890 research outputs found
Prospects for the habitability of OGLE-2006-BLG-109L
The extrasolar system OGLE-2006-BLG-109L is the first multiple-planet system
to be discovered by gravitational microlensing (Gaudi et al., 2008); the two
large planets that have been detected have mass ratios, semimajor axis ratios,
and equilibrium temperatures that are similar to those of Jupiter and Saturn;
the mass of the host star is only 0.5 M_sun, and the system is more compact
than our own Solar system. We find that in the habitable zone of the host star,
the two detected planets resonantly excite large orbital eccentricities on a
putative earth-mass planet, driving such a planet out of the habitable zone. We
show that an additional inner planet of ~>0.3M_earth at <~0.1 AU would suppress
the eccentricity perturbation and greatly improve the prospects for
habitability of the system. Thus, the planetary architecture of a potentially
habitable OGLE-2006-BLG-109L planetary system -- with two ``terrestrial''
planets and two jovian planets -- could bear very close resemblance to our own
Solar system.Comment: 11 pages including 4 figures; accepted for publication in ApJ-Letter
The origin of planetary impactors in the inner solar system
New insights into the history of the inner solar system are derived from the
impact cratering record of the Moon, Mars, Venus and Mercury, and from the size
distributions of asteroid populations. Old craters from a unique period of
heavy bombardment that ended 3.8 billion years ago were made by asteroids
that were dynamically ejected from the main asteroid belt, possibly due to the
orbital migration of the giant planets. The impactors of the past 3.8
billion years have a size distribution quite different from the main belt
asteroids, but very similar to the population of near-Earth asteroids.Comment: 12 pages (including 4 figures
Endothelial and myocardial injury during ischemia and reperfusion: Pathogenesis and therapeutic implications
AbstractEarly reperfusion remains the most effective way of limiting myocardial necrosis and improving ventricular function in experimental models and human patients. However, the introduction of oxygen and cellular elements, especially the neutrophil, into the ischemic zone may initiate a deleterious cascade of events that limits myocardial salvage after reperfusion. Although the pathogenesis of reperfusion injury remains controversial, recent studies have suggested that the endothelium may play a critical role.Endothelial cells maintain flow in the microcirculation by secreting a number of vasodilatory compounds and substances that prevent plugging of capillaries by inhibiting neutrophil adherence and platelet aggregation. Reperfusion of ischemic myocardium accelerates structural and functional changes in endothelial cells, resulting in a progressive decrease in microcirculatory flow (“no reflow” phenomenon). Numerous studies suggest that activated neutrophils mediate vascular damage by releasing reactive oxygen species and potent proteolytic enzymes. The administration of therapeutic agents that limit endothelial disruption and neutrophil plugging has shown promising results in limiting myocardial reperfusion injury in experimental models
Secular resonance sweeping of the main asteroid belt during planet migration
We calculate the eccentricity excitation of asteroids produced by the
sweeping secular resonance during the epoch of planetesimal-driven
giant planet migration in the early history of the solar system. We derive
analytical expressions for the magnitude of the eccentricity change and its
dependence on the sweep rate and on planetary parameters; the sweeping
leads to either an increase or a decrease of eccentricity depending on an
asteroid's initial orbit. Based on the slowest rate of sweeping that
allows a remnant asteroid belt to survive, we derive a lower limit on Saturn's
migration speed of \sim0.15\AU\My^{-1} during the era that the
resonance swept through the inner asteroid belt (semimajor axis range
2.1--2.8\AU). This rate limit is for Saturn's current eccentricity, and
scales with the square of Saturn's eccentricity; the limit on Saturn's
migration rate could be lower if Saturn's eccentricity were lower during its
migration. Applied to an ensemble of fictitious asteroids, our calculations
show that a prior single-peaked distribution of asteroid eccentricities would
be transformed into a double-peaked distribution due to the sweeping of the
. Examination of the orbital data of main belt asteroids reveals that
the proper eccentricities of the known bright () asteroids may be
consistent with a double-peaked distribution. If so, our theoretical analysis
then yields two possible solutions for the migration rate of Saturn and for the
dynamical states of the pre-migration asteroid belt: a dynamically cold state
(single-peaked eccentricity distribution with mean of ) linked with
Saturn's migration speed \sim 4\AU\My^{-1}, or a dynamically hot state
(single-peaked eccentricity distribution with mean of ) linked with
Saturn's migration speed \sim 0.8\AU\My^{-1}.Comment: 32 pages, 7 figures. Accepted for publication in ApJ on Mar. 1, 201
The Program for climate Model diagnosis and Intercomparison: 20-th anniversary Symposium
Twenty years ago, W. Lawrence (Larry) Gates approached the U.S. Department of Energy (DOE) Office of Energy Research (now the Office of Science) with a plan to coordinate the comparison and documentation of climate model differences. This effort would help improve our understanding of climate change through a systematic approach to model intercomparison. Early attempts at comparing results showed a surprisingly large range in control climate from such parameters as cloud cover, precipitation, and even atmospheric temperature. The DOE agreed to fund the effort at the Lawrence Livermore National Laboratory (LLNL), in part because of the existing computing environment and because of a preexisting atmospheric science group that contained a wide variety of expertise. The project was named the Program for Climate Model Diagnosis and Intercomparison (PCMDI), and it has changed the international landscape of climate modeling over the past 20 years. In spring 2009 the DOE hosted a 1-day symposium to celebrate the twentieth anniversary of PCMDI and to honor its founder, Larry Gates. Through their personal experiences, the morning presenters painted an image of climate science in the 1970s and 1980s, that generated early support from the international community for model intercomparison, thereby bringing PCMDI into existence. Four talks covered GatesÃÂâÃÂÃÂÃÂÃÂs early contributions to climate research at the University of California, Los Angeles (UCLA), the RAND Corporation, and Oregon State University through the founding of PCMDI to coordinate the Atmospheric Model Intercomparison Project (AMIP). The speakers were, in order of presentation, Warren Washington [National Center for Atmospheric Research (NCAR)], Kelly Redmond (Western Regional Climate Center), George Boer (Canadian Centre for Climate Modelling and Analysis), and Lennart Bengtsson [University of Reading, former director of the European Centre for Medium-Range Weather Forecasts (ECMWF)]. The afternoon session emphasized the scientific ideas that are the basis of PCMDIÃÂâÃÂÃÂÃÂÃÂs success, summarizing their evolution and impact. Four speakers followed the various PCMDI-supported climate model intercomparison projects, beginning with early work on cloud representations in models, presented by Robert D. Cess (Distinguished Professor Emeritus, Stony Brook University), and then the latest Cloud Feedback Model Intercomparison Projects (CFMIPs) led by Sandrine Bony (Laboratoire de MÃÂÃÂÃÂétÃÂÃÂÃÂéorologie Dynamique). Benjamin Santer (LLNL) presented a review of the climate change detection and attribution (D & A) work pioneered at PCMDI, and Gerald A. Meehl (NCAR) ended the day with a look toward the future of climate change research
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