5,713 research outputs found
Thermal evolution and lifetime of intrinsic magnetic fields of Super Earths in habitable zones
We have numerically studied the thermal evolution of various-mass terrestrial
planets in habitable zones, focusing on duration of dynamo activity to generate
their intrinsic magnetic fields, which may be one of key factors in
habitability on the planets. In particular, we are concerned with super-Earths,
observations of which are rapidly developing. We calculated evolution of
temperature distributions in planetary interior, using Vinet equations of
state, Arrhenius-type formula for mantle viscosity, and the astrophysical
mixing length theory for convective heat transfer modified for mantle
convection. After calibrating the model with terrestrial planets in the Solar
system, we apply it for 0.1-- rocky planets with surface
temperature of 300~\mbox{K} (in habitable zones) and the Earth-like
compositions. With the criterion for heat flux at the CMB (core-mantle
boundary), the lifetime of the magnetic fields is evaluated from the calculated
thermal evolution. We found that the lifetime slowly increases with the
planetary mass () independent of initial temperature gap at the
core-mantle boundary () but beyond a critical value
() it abruptly declines by the mantle viscosity
enhancement due to the pressure effect. We derived as a function of
and a rheological parameter (activation volume, ).
Thus, the magnetic field lifetime of super-Earths with
sensitively depends on , which reflects planetary
accretion, and , which has uncertainty at very high pressure. More
advanced high-pressure experiments and first-principle simulation as well as
planetary accretion simulation are needed to discuss habitability of
super-Earths.Comment: 19pages, 15 figures, accepted for publication in Ap
Quantum Creation of the Randall-Sundrum Bubble
We investigate the semiclassical instability of the Randall-Sundrum brane
world. We carefully analyze the bubble solution with the Randall-Sundrum
background, which expresses the decay of the brane world. We evaluate the decay
probability following the Euclidean path integral approach to quantum gravity.
Since a bubble rapidly expands after the nucleation, the entire spacetime will
be occupied by such bubbles.Comment: 13 pages, 6 figures, To appear in Prog. Theor. Phy
Intensi Kewirausahaan pada Mahasiswa-mahasiswa di Indonesia
Tingginya tingkat pengangguran intelektual, khususnya di kalangan para lulusan perguruan tinggi menjadipermasalahan yang berat. Jika hal ini tidak dicegah maka jumlah pengangguran intelektual akan semakinbertambah seiring banyaknya jumlah lulusan perguruan tinggi yang akan memasuki dunia kerja. Permasalahanini menjadi tanggung jawab bersama baik bagi pemerintah maupun masyarakat itu sendiri. Kontribusi yangdiberikan dunia pendidikan juga menjadi sangat penting untuk memberikan informasi yang komprehensifmengenai masalah pengangguran tersebut. Peran dunia pendidikan menjadi sangat vital, karena prosespembentukan sikap dan kompetensi calon angkatan kerja, terakhir berada di dunia pendidikan khususnyaperguruan tinggi. Pendidikan kewirausahaan memegang peranan penting dalam mencegah bahkan mengatasitingginya jumlah pengangguran intelektual. Salah satu upaya yang bisa dilakukan adalah meningkatkan intensikewirausahaan calon lulusan perguruan tinggi atau mahasiswa. Karena setelah lulus diharapkan mereka memilikipekerjaan sendiri tanpa bergantung pada lowongan pekerjaan yang ditawarkan Perusahaan, yang manajumlahnya tidak sebanding dengan jumlah angkatan kerja. Selain itu diharapkan jika membuka USAha sendiri,dapat menciptakan lapangan pekerjaan bagi orang lain. Dalam penelitian ini penulis menguji beberapa faktoryang dapat mempengaruhi intensi kewirausahaan, antara lain personality traits, portfolio choice of risk andambiguity aversion dan gender. Metode yang digunakan untuk menguji pengaruh variabel Independen terhadapvariabel dependen menggunakan uji regresi. Sedangkan untuk menguji pengaruh moderasi gender menggunakanchow test. Hasil penelitian menunjukkan terdapat pengaruh personality traits (antagonis, emotional stability danopenness to experience) dan portfolio choice of risk aversion terhadap intensi kewirausahaan. Selain itu, terdapatpengaruh portfolio choice of risk and ambiguity aversion terhadap intensi kewirausahaan berdasarkan gender
3D MHD Simulations of Planet Migration in Turbulent Stratified Disks
We performed 3D MHD simulations of planet migration in stratified disks using
the Godunov code PLUTO, where the disk is turbulent due to the
magnetorotational instability. We study the migration for planets with
different planet-star mass ratios . In agreement with previous
studies, for the low-mass planet cases ( and ),
migration is dominated by random fluctuations in the torque. For a Jupiter-mass
planet for , we find a reduction of
the magnetic stress inside the orbit of the planet and around the gap region.
After an initial stage where the torque on the planet is positive, it reverses
and we recover migration rates similar to those found in disks where the
turbulent viscosity is modelled by an viscosity. For the
intermediate-mass planets ( and ) we
find a new and so far unexpected behavior. In some cases they experience
sustained and systematic outwards migration for the entire duration of the
simulation. For this case, the horseshoe region is resolved and torques coming
from the corotation region can remain unsaturated due to the stresses in the
disk. These stresses are generated directly by the magnetic field. The
magnitude of the horseshoe drag can overcome the negative Lindblad contribution
when the local surface density profile is flat or increasing outwards, which we
see in certain locations in our simulations due to the presence of a zonal
flow. The intermediate-mass planet is migrating radially outwards in locations
where there is a positive gradient of a pressure bump (zonal flow).Comment: Accepted for publication in Ap
Reinventing spacetime on a dynamical hypersurface
In braneworld models, Space-Time-Matter and other Kaluza-Klein theories, our
spacetime is devised as a four-dimensional hypersurface {\it orthogonal} to the
extra dimension in a five-dimensional bulk. We show that the FRW line element
can be "reinvented" on a dynamical four-dimensional hypersurface, which is {\it
not} orthogonal to the extra dimension, without any internal contradiction.
This hypersurface is selected by the requirement of continuity of the metric
and depends explicitly on the evolution of the extra dimension. The main
difference between the "conventional" FRW, on an orthogonal hypersurface, and
the new one is that the later contains higher-dimensional modifications to the
regular matter density and pressure in 4D. We compare the evolution of the
spacetime in these two interpretations. We find that a wealth of "new" physics
can be derived from a five-dimensional metric if it is interpreted on a
dynamical (non-orthogonal) 4D hypersurface. In particular, in the context of a
well-known cosmological metric in , we construct a FRW model which is
consistent with the late accelerated expansion of the universe, while fitting
simultaneously the observational data for the deceleration parameter. The model
predicts an effective equation of state for the universe, which is consistent
with observations.Comment: References added to the Introduction, and Abstract modified. Accepted
for publication in Mod. Phys. Lett.
A coding problem for pairs of subsets
Let be an --element finite set, an integer. Suppose that
and are pairs of disjoint -element subsets of
(that is, , , ). Define the distance of these pairs by . This is the
minimum number of elements of one has to move to obtain the other
pair . Let be the maximum size of a family of pairs of
disjoint subsets, such that the distance of any two pairs is at least .
Here we establish a conjecture of Brightwell and Katona concerning an
asymptotic formula for for are fixed and . Also,
we find the exact value of in an infinite number of cases, by using
special difference sets of integers. Finally, the questions discussed above are
put into a more general context and a number of coding theory type problems are
proposed.Comment: 11 pages (minor changes, and new citations added
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