6,146 research outputs found
Market failure, government inefficiency, and optimal R&D policy
This paper presents a growth model that can explain the coexistence of intellectual property rights and R&D subsidies as a response to the presence of both market and government failures. The framework can also generate the observed positive correlation between these two policy tools
Cooling in the shade of warped transition disks
The mass of the gaseous reservoir in young circumstellar disks is a crucial
initial condition for the formation of planetary systems, but estimates vary by
orders of magnitude. In some disks with resolvable cavities, sharp inner disk
warps cast two-sided shadows on the outer rings; can the cooling of the gas as
it crosses the shadows bring constraints on its mass? The finite cooling
timescale should result in dust temperature decrements shifted ahead of the
optical/IR shadows in the direction of rotation. However, some systems show
temperature drops, while others do not. The depth of the drops and the
amplitude of the shift depend on the outer disk surface density Sigma through
the extent of cooling during the shadow crossing time, and also on the
efficiency of radiative diffusion. These phenomena may bear observational
counterparts, which we describe with a simple one-dimensional model. An
application to the HD142527 disk suggests an asymmetry in its shadows, and
predicts a >~10deg shift for a massive gaseous disk, with peak Sigma > 8.3
g/cm2. Another application to the DoAr44 disk limits the peak surface density
to Sigma < 13g/cm2Comment: accepted to MNRAS Letter
SS433's circumbinary ring and accretion disc viewed through its attenuating disc wind
We present optical spectroscopy of the microquasar SS433 covering a
significant fraction of a precessional cycle of its jet axis. The components of
the prominent stationary H-alpha and H-beta lines are mainly identified as
arising from three emitting regions: (i) a super-Eddington accretion disc wind,
in the form of a broad component accounting for most of the mass loss from the
system, (ii) a circumbinary disc of material that we presume is being excreted
through the binary's L2 point, and (iii) the accretion disc itself as two
remarkably persistent components. The accretion disc components move with a
Keplerian velocity of ~600 km/s in the outer region of the disc. A direct
result of this decomposition is the determination of the accretion disc size,
whose outer radius attains ~8 R_sun in the case of Keplerian orbits around a
black hole mass of 10 M_sun. We determine an upper limit for the accretion disc
inner to outer radius ratio in SS433, R_in/R_out ~ 0.2, independent of the mass
of the compact object. The Balmer decrements, H-alpha/H-beta, are extracted
from the appropriate stationary emission lines for each component of the
system. The physical parameters of the gaseous components are derived. The
circumbinary ring decrement seems to be quite constant throughout precessional
phase, implying a constant electron density of log N_e(cm^-3) ~ 11.5 for the
circumbinary disc. The accretion disc wind shows a larger change in its
decrements exhibiting a clear dependence on precessional phase, implying a
sinusoid variation in its electron density log N_e(cm^-3) along our
line-of-sight between 10 and 13. This dependence of density on direction
suggests that the accretion disc wind is polloidal in nature.Comment: 7 pages, 5 figures. Accepted for publication in MNRAS Main Journal
Unification and Local Baryon Number
We investigate the possibility to find an ultraviolet completion of the
simple extensions of the Standard Model where baryon number is a local
symmetry. In the context of such theories one can understand the spontaneous
breaking of baryon number at the low scale and the proton stability. We find a
simple theory based on SU(4)_C x SU(3)_L x SU(3)_R where baryon number is
embedded in a non-Abelian gauge symmetry. We discuss the main features of the
theory and the possible implications for experiments. This theory predicts
stable colored and/or fractional electric charged fields which can give rise to
very exotic signatures at the Large Hadron Collider experiments such as CMS and
ATLAS. We further discuss the embedding in a gauge theory based on SU(4)_C x
SU(4)_L x SU(4)_R which could define the way to achieve the unification of the
gauge interactions at the low scale.Comment: new references, minor corrections, to appear in Physics Letters
Low Scale Unification of Gauge Interactions
We investigate the possibility to realize the unification of the Standard
Model gauge interactions at the low scale in four dimensions. We find that the
fields needed to define a minimal theory where baryon and lepton numbers are
local symmetries, allow for unification at the low scale in agreement with
experiments. In these scenarios the proton is stable and we briefly discuss the
implications for cosmology
Health cycles and health transitions
We study the dynamics of poverty and health in a model of endogenous growth and rational health behavior. Population health depends on the prevalence of infectious diseases that can be avoided through costly prevention. The incentive to do so comes from the negative effects of ill health on the quality and quantity of life. The model can generate a poverty trap where infectious diseases cycle between high and low prevalence. These cycles originate from the rationality of preventive behavior in contrast to the predator-prey dynamics of epidemiological models. We calibrate the model to reflect sub-Saharan Africa's recent economic recovery and analyze policy alternatives. Unconditional transfers are found to improve welfare relative to conditional health-based transfers: at low income levels, income growth (quality of life) is valued more than improvements to health (quantity of life)
Simple Left-Right Theory: Lepton Number Violation at the LHC
We propose a simple left-right symmetric theory where the neutrino masses are
generated at the quantum level. In this context the neutrinos are Majorana
fermions and the model has the minimal degrees of freedom in the scalar sector
needed for symmetry breaking and mass generation. We discuss the lepton number
violating signatures with two charged leptons of different flavor and missing
energy at the Large Hadron Collider in order to understand the testability of
the theory.Comment: to appear as a Rapid Communication in Physical Review
Spiral waves triggered by shadows in transition disks
Circumstellar asymmetries such as central warps have recently been shown to
cast shadows on outer disks. We investigate the hydrodynamical consequences of
such variable illumination on the outer regions of a transition disk, and the
development of spiral arms. Using 2D simulations, we follow the evolution of a
gaseous disk passively heated by the central star, under the periodic forcing
of shadows with an opening angle of 28. With a lower pressure
under the shadows, each crossing results in a variable azimuthal acceleration,
which in time develops into spiral density waves. Their pitch angles evolve
from at the onset, to 11-14,
over 65~AU to 150~AU. Self-gravity enhances the density contrast of the
spiral waves, as also reported previously for spirals launched by planets. Our
control simulations with unshadowed irradiation do not develop structures,
except for a different form of spiral waves seen at later times only in the
gravitationally unstable control case. Scattered light predictions in the
-band show that such illumination spirals should be observable. We suggest
that spiral arms in the case-study transition disk HD~142527 could be explained
as a result of shadowing from the tilted inner disk.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in ApJ
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