Vega's hot dust from icy planetesimals scattered inward by an outward-migrating planetary system

Vega has been shown to host multiple dust populations, including both hot exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100 AU. We use dynamical simulations to show how Vega's hot dust can be created by long-range gravitational scattering of planetesimals from its cold outer regions. Planetesimals are scattered progressively inward by a system of 5-7 planets from 30-60 AU to very close-in. In successful simulations the outermost planets are typically Neptune-mass. The back-reaction of planetesimal scattering causes these planets to migrate outward and continually interact with fresh planetesimals, replenishing the source of scattered bodies. The most favorable cases for producing Vega's exo-zodi have negative radial mass gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized planet exists beyond ~15 AU because the planet preferentially ejects planetesimals before they can reach the inner system. Direct-imaging planet searches can therefore directly test this mechanism.Comment: Updated references. Accepted to MNRAS Letters. 5 pages, 4 figures. Blog post about the paper at http://planetplanet.net/2014/03/31/vega-a-planetary-poem

Predicting Planets in Known Extra-Solar Planetary Systems II: Testing for Saturn-mass Planets

Recent results have shown that many of the known extrasolar planetary systems contain regions which are stable for massless test particles. We examine the possibility that Saturn-mass planets exist in these systems, just below the detection threshold, and attempt to predict likely orbital parameters for such unseen planets. To do this, we insert a Saturn-mass planet into the stable regions of these systems and integrate its orbit for 100 million years. We conduct 200-600 of these experiments to test parameter space in HD37124, HD38529, 55Cnc, and HD74156. In HD37124 the global maximum of the survival rate of Saturns in parameter space is at semimajor axis a = 1.03 AU, eccentricity e=0.1. In HD38529, only 5% of Saturns are unstable, and the region in which a Saturn could survive is very broad, centered on 0.5<a<0.6, e<0.2. In 55Cnc we find three maxima at (a,e) = (1.0 AU, 0.02), (2.0 AU, 0.08), and (3.0 AU, 0.17). In HD74156 we find a broad maximum with $a$ = 0.9-1.2 AU, e<=0.15. Several of these maxima are located in the habitable zones of their parent stars and are therefore of astrobiological interest. We suggest the possibility that companions may lie in these locations of parameter space, and encourage further observational investigation of these systems.Comment: submitted to ApJ 9 pages, 8 figures, 3 table

Flight evaluation of a computer aided low-altitude helicopter flight guidance system

The Flight Systems Development branch of the U.S. Army's Avionics Research and Development Activity (AVRADA) and NASA Ames Research Center developed for flight testing a Computer Aided Low-Altitude Helicopter Flight (CALAHF) guidance system. The system includes a trajectory-generation algorithm which uses dynamic programming and a helmet-mounted display (HMD) presentation of a pathway-in-the-sky, a phantom aircraft, and flight-path vector/predictor guidance symbology. The trajectory-generation algorithm uses knowledge of the global mission requirements, a digital terrain map, aircraft performance capabilities, and precision navigation information to determine a trajectory between mission waypoints that seeks valleys to minimize threat exposure. This system was developed and evaluated through extensive use of piloted simulation and has demonstrated a 'pilot centered' concept of automated and integrated navigation and terrain mission planning flight guidance. This system has shown a significant improvement in pilot situational awareness, and mission effectiveness as well as a decrease in training and proficiency time required for a near terrain, nighttime, adverse weather system

Orbital Dynamics of Multi-Planet Systems with Eccentricity Diversity

Since exoplanets were detected using the radial velocity method, they have revealed a diverse distribution of orbital configurations. Amongst these are planets in highly eccentric orbits (e > 0.5). Most of these systems consist of a single planet but several have been found to also contain a longer period planet in a near-circular orbit. Here we use the latest Keplerian orbital solutions to investigate four known systems which exhibit this extreme eccentricity diversity; HD 37605, HD 74156, HD 163607, and HD 168443. We place limits on the presence of additional planets in these systems based on the radial velocity residuals. We show that the two known planets in each system exchange angular momentum through secular oscillations of their eccentricities. We calculate the amplitude and timescale for these eccentricity oscillations and associated periastron precession. We further demonstrate the effect of mutual orbital inclinations on the amplitude of high-frequency eccentricity oscillations. Finally, we discuss the implications of these oscillations in the context of possible origin scenarios for unequal eccentricities.Comment: 12 pages, 9 figures, accepted for publication in the Astrophysical Journa

Loop interaction in the visible emission corona: Morphological details

Coronagraph observations of two post flare loop systems, recorded photographically in the emissions of Fe 14 (5303 A) and Fe 10 (6374 A), show occasional enhancements at the intersections of some loops. The brightness of such enhancements in the green line gradually increases to a maximum value several times greater than that of the legs of the loops and then declines with a typical lifetime approx. 30 to 60 min. In red line emission the loop systems are usually very faint, but show the same overall type of enhancement, with a lag in maximum brightness relative to that of the green line approx. 10 min. The electron density, derived from the cooling time, is approx. 10 to the 12th power/cu cm

High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability

The water content and habitability of terrestrial planets are determined during their final assembly, from perhaps a hundred 1000-km "planetary embryos" and a swarm of billions of 1-10 km "planetesimals." During this process, we assume that water-rich material is accreted by terrestrial planets via impacts of water-rich bodies that originate in the outer asteroid region. We present analysis of water delivery and planetary habitability in five high-resolution simulations containing about ten times more particles than in previous simulations (Raymond et al 2006a, Icarus, 183, 265-282). These simulations formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five planets in the habitable zone. Every planet from each simulation accreted at least the Earth's current water budget; most accreted several times that amount (assuming no impact depletion). Each planet accreted at least five water-rich embryos and planetesimals from past 2.5 AU; most accreted 10-20 water-rich bodies. We present a new model for water delivery to terrestrial planets in dynamically calm systems, with low-eccentricity or low-mass giant planets -- such systems may be very common in the Galaxy. We suggest that water is accreted in comparable amounts from a few planetary embryos in a "hit or miss" way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects such as planetary mass and location, and giant planet properties.Comment: Astrobiology, in pres

Dynamical photo-induced electronic properties of molecular junctions

Nanoscale molecular-electronic devices and machines are emerging as promising functional elements, naturally flexible and efficient, for next generation technologies. A deeper understanding of carrier dynamics in molecular junctions is expected to benefit many fields of nanoelectronics and power-devices. We determine time-resolved charge current flowing at donor- acceptor interface in molecular junctions connected to metallic electrodes by means of quantum transport simulations. The current is induced by the interaction of the donor with a Gaussian-shape femtosecond laser pulse. Effects of the molecular internal coupling, metal- molecule tunneling and light-donor coupling on photocurrent are discussed. We then examine the junction working through the time-resolved donor density of states. Non-equilibrium reorganization of hybridized molecular orbitals through the light-donor interaction gives rise to two phenomena: the dynamical Rabi shift and the appearance of Floquet-like states. Such insights into the dynamical photoelectronic structure of molecules are of strong interest for ultrafast spectroscopy, and open avenues toward the possibility of analyzing and controlling the internal properties of quantum nanodevices with pump-push photocurrent spectroscopy