2,102 research outputs found
The existence of a 2Po excited state for the e+Ca system
The Configuration Interaction method is used to demonstrate that there is an
electronically stable state of positronic calcium with an orbital angular
momentum of L=1. This prediction relies on the use of an asymptotic series to
estimate the variational limit of the energy. The best estimate of the binding
energy is 37 meV. A discussion of the structure of the system is also
presented.Comment: 4 pages, 2 figures, in press PR
A Hybrid N-body--Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planets from an
initial ensemble of planetesimals. The algorithm uses a coagulation code to
treat the growth of planetesimals into oligarchs and explicit N-body
calculations to follow the evolution of oligarchs into planets. To validate the
N-body portion of the algorithm, we use a battery of tests in planetary
dynamics. Several complete calculations of terrestrial planet formation with
the hybrid code yield good agreement with previously published calculations.
These results demonstrate that the hybrid code provides an accurate treatment
of the evolution of planetesimals into planets.Comment: Astronomical Journal, accepted; 33 pages + 11 figure
Kinematics of the swimming of Spiroplasma
\emph{Spiroplasma} swimming is studied with a simple model based on
resistive-force theory. Specifically, we consider a bacterium shaped in the
form of a helix that propagates traveling-wave distortions which flip the
handedness of the helical cell body. We treat cell length, pitch angle, kink
velocity, and distance between kinks as parameters and calculate the swimming
velocity that arises due to the distortions. We find that, for a fixed pitch
angle, scaling collapses the swimming velocity (and the swimming efficiency) to
a universal curve that depends only on the ratio of the distance between kinks
to the cell length. Simultaneously optimizing the swimming efficiency with
respect to inter-kink length and pitch angle, we find that the optimal pitch
angle is 35.5 and the optimal inter-kink length ratio is 0.338, values
in good agreement with experimental observations.Comment: 4 pages, 5 figure
Line Emission from an Accretion Disk around a Rotating Black Hole: Toward a Measurement of Frame Dragging
Line emission from an accretion disk and a corotating hot spot about a
rotating black hole are considered for possible signatures of the
frame-dragging effect. We explicitly compare integrated line profiles from a
geometrically thin disk about a Schwarzschild and an extreme Kerr black hole,
and show that the line profile differences are small if the inner radius of the
disk is near or above the Schwarzschild stable-orbit limit of radius 6GM/c^2.
However, if the inner disk radius extends below this limit, as is possible in
the extreme Kerr spacetime, then differences can become significant, especially
if the disk emissivity is stronger near the inner regions. We demonstrate that
the first three moments of a line profile define a three-dimensional space in
which the presence of material at small radii becomes quantitatively evident in
broad classes of disk models. In the context of the simple, thin disk paradigm,
this moment-mapping scheme suggests formally that the iron line detected by the
Advanced Satellite for Cosmology and Astrophysics mission from MCG-6-30-15
(Tanaka et al. 1995) is 3 times more likely to originate from a disk about a
rotating black hole than from a Schwarzschild system. A statistically
significant detection of black hole rotation in this way may be achieved after
only modest improvements in the quality of data. We also consider light curves
and frequency shifts in line emission as a function of time for corotating hot
spots in extreme Kerr and Schwarzschild geometries. Both the frequency-shift
profile and the light curve from a hot spot are valuable measures of orbital
parameters and might possibly be used to detect frame dragging even at radii
approaching 6GM/c^2 if the inclination angle of the orbital plane is large.Comment: 15 pages (LaTex), 7 postscript figures; color plot (Figure 1)
available at http://cfata2.harvard.edu/bromley/nu_nofun.html (This version
contains a new subsection as well as minor corrections.
On the Doppler effect for light from orbiting sources in Kerr-type metrics
A formula is derived for the combined motional and gravitational Doppler
effect in general stationary axisymmetric metrics for a photon emitted parallel
or antiparallel to the assumed circular orbital motion of its source. The same
formula is derived from eikonal approximation and Killing vector approaches to
elucidate connections between observational astronomy and modern Relativity.
The formula yields expected results in the limits of a moving or stationary
source in the exterior Kerr and Schwarzschild metrics and a moving source in
flat space.Comment: Accepted for publication in in Monthly Notices of the Royal
Astronomical Society Main Journal 1.23.15. This version has substantially
shortened and clarified derivations and added content regarding applicability
of the derivation
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.Comment: 21 pages including 8 figures; LaTeX; ApJ format; accepted by ApJ;
short results of this paper appeared before as a conference proceedings
(astro-ph/9711214
The Rise and Fall of Debris Disks: MIPS Observations of h and chi Persei and the Evolution of Mid-IR Emission from Planet Formation
We describe Spitzer/MIPS observations of the double cluster, h and
Persei, covering a 0.6 square-degree area surrounding the cores of both
clusters. The data are combined with IRAC and 2MASS data to investigate
616 sources from 1.25-24 . We use the long-baseline -[24] color
to identify two populations with IR excess indicative of circumstellar
material: Be stars with 24 excess from optically-thin free free
emission and 17 fainter sources (J 14-15) with [24] excess consistent
with a circumstellar disk. The frequency of IR excess for the fainter sources
increases from 4.5 through 24 . The IR excess is likely due to
debris from the planet formation process. The wavelength-dependent behavior is
consistent with an inside-out clearing of circumstellar disks. A comparison of
the 24 excess population in h and Per sources with results for
other clusters shows that 24 emission from debris disks 'rises' from 5
to 10 Myr, peaks at 10-15 Myr, and then 'falls' from 15/20 Myr to
1 Gyr.Comment: 48 pages, 15 figures, accepted for publication in The Astrophysical
Journa
Quantitative complementarity in two-path interferometry
The quantitative formulation of Bohr's complementarity proposed by
Greenberger and Yasin is applied to some physical situations for which
analytical expressions are available. This includes a variety of conventional
double-slit experiments, but also particle oscillations, as in the case of the
neutral-kaon system, and Mott scattering of identical nuclei. For all these
cases, a unified description can be achieved including a new parameter, ,
which quantifies the effective number of fringes one can observe in each
specific interferometric set-up.Comment: 11 RevTex pages, 5 figure
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