1,162 research outputs found
Second bound state of PsH
The existence of a second bound state of PsH that is electronically stable
and also stable against positron annihilation by the normal 2gamma and 3gamma
processes is demonstrated by explicit calculation. The state can be found in
the 2,4So symmetries with the two electrons in a spin triplet state. The
binding energy against dissociation into the H(2p) + Ps(2p) channel was
6.06x10-4 Hartree. The dominant decay mode of the states will be radiative
decay into a configuration that autoionizes or undergoes positron annihilation.
The NaPs system of the same symmetry is also electronically stable with a
binding energy of 1.553x10-3 Hartree with respect to the Na(3p) + Ps(2p)
channel.Comment: 4 pages, 2 figures, RevTex styl
Kinematic Effects of Tidal Interaction on Galaxy Rotation Curves
We use self-consistent N-body models, in conjunction with models of test
particles moving in galaxy potentials, to explore the initial effects of
interactions on the rotation curves of spiral galaxies. Using nearly
self-consistent disk/bulge/halo galaxy models (Kuijken & Dubinski 1995), we
simulate the first pass of galaxies on nearly parabolic orbits; we vary orbit
inclinations, galaxy halo masses and impact parameters. For each simulation, we
mimic observed rotation curves of the model galaxies. Transient
interaction-induced features of the curves include distinctly rising or falling
profiles at large radii and pronounced bumps in the central regions. Remarkably
similar features occur in our statistical sample of optical emission-line
rotation curves of spiral galaxies in tight pairs and n-tuples.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter
Planet formation around low mass stars: the moving snow line and super-Earths
We develop a semi-analytic model for planet formation during the pre-main
sequence contraction phase of a low mass star. During this evolution, the
stellar magnetosphere maintains a fixed ratio between the inner disk radius and
the stellar radius. As the star contracts at constant effective temperature,
the `snow line', which separates regions of rocky planet formation from regions
of icy planet formation, moves inward. This process enables rapid formation of
icy protoplanets that collide and merge into super-Earths before the star
reaches the main sequence. The masses and orbits of these super-Earths are
consistent with super-Earths detected in recent microlensing experiments.Comment: accepted by ApJ Letter
The Size Distribution of Kuiper Belt Objects
We describe analytical and numerical collisional evolution calculations for
the size distribution of icy bodies in the Kuiper Belt. For a wide range of
bulk properties, initial masses, and orbital parameters, our results yield
power-law cumulative size distributions, N_C propto r^{-q}, with q_L = 3.5 for
large bodies with radii of 10-100 km, and q_s = 2.5-3 for small bodies with
radii lesss than 0.1-1 km. The transition between the two power laws occurs at
a break radius of 1-30 km. The break radius is more sensitive to the initial
mass in the Kuiper Belt and the amount of stirring by Neptune than the bulk
properties of individual Kuiper Belt objects (KBOs). Comparisons with
observations indicate that most models can explain the observed sky surface
density of KBOs for red magnitudes, R = 22-27. For R 28, the model
surface density is sensitive to the amount of stirring by Neptune, suggesting
that the size distribution of icy planets in the outer solar system provides
independent constraints on the formation of Neptune.Comment: 24 pages of text, 12 figures; to appear in the Astronomical Journal,
October 200
Dynamic Stark shift of the Li-7(2s -> 3s) transition
The dynamic dipole polarizabilities for the 3s state of the Li atom are computed using Hylleraas and semiempirical approaches. This enables the calculation of the 7Li(2s→3s) Stark shift at photon wavelengths of 610.5 nm and 735.1 nm. They are calculated to be 834.7(5) and -686.7(25) a03, respectively. The first two tune-out frequencies for the 7Li(2s) ground state are determined to be 670.971626(1) nm and 324.192(2) nm, respectively
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
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
Configuration-interaction calculations of PsH and e(+)Be
The configuration-interaction (CI) method is applied to the study of the positronium-hydride (PsH) and positronic-beryllium (e+Be) systems. The binding energy and other properties are slowly convergent with respect to the angular momentum of the orbitals used to construct the CI basis states. The largest calculations recover 94% and 80% of the binding energy against dissociation when compared with existing calculations of PsH and e+ Be. Extrapolating using Cl convergence trends improves these results to 99% and 98%, respectively. Convergence is not so good for the electron-positron annihilation rates, but the extrapolated annihilation rates were within 10% of the best calculations. Two different schemes have been used to construct the CI basis, and it is found that it is possible to discard roughly half the CI basis with almost no degradation in the binding energy and the annihilation rate. These investigations demonstrate the feasibility of using single particle orbitals centred on the nucleus to represent positronic systems with two valence electrons
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