Kinematic Density Waves in Accretion Disks

When thin accretion disks around black holes are perturbed, the main restoring force is gravity. If gas pressure, magnetic stresses, and radiation pressure are neglected, the disk remains thin as long as orbits do not intersect. Intersections would result in pressure forces which limit the growth of perturbations. We find that a discrete set of perturbations is possible for which orbits remain non-intersecting for arbitrarily long times. These modes define a discrete set of frequencies. We classify all long-lived perturbations for arbitrary potentials and show how their mode frequencies are related to pattern speeds computed from the azimuthal and epicyclic frequencies. We show that modes are concentrated near radii where the pattern speed has vanishing radial derivative. We explore these modes around Kerr black holes as a possible explanation for the high-frequency quasi-periodic oscillations of black hole binaries such as GRO J1655-40. The long-lived modes are shown to coincide with diskoseismic waves in the limit of small sound speed. While the waves have long lifetime, they have the wrong frequencies to explain the pairs of high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added reference to astro-ph/060368

Anomalously Weak Dynamical Friction in Halos

A bar rotating in a pressure-supported halo generally loses angular momentum and slows down due to dynamical friction. Valenzuela & Klypin report a counter-example of a bar that rotates in a dense halo with little friction for several Gyr, and argue that their result invalidates the claim by Debattista & Sellwood that fast bars in real galaxies require a low halo density. We show that it is possible for friction to cease for a while should the pattern speed of the bar fluctuate upward. The reduced friction is due to an anomalous gradient in the phase-space density of particles at the principal resonance created by the earlier evolution. The result obtained by Valenzuela & Klypin is probably an artifact of their adaptive mesh refinement method, but anyway could not persist in a real galaxy. The conclusion by Debattista & Sellwood still stands.Comment: To appear in "Island Universes - Structure and Evolution of Disk Galaxies" ed. R. S. de Jong, 8 pages, 4 figures, .cls and .sty files include

ウィスコンシン ダイガク マディソンコウ ガ ジッシ シテイル ナンキョク ムジン キショウ カンソク (AWS) ケイカク ノ 2011-2012 ネン カキ ノ カツドウ

ウィスコンシン大学マディソン校で推進している南極無人気象観測計画(Antarctic Automatic Weather Station(AWS)program)の32 年目の観測が,2011/2012年の南半球夏期に完了した.無人気象観測網を利用して南極の気象と気候の研究が行われている.今シーズンはロス島周辺域,ロス棚氷,西南極,東南極にわたる領域で活動した.基本的に観測点のデータはアルゴス衛星を中継して配信されるが,今年はロス島周辺域の多くの観測点で,マクマード基地を中継して"Freewave modem"を通して配信された.各無人気象観測点報告には,現在設置されている測器と動作状況が含まれる.また,無人気象観測計画の全体像を,野外活動の実施状況に沿って示す.During the 2011-2012 austral summer, the Antarctic Automatic Weather Station (AWS) program at the University of Wisconsin?Madison completed its 32nd year of observations. Ongoing studies utilizing the network include topics in Antarctic meteorology and climate studies. This field season consisted of work throughout the Ross Island area, the Ross Ice Shelf, West Antarctica, and East Antarctica. Argos satellite transmissions are the primary method for relaying station data, but throughout this year, a number of stations in the Ross Island area have been converted to Freewave modems, with their data being relayed through McMurdo station. Each AWS station report contains information regarding the instrumentation currently installed and the work performed at each site. An overview of the AWS applications is included along with field work accomplished

An inner ring and the micro lensing toward the Bulge

All current Bulge-Disk models for the inner Galaxy fall short of reproducing self-consistently the observed micro-lensing optical depth by a factor of two ($> 2\sigma$). We show that the least mass-consuming way to increase the optical depth is to add density roughly half-way the observer and the highest micro-lensing-source density. We present evidence for the existence of such a density structure in the Galaxy: an inner ring, a standard feature of barred galaxies. Judging from data on similar rings in external galaxies, an inner ring can contribute more than 50% of a pure Bulge-Disk model to the micro-lensing optical depth. We may thus eliminate the need for a small viewing angle of the Bar. The influence of an inner ring on the event-duration distribution, for realistic viewing angles, would be to increase the fraction of long-duration events toward Baade's window. The longest events are expected toward the negative-longitude tangent point at $\ell\sim$ -22\degr . A properly sampled event-duration distribution toward this tangent point would provide essential information about viewing angle and elongation of the over-all density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted

Halo properties and secular evolution in barred galaxies

The halo plays a crucial role in the evolution of barred galaxies. Its near-resonant material absorbs angular momentum emitted from some of the disc particles and helps the bar become stronger. As a result, a bar (oval) forms in the inner parts of the halo of strongly barred disc galaxies. It is thinner in the inner parts (but still considerably fatter than the disc bar) and tends to spherical at larger radii. Its length increases with time, while always staying shorter than the disc bar. It is roughly aligned with the disc bar, which it trails only slightly, and it turns with roughly the same pattern speed. The bi-symmetric component of the halo density continues well outside the halo bar, where it clearly trails behind the disc bar. The length and strength of the disc and halo bars correlate; the former being always much stronger than the latter. If the halo is composed of weakly interacting massive particles, then the formation of the halo bar, by redistributing the matter in the halo and changing its shape, could influence the expected annihilation signal. This is indeed found to be the case if the halo has a core, but not if it has a steep cusp. The formation and evolution of the bar strongly affect the halo orbits. A fraction of them becomes near-resonant, similar to the disc near-resonant orbits at the same resonance, while another fraction becomes chaotic. Finally, a massive and responsive halo makes it harder for a central mass concentration to destroy the disc bar.Comment: 6 pages, 3 figures, to appear in "Island Universes - Structure and Evolution of Disk Galaxies" ed. R. S. de Jon

Automatic Weather Station (AWS) Program operated by the University of Wisconsin-Madison during the 2012-2013 field season: Challenges and Successes

ウィスコンシン大学マディソン校で推進している南極無人気象観測計画（Antarctic Automatic Weather Station（AWS）program）の，2012-2013年のフィールド調査および結果の概要を報告する．今期はAWS 観測網の歴史上，特異なシーズンであった．ロス島地域が温暖であったことは氷上滑走路の利用に影響を及ぼし，いくつかの設営面での制約に直面した． 柔軟な計画により，限られた条件下でAWS サービルを最大化し，自動観測ネットワークへの要求に対応する最善の手段をとることができた．This report reviews 2012-2013 field season activities of the University of Wisconsin-Madison＇s Antarctic Automatic Weather Station (AWS) program, summarizes the science that these sites are supporting, and outlines the factors that impact the number of AWS sites serviced in any given field season. The 2012-2013 austral summer season was unusual in the AWS network history. Challenges encountered include, but are not limited to, warmer than normal conditions in the Ross Island area impacting airfield operations, changes to logistical procedures, and competition for shared resources. A flexible work plan provides the best means for taking on these challenges while maximizing AWS servicing efforts under restricted conditions and meeting the need for routine servicing that maintaining an autonomous observing network demands

Leading Wave as a Component of the Spiral Pattern of the Galaxy

The spiral pattern of the Galaxy identified by analyzing the kinematics of young stars within 3 kpc of the Sun is Fourier decomposed into spiral harmonics. The spiral pattern of the Galaxy is shown to be representable as a superposition of trailing and leading waves with interarm distances of 1.8(+/-0.4) kpc and 4(+/-2) kpc, respectively. Shock waves are probably present only in the portions of the trailing spiral pattern where it crosses the crest of the leading wave. The small interarm distance of the trailing spiral wave (1.8 kpc) can be explained by its evolution - by the decrease in the interarm distance as the wave is displaced toward the inner Lindblad resonance. The Carina arm may be part of this resonance ring.Comment: 17 pages, 4 figures, to be published in Astronomy Letters, 200

Singular Isothermal Disks: II. Nonaxisymmetric Bifurcations and Equilibria

We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance from the rotation axis of the system. In agreement with earlier analysis by Syer and Tremaine, we find that lopsided (M=1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into M=2, 3, 4, ... separate bodies. On the basis of our experience in this paper, and the preceding Paper I, we advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse.Comment: 49 pages, 11 figures, LaTeX, needs aaspp4.sty. The Astrophysical Journal, in pres

Extracting science from surveys of our Galaxy

Our knowledge of the Galaxy is being revolutionised by a series of photometric, spectroscopic and astrometric surveys. Already an enormous body of data is available from completed surveys, and data of ever increasing quality and richness will accrue at least until the end of this decade. To extract science from these surveys we need a class of models that can give probability density functions in the space of the observables of a survey -- we should not attempt to "invert" the data from the space of observables into the physical space of the Galaxy. Currently just one class of model has the required capability, so-called "torus models". A pilot application of torus models to understanding the structure of the Galaxy's thin and thick discs has already produced two significant results: a major revision of our best estimate of the Sun's velocity with respect to the Local Standard of Rest, and a successful prediction of the way in which the vertical velocity dispersion in the disc varies with distance from the Galactic plane.Comment: 13 pages. Invited review to appear in Pramana - journal of physics (Indian Academy of Sciences

A Recent Lindblad Resonance in the Solar Neighbourhood

Stars in the solar neighbourhood do not have a smooth distribution of velocities. Instead, the distribution of velocity components in the Galactic plane manifests a great deal of kinematic substructure. Here I present an analysis in action-angle variables of the Geneva-Copenhagen survey of ~14,000 nearby F & G dwarfs with distances and full space motions. I show that stars in the so-called "Hyades stream" have both angle and action variables characteristic of their having been scattered at an inner Lindblad resonance of a rotating disturbance potential. This discovery seems to favour spiral patterns as recurrent, short-lived instabilities.Comment: 12 pages, 10 figures to appear in MNRAS. Minor revisions from original versio