Annual Review of Astronomy and Astrophysics - Volume 54, 2016
Volume 54, 2016
- Preface
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A Fortunate Half-Century
Vol. 54 (2016), pp. 1–17More LessThe author has had the happy opportunity to work and study in the field of theoretical astrophysics during a half-century of unparalleled observational discovery. Quasars, pulsars, large-scale cosmic structure, galaxy evolution, and many other phenomena were discovered using revolutionary new observational techniques. In the same period of time, analytical tools ranging from new disciplines like plasma physics to enormously enhanced computational facilities became available, giving theoreticians the machinery to make some sense of this new world. At the start of the era the extragalactic world was largely framed as a quasi-homogeneous Universe of static galaxies formed by unknown and in fact unexamined processes. By the end of the period there was widespread acceptance of a self-consistent, evolving cosmological model amenable to calculation and comprehension. The author was fortunate indeed to have had the chance to collaborate with many wonderfully talented colleagues during this exciting time, attempting to unravel the multiple cosmic puzzles that nature delivered to us.
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The Remnant of Supernova 1987A
Vol. 54 (2016), pp. 19–52More LessAlthough it has faded by a factor of ∼107, SN 1987A is still bright enough to be observed in almost every band of the electromagnetic spectrum. Today, the bolometric luminosity of the debris is dominated by a far-infrared (∼200μm) continuum from ∼0.5 M⊙ of dust grains in the interior debris. The dust is heated by UV, optical, and near-infrared (NIR) emission resulting from radioactive energy deposition by 44Ti.
The optical light of the supernova debris is now dominated by illumination of the debris by X-rays resulting from the impact of the outer supernova envelope with an equatorial ring (ER) of gas that was expelled some 20,000 years before the supernova explosion. X-ray and optical observations trace a complex system of shocks resulting from this impact, whereas radio observations trace synchrotron radiation from relativistic electrons accelerated by these shocks. The luminosity of the remnant is dominated by an NIR (∼20μm) continuum from dust grains in the ER heated by collisions with ions in the X-ray emitting gas.
With the Atacama Large Millimeter Array (ALMA), we can observe the interior debris at millimeter/submillimeter wavelengths, which are not absorbed by the interior dust. The ALMA observations reveal bright emission lines from rotational transitions of CO and SiO lines that provide a new window into the interior structure of the supernova debris. Optical, NIR, and ALMA observations all indicate strongly asymmetric ejecta.
Intensive searches have failed to yield any evidence for the compact object expected to reside at the center of the remnant. The current upper limit to the luminosity of such an object is a few tens of solar luminosities.
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Astrophysics with Extraterrestrial Materials
Vol. 54 (2016), pp. 53–93More LessExtraterrestrial materials, including meteorites, interplanetary dust, and spacecraft-returned asteroidal and cometary samples, provide a record of the starting materials and early evolution of the Solar System. We review how laboratory analyses of these materials provide unique information, complementary to astronomical observations, about a wide variety of stellar, interstellar and protoplanetary processes. Presolar stardust grains retain the isotopic compositions of their stellar sources, mainly asymptotic giant branch stars and Type II supernovae. They serve as direct probes of nucleosynthetic and dust formation processes in stars, galactic chemical evolution, and interstellar dust processing. Extinct radioactivities suggest that the Sun's birth environment was decoupled from average galactic nucleosynthesis for some tens to hundreds of Myr but was enriched in short-lived isotopes from massive stellar winds or explosions shortly before or during formation of the Solar System. Radiometric dating of meteorite components tells us about the timing and duration over which solar nebula solids were assembled into the building blocks of the planets. Components of the most primitive meteoritical materials provide further detailed constraints on the formation, processing, and transport of material and associated timescales in the Sun's protoplanetary disk as well as in other forming planetary systems.
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Red Clump Stars
Vol. 54 (2016), pp. 95–133More LessLow-mass stars in their core-helium-burning stage define the sharpest feature present in the color-magnitude diagrams of nearby galaxy systems: the red clump (RC). This feature has given rise to a series of methods aimed at measuring the distributions of stellar distances and extinctions, especially in the Magellanic Clouds and Milky Way Bulge. Because the RC is easily recognizable within the data of large spectroscopic and asteroseismic surveys, it is a useful probe of stellar densities, kinematics, and chemical abundances across the Milky Way disk; it can be applied up to larger distances than that allowed by dwarfs; and it has better accuracy than is possible with other kinds of giants. Here, we discuss the reasons for the RC narrowness in several sets of observational data, its fine structure, and the presence of systematic changes in the RC properties as regards age, metallicity, and the observed passband. These factors set the limits on the validity and accuracy of several RC methods defined in the literature.
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Accretion onto Pre-Main-Sequence Stars
Vol. 54 (2016), pp. 135–180More LessAccretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-main-sequence stars of masses ≲1M⊙ (and in at least some 2–3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ∼3 Myr is modest in terms of stellar evolution, but is comparable to total disk reservoirs as estimated from millimeter-wave dust emission (∼10−2 M⊙). Pre-main-sequence accretion is not steady, encompassing timescales ranging from approximately hours to a century, with longer-timescale variations tending to be the largest. Accretion during the protostellar phase—while the protostellar envelope is still falling onto the disk—is much less well understood, mostly because the properties of the central obscured protostar are difficult to estimate. Kinematic measurements of protostellar masses with new interfometric facilities should improve estimates of accretion rates during the earliest phases of star formation.
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Interstellar Hydrides
Vol. 54 (2016), pp. 181–225More LessInterstellar hydrides—that is, molecules containing a single heavy element atom with one or more hydrogen atoms—were among the first molecules detected outside the solar system. They lie at the root of interstellar chemistry, being among the first species to form in initially atomic gas, along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to FIR and submillimeter observations performed with the Herschel Space Observatory. In this review, we discuss observations of interstellar hydrides, along with the advanced modeling approaches that have been used to interpret them and the unique information that has thereby been obtained.
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The Quest for B Modes from Inflationary Gravitational Waves
Vol. 54 (2016), pp. 227–269More LessThe search for the curl component (B mode) in the cosmic microwave background (CMB) polarization induced by inflationary gravitational waves is described. The canonical single-field slow-roll model of inflation is presented, and we explain the quantum production of primordial density perturbations and gravitational waves. It is shown how these gravitational waves then give rise to polarization in the CMB. We then describe the geometric decomposition of the CMB polarization pattern into a curl-free component (E mode) and curl component (B mode) and show explicitly that gravitational waves induce B modes. We discuss the B modes induced by gravitational lensing and by Galactic foregrounds and show how both are distinguished from those induced by inflationary gravitational waves. Issues involved in the experimental pursuit of these B modes are described, and we summarize some of the strategies being pursued. We close with a brief discussion of some other avenues toward detecting/characterizing the inflationary gravitational-wave background.
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Gravitational Instabilities in Circumstellar Disks
Vol. 54 (2016), pp. 271–311More LessStar and planet formation are the complex outcomes of gravitational collapse and angular momentum transport mediated by protostellar and protoplanetary disks. In this review, we focus on the role of gravitational instability in this process. We begin with a brief overview of the observational evidence for massive disks that might be subject to gravitational instability and then highlight the diverse ways in which the instability manifests itself in protostellar and protoplanetary disks: the generation of spiral arms, small-scale turbulence-like density fluctuations, and fragmentation of the disk itself. We present the analytic theory that describes the linear growth phase of the instability supplemented with a survey of numerical simulations that aim to capture the nonlinear evolution. We emphasize the role of thermodynamics and large-scale infall in controlling the outcome of the instability. Despite apparent controversies in the literature, we show a remarkable level of agreement between analytic predictions and numerical results. In the next part of our review, we focus on the astrophysical consequences of the instability. We show that the disks most likely to be gravitationally unstable are young and relatively massive compared with their host star, Md/M*≥0.1. They will develop quasi-stable spiral arms that process infall from the background cloud. Although instability is less likely at later times, once infall becomes less important, the manifestations of the instability are more varied. In this regime, the disk thermodynamics, often regulated by stellar irradiation, dictates the development and evolution of the instability. In some cases the instability may lead to fragmentation into bound companions. These companions are more likely to be brown dwarfs or stars than planetary mass objects. Finally, we highlight open questions related to the development of a turbulent cascade in thin disks and the role of mode-mode coupling in setting the maximum angular momentum transport rate in thick disks.
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The Evolution of the Intergalactic Medium
Vol. 54 (2016), pp. 313–362More LessThe bulk of cosmic matter resides in a dilute reservoir that fills the space between galaxies, the intergalactic medium (IGM). The history of this reservoir is intimately tied to the cosmic histories of structure formation, star formation, and supermassive black hole accretion. Our models for the IGM at intermediate redshifts (2≲z≲5) are a tremendous success, quantitatively explaining the statistics of Lyα absorption of intergalactic hydrogen. However, at both lower and higher redshifts (and around galaxies) much is still unknown about the IGM. We review the theoretical models and measurements that form the basis for the modern understanding of the IGM, and we discuss unsolved puzzles (ranging from the largely unconstrained process of reionization at high z to the missing baryon problem at low z), highlighting the efforts that have the potential to solve them.
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The Magellanic Stream: Circumnavigating the Galaxy
Vol. 54 (2016), pp. 363–400More LessThe Magellanic Clouds are surrounded by an extended network of gaseous structures. Chief among these is the Magellanic Stream, an interwoven tail of filaments trailing the Clouds in their orbit around the Milky Way. When considered in tandem with its Leading Arm, the Stream stretches over 200° on the sky. The Stream is thought to represent the result of tidal interactions between the Clouds and ram-pressure forces exerted by the Galactic corona, and its kinematic properties reflect the dynamical history of the pair of dwarf galaxies closest to the Milky Way. The Stream is a benchmark for hydrodynamical simulations of accreting gas and cloud/corona interactions. If the Stream survives these interactions and arrives safely in the Galactic disk, its cargo of over a billion solar masses of gas has the potential to maintain or elevate the Galactic star-formation rate. In this article, we review the current state of knowledge of the Stream, including its chemical composition, physical conditions, origin, and fate. We also review the dynamics of the Magellanic System, including the proper motions and orbital history of the Large and Small Magellanic Clouds, the first-passage and second-passage scenarios, and the evidence for a Magellanic Group of galaxies.
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Masses, Radii, and the Equation of State of Neutron Stars
Feryal Özel, and Paulo FreireVol. 54 (2016), pp. 401–440More LessWe summarize our current knowledge of neutron-star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the past few years, leading to a large number of mass measurements. These discoveries show that the neutron-star mass distribution is much wider than previously thought, with three known pulsars now firmly in the 1.9–2.0-M⊙ mass range. For radii, large, high-quality data sets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 10–11.5-km range and shrinking their uncertainties, owing to a better understanding of the sources of systematic errors. The combination of the massive-neutron-star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.
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The Eccentric Kozai-Lidov Effect and Its Applications
Vol. 54 (2016), pp. 441–489More LessThe hierarchical triple-body approximation has useful applications to a variety of systems from planetary and stellar scales to supermassive black holes. In this approximation, the energy of each orbit is separately conserved, and therefore the two semimajor axes are constants. On timescales much larger than the orbital periods, the orbits exchange angular momentum, which leads to eccentricity and orientation (i.e., inclination) oscillations. The orbits' eccentricity can reach extreme values, leading to a nearly radial motion, which can further evolve into short orbit periods and merging binaries. Furthermore, the orbits' mutual inclinations may change dramatically from pure prograde to pure retrograde, leading to misalignment and a wide range of inclinations. This dynamical behavior is coined the “eccentric Kozai-Lidov mechanism.” The behavior of such a system is exciting, rich, and chaotic in nature. Furthermore, these dynamics are accessible from a large part of the triple-body parameter space and can be applied to a diverse range of astrophysical settings and used to gain insights into many puzzles.
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Protostellar Outflows
Vol. 54 (2016), pp. 491–528More LessOutflows from accreting, rotating, and magnetized systems are ubiquitous. Protostellar outflows can be observed from radio to X-ray wavelengths in the continuum and a multitude of spectral lines that probe a wide range of physical conditions, chemical phases, radial velocities, and proper motions. Wide-field visual and near-IR data, mid-IR observations from space, and aperture synthesis with centimeter- and millimeterwave interferometers are revolutionizing outflow studies. Many outflows originate in multiple systems and clusters. Although most flows are bipolar and some contain highly collimated jets, others are wide-angle winds, and a few are nearly isotropic and exhibit explosive behavior. Morphologies and velocity fields indicate variations in ejection velocity, mass-loss rate, and in some cases, flow orientation and degree of collimation. These trends indicate that stellar accretion is episodic and often occurs in a complex dynamical environment. Outflow power increases with source luminosity but decreases with evolutionary stage. The youngest outflows are small and best traced by molecules such as CO, SiO, H2O, and H2. Older outflows can grow to parsec scales and are best traced by shock-excited atoms and ions such as hydrogen-recombination lines, [Sii], and [Oii]. Outflows inject momentum and energy into their surroundings and provide an important mechanism in the self-regulation of star formation. However, momentum injection rates remain uncertain with estimates providing lower bounds.
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The Galaxy in Context: Structural, Kinematic, and Integrated Properties
Vol. 54 (2016), pp. 529–596More LessOur Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from faint dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L⋆) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated “green valley” region of the galaxy color-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.
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Structure and Kinematics of Early-Type Galaxies from Integral Field Spectroscopy
Vol. 54 (2016), pp. 597–665More LessObservations of galaxy isophotes, long-slit kinematics, and high-resolution photometry suggested a possible dichotomy between two distinct classes of elliptical galaxies. But these methods are expensive for large galaxy samples. Instead, integral field spectroscopy can efficiently recognize the shape, dynamics, and stellar population of complete samples of early-type galaxies (ETGs). These studies showed that the two main classes, the fast and slow rotators, can be separated using stellar kinematics. I show that there is a dichotomy in the dynamics of the two classes. The slow rotators are weakly triaxial and dominate above
. Below Mcrit, the structure of fast rotators parallels that of spiral galaxies. There is a smooth sequence along which the age, the metal content, the enhancement in α-elements, and the weight of the stellar initial mass function all increase with the central mass density slope, or bulge mass fraction, while the molecular gas fraction correspondingly decreases. The properties of ETGs on galaxy scaling relations, in particular the
diagram, and their dependence on environment, indicate two main independent channels for galaxy evolution. Fast-rotator ETGs start as star-forming disks and evolve through a channel dominated by gas accretion, bulge growth, and quenching, whereas slow rotators assemble near the centers of massive halos via intense star formation at high redshift and remain as such for the rest of their evolution via a channel dominated by gas poor mergers. This is consistent with independent studies of the galaxies redshift evolution.
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Six Decades of Spiral Density Wave Theory
Vol. 54 (2016), pp. 667–724More LessThe theory of spiral density waves had its origin approximately six decades ago in an attempt to reconcile the winding dilemma of material spiral arms in flattened disk galaxies. We begin with the earliest calculations of linear and nonlinear spiral density waves in disk galaxies, in which the hypothesis of quasi-stationary spiral structure (QSSS) plays a central role. The earliest success was the prediction of the nonlinear compression of the interstellar medium and its embedded magnetic field; the earliest failure, seemingly, was not detecting color gradients associated with the migration of OB stars whose formation is triggered downstream from the spiral shock front. We give the reasons for this apparent failure with an update on the current status of the problem of OB star formation, including its relationship to the feathering substructure of galactic spiral arms. Infrared images can show two-armed, grand design spirals, even when the optical and UV images show flocculent structures. We suggest how the nonlinear response of the interstellar gas, coupled with overlapping subharmonic resonances, might introduce chaotic behavior in the dynamics of the interstellar medium and Population I objects, even though the underlying forces to which they are subject are regular. We then move to a discussion of resonantly forced spiral density waves in a planetary ring and their relationship to the ideas of disk truncation, and the shepherding of narrow rings by satellites orbiting nearby. The back reaction of the rings on the satellites led to the prediction of planet migration in protoplanetary disks, which has had widespread application in the exploding data sets concerning hot Jupiters and extrasolar planetary systems. We then return to the issue of global normal modes in the stellar disk of spiral galaxies and its relationship to the QSSS hypothesis, where the central theoretical concepts involve waves with negative and positive surface densities of energy and angular momentum in the regions interior and exterior, respectively, to the corotation circle; the consequent transmission and overreflection of propagating spiral density waves incident on the corotation circle; and the role of feedback from the central regions.
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Gamma-Ray Observations of Active Galactic Nuclei
Vol. 54 (2016), pp. 725–760More LessThis article reviews the recent observational results regarding γ-ray emission from active galaxies. The most numerous discrete extragalactic γ-ray sources are AGNs dominated by relativistic jets pointing in our direction (commonly known as blazars), and they are the main subject of the review. They are detected in all observable energy bands and are highly variable. The advent of the sensitive γ-ray observations, afforded by the launch and continuing operation of the Fermi Gamma-ray Space Telescope and the AGILE Gamma-ray Imaging Detector, as well as by the deployment of current-generation Air Cerenkov Telescope arrays such as VERITAS, MAGIC, and HESS-II, continually provides sensitive γ-ray data over the energy range of ∼100 MeV to multi-TeV. Importantly, it has motivated simultaneous, monitoring observations in other bands, resulting in unprecedented time-resolved broadband spectral coverage. After an introduction, in Sections 3, 4, and 5, we cover the current status and highlights of γ-ray observations with (mainly) Fermi but also AGILE and put those in the context of broadband spectra in Section 6. We discuss the radiation processes operating in blazars in Section 7, and we discuss the content of their jets and the constraints on the location of the energy dissipation regions in, respectively, Sections 8 and 9. Section 10 covers the current ideas for particle acceleration processes in jets, and Section 11 discusses the coupling of the jet to the accretion disk in the host galaxy. Finally, Sections 12, 13, and 14 cover, respectively, the contribution of blazars to the diffuse γ-ray background, the utility of blazars to study the extragalactic background light, and the insight they provide for study of populations of supermassive black holes early in the history of the Universe.
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Galaxies in the First Billion Years After the Big Bang
Vol. 54 (2016), pp. 761–803More LessIn the past five years, deep imaging campaigns conducted with the Hubble Space Telescope (HST) and ground-based observatories have delivered large samples of galaxies at 6.5<z<10, providing our first glimpse of the census of star formation activity in what is thought to be the heart of the reionization era. The space density of luminous galaxies has been shown to decrease by 15–20× over 4<z<8. Over this same redshift interval, the faint-end slope of the UV luminosity function becomes steeper (α≃−2.0 at z≃7−8), revealing a dominant population of low-luminosity galaxies. Analysis of multiwavelength imaging from HST and the Spitzer Space Telescope demonstrates that z>6 UV-selected galaxies are relatively compact with blue UV continuum slopes, low stellar masses, and large specific star formation rates. In the last year, ALMA (the Atacama Large Millimeter Array) and ground-based infrared spectrographs have begun to complement this picture, revealing minimal dust obscuration and hard radiation fields, and providing evidence for metal-poor ionized gas. Weak low-ionization absorption lines suggest a patchy distribution of neutral gas surrounds O and B stars, possibly aiding in the escape of ionizing radiation. Gamma ray burst afterglows and Lyman-α surveys have provided evidence that the intergalactic medium (IGM) evolves from mostly ionized at z≃6−6.5 (
) to considerably neutral at z≃7−8 (
). The reionization history that emerges from considering the UV output of galaxies over 6<z<10 is consistent with these constraints on the IGM ionization state. The latest measurements suggest that galaxies can complete reionization by z≃6 and reproduce the Thomson scattering optical depth faced by cosmic microwave background photons if the luminosity function extends ≃4 mag below current surveys and a moderate fraction (
) of ionizing radiation escapes from galaxies.
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Previous Volumes
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Volume 62 (2024)
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Volume 61 (2023)
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Volume 60 (2022)
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Volume 59 (2021)
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Volume 58 (2020)
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Volume 57 (2019)
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Volume 56 (2018)
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Volume 55 (2017)
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Volume 54 (2016)
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Volume 53 (2015)
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Volume 52 (2014)
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Volume 51 (2013)
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Volume 50 (2012)
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Volume 49 (2011)
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Volume 48 (2010)
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Volume 47 (2009)
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Volume 46 (2008)
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Volume 45 (2007)
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Volume 44 (2006)
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Volume 43 (2005)
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Volume 42 (2004)
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Volume 41 (2003)
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Volume 40 (2002)
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Volume 39 (2001)
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Volume 38 (2000)
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Volume 37 (1999)
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Volume 36 (1998)
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Volume 35 (1997)
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Volume 34 (1996)
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Volume 33 (1995)
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Volume 32 (1994)
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Volume 31 (1993)
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Volume 30 (1992)
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Volume 29 (1991)
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Volume 28 (1990)
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Volume 27 (1989)
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Volume 26 (1988)
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Volume 25 (1987)
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Volume 24 (1986)
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Volume 23 (1985)
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Volume 22 (1984)
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Volume 21 (1983)
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Volume 20 (1982)
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Volume 19 (1981)
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Volume 18 (1980)
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Volume 17 (1979)
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Volume 16 (1978)
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Volume 15 (1977)
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Volume 14 (1976)
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Volume 13 (1975)
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Volume 12 (1974)
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Volume 11 (1973)
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Volume 10 (1972)
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Volume 9 (1971)
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Volume 8 (1970)
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Volume 7 (1969)
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Volume 6 (1968)
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Volume 5 (1967)
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Volume 4 (1966)
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Volume 3 (1965)
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Volume 2 (1964)
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Volume 1 (1963)
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Volume 0 (1932)