1,065 research outputs found
New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations
One aspect of the quantum nature of spacetime is its "foaminess" at very
small scales. Many models for spacetime foam are defined by the accumulation
power , which parameterizes the rate at which Planck-scale spatial
uncertainties (and thephase shifts they produce) may accumulate over large
path-lengths. Here is defined by theexpression for the path-length
fluctuations, , of a source at distance , wherein , with being the Planck
length. We reassess previous proposals to use astronomical observations
ofdistant quasars and AGN to test models of spacetime foam. We show explicitly
how wavefront distortions on small scales cause the image intensity to decay to
the point where distant objects become undetectable when the path-length
fluctuations become comparable to the wavelength of the radiation. We use X-ray
observations from {\em Chandra} to set the constraint ,
which rules out the random walk model (with ). Much firmer
constraints canbe set utilizing detections of quasars at GeV energies with {\em
Fermi}, and at TeV energies with ground-based Cherenkovtelescopes: and , respectively. These limits on
seem to rule out , the model of some physical interest.Comment: 11 pages, 9 figures, ApJ, in pres
Privatization Versus Competition: Changing Enterprise Behavior in Russia
We investigate whether competitive forces and privatization have yet began to play an efficiency-enhancing role in Russia. We also explore the economic effects of harder budget constraints on enterprise behavior. The empirical work is based on a large enterprise panel of Russian firms 1990-94, representing around 10% of Russian manufacturing output. We conclude that privatization is having an impact on enterprise efficiency and restructuring but domestic market structure and harder budget constraints for the most part are not. Intriguingly, Russian firms are found to be sensitive to the degree of import penetration.http://deepblue.lib.umich.edu/bitstream/2027.42/39460/3/wp70.pd
Multiwavelength transit observations of the candidate disintegrating planetesimals orbiting WD 1145+017
We present multiwavelength, ground-based follow-up photometry of the white dwarf WD 1145+017, which has recently been suggested to be orbited by up to six or more short-period, low-mass, disintegrating planetesimals. We detect nine significant dips in flux of between 10% and 30% of the stellar flux in our ~32 hr of photometry, suggesting that WD 1145+017 is indeed being orbited by multiple, short-period objects. Through fits to the asymmetric transits that we observe, we confirm that the transit egress is usually longer than the ingress, and that the transit duration is longer than expected for a solid body at these short periods, all suggesting that these objects have cometary tails streaming behind them. The precise orbital periods of the planetesimals are unclear, but at least one object, and likely more, have orbital periods of ~4.5 hr. We are otherwise unable to confirm the specific periods that have been reported, bringing into question the long-term stability of these periods. Our high-precision photometry also displays low-amplitude variations, suggesting that dusty material is consistently passing in front of the white dwarf, either from discarded material from these disintegrating planetesimals or from the detected dusty debris disk. We compare the transit depths in the V- and R-bands of our multiwavelength photometry, and find no significant difference; therefore, for likely compositions, the radius of single-size particles in the cometary tails streaming behind the planetesimals must be ~0.15 μm or larger, or ~0.06 μm or smaller, with 2σ confidence
Privatization Versus Competition: Changing Enterprise Behavior in Russia
We investigate whether competitive forces and privatization have yet began to play an efficiency-enhancing role in Russia. We also explore the economic effects of harder budget constraints on enterprise behavior. The empirical work is based on a large enterprise panel of Russian firms 1990-94, representing around 10% of Russian manufacturing output. We conclude that privatization is having an impact on enterprise efficiency and restructuring but domestic market structure and harder budget constraints for the most part are not. Intriguingly, Russian firms are found to be sensitive to the degree of import penetration.privatization, competition, economic transition
Collisions of Free Floating Planets with Evolved Stars in Globular Clusters
We estimate the rate of collisions between stars and free-floating planets
(FFPs) in globular clusters, in particular the collision of FFPs with red giant
branch (RGB) stars. Recent dynamical simulations imply that the density of such
objects could exceed million per cubic parsec near the cores of rich globular
clusters. We show that in these clusters 5-10 per cents of all RGB stars near
the core would suffer a collision with a FFP, and that such a collision can
spin up the RGB star's envelope by an order of magnitude. In turn, the higher
rotation rates may lead to enhanced mass-loss rates on the RGB, which could
result in bluer horizontal branch (HB) stars. Hence, it is plausible that the
presence of a large population of FFPs in a globular cluster can influence the
distribution of stars on the HB of that cluster to a detectable degree.Comment: 10 pages, Accepted by ApJ Letter
Multiwavelength Transit Observations of the Candidate Disintegrating Planetesimals Orbiting WD 1145+017
We present multiwavelength, multi-telescope, ground-based follow-up
photometry of the white dwarf WD 1145+017, that has recently been suggested to
be orbited by up to six or more, short-period, low-mass, disintegrating
planetesimals. We detect 9 significant dips in flux of between 10% and 30% of
the stellar flux from our ground-based photometry. We observe transits deeper
than 10% on average every ~3.6 hr in our photometry. This suggests that WD
1145+017 is indeed being orbited by multiple, short-period objects. Through
fits to the multiple asymmetric transits that we observe, we confirm that the
transit egress timescale is usually longer than the ingress timescale, and that
the transit duration is longer than expected for a solid body at these short
periods, all suggesting that these objects have cometary tails streaming behind
them. The precise orbital periods of the planetesimals in this system are
unclear from the transit-times, but at least one object, and likely more, have
orbital periods of ~4.5 hours. We are otherwise unable to confirm the specific
periods that have been reported, bringing into question the long-term stability
of these periods. Our high precision photometry also displays low amplitude
variations suggesting that dusty material is consistently passing in front of
the white dwarf, either from discarded material from these disintegrating
planetesimals or from the detected dusty debris disk. For the significant
transits we observe, we compare the transit depths in the V- and R-bands of our
multiwavelength photometry, and find no significant difference; therefore, for
likely compositions the radius of single-size particles in the cometary tails
streaming behind the planetesimals in this system must be ~0.15 microns or
larger, or ~0.06 microns or smaller, with 2-sigma confidence.Comment: 16 pages, 12 figures, submitted to ApJ on October 8th, 201
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