2,906 research outputs found
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Developmental Exposure of Rats to Chlorpyrifos Elicits Sex-Selective Hyperlipidemia and Hyperinsulinemia in Adulthood
Developmental exposure to chlorpyrifos alters cell signaling both in the brain and in peripheral tissues, affecting the responses to a variety of neurotransmitters and hormones. We administered 1 mg/kg/day chlorpyrifos to rats on postnatal days 1â4, a regimen below the threshold for systemic toxicity. When tested in adulthood, chlorpyrifos-exposed animals displayed elevations in plasma cholesterol and triglycerides, without underlying alterations in nonesterified free fatty acids and glycerol. This effect was restricted to males. Similarly, in the postprandial state, male rats showed hyperinsulinemia in the face of normal circulating glucose levels but demonstrated appropriate reduction of circulating insulin concentrations after fasting. These outcomes and sex selectivity resemble earlier findings at the cellular level, which identified hepatic hyperresponsiveness to gluconeogenic inputs from ÎČ-adrenoceptors or glucagon receptors. Our results thus indicate that apparently subtoxic neonatal chlorpyrifos exposure, devoid of effects on viability or growth but within the parameters of human fetal or neonatal exposures, produce a metabolic pattern for plasma lipids and insulin that resembles the major adult risk factors for atherosclerosis and type 2 diabetes mellitus
High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism
One of the most remarkable properties of extrasolar planets is their high
orbital eccentricities. Observations have shown that at least 20% of these
planets, including some with particularly high eccentricities, are orbiting a
component of a wide binary star system. The presence of a distant binary
companion can cause significant secular perturbations to the orbit of a planet.
In particular, at high relative inclinations, a planet can undergo a
large-amplitude eccentricity oscillation. This so-called "Kozai mechanism" is
effective at a very long range, and its amplitude is purely dependent on the
relative orbital inclination. In this paper, we address the following simple
question: assuming that every host star with a detected giant planet also has a
(possibly unseen, e.g., substellar) distant companion, with reasonable
distributions of orbital parameters and masses, how well could secular
perturbations reproduce the observed eccentricity distribution of planets? Our
calculations show that the Kozai mechanism consistently produces an excess of
planets with very high (e >0.6) and very low (e < 0.1) eccentricities. The
paucity of near-circular orbits in the observed sample cannot be explained
solely by the Kozai mechanism, because, even with high enough inclinations, the
Kozai mechanism often fails to produce significant eccentricity perturbations
when there are other competing sources of orbital perturbations on secular
timescales, such as general relativity. On the other hand, the Kozai mechanism
can produce many highly eccentric orbits. Indeed the overproduction of high
eccentricities observed in our models could be combined with plausible
circularizing mechanisms (e.g., friction from residual gas) to create more
intermediate eccentricities (e=0.1-0.6).Comment: 24 pages, 6 figures, ApJ, in press, minor changes to reflect the
accepted versio
MONETARY POLICY UNDER INFLATION TARGETING: AN INTRODUCTION
This brief review takes stock of the recent literature on monetary policy under inflation targeting and introduces new analytical and empirical research in this field. Six key areas of previous research are reviewed: the practice and optimality of inflation targeting regime features; optimal monetary policy; uncertainty, learning, and monetary policy; transparency, communication, and accountability; asset prices and monetary policy; and economic performance under inflation targeting and in comparison to non-targeting regimes. The review suggests a significant number of open issues that are addressed in 13 new contributions presented at the 2005 Annual Conference of the Central Bank of Chile, which are summarized here.
Monte Carlo Simulations of Globular Cluster Evolution - II. Mass Spectra, Stellar Evolution and Lifetimes in the Galaxy
We study the dynamical evolution of globular clusters using our new 2-D Monte
Carlo code, and we calculate the lifetimes of clusters in the Galactic
environment. We include the effects of a mass spectrum, mass loss in the
Galactic tidal field, and stellar evolution. We consider initial King models
containing N = 10^5 - 3x10^5 stars, and follow the evolution up to core
collapse, or disruption, whichever occurs first. We find that the lifetimes of
our models are significantly longer than those obtained using 1-D Fokker-Planck
(F-P) methods. We also find that our results are in very good agreement with
recent 2-D F-P calculations, for a wide range of initial conditions. Our
results show that the direct mass loss due to stellar evolution can
significantly accelerate the mass loss through the tidal boundary, causing most
clusters with a low initial central concentration (Wo <~ 3) to disrupt quickly
in the Galactic tidal field. Only clusters born with high initial central
concentrations (Wo >~ 7) or steep initial mass functions are likely to survive
to the present and undergo core collapse. We also study the orbital
characteristics of escaping stars, and find that the velocity distribution of
escaping stars in collapsing clusters looks significantly different from the
distribution in disrupting clusters. We calculate the lifetime of a cluster on
an eccentric orbit in the Galaxy, such that it fills its Roche lobe only at
perigalacticon. We find that such an orbit can extend the lifetime by at most a
factor of a few compared to a circular orbit in which the cluster fills its
Roche lobe at all times.Comment: 32 pages, including 10 figures, to appear in ApJ, minor corrections
onl
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
Rapid formation of exponential disks and bulges at high redshift from the dynamical evolution of clump cluster and chain galaxies
Many galaxies at high redshift have peculiar morphologies dominated by
10^8-10^9 Mo kpc-sized clumps. Using numerical simulations, we show that these
"clump clusters" can result from fragmentation in gravitationally unstable
primordial disks. They appear as "chain galaxies" when observed edge-on. In
less than 1 Gyr, clump formation, migration, disruption, and interaction with
the disk cause these systems to evolve from initially uniform disks into
regular spiral galaxies with an exponential or double-exponential disk profile
and a central bulge. The inner exponential is the initial disk size and the
outer exponential is from material flung out by spiral arms and clump torques.
A nuclear black hole may form at the same time as the bulge from smaller black
holes that grow inside the dense cores of each clump. The properties and
lifetimes of the clumps in our models are consistent with observations of the
clumps in high redshift galaxies, and the stellar motions in our models are
consistent with the observed velocity dispersions and lack of organized
rotation in chain galaxies. We suggest that violently unstable disks are the
first step in spiral galaxy formation. The associated starburst activity gives
a short timescale for the initial stellar disk to form.Comment: ApJ Accepted, 13 pages, 9 figure
- âŠ