ASTR 565: Compact Stellar Exotica

ASTR 565 Course Profile: Compact Stellar Exotica

As a brief introduction to ASTR 565, to supplement the Course Description, here we survey the course, which provides an in-depth, basic study of the astrophysics and pertinent physics of some of the most exotic and fascinating objects that are found in the Milky Way and in distant galaxies. This gallery is a representative (but of course incomplete) sampling of the material covered in the course.


[Click on the images below for larger versions]

Compact, post-main sequence stars in the Milky Way and beyond display a remarkable array of phenomena that have fascinated astronomers and physicists since the discovery of white dwarfs in the middle of the 19th Century. Neutron stars and black holes were envisaged first as theoretical concepts, spawned by the advances in relativity, quantum mechanics and nuclear physics in the ``golden age'' of physics in the early part of the 20th Century. Their observational confirmations over the last 4 decades has been spawned by technological advances in various wavebands. This course explores our current understanding of these objects, with a principal focus being on the physics of their galactic manifestations, but also providing discussion of the extragalactic environments of supermassive black holes and gamma-ray bursts. At left is a Chandra Observatory X-ray image of the wind nebula and jet surrounding the pulsar PSR 1509-58.

The course begins with an astrophysical overview of the origin and occurrence of compact objects in the cosmos, highlighting specific examples that are have been particularly interesting and those that are topical at the moment. Then it focuses on the details of condensed stellar forms, starting with white dwarf stars. This material then addresses the most pertinent issue of electron degeneracy, and how this impacts the equation of state and the hydrostatic structure in the interior of white dwarfs. Signature properties such as the mass-radius relation and the Chandrasekhar mass limit are highlighted. Then our attention turns to neutronization and pyconuclear reactions in condensed dwarf interiors. Subsequently, properties of white dwarf cooling and crystallization are explored, noting how these can lead to using white dwarfs as a chronometer for our universe. To the right is an artist's depiction using optical Hubble Space Telescope images of the first known white dwarf, Sirius B (small blue star) together with its brighter main sequence companion Sirius A (the Dog Star).

After a brief encounter with stellar stability, the subject matter turns to general relativity, a necessary prerequisite for our studies of both neutron stars and black holes. The lectures summarize metrics, the Equivalence principal, Einstein's field equations, gravitational redshift, and introduce the Schwarzschild solution for static, spherically symmetric systems. The lynchpin physical tests of Einstein's theory are addressed, specifically light deflection and precession of the perihelia. Exploration of gravitational radiation is deferred to later in the course, when binary pulsars are discussed. An artist's conception of an infalling body (and it's own potential) under the influence of the extremely strong gravitational potential of a black hole is depicted on the left. Such coalescences are prime candidates for the gravity waves that are the holy grail of LIGO and LISA. [Courtesy: LISA/SRL Caltech]

Prior to our neutron star studies will be the consideration of stellar rotation. and strong magnetic field development. Oblateness of rapidly rotating stars will be addressed, specifically Maclaurin spheroids, which serves both to investigate stability of white dwarfs and neutron stars, and also as a preparation for gravitational quadrupole radiation later in the course. The neutron star theme will begin with the exploration of nuclear/particle physics of their interiors, including the Baym-Bethe-Pethick equation of state, nucleon-nucleon interactions and pion condensation. Neutron star models, mass determinations and their maximum mass will then be examined. A depiction of neutron star magnetospheric geometry and putative particle acceleration locales (electrodynamic potential gaps) is given at left [Credit: Alice Harding, NASA/GSFC].

The neutron star studies will then turn our attention to their observational manifestations, pulsars. Starting with the radio pulsar discovery and various radio properties, the course will explore pulsar electrodynamics, their winds, characteristics of the magnetospheric geometry, and spin-down. The high energy astrophysics of pulsars, including magnetospheric emission mechanisms and pair cascades, and the relationship to the radio pulsar death line, will all be addressed. Simple models of glitch activity will be treated, in particular angular momentum transfer between the superfluid core and the crust in starquakes. All along, recent observational highlights will embellish the pedagogy, including new insights provided by NASA's Fermi Gamma-ray Space Telescope. The sky distribution, as of early 2006, for isolated pulsars is illustrated to the left. This was obtained from the Australia Telescope National Facility (ATNF) Pulsar Catalogue.

As part of the pulsar material, next is a sub-focus on the topical magnetars, with their phenomenal power and intense fields; this will excite our imaginations and blow our minds. Providing the brightest transients in the galaxy after supernovae, following an observational overview, our discussion will center on the possible magnetic origin of their dissipation, their giant flares and associated evidence for magnetic field restructing. We will also touch upon some of the exotic QED processes that can be active in the atmospheres and magnetospheres. Then we will move to an examination neutron star cooling and how surface emission observations from X-ray telescopes can probe the equations of state in their interiors, by constraining the mass-radius phase space. The light curve for the August 1998 giant flare of the soft gamma repeater SGR1900+14, is depicted to the right, exhibiting the periodicity that is the hallmark of these powerful outbursts from magnetars. This data was obtained from the gamma-ray detector on the Ulysses Mission, whose principal objective was to probe the solar wind. [Courtesy: Kevin Hurley, UC Berkeley]

Up until this point, we will have been sampling general relativity, but not "devouring" it. At this juncture, we will explore the properties of Schwarzschild black holes, focusing on particle motions and photon orbits in their environs, and the nonsingularity of the event horizon. Rotating Kerr black holes will be next, including how X-ray line diagnostics can be used to infer spin-to-mass ratios. The more formal material of the area theorem and black hole evaporation will also be briefly addressed. Then we will explore gravitational radiation, its generation and impact upon rotating systems, and its detection by current laser interferometer initiatives such as LIGO and LISA. To encapsulate the exotic nature of gravity waves, we summarize potential astrophysical sources, in particular the binary pulsar systems that are stunningly powerful probes of Einstein's landmark theoretical predictions. At right is an artist's visualization of gravitational waves from a black hole together with the planned Laser Interferometer Space Antenna (LISA), NASA's space initiative to detect gravitational radiation from compact objects.

The course now evolves into another "astrophysical manifestation" phase, focusing first on gamma-ray bursts, the exotic transients that have captured the imaginations of astrophysicists for over three decades. The development of their paradigm is followed, from the galactic neutron star hypothesis to a cosmological population with measured redshifts that are believed to originate from massive supernovae or coalescence of binary compact objects. Then we explore X-ray binaries, accreting systems that emit prominently in X-rays. Key signatures of these sources that elucidate their environs are cyclotron emission lines, which probe their fields, and quasi-periodic oscillations that probe the dynamics of their accretion disks. As an entree into the world of accreting black holes that expel matter through jets, the course then summarizes the properties of microquasars, a comparatively recent addition to the zoo of compact objects, which exhibit superluminal motion within structured jets. A time sequence of radio images of the galactic microquasar GRS1915+105 is depicted at left. These images were obtained using the Very Large Array of radio telescopes, and illustrates the motion of blobs of luminous material that are expanding superluminally from a central compact object (black hole).

Our final focal points will be supermassive black holes and elements of accretion theory pertaining to them. After exploring some of the observational characteristics of active galactic nuclei, including a brief outline of Seyferts, blazars and unification scenarios for their morphology, the material will briefly cover spherical Bondi accretion, angular momentum characteristics and elements of accretion disk structure. At right is a 6cm radio image (from the Very Large Array) of the radio galaxy Cygnus A, displaying both its beautiful asymmetric jets emanating from a compact nucleus, and its Mpc-scale lobes with prominent hotspots.


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