Η μελέτη των διπλών εκλειπτικών συστημάτων παραμένει ένα από τα πιο ενδιαφέροντα ερευνητικά πεδία της αστροφυσικής γιατί παρέχει την πιο ακριβή μέθοδο υπολογισμού των βασικών αστρικών μεγεθών (μαζών, ακτίνων, θερμοκρασιών) αλλά και κάθε μορφής αστρικής δραστηριότητας που αναπτύσσεται (κηλίδες, δίσκοι προαύξησης ύλης, αναπάλσεις) μέσα από τη μακρόχρονη μεταβολή της περιόδου τους. Επιπλέον επιτρέπει τον έλεγχο των θεωριών αστρικής εξέλιξης μέσα από την αλληλεπίδρασή τους αλλά και την πρόβλεψη παρουσίας τρίτων σωμάτων αστρικής ή μη φύσης. Σε αυτήν τη διατριβή μελετάται μία ιδιαίτερη κατηγορία διπλών εκλειπτικών συστημάτων σε επαφή, τα W UMa τα οποία αποτελούνται από αστέρες κύριας ακολουθίας οι οποίοι βρίσκονται σε υπερεπαφή, και χαρακτηρίζονται από τις μικρότερες περιόδους κι άρα από τη μικρότερη στροφορμή. Σκοπός της είναι τόσο η παρατηρησιακή μελέτη και ανάλυση συστημάτων W UMa που παρουσιάζουν ιδιαίτερο ενδιαφέρον όσο και η ανάγκη ανάπτυξης μεθοδολογίας μοντελοποίησης για την εξαγωγή της μέγιστης πληροφορίας και ελέγχου των εξαγόμενων φυσικών παραμέτρων κάτω από την εφαρμογή σύγχρονων στατιστικών μεθόδων. Το πρώτο μέρος της επικεντρώνεται στη σκιαγράφηση του θεωρητικού επιστημονικού πλαισίου περιγραφής των διπλών εκλειπτικών αστρικών συστημάτων σε επαφή. Το Κεφάλαιο1 αναφέρεται στη γεωμετρία των τροχιών και στη φυσική των αστέρων του συστήματος, στην περιγραφή του μοντέλου Roche και στην αλληλεπίδραση των μελών καθώς και στη δυναμική των συστημάτων όπως αυτή εκδηλώνεται από την παρατηρούμενη μεταβολή της περιόδου του. Το Κεφάλαιο 2 αναφέρεται στις φωτομετρικές παρατηρήσεις που έγιναν κατά την διάρκεια της παρούσας διατριβής καθώς και στις αυτοματοποιημένες μεθόδους επεξεργασίας και ανάλυσης των παρατηρησιακών αστρονομικών δεδομένων οι οποίες αναπτύχθηκαν. Το Κεφάλαιο 3 περιγράφει τις σύγχρονες τεχνικές μοντελοποίησης και τα προγράμματα ανάλυσης για τηλύση του αντίστροφου προβλήματος. Το Κεφάλαιο 4 περιγράφει την αναζήτηση, εξόρυξη και αυτόματη ανάλυση δεδομένων από παρατηρησιακές αστρονομικές επισκοπήσεις και την εφαρμογή του κώδικα τεχνητών νευρωνικών δικτύων EBAI. Το δεύτερο μέρος επικεντρώνεται στην αναλυτική μελέτη επιλεγμένων στενών διπλών συστημάτων W UMa με ιδιαίτερο ενδιαφέρον. Σε όλα τα επιλεγμένα συστήματα παρουσιάζεται η φωτομετρική τους μελέτη κάτω από το φως νέων BV RcIc παρατηρήσεων, η μελέτη της περιόδου, το εξαγόμενο μοντέλο σε συνδυασμό με φασματοσκοπικά δεδομένα, ο προσδιορισμός τροχιακών και φυσικών παραμέτρων και η θέση τους σε εξελικτικά διαγράμματα. Επίσης γίνεται η διερεύνηση του προτεινόμενου μοντέλου για μοναδικότητα στο χώρο των λύσεων μέσα από σάρωση με εισαγωγή διαταραχών ή άλλων στατιστικών μεθόδων και ο στατιστικός προσδιορισμός των σφαλμάτων των παραμέτρων. Συγκεκριμένα στο Κεφάλαιο5 παρουσιάζεται το ενοποιημένο μοντέλο του συστήματος TY Boo δύο κηλίδων που ερμηνεύει τη συμπεριφορά του την περίοδο 1969-2011 και τη μελέτη της περιόδου του. Σύμφωνα με αυτό το σύστημα TY Boo ανήκει στην υποκατηγορία W των W UMa με μικρό βαθμό επαφής f = (7.6 ±0.8)% και η μακρόχρονη μελέτη της περιόδου του δείχνει μακροχρόνια μείωση (dP/dt = −3.65x10^−8 d yr^−1 ) και μία περιοδικότητα (P3 = 58.9yrs, A = 0.0254 days)η οποία, χωρίς να αποκλείεται η παρουσία τρίτου σώματος, ερμηνεύεται με ενεργό μαγνητικό κύκλο που προκαλεί την εμφάνιση κηλίδων.Στο Κεφάλαιο 6 παρουσιάζεται για πρώτη φορά η λεπτομερής ανάλυση του συστήματος WUMa, FI Boo το οποίο ταξινομείται στην υποκατηγορία W, με βαθμό επαφής f = (50.15 ±8.10) %, κάτω από την παρουσία τρίτου σώματος το οποίο μπορεί να παίζει σημαντικό ρόλο στη δημιουργία και την εξέλιξή του. Σύμφωνα με το προτεινόμενο μοντέλο προσδιορίζονται οι μάζες Mh =0.40 ± 0.05 Msun , Mc =1.07 ± 0.05 Msun , και οι θερμοκρασίες Th = 5746±33Κ, Tc = 5420±46 Κ των αστέρων-μελών του καθώς και των προγεννητόρων τους (1.71 ±0.10 Msun και 0.63 ± 0.01 Msun , αντίστοιχα) και διερευνάται το εξελικτικό στάδιο του αστέρα με τη μεγαλύτερη μάζα μέσα από ισόχρονες.Στο Κεφάλαιο 7 διερευνάται σε βάθος το σύστημα σε επαφή V1003 Her, το οποίο χαρακτηρίζεται από μεταβολή μικρού πλάτους, με πολλαπλά μοντέλα για τα οποία ελέγχεται η σταθερότητα των με ανεξάρτητες μεθόδους. Το επικρατέστερο από αυτά δείχνει ότι το σύστημα είναι W UMa με βαθμό επαφής f = (36±10) %, με ασυνήθιστα άνισες θερμοκρασίες μεταξύ των αστέρων του 550 Κ, και λόγο μαζών q = 0.373. Η μικρή του κλίση i = 38^o ±1^ο το καθιστά ένα από τα μεγαλύτερης μάζας στην υποκατηγορίας W.Στο Κεφάλαιο 8 εξετάζεται η φύση του διπλού συστήματος HI Dra, η λεπτομερής ανάλυση του οποίου δείχνει ότι είναι W UMa με μικρό βαθμό επαφής f = (24 ± 4) % και διαφορά θερμοκρασίας μεταξύ των αστέρων του 330 Κ. Προτείνονται και διερευνώνται εξονυχιστικά δύο μοντέλα με κηλίδες από τα οποία επικρατέστερο, με ψυχρή κηλίδα πάνω στον μικρότερης μάζας και ψυχρότερο αστέρα- τον τοποθετεί στην υποκατηγορία Α. Προσδιορίζονται οι φυσικές παράμετροι των μελών του (M = 1.72 ± 0.08Msun , Mc = 0.43 ± 0.02 Msun , Rh =1.98 ± 0.03Rsun , Rc = 1.08 ± 0.02 Rsun , Lh = 9.6 ± 0.1 Lsun , Lc = 2.4 ± 0.1 Lsun ) και των προγεννητόρων τους (1.11 ± 0.03 Msun και 2.25 ± 0.07 Msun, αντίστοιχα) και προσεγγίζεται η ηλικία του συστήματος σε 2.4 Gyr .Τέλος στο Κεφάλαιο 8 παρουσιάζονται τα πρώτα αποτελέσματα από τις φωτομετρικές παρατηρήσεις με το τηλεσκόπιο Αρίσταρχος 2.3 m, διπλών εκλειπτικών συστημάτων με ιδιαίτερο ενδιαφέρον που ανακαλύφθηκαν ή παρατηρήθηκαν από την διαστημική αποστολή Kepler με περιόδους περιφοράς < 0.45 d, τα οποία παρουσιάζουν ασυμμετρίες στην καμπύλη φωτός, χρονικές μεταβολές των εκλείψεων των μελών τους ή είναι εν δυνάμει τριπλά συστήματα. Τα πρώτα αποτελέσματα αφορούν τα συστήματα KIC 11246163 και KIC 4563150τα οποία ταξινομούνται ως συστήματα τύπου W UMa, W υποκατηγορίας με πιθανό τριτο συνοδό. / The study of eclipsing binary systems remains one of the most powerful research fields in stellar astrophysics because it provides the primary source of calculating fundamentals properties of stars (masses, radii, temperatures) and every form of stellar activity (spots, accreting discs, pulsations) through photometry and spectroscopy or/and through the study of long-term variability of their period. Additionally it allows testing of stellar structure and evolution theories and the prediction of third body companions of stellar or sub-stellar origin. This thesis is focused on the study of W UMa type eclipsing binaries containing main sequence stars in overcontact configuration with short periods and,therefore the small angular momentum. The aim is both the multiband observational study, analysis and investigation of WUMa systems of particular interest but also the development of modeling methodology in order to extract the maximum information and to determine a detailed assessment of the parameter uncertainties through the application of modern statistical methods. The first part of the Doctoral Thesis outlines the theoretical framework for describing eclipsing binary stars. Chapter 1 refers to the physics and geometry of orbits and components, the description of the Roche model that led to the classification based on equipotential surfaces, the computation of the total radiated flux in the direction of the observer by including all the corrections needed and the description of systems dynamics as manifested by the observed change of their period. Chapter2 presents the photometric observations made during this research and the automated methods of processing and analysis (pipelines) which were developed. Chapter 3 sketches the modern modeling and analysis software techniques for the solution of inverse problem(Differential Corrections, Levenberg-Marquardt, Downhill Simplex, Genetic Algorithms,Heuristic Scanning, Bootstrap resampling, Metropolis –Hasting Markov Chain MonteCarlo), their implementation to programs developed as part of the present thesis and their application to real data. These also represent the methodology of the analyses implemented for the study of the WUMa systems, described in Part 2. Finally, Chapter4 describes the development of a pipeline for the exploration, extraction and automatic analysis of data from astronomical surveys and its application to ASAS database. In the same chapter the first results from the training and validation of the Artificial Neural Network (ANN), EBAI, and the best topology for the ANN are presented.The second part focuses on the detailed study of selected W UMa systems with particular interest. This includes: the new multiband photometric observations, the longterm period variation, the exported model from photometric and spectroscopic data (from the literature), the determination of orbital and physical parameters and their position in evolutionary diagrams, the investigation of the proposed model for uniqueness in the parameter space through heuristic scanning with parameter kicking or other statistical methods- and the statistical determination of the uncertainties of the derived parameters.In Chapter 5, new CCD four-color light curves of TY Boo made on eight nights over2010–2011 were analyzed in comparison with historical light curves obtained from 1969through 2011. The light curves could all be represented by a unique geometry and by wavelength consistent phototometric parameters of a two-spot model on either stellar component. It is confirmed that TY Boo is a shallow W-type contact binary system with a degree of contact factor of f = 7.6 ± 0.8%. A period investigation based on all available data shows a long-term decrease (dP/dt = −3.65 ×10−8 days yr−1 ) and an oscillation (P3 = 58.9 yr, A = 0.0254 days). Without ruling out the presence of a tertiary companion, the weight of evidence points to an active cyclic magnetic activity that causes spot formation rather than an unseen companion. Mass transfer between the components and angular momentum loss are also considered as possible mechanisms.In Chapter 6, a detailed analysis of the interesting W UMa binary FI Boo in view of the spectroscopic signature of a third body through photometry, period variation, and a thorough investigation of solution uniqueness is presented. We obtained new BVRcIc photometric data that, when combined with spectroscopic data, enable us to analyze the system FI Boo and determine its basic orbital and physical properties through PHOEBE,as well as the period variation by studying the times of the minima. This combined approach allows us to study the long-term period changes in the system for the first time in order to investigate the presence of a third body and to check extensively the solution uniqueness and the uncertainties of derived parameters. Our modeling indicates that FIBoo is a W-type moderate (f = 50.15% ± 8.10%) overcontact binary with component masses of Mh = 0.40 ± 0.05 M and Mc =1.07 ± 0.05 M , temperatures of Th = 5746 ±33 K and Tc = 5420 ± 56 K, and a third body, which may play an important role in the formation and evolution. The results were tested by heuristic scanning and parameter kicking to provide the consistent and reliable set of parameters that was used to obtain the initial masses of the progenitors (1.71 ± 0.10 M and 0.63 ± 0.01 M , respectively). We also investigated the evolutionary status of massive components with several sets of widely used isochrones.In Chapter 7, an extensive analysis of the low amplitude, contact binary V1003 Her is presented, based on the new VRc Ic , CCD photometric light curves in combination with published radial velocity (RV) curves. We investigate the stable configurations for the system with two independent methods and modeling tools: PHOEBE, ROCHE, via heuristic scanning and genetic algorithms, although the very low inclination of the system can place limitations. All methods indicate that V1003 Her is most likely in overcontact state with unequal components with temperature difference of 550 K, a mass ratio of q= 0.373 and a contact degree of 36 ± 10 %. As it is viewed at the very low inclination of i ∼ 38^◦ ± 1^◦ , if its configuration is confirmed, it will be among the most massive W-subtype of W UMa systems. This conclusion is also supported by other published models.However, in order to conclude reliable physical properties of the system, the high-precise based-ground photometry (or satellite photometry) and spectroscopic follow-up of V1003Her is required.In Chapter 8, a detailed investigation of the low-amplitude contact binary HI Dra is presented, based on the new VRc Ic CCD photometric light curves (LCs) combined with published radial velocity (RV) curves. Our completely covered LCs were analyzed using PHOEBE and revealed that HI Dra is an overcontact binary with low fill-out factor f = 24 ± 4 (%) and temperature difference between the components of 330 K.Two spotted models are proposed to explain the LC symmetry, between which the A subtype of W UMa type eclipsing systems, with a cool spot on the less massive and cooler component, proves to be more plausible on evolutionary grounds. The results and stability of the solutions were explored by heuristic scan and parameter perturbation to provide a consistent and reliable set of parameters and their errors. Our photometric modeling and RV curve solution give the following absolute parameters of the hot and cool components, respectively: Mh = 1.72 ± 0.08 Msun and Mc = 0.43 ± 0.02 Msun , Rh =1.98 ± 0.03 Rsun and Rc = 1.08 ± 0.02 Rsun , and Lh = 9.6 ± 0.1 Lsun and Lc = 2.4 ± 0.1Lsun . Based on these results the initial masses of the progenitors (1.11 ± 0.03 Msun and2.25 ± 0.07 Msun , respectively) and a rough estimate of the age of the system of 2.4 Gyr are discussed.Finally, in Chapter 9, our first results from ground based follow up photometric observation of interesting eclipsing binary systems (EBs) from Kepler field are presented.The program was launched in 2013 with the 2.3 m Aristarchos telescope at Helmos Observatory, Greece including eclipsing binary systems with periods < 0.45d and Kp (mag)=12.6-16 mag. The included targets in this program show light curve asymmetries,Eclipse Timings Variation or they are third body candidates. Modern analysis techniques such as heuristic scanning with parameter perturbation and genetic algorithm(PIKAIA), enable to reveal and optimize the astrophysical parameters of selected EBs.The results from BVRI photometry for the third body candidates, WUMa eclipsing binary systems of W-subtype, KIC11246163 and KIC4563150, are presented for the first time, as well as conclusions derived so far.

Paper discusses the expected uncertainty of orbital parameters of binary stars as measured by the space-based gravitational wave observatory LISA (Laser Interferometer Space Antenna) and how the inclusion of spin in the model of the binary stars affects the uncertainty. The uncertainties are found by calculating the received gravitational wave from a binary pair and then performing a linear least-squares parameter estimation. The case of a 1500 solar mass black hole that is 20 years from coalescing with a 1000 solar mass black hole--both of which are 50 x 10^6 light years away--is analyzed, and the results show that the inclusion of spin has a negligible effect upon the angular resolution of LISA but can increase the accuracy in mass and distance measurements by factors of 15 and 65, respectively.

abstract: The lowest-mass stars, known as M-dwarfs, form target samples for upcoming exoplanet searches, and together with lower-mass substellar objects known as brown dwarfs, are among prime targets for detailed study with high-contrast adaptive optics (AO) imaging and sub-millimeter interferometry. In this thesis, I describe results from three studies investigating the companion properties and environments of low-mass systems: (1) The 245-star M-dwarfs in Multiples (MinMs) Survey, a volume-limited survey of field M-dwarf companions within 15 pc, (2) the Taurus Boundary of Stellar/Substellar (TBOSS) Survey, an ongoing study of disk properties for low-mass members within the Taurus star-forming region, and (3) spectroscopy of a brown dwarf companion using the Gemini Planet Imager (GPI). Direct imaging of M-dwarfs is a sensitive technique to identify low-mass companions over a wide range of orbital separation, and the high proper motion of nearby M-dwarfs eases confirmation of new multiple stars. Combining AO and wide-field imaging, the MinMs Survey provides new measurements of the companion star fraction (CSF), separation distribution, and mass ratio distribution for the nearest K7-M6 dwarfs. These results demonstrate the closer orbital separations (~6 AU) and lower frequency (~23% CSF) of M-dwarf binaries relative to higher-mass stars. From the TBOSS project, I report 885µm Atacama Large Millimeter/sub-millimeter Array continuum measurements for 24 Taurus members spanning the stellar/substellar boundary (M4-M7.75). Observations of submillimeter emission from dust grains around the lowest-mass hosts show decreasing disk dust mass for decreasing host star mass, consistent with low frequencies of giant planets around M-dwarfs. Compared to the older stellar association of Upper Scorpius, Taurus disks have a factor of four higher mass in submillimeter-sized grains. From the GPI Exoplanet Survey, I describe near-infrared spectroscopy of an unusually red companion orbiting inside the debris disk of an F5V star. As the second brown dwarf discovered within the innermost region of a debris disk, the properties of this system offer important dynamical constraints for companion-disk interaction and a useful benchmark for brown dwarf and giant planet atmospheric study. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2017

Beta Lyrae is a complex binary star system with a 13-day orbital period containing two massive stars that are in the process of mass reversal accretion. The primary star is the higher mass star which is gaining mass from the secondary star. This reversal mass accretion causes gas to build and form a disk around the primary star. The disk is geometrically and optically thick. Previous interferometric studies in Optical and Infrared wavelengths have shown that a bipolar jet exists in the system and suggest that the jet contributes to the H-alpha emission. Meanwhile, other studies have suggested that the disk contributes to the H-alpha emission. We have taken into account various factors to model the emission of H-alpha from Beta Lyrae. The observed profile is double-peaked and varies with orbital phase. We found that the jet produces a single-peak for H-alpha emission. Meanwhile, the disk produces a double-peak for H-alpha emission if it is based on Keplerian motion. We use our model to interpret the observed H-alpha emission variations in the line shape with orbital phase.

In this Thesis, I explore transiting exoplanets: what we can learn from modeling their light curves, and what we can learn from their arrangement in planetary systems. I begin in Chapter 1 by briefly reviewing the history of transit modeling, from the earliest theoretical models of eclipsing binary stars to the models in current widespread use to model exoplanet transits. In Chapter 2, I model the transits of a sample of Kepler exoplanets with strong prior eccentricity constraints in order to derive correspondingly strong constraints on the density of their host stars, and find that the density constraints I derive are as precise as density constraints from asteroseismology if the transits are observed at high signal-to-noise. In Chapter 3, I apply the same methodology in reverse: using prior knowledge of the stellar density based on Gaia parallax measurements, I model the transits of twelve singly-transiting planets observed by K2 and derive constraints on their periods. In Chapter 4, I consider the general problem of deducing the shape of a transiting object from its light curve alone, which I term ``shadow imaging;'' I explore the mathematical degeneracies of the problem and construct shadow images to explain Dips 5 and 8 of Boyajian's Star. I next turn to multi-planet systems: in Chapter 5, I investigate the underlying multiplicity distribution of planetary systems orbiting FGK dwarfs observed by Kepler. I find that we can explain the multiplicities of these systems with a single Zipfian multiplicity distribution, without invoking a dichotomous population. In Chapter 6, I consider the arrangement of planets in those systems, and use neural networks inspired by models used for part-of-speech tagging in computational linguistics to model the relationship between exoplanets and their surrounding "context," i.e. their host star and sibling planets. I find that our trained regression model is able to predict the period and radius of an exoplanet to a factor of two better than a naive model which only takes into account basic dynamical stability. I also find that our trained classification model identifies consistent classes of planets in the period-radius plane, and that it is rare for multi-planet systems to contain a neighboring pair of planets from non-contiguous classes. In Chapter 7, I summarize these results and briefly discuss avenues for future work, including the application of our methods to planets and planetary systems discovered by TESS.

No description available.

Globular clusters are not simple stellar populations. Practically all globular clusters show multiple populations (MPs), where at the same metallicity [Fe/H], approximately half of their stars are enriched by the products of high-temperature hydrogen burning relative to the rest that show field-like abundances. The source of enrichment for forming the enriched population is an unresolved problem. Interacting massive binaries are an underexplored proposed source of enrichment. Many assessments of the theory are based on only one modelled binary. We simulate a suite of metal-intermediate, [Fe/H]=-1.44, interacting binaries with initial primary masses of 10 to 40 solar masses, with mass ratios ranging from 0.15 to 0.9, over periods ranging from about 2 to 700 days using MESA. Our simulations show that binaries at higher masses, higher mass ratios, and near our upper period limit tend to be the most enriching with ejecta showing HeNaCNOAlMg variations consistent with hot-H burning. Some binaries do not eject material, suggesting binary mass loss can contribute to the dilution of enrichment. As a realistic population, binaries within our parameter space eject about ten times as much mass as they would as single stars. Ejection occurs on timescales of about 11 Myr, consistent with observed and theoretical limits on the age spreads for MPs. Our systems are rare, making them more suited to explaining the stochastic nature of MPs but not the large fraction of enriched stars. Spreads in He, N, Na, C, and Al for our ejecta could reasonably explain the observed spreads in clusters. Reduced variation in O and Mg suggests more massive binaries should be investigated. A multi-scale approach to cluster formation with multiple types of enrichment sources is a necessary next step for validating MP formation scenarios. / Thesis / Master of Science (MSc) / The majority of stars form in star clusters. Globular clusters are the oldest and most massive type of star cluster. Formerly thought to be made of stars of the same age and chemical composition, nearly all observed globular clusters are now known to host multiple populations. About half of their stars form from similar material as isolated stars. The other half show signs of enrichment. How enriched stars get their enriching material is an open problem in cluster formation. Pairs of stars orbiting each other as binaries were proposed to eject the material needed to form these stars. We model 408 binaries to find that some systems eject large amounts of enrichment, especially when the stars are more massive. The rarity of these systems suggests binaries can explain the variations seen in multiple populations between clusters but cannot fully explain the large fraction of enriched stars seen.

Dwarf carbon (dC) stars are main-sequence stars with carbon molecular bands (C_2, CN, CH) in their optical spectra. They are an important class of post-mass transfer binaries since, as main-sequence stars, dCs cannot have produced carbon themselves. Rather, the excess carbon originated in an evolved companion, now a white dwarf, and was transferred to the dC. Because of their complex histories, dCs are an excellent sample for testing stellar physics, including common-envelope evolution, wind accretion, mass transfer efficiencies, and accretion spin-up. However, their fundamental properties remain a mystery, and this impedes efforts to use dCs to constrain the evolution of binary systems. Here, I have investigated the observed properties of dCs, both as a population and as individual objects. Using multi-epoch spectroscopy, I constrained the dC binary fraction to be consistent with 100% binarity. The best-fit orbital separation distribution agrees with the few known dC orbital periods, and suggests a bimodal distribution (one sample with mean periods of hundreds of days, the other thousands of days). I also built a set of optical templates to find and classify additional dCs in spectroscopic surveys. Further, I discovered periodic variability in photometry of 34 dCs, dramatically increasing the number of measured periods. This allowed me to investigate mass transfer mechanisms that are likely to be important in the formation of dCs. Interestingly, some of these objects have short periods (P < 2d), indicating they have gone through a common-envelope phase. I explored the implications of these short-period dCs and how they will allow for constraints to be placed on the physics of common-envelope evolution. Finally, I searched for signs of spin-up and activity in dCs using X-ray emission. From this, I found that dCs are consistent with being rapid rotators, similar to what is observed in samples of normal young dwarfs. In summary, this dissertation presents the most extensive set of dC observational properties that has been compiled to date. I have confirmed the binary origin of dCs and linked some to post-common-envelope binaries. My work has provided a firmer foundation for the use of dCs to explore many essential astrophysical phenomena.

No description available.

No description available.