2,568 research outputs found
The Formation of Ice Giants in a Packed Oligarchy: Instability and Aftermath
As many as 5 ice giants--Neptune-mass planets composed of 90% ice and rock
and 10% hydrogen--are thought to form at heliocentric distances of 10-25 AU on
closely packed orbits spaced ~5 Hill radii apart. Such oligarchies are
ultimately unstable. Once the parent disk of planetesimals is sufficiently
depleted, oligarchs perturb one another onto crossing orbits. We explore both
the onset and the outcome of the instability through numerical integrations,
including dynamical friction cooling of planets by a planetesimal disk whose
properties are held fixed. To trigger instability and the ejection of the first
ice giant in systems having an original surface density in oligarchs of Sigma ~
1 g/cm^2, the disk surface density s must fall below 0.1 g/cm^2. Ejections are
predominantly by Jupiter and occur within 10 Myr. To eject more than 1 oligarch
requires s < 0.03 g/cm^2. Systems starting with up to 4 oligarchs in addition
to Jupiter and Saturn can readily yield solar-system-like outcomes in which 2
surviving ice giants lie inside 30 AU and have their orbits circularized by
dynamical friction. Our numerical simulations support the idea that planetary
systems begin in more crowded and compact configurations, like those of
shear-dominated oligarchies. In contrast to previous studies, we identify s <
0.1 Sigma as the regime relevant for understanding the evolution of the outer
solar system, and we encourage future studies to concentrate on this regime
while relaxing our assumption of a fixed planetesimal disk.Comment: Accepted to ApJ Jan 27. Incorporates comments from the referee and
community at large. 15 pages, 14 figures, including 7 colo
Analyzing large-scale conservation interventions with Bayesian hierarchical models: a case study of supplementing threatened Pacific salmon.
Myriad human activities increasingly threaten the existence of many species. A variety of conservation interventions such as habitat restoration, protected areas, and captive breeding have been used to prevent extinctions. Evaluating the effectiveness of these interventions requires appropriate statistical methods, given the quantity and quality of available data. Historically, analysis of variance has been used with some form of predetermined before-after control-impact design to estimate the effects of large-scale experiments or conservation interventions. However, ad hoc retrospective study designs or the presence of random effects at multiple scales may preclude the use of these tools. We evaluated the effects of a large-scale supplementation program on the density of adult Chinook salmon Oncorhynchus tshawytscha from the Snake River basin in the northwestern United States currently listed under the U.S. Endangered Species Act. We analyzed 43Â years of data from 22 populations, accounting for random effects across time and space using a form of Bayesian hierarchical time-series model common in analyses of financial markets. We found that varying degrees of supplementation over a period of 25Â years increased the density of natural-origin adults, on average, by 0-8% relative to nonsupplementation years. Thirty-nine of the 43Â year effects were at least two times larger in magnitude than the mean supplementation effect, suggesting common environmental variables play a more important role in driving interannual variability in adult density. Additional residual variation in density varied considerably across the region, but there was no systematic difference between supplemented and reference populations. Our results demonstrate the power of hierarchical Bayesian models to detect the diffuse effects of management interventions and to quantitatively describe the variability of intervention success. Nevertheless, our study could not address whether ecological factors (e.g., competition) were more important than genetic considerations (e.g., inbreeding depression) in determining the response to supplementation
A Third Planet Orbiting HIP 14810
We present new precision radial velocities and a three-planet Keplerian orbit
fit for the V = 8.5, G5 V star HIP 14810. We began observing this star at Keck
Observatory as part of the N2K Planet Search Project. Wright et al. (2007)
announced the inner two planets to this system, and subsequent observations
have revealed the outer planet planet and the proper orbital solution for the
middle planet. The planets have minimum masses of 3.9, 1.3, and 0.6 M_Jup and
orbital periods of 6.67, 147.7, and 952 d, respectively. We have numerically
integrated the family of orbital solutions consistent with the data and find
that they are stable for at least 10^6 yr. Our photometric search shows that
the inner planet does not transit.Comment: ApJL, accepte
Information Systems and Healthcare XXIX: Information Technology Investments and Returns -- Uniqueness in the Healthcare Industry
The way in which information technology (IT) impacts firm productivity is an enduring question in organizational research and practice. Rather than adopting the common explanation that IT spending improves organizational performance, we hypothesize that IT spending determines the amount of IT assets, such as IT hardware, IT personnel, IT systems, and IT outsourcing, that can be acquired. These IT assets, in turn, affect productivity. The context of our study is the healthcare industry. Because of the unique set of managerial values, incentives, and constraints in this industry, we also hypothesize that IT personnel play a key role in determining hospital productivity. Analysis of panel data on acute-care centers provides support for our hypotheses. This paper contributes to literature by (1) refining earlier research that explains that IT spending improves organizational productivity; (2) examining the interrelationships between various types of IT assets; and (3) providing initial indications that outsourcing of the IT function may not have the beneficial effects in healthcare that it does in other industries
The California Planet Survey IV: A Planet Orbiting the Giant Star HD 145934 and Updates to Seven Systems with Long-Period Planets
We present an update to seven stars with long-period planets or planetary
candidates using new and archival radial velocities from Keck-HIRES and
literature velocities from other telescopes. Our updated analysis better
constrains orbital parameters for these planets, four of which are known
multi-planet systems. HD 24040 b and HD 183263 c are super-Jupiters with
circular orbits and periods longer than 8 yr. We present a previously unseen
linear trend in the residuals of HD 66428 indicative on an additional planetary
companion. We confirm that GJ 849 is a multi-planet system and find a good
orbital solution for the c component: it is a planet in a 15 yr
orbit (the longest known for a planet orbiting an M dwarf). We update the HD
74156 double-planet system. We also announce the detection of HD 145934 b, a planet in a 7.5 yr orbit around a giant star. Two of our stars, HD
187123 and HD 217107, at present host the only known examples of systems
comprising a hot Jupiter and a planet with a well constrained period yr,
and with no evidence of giant planets in between. Our enlargement and
improvement of long-period planet parameters will aid future analysis of
origins, diversity, and evolution of planetary systems.Comment: 16 pages, 13 figures. Accepted for publication in Ap
The efficiency of geometric samplers for exoplanet transit timing variation models
Transit timing variations (TTVs) are a valuable tool to determine the masses and orbits of transiting planets in multi-planet systems. TTVs can be readily modeled given knowledge of the interacting planets’ orbital configurations and planet-star mass ratios, but such models are highly nonlinear and difficult to invert. Markov chain Monte Carlo (MCMC) methods are often used to explore the posterior distribution for model parameters, but, due to the high correlations between parameters, nonlinearity, and potential multi-modality in the posterior, many samplers perform very inefficiently. Therefore, we assess the performance of several MCMC samplers that use varying degrees of geometric information about the target distribution. We generate synthetic datasets from multiple models, including the TTVFaster model and a simple sinusoidal model, and test the efficiencies of various MCMC samplers. We find that sampling efficiency can be greatly improved for all models by sampling from a parameter space transformed using an estimate of the covariance and means of the target distribution. No one sampler performs the best for all datasets. For datasets with near Gaussian posteriors, the Hamiltonian Monte Carlo sampler obtains the highest efficiencies when the step size and number of steps are properly tuned. Two samplers — Differential Evolution Monte Carlo and Geometric adaptive Monte Carlo, have consistently efficient performance for each dataset. Based on differences in effective sample sizes per time, we show that the right choice of sampler can improve sampling efficiencies by several orders of magnitude
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Using the surface brightness fluctuations in HST WFPC-2 images, we determine
that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC
4476 lies in the background at ~21 Mpc. We also examine the globular clusters
of M87 using archived HST fields. We detect the bimodal color distribution, and
find that the amplitude of the red peak relative to the blue peak is greatest
near the center. This feature is in good agreement with the merger model of
elliptical galaxy formation, where some of the clusters originated in
progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure
Assessing Differences Between Physician\u27s Realized And Anticipated Gains From Electronic Health Record Adoption
Return on investment (ROI) concerns related to Electronic Health Records (EHRs) are a major barrier to the technology’s adoption. Physicians generally rely upon early adopters to vet new technologies prior to putting them into widespread use. Therefore, early adopters’ experiences with EHRs play a major role in determining future adoption patterns. The paper’s purposes are: (1) to map the EHR value streams that define the ROI calculation; and (2) to compare Current Users’ and Intended Adopters’ perceived value streams to identify similarities, differences and governing constructs. Primary data was collected by the Texas Medical Association, which surveyed 1,772 physicians on their use and perceptions of practice gains from EHR adoption. Using Bayesian Belief Network Modeling, value streams are constructed for both current EHR users and Intended Adopters. Current Users and Intended Adopters differ significantly in their perceptions of the EHR value stream. Intended Adopters’ value stream displays complex relationships among the potential gains compared to the simpler, linear relationship that Current Users identified. The Current Users identify “Reduced Medical Records Costs” as the gain that governs the value stream while Intended Adopters believe “Reduced Charge Capture Costs” define the value stream’s starting point. Current Users’ versus Intended Adopters’ assessments of EHR benefits differ significantly and qualitatively from one another
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