THE TAX REVOLT

Public Economics,

Angular Momentum Transport and Variability in Boundary Layers of Accretion Disks Driven by Global Acoustic Modes

Disk accretion onto a weakly magnetized central object, e.g. a star, is inevitably accompanied by the formation of a boundary layer near the surface, in which matter slows down from the highly supersonic orbital velocity of the disk to the rotational velocity of the star. We perform high resolution 2D hydrodynamical simulations in the equatorial plane of an astrophysical boundary layer with the goal of exploring the dynamics of non-axisymmetric structures that form there. We generically find that the supersonic shear in the boundary layer excites non-axisymmetric quasi-stationary acoustic modes that are trapped between the surface of the star and a Lindblad resonance in the disk. These modes rotate in a prograde fashion, are stable for hundreds of orbital periods, and have a pattern speed that is less than and of order the rotational velocity at the inner edge of the disk. The origin of these intrinsically global modes is intimately related to the operation of a corotation amplifier in the system. Dissipation of acoustic modes in weak shocks provides a universal mechanism for angular momentum and mass transport even in purely hydrodynamic (i.e. non-magnetized) boundary layers. We discuss the possible implications of these trapped modes for explaining the variability seen in accreting compact objects.Comment: 41 pages, 19 figures, accepted to Ap

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.Comment: 14 pages including 3 figure

Computer-Modeling of Stars

A human being experiences his immediate environment on the scale of meters, seconds and grams. These are also the natural scales of his actions. Thus, as soon as he starts to explore the laws of physics, he can easily move around masses at the scale of grams, objects on the scale of meters and perform experiments on the scale of seconds. On these scales, the experimentator has full control on the setup of an experiment and direct access to all degrees of freedom during the evolution of the experiment. This direct access is lost in experiments that explore the physics on scales that are many orders of magnitude smaller. The experimentator still has full control on the setup, for example, by putting a specific target into a properly designed accelerator beam. But the measurements are then limited to the far field, where only a superposition of the effects of the microscopic physics becomes detectable. The large number of degrees of freedom that may be present in the microscopic physics must be explored by clever variations of the experimental setup. Most astronomical observations are obviously also taken from the far field, because the distance to the observed source is so much larger than the length scale of the source. Hence, many degrees of freedom of the dynamics on the length scale of the source are only indirectly accessible for the observer. Moreover, it is not possible to efficiently manipulate and prepare matter outside the solar system in order to produce systematic variations in the setup as in terrestrial experiments

swcarpentry/shell-novice: Software Carpentry: the UNIX shell, June 2019

Software Carpentry lesson on how to use the shell to navigate the filesystem and write simple loops and scripts