50 research outputs found
Negative Energy and Angular Momentum Modes of Thin Accretion Disks
This work derives the linearized equations of motion, the Lagrangian density,
the Hamiltonian density, and the canonical angular momentum density for general
perturbations [ with ] of a geometrically
thin self-gravitating, homentropic fluid disk including the pressure. The
theory is applied to ``eccentric,'' perturbations of a geometrically
thin Keplerian disk. We find modes at low frequencies relative to the
Keplerian frequency. Further, it shown that these modes can have negative
energy and negative angular momentum. The radial propagation of these low
frequency modes can transport angular momentum away from the inner region
of a disk and thus increase the rate of mass accretion. Depending on the radial
boundary conditions there can be discrete low-frequency, negative-energy,
modes.Comment: 24 pages, 8 figure
Gravitational Collapse and Disk Formation in Magnetized Cores
We discuss the effects of the magnetic field observed in molecular clouds on
the process of star formation, concentrating on the phase of gravitational
collapse of low-mass dense cores, cradles of sunlike stars. We summarize recent
analytic work and numerical simulations showing that a substantial level of
magnetic field diffusion at high densities has to occur in order to form
rotationally supported disks. Furthermore, newly formed accretion disks are
threaded by the magnetic field dragged from the parent core during the
gravitational collapse. These disks are expected to rotate with a sub-Keplerian
speed because they are partially supported by magnetic tension against the
gravity of the central star. We discuss how sub-Keplerian rotation makes it
difficult to eject disk winds and accelerates the process of planet migration.
Moreover, magnetic fields modify the Toomre criterion for gravitational
instability via two opposing effects: magnetic tension and pressure increase
the disk local stability, but sub-Keplerian rotation makes the disk more
unstable. In general, magnetized disks are more stable than their nonmagnetic
counterparts; thus, they can be more massive and less prone to the formation of
giant planets by gravitational instability.Comment: Chapter 16 in "Magnetic Fields in Diffuse Media", Springer-Verlag,
eds. de Gouveia Dal Pino, E., Lazarian, A., Melioli,
The Physics of Star Cluster Formation and Evolution
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe
An integrative multi-omics analysis to identify candidate DNA methylation biomarkers related to prostate cancer risk
It remains elusive whether some of the associations identified in genome-wide association studies of prostate cancer (PrCa) may be due to regulatory effects of genetic variants on CpG sites, which may further influence expression of PrCa target genes. To search for Cp
A law of large numbers and central limit theorem for the logarithm of an autoregressive process with a stationary driving sequence
Let be a stationary and ergodic sequence in . Consider the autoregressive process defined by, R0([xi],[eta],[nu])=[eta]0 and Rn([xi],[eta],[nu])=[xi]nRn-1([xi],[eta],[nu])+[eta]n[nu]n[dot operator]...[dot operator][nu]1,n>=1. We give conditions under which exists a.s. and at the same time, . We also generalize a Central Limit Theorem of Szekely for this process.
Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6
Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu2PO6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b-and a-axes respectively. Here, we investigate thermal conductivity Îș of BiCu2PO6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in Îș, located at âŒ15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of âŒ30 near the critical magnetic field of Hc â° 23.5 tesla. The giant H-and T-dependent suppression of Îș can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at Hc, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at Hc. © 2016 The Author(s)1441sciescopu
Incommensurate dynamic correlations in the quasi-two-dimensional spin liquid BiCu2PO6
We report detailed inelastic neutron-scattering measurements on single crystals of the frustrated two-leg ladder BiCu2PO6, whose ground state is described as a spin liquid phase with no long-range order down to 6 K. Two branches of steeply dispersing long-lived spin excitations are observed with excitation gaps of \u3941=1.90(9) meV and \u3942=3.95(8) meV. Significant frustrating next-nearest-neighbor interactions along the ladder leg drive the minimum of each excitation branch to incommensurate wave vectors \u3b61=0.574\u3c0 and \u3b62=0.553\u3c0 for the lower and upper energy branches, respectively. The temperature dependence of the excitation spectrum near the gap energy is consistent with thermal activation into singly and doubly degenerate excited states. The observed magnetic excitation spectrum as well as earlier thermodynamic data could be consistently explained by the presence of strong anisotropic interactions in the ground-state Hamiltonian. \ua9 2013 American Physical Society.Peer reviewed: YesNRC publication: Ye
Scale-free equilibria of isopedic polytropic clouds
Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal