14,331 research outputs found
Survival Rates of Planets in Open Clusters: the Pleiades, Hyades, and Praesepe clusters
In clustered environments, stellar encounters can liberate planets from their
host stars via close encounters. Although the detection probability of planets
suggests that the planet population in open clusters resembles that in the
field, only a few dozen planet-hosting stars have been discovered in open
clusters. We explore the survival rates of planets against stellar encounters
in open clusters similar to the Pleiades, Hyades, and Praesepe and embedded
clusters. We performed a series of N-body simulations of high-density and
low-density open clusters, open clusters that grow via mergers of subclusters,
and embedded clusters. We semi-analytically calculated the survival rate of
planets in star clusters up to 1Gyr using relative velocities, masses, and
impact parameters of intruding stars. Less than 1.5% of close-in planets within
1 AU and at most 7% of planets with 1-10 AU are ejected by stellar encounters
in clustered environments after the dynamical evolution of star clusters. If a
planet population from 0.01-100 AU in an open cluster initially follows the
probability distribution function of exoplanets with semi-major axis ()
between 0.03-3 AU in the field discovered by RV surveys, the PDF of surviving
planets beyond ~10 AU in open clusters can be slightly modified to . The production rate of free-floating planets (FFPs) per star is
0.0096-0.18, where we have assumed that all the stars initially have one giant
planet with a mass of 1--13 MJ in a circular orbit. The expected frequency of
FFPs is compatible with the upper limit on that of FFPs indicated by recent
microlensing surveys. Our survival rates of planets in open clusters suggest
that planets within 10 AU around FGKM-type stars are rich in relatively-young
(<~10-100 Myr for open clusters and ~1-10 Myr for embedded clusters), less
massive open clusters, which are promising targets for planet searches.Comment: 23 pages, 15 figures, A&A accepte
The unitary-model-operator approach to nuclear many-body problems
Microscopic nuclear structure calculations have been performed within the
framework of the unitary-model-operator approach. Ground-state and
single-particle energies are calculated for nuclei around ^{14}C, ^{16}O and
^{40}Ca with modern nucleon-nucleon interactions.Comment: 6 pages, 4 figures, Talk presented at the International Symposium on
Correlation Dynamics in Nuclei (CDN05), Jan. 1 - Feb. 4, 2005, Tokyo, Japa
The unitary-model-operator approach to nuclear many-body problems
Microscopic nuclear structure calculations have been performed within the
framework of the unitary-model-operator approach. Ground-state and
single-particle energies are calculated for nuclei around ^{14}C, ^{16}O and
^{40}Ca with modern nucleon-nucleon interactions.Comment: 6 pages, 4 figures, Talk presented at the International Symposium on
Correlation Dynamics in Nuclei (CDN05), Jan. 1 - Feb. 4, 2005, Tokyo, Japa
The unitary-model-operator approach to nuclear many-body problems
Microscopic nuclear structure calculations have been performed within the
framework of the unitary-model-operator approach. Ground-state and
single-particle energies are calculated for nuclei around ^{14}C, ^{16}O and
^{40}Ca with modern nucleon-nucleon interactions.Comment: 6 pages, 4 figures, Talk presented at the International Symposium on
Correlation Dynamics in Nuclei (CDN05), Jan. 1 - Feb. 4, 2005, Tokyo, Japa
The Origin of OB Runaway Stars
About 20% of all massive stars in the Milky Way have unusually high
velocities, the origin of which has puzzled astronomers for half a century. We
argue that these velocities originate from strong gravitational interactions
between single stars and binaries in the centers of star clusters. The ejecting
binary forms naturally during the collapse of a young (\aplt 1\,Myr) star
cluster. This model replicates the key characteristics of OB runaways in our
galaxy and it explains the \apgt 100\,\Msun\, runaway stars around young star
clusters, e.g. R136 and Westerlund~2. The high proportion and the distributions
in mass and velocity of runaways in the Milky Way is reproduced if the majority
of massive stars are born in dense and relatively low-mass (5000-10000 \Msun)
clusters.Comment: to appear in Scienc
Baryogenesis and Gravitino Dark Matter in Gauge-Mediated Supersymmetry-Breaking Models
We discuss two cosmological issues in a generic gauge-mediated supersymmetry
(SUSY)-breaking model, namely the Universe's baryon asymmetry and the gravitino
dark-matter density. We show that both problems can be simultaneously solved if
there exist extra matter multiplets of a SUSY-invariant mass of the order of
the ``-term'', as suggested in several realistic SUSY grand-unified
theories. We propose an attractive scenario in which the observed baryon
asymmetry is produced in a way totally independent of the reheating temperature
of inflation without causing any cosmological gravitino problem. Furthermore,
in a relatively wide parameter space, we can also explain the present mass
density of cold dark matter by the thermal relics of the gravitinos without an
adjustment of the reheating temperature of inflation. We point out that there
is an interesting relation between the baryon asymmetry and the dark-matter
density.Comment: 20 pages, 2 figure
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