1,080 research outputs found
Particle-Particle Particle-Tree: A Direct-Tree Hybrid Scheme for Collisional N-Body Simulations
In this paper, we present a new hybrid algorithm for the time integration of
collisional N-body systems. In this algorithm, gravitational force between two
particles is divided into short-range and long-range terms, using a
distance-dependent cutoff function. The long-range interaction is calculated
using the tree algorithm and integrated with the constant-timestep leapfrog
integrator. The short-range term is calculated directly and integrated with the
high-order Hermite scheme. We can reduce the calculation cost per orbital
period from O(N^2) to O(N log N), without significantly increasing the
long-term integration error. The results of our test simulations show that
close encounters are integrated accurately. Long-term errors of the total
energy shows random-walk behaviour, because it is dominated by the error caused
by tree approximation.Comment: 22 pages, 15 figure
Ejection of close-in super-Earths around low-mass stars in the giant impact stage
Earth-sized planets were observed in close-in orbits around M dwarfs. While
more and more planets are expected to be uncovered around M dwarfs, theories of
their formation and dynamical evolution are still in their infancy. We
investigate the giant impact growth of protoplanets, which includes strong
scattering around low-mass stars. The aim is to clarify whether strong
scattering around low-mass stars affects the orbital and mass distributions of
the planets. We perform -body simulation of protoplanets by systematically
surveying the parameter space of the stellar mass and surface density of
protoplanets. We find that protoplanets are often ejected after twice or three
times close-scattering around late M dwarfs. The ejection sets the upper limit
of the largest planet mass. Adopting the surface density scaling linearly with
the stellar mass, we find that as the stellar mass decreases less massive
planets are formed in orbits with higher eccentricities and inclinations. Under
this scaling, we also find that a few close-in protoplanets are generally
ejected. The ejection of protoplanets plays an important role in the mass
distribution of super-Earths around late M dwarfs. The mass relation of
observed close-in super-Earths and their central star mass is well reproduced
by ejection.Comment: accepted for publication in A&
Potassium Abundances in Red Giants of Mildly to Very Metal-Poor Globular Clusters
A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 A was
carried out for 15 red giants belonging to three globular clusters of different
metallicity (M 4, M 13, and M 15) along with two reference early-K giants (rho
Boo and alpha Boo), in order to check whether the K abundances are uniform
within a cluster and to investigate the behavior of [K/Fe] ratio at the
relevant metallicity range of -2.5 <[Fe/H] < -1. We confirmed that [K/H] (as
well as [Fe/H]) is almost homogeneous within each cluster to a precision of <
~0.1 dex, though dubiously large deviations are exceptionally seen for two
peculiar stars showing signs of considerably increased turbulence in the upper
atmosphere. The resulting [K/Fe] ratios are mildly supersolar by a few tenths
of dex for three clusters, tending to gradually increase from ~+0.1-0.2 at
[Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connects reasonably well with
the [K/Fe] trend of disk stars (-1 < [Fe/H]) and that of extremely metal-poor
stars (-4 <[Fe/H] < -2.5). That is, [K/Fe] appears to continue a gradual
increase from [Fe/H]~0 toward a lower metallicity regime down to [Fe/H]~-3,
where a broad maximum of [K/Fe]~+0.3-0.4 is attained, possibly followed by a
slight downturn at [Fe/H]<~-3.Comment: 13 pages, 4 tables, 6 figures, and 1 electronic table (accepted for
publication in Publ. Astron. Soc. Japan
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