16,261 research outputs found
Planet formation around stars of various masses: The snow line and the frequency of giant planets
We use a semi-analytic circumstellar disk model that considers movement of
the snow line through evolution of accretion and the central star to
investigate how gas giant frequency changes with stellar mass. The snow line
distance changes weakly with stellar mass; thus giant planets form over a wide
range of spectral types. The probability that a given star has at least one gas
giant increases linearly with stellar mass from 0.4 M_sun to 3 M_sun. Stars
more massive than 3 M_sun evolve quickly to the main-sequence, which pushes the
snow line to 10-15 AU before protoplanets form and limits the range of disk
masses that form giant planet cores. If the frequency of gas giants around
solar-mass stars is 6%, we predict occurrence rates of 1% for 0.4 M_sun stars
and 10% for 1.5 M_sun stars. This result is largely insensitive to our assumed
model parameters. Finally, the movement of the snow line as stars >2.5 M_sun
move to the main-sequence may allow the ocean planets suggested by Leger et.
al. to form without migration.Comment: Accepted to ApJ. 12 pages of emulateap
Giant nonlinear conduction and thyristor-like negative derivative resistance in BaIrO3 single crystals
We synthesized single-crystalline samples of monoclinic BaIrO3 using a molten
flux method, and measured their magnetization, resistivity, Seebeck coefficient
and nonlinear voltage-current characteristics. The magnetization rapidly
increases below a ferromagnetic transition temperature TC of 180 K, where the
resistivity concomitantly shows a hump-type anomaly, followed by a sharp
increase below 30 K. The Seebeck coefficient suddenly increases below TC, and
shows linear temperature dependence below 50 K. A most striking feature of this
compound is that the anomalously giant nonlinear conduction is observed below
30 K, where a small current density of 20 A/cm2 dramatically suppresses the
sharp increase in resistivity to induce a metallic conduction down to 4 K.Comment: 10 pages, 4 figures Submitted to Physical Review Letter
Comments on differential cross section of phi-meson photoproduction at threshold
We show that the differential cross section d_sigma/d_t of gamma p --> \phi p
reaction at the threshold is finite and its value is crucial to the mechanism
of the phi meson photoproduction and for the models of phi-N interaction.Comment: 8 pages, 2 figure
Drude Weight of the Two-Dimensional Hubbard Model -- Reexamination of Finite-Size Effect in Exact Diagonalization Study --
The Drude weight of the Hubbard model on the two-dimensional square lattice
is studied by the exact diagonalizations applied to clusters up to 20 sites. We
carefully examine finite-size effects by consideration of the appropriate
shapes of clusters and the appropriate boundary condition beyond the imitation
of employing only the simple periodic boundary condition. We successfully
capture the behavior of the Drude weight that is proportional to the squared
hole doping concentration. Our present result gives a consistent understanding
of the transition between the Mott insulator and doped metals. We also find, in
the frequency dependence of the optical conductivity, that the mid-gap
incoherent part emerges more quickly than the coherent part and rather
insensitive to the doping concentration in accordance with the scaling of the
Drude weight.Comment: 9 pages with 10 figures and 1 table. accepted in J. Phys. Soc. Jp
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
S wave superconductivity in newly discovered superconductor BaTiSbO revealed by Sb-NMR/Nuclear Quadrupole Resonance measurements
We report the Sb-NMR/nuclear quadrupole resonance (NQR)
measurements on the newly-discovered superconductor BaTiSbO with a
two-dimensional TiO square-net layer formed with Ti (3). NQR
measurements revealed that the in-plane four-fold symmetry is broken at the Sb
site below 40 K, without an internal field appearing at the Sb
site. These exclude a spin-density wave (SDW)/ charge density wave (CDW)
ordering with incommensurate correlations, but can be understood with the
commensurate CDW ordering at . The spin-lattice relaxation rate
, measured at the four-fold symmetry breaking site, decreases below
superconducting (SC) transition temperature , indicative of the
microscopic coexistence of superconductivity and the CDW/SDW phase below
. Furthermore, of Sb-NQR shows a coherence peak just
below and decreases exponentially at low temperatures. These
results are in sharp contrast with those in cuprate and iron-based
superconductors, and strongly suggest that its SC symmetry is classified to an
ordinary s-wave state.Comment: 5 pages, 6 figure
Resonance Contributions to Photoproduction on Protons Found Using Dispersion Relations and an Isobar Model
The contributions of the resonances , ,
, , , , ,
to are found from the data on cross
sections, beam and target asymmetries using two approaches: fixed-t dispersion
relations and an isobar model. Utilization of the two approaches and comparison
of the results obtained with different parametrizations of the resonance
contributions allowed us to make conclusions about the model-dependence of
these contributions. We conclude that the results for the contributions of the
resonances , , to corresponding
multipole amplitudes are stable. With this the results for and
, combined with their PDG photoexcitation helicity amplitudes,
allowed us to find the branching ratios , which have significantly
better accuracy than the PDG data. The total Breit-Wigner width of the
is model-dependent, we have obtained and using dispersion relations and the isobar model,
respectively. The results for the , ,
, are model dependent, only the signs and orders
of magnitude of their contributions to multipole amplitudes are determined. The
results for the are strongly model-dependent.Comment: 26 pages, 6 figure
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
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