8,368 research outputs found
Revisiting the Parallax of the Isolated Neutron Star RX J185635-3754 Using HST/ACS Imaging
We have redetermined the parallax and proper motion of the nearby isolated
neutron star RX~J185635-3754. We used eight observations with the high
resolution camera of the HST/ACS taken from 2002 through 2004. We performed the
astrometric fitting using five independent methods, all of which yielded
consistent results. The mean estimate of the distance is 123 (+11, -15) pc (1
sigma), in good agreement with our earlier published determination
The Stellar and Gaseous Contents of the Orion Dwarf Galaxy
We present new KPNO 0.9-m optical and VLA HI spectral line observations of
the Orion dwarf galaxy. This nearby (D ~ 5.4 Mpc), intermediate-mass (M_dyn =
1.1x10^10 Solar masses) dwarf displays a wealth of structure in its neutral
ISM, including three prominent "hole/depression" features in the inner HI disk.
We explore the rich gas kinematics, where solid-body rotation dominates and the
rotation curve is flat out to the observed edge of the HI disk (~6.8 kpc). The
Orion dwarf contains a substantial fraction of dark matter throughout its disk:
comparing the 4.7x10^8 Solar masses of detected neutral gas with estimates of
the stellar mass from optical and near-infrared imaging (3.7x10^8 Solar masses)
implies a mass-to-light ratio of ~13. New H alpha observations show only
modest-strength current star formation (~0.04 Solar masses per year); this star
formation rate is consistent with our 1.4 GHz radio continuum non-detection.Comment: Astronomical Journal, in press. Full-resolution version available
from http://www.macalester.edu/~jcannon/pubs.htm
An open--quantum--system formulation of particle decay
We consider an open quantum system which contains unstable states. The time
evolution of the system can be described by an effective non-hermitian
Hamiltonian H_{eff}, in accord with the Wigner--Weisskopf approximation, and an
additional term of the Lindblad form, the socalled dissipator. We show that,
after enlarging the original Hilbert space by states which represent the decay
products of the unstable states, the non-hermitian part of H_{eff} --the
``particle decay''-- can be incorporated into the dissipator of the enlarged
space via a specific Lindblad operator. Thus the new formulation of the time
evolution on the enlarged space has a hermitian Hamiltonian and is probability
conserving. The equivalence of the new formulation with the original one
demonstrates that the time evolution which is governed by a non-hermitian
Hamiltonian and a dissipator of the Lindblad form is nevertheless completely
positive, just as systems with hermitian Hamiltonians.Comment: 8 page
Root Coverage Procedure With Connective Tissue Graft Harvested From a Distal Wedge: A Case Report
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141955/1/cap0134.pd
Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I
We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW.open114sciescopu
Evidence of Volatile-Induced Melting in the Northeast Asian Upper Mantle
A seismic low velocity layer (LVL) above the mantle transition zone (MTZ), often thought to be caused by volatile-induced melting, can significantly modulate planetary volatile cycles. In this work, we show that an LVL observed beneath northeast Asia is characterized by small, 0.8 (Formula presented.) 0.5 vol%, average degrees of partial melting. Seismically derived P-T conditions of the LVL indicate that slab-derived (Formula presented.), possibly combined with small amounts of (Formula presented.) O, is necessary to induce melting. Modeling the reactive infiltration instability of the melt in a stationary mantle above a stalled slab, we demonstrate that the volatile-rich melt slowly rises above the stalled slab in the MTZ, with percolation velocities of 200–500 (Formula presented.) m/yr. The melt remains stable within the LVL for this geologically significant period of time, potentially transferring up to 52 Mt/yr of (Formula presented.) from the subducting slab to the mantle for an LVL similar in areal extent ((Formula presented.)) to the northeast Asian LVL. Reaction between the melt channels and the LVL mantle precipitates up to 200 ppmw solid C in localized zones. Using the inferred small melt volume fraction to model trace element abundances and isotopic signatures, we show that interaction between this melt and the surrounding mantle can over the long-term produce rocks bearing a HIMU like geochemical signature
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
We compare the stellar populations and complex neutral gas dynamics of the
M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I
identified two kinematically distinct HI components, multiple localized high
velocity gas features, and eight HI holes and shells (the largest of which
spans ~2.2x1.1 kpc). Using the spatial and temporal information from the
stellar populations in DDO 165, we compare the patterns of star formation over
the past 500 Myr with the HI dynamics. We extract localized star formation
histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other
regions that sample a range of stellar densities and neutral gas properties.
From population synthesis modeling, we derive the energy outputs (from stellar
winds and supernovae) of the stellar populations within these regions over the
last 100 Myr, and compare with refined estimates of the energies required to
create the HI holes. In all cases, we find that "feedback" is energetically
capable of creating the observed structures in the ISM. Numerous regions with
significant energy inputs from feedback lack coherent HI structures but show
prominent localized high velocity gas features; this feedback signature is a
natural product of temporally and spatially distributed star formation. In DDO
165, the extended period of heightened star formation activity (lasting more
than 1 Gyr) is energetically capable of creating the observed holes and high
velocity gas features in the neutral ISM.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
Model for Glass Transition in a Binary fluid from a Mode Coupling approach
We consider the Mode Coupling Theory (MCT) of Glass transition for a Binary
fluid. The Equations of Nonlinear Fluctuating Hydrodynamics are obtained with a
proper choice of the slow variables corresponding to the conservation laws. The
resulting model equations are solved in the long time limit to locate the
dynamic transition. The transition point from our model is considerably higher
than predicted in existing MCT models for binary systems. This is in agreement
with what is seen in Computer Simulation of binary fluids. fluids.Comment: 9 Pages, 3 Figure
Angle-Resolved Photoemission Spectroscopy of Tetragonal CuO: Evidence for Intralayer Coupling Between Cupratelike Sublattices
We investigate by angle-resolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasi-two-dimensional edge-sharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the CuO2 layers of the high-T-c cuprates, the low-energy electronic excitations are surprisingly similar, with a Zhang-Rice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the high-T-c cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.open1199sciescopu
- …