418 research outputs found
Comment on ``Inflation and flat directions in modular invariant superstring effective theories''
The inflation model of Gaillard, Lyth and Murayama is revisited, with a
systematic scan of the parameter space for dilaton stabilization during
inflation.Comment: 7 pages, 2 figure
Linearly and Circularly Polarized Emission in Sagittarius A*
We perform general relativistic ray-tracing calculations of the transfer of
polarized synchrotron radiation through the relativistic accretion flow in
Sagittarius (Sgr) A*. Based on a two-temperature magneto-rotational-instability
(MRI) induced accretion mode, the birefringence effects are treated
self-consistently. By fitting the spectrum and polarization of Sgr A* from
millimeter to near-infrared bands, we are able to not only constrain the basic
parameters related to the MRI and the electron heating rate, but also limit the
orientation of the accretion torus. These constraints lead to unique
polarimetric images, which may be compared with future millimeter and
sub-millimeter VLBI observations. In combination with general relativistic MHD
simulations, the model has the potential to test the MRI with observations of
Sgr A*.Comment: 12 pages, 2 figures, ApJL accepte
Migration of Extrasolar Planets: Effects from X-Wind Accretion Disks
Magnetic fields are dragged in from the interstellar medium during the
gravitational collapse that forms star/disk systems. Consideration of mean
field magnetohydrodynamics (MHD) in these disks shows that magnetic effects
produce subkeplerian rotation curves and truncate the inner disk. This letter
explores the ramifications of these predicted disk properties for the migration
of extrasolar planets. Subkeplerian flow in gaseous disks drives a new
migration mechanism for embedded planets and modifies the gap opening processes
for larger planets. This subkeplerian migration mechanism dominates over Type I
migration for sufficiently small planets (m_P < 1 M_\earth) and/or close orbits
(r < 1 AU). Although the inclusion of subkeplerian torques shortens the total
migration time by only a moderate amount, the mass accreted by migrating
planetary cores is significantly reduced. Truncation of the inner disk edge
(for typical system parameters) naturally explains final planetary orbits with
periods P=4 days. Planets with shorter periods P=2 days can be explained by
migration during FU-Ori outbursts, when the mass accretion rate is high and the
disk edge moves inward. Finally, the midplane density is greatly increased at
the inner truncation point of the disk (the X-point); this enhancement, in
conjunction with continuing flow of gas and solids through the region, supports
the in situ formation of giant planets.Comment: 15 pages, 2 figures, accepted to ApJ Letter
Residence times of air in a mature forest:observational evidence from a free-air CO2 enrichment experiment
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