4,579 research outputs found
The Origin of OB Runaway Stars
About 20% of all massive stars in the Milky Way have unusually high
velocities, the origin of which has puzzled astronomers for half a century. We
argue that these velocities originate from strong gravitational interactions
between single stars and binaries in the centers of star clusters. The ejecting
binary forms naturally during the collapse of a young (\aplt 1\,Myr) star
cluster. This model replicates the key characteristics of OB runaways in our
galaxy and it explains the \apgt 100\,\Msun\, runaway stars around young star
clusters, e.g. R136 and Westerlund~2. The high proportion and the distributions
in mass and velocity of runaways in the Milky Way is reproduced if the majority
of massive stars are born in dense and relatively low-mass (5000-10000 \Msun)
clusters.Comment: to appear in Scienc
A large-deviations analysis of the GI/GI/1 SRPT queue
We consider a GI/GI/1 queue with the shortest remaining processing time
discipline (SRPT) and light-tailed service times. Our interest is focused on
the tail behavior of the sojourn-time distribution. We obtain a general
expression for its large-deviations decay rate. The value of this decay rate
critically depends on whether there is mass in the endpoint of the service-time
distribution or not. An auxiliary priority queue, for which we obtain some new
results, plays an important role in our analysis. We apply our SRPT-results to
compare SRPT with FIFO from a large-deviations point of view.Comment: 22 page
Competition Leverage: How the Demand Side Affects Optimal Risk Adjustment
We study optimal risk adjustment in imperfectly competitive health insurance markets when high-risk consumers are less likely to switch insurer than low-risk consumers. First, we find that insurers still have an incentive to select even if risk adjustment perfectly corrects for cost differences among consumers. Consequently, the outcome is not efficient even if cost differences are fully compensated. To achieve first best, risk adjustment should overcompensate for serving high-risk agents to take into account the difference in mark- ups among the two types. Second, the difference in switching behavior creates a trade off between efficiency and consumer welfare. Reducing the difference in risk adjustment subsidies to high and low types increases consumer welfare by leveraging competition from the elastic low-risk market to the less elastic high-risk market. Finally, mandatory pooling can increase consumer surplus even further, at the cost of efficiency.health insurance;risk adjustment;imperfect competition;leverage
Selective Contracting and Foreclosure in Health Care Markets
We analyze exclusive contracts between health care providers and insurers in a model where some consumers choose to stay uninsured. In case of a monopoly insurer, exclusion of a provider changes the distribution of consumers who choose not to insure. Although the foreclosed care provider remains active in the market for the non-insured, we show that exclusion leads to anti-competitive effects on this non-insured market. As a consequence exclusion can raise industry profits, and then occurs in equilibrium. Under competitive insurance markets, the anticompetitive exclusive equilibrium survives. Uninsured consumers, however, are now not better off without exclusion. Competition among insurers raises prices in equilibria without exclusion, as a result of a horizontal analogue to the double marginalization effect. Instead, under competitive insurance markets exclusion is desirable as long as no provider is excluded by all insurers.health insurance;uninsured;selective contracting;exclusion;foreclosure;anti-competitive effects
Competition for Traders and Risk
Abstract: The financial crisis has been attributed partly to perverse incentives for traders at banks and has led policy makers to propose regulation of banks’ remuneration packages. We explain why poor incentives for traders cannot be fully resolved by only regulating the bank’s top executives, and why direct intervention in trader compensation is called for. We present a model with both trader moral hazard and adverse selection on trader abilities. We demonstrate that as competition on the labour market for traders intensifies, banks optimally offer top traders contracts inducing them to take more risk, even if banks fully internalize the costs of negative outcomes. In this way, banks can reduce the surplus they have to offer to lower ability traders. In addition, we find that increasing banks’ capital requirements does not unambiguously lead to reduced risk-taking by their top traders.optimal contracts;remuneration policy;imperfect competition;financial institutions;risk
The Relation Between the Globular Cluster Mass and Luminosity Functions
The relation between the globular cluster luminosity function (GCLF,
dN/dlogL) and globular cluster mass function (GCMF, dN/dlogM) is considered.
Due to low-mass star depletion, dissolving GCs have mass-to-light (M/L) ratios
that are lower than expected from their metallicities. This has been shown to
lead to an M/L ratio that increases with GC mass and luminosity. We model the
GCLF and GCMF and show that the power law slopes inherently differ (1.0 versus
0.7, respectively) when accounting for the variability of M/L. The observed
GCLF is found to be consistent with a Schechter-type initial cluster mass
function and a mass-dependent mass-loss rate.Comment: 4 pages, 2 figures. To appear in the proceedings of "Galaxy Wars:
Stellar Populations and Star Formation in Interacting Galaxies" (Tennessee,
July 2009
Selection of the SIM Astrometric Grid
We investigate the choice of stellar population for use as the Astrometric
Grid for the Space Interferometry Mission (SIM). SIM depends on the astrometric
stability of about 2000 stars, the so called Grid, against which the science
measures are referenced. Low metallicity, and thus relatively high luminosity K
giants are shown to be the population of choice, when available. The
alternative, nearby G dwarfs, are shown to be suseptable to unmodeled motions
induced by gas-giant planetary companions, should there be a significant
population of such companions.
Radial velocity filtering is quite efficient in selecting Grid members from
the K giants with yields exceeding 50% if filtering at 30m/s (1-sigma) is
available. However if the binary fraction of the G dwarfs approaches 100% as
some studies suggest, the yield of stable systems would be in the range of 15%
at best (with 10m/s filtering). Use of the initial SIM measurement as a final
filter is shown not to be critical in either case, although it could improve
the yield of stable grid members.
For a Grid composed of weak-lined K giants, the residual contamination by
large unmodeled motions will amount to about 3% (and rises to about 6% if a
60m/s radial velocity criterion is used). The selective introduction of
quadratic terms in the proper motion solutions during the post-mission phase of
data reduction can reduce contamination to a remarkable 1% or better in either
case.
Analytic estimates based on circular orbits are developed which show how
these results come about.Comment: 42 pages including 13 eps figures. To be published Sept 2002 in PAS
Black hole mergers in the universe
Mergers of black-hole binaries are expected to release large amounts of
energy in the form of gravitational radiation. However, binary evolution models
predict merger rates too low to be of observational interest. In this paper we
explore the possibility that black holes become members of close binaries via
dynamical interactions with other stars in dense stellar systems. In star
clusters, black holes become the most massive objects within a few tens of
millions of years; dynamical relaxation then causes them to sink to the cluster
core, where they form binaries. These black-hole binaries become more tightly
bound by superelastic encounters with other cluster members, and are ultimately
ejected from the cluster. The majority of escaping black-hole binaries have
orbital periods short enough and eccentricities high enough that the emission
of gravitational radiation causes them to coalesce within a few billion years.
We predict a black-hole merger rate of about per year per
cubic megaparsec, implying gravity wave detection rates substantially greater
than the corresponding rates from neutron star mergers. For the first
generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), we
expect about one detection during the first two years of operation. For its
successor LIGO-II, the rate rises to roughly one detection per day. The
uncertainties in these numbers are large. Event rates may drop by about an
order of magnitude if the most massive clusters eject their black hole binaries
early in their evolution.Comment: 12 pages, ApJL in pres
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