On the formation time scale and core masses of gas giant planets

Numerical simulations show that the migration of growing planetary cores may be dominated by turbulent fluctuations in the protoplanetary disk, rather than by any mean property of the flow. We quantify the impact of this stochastic core migration on the formation time scale and core mass of giant planets at the onset of runaway gas accretion. For standard Solar Nebula conditions, the formation of Jupiter can be accelerated by almost an order of magnitude if the growing core executes a random walk with an amplitude of a few tenths of an au. A modestly reduced surface density of planetesimals allows Jupiter to form within 10 Myr, with an initial core mass below 10 Earth masses, in better agreement with observational constraints. For extrasolar planetary systems, the results suggest that core accretion could form massive planets in disks with lower metallicities, and shorter lifetimes, than the Solar Nebula.Comment: ApJL, in pres

Measurement of Enthalpies of Evaporation of Bi and Bi2 by an Optical Absorption Technique

The enthalpies of evaporation of Bi and Bi2 were determined by the second‐law method using an optical absorption technique to measure the change of vapor density with temperature for each species. The 3067‐Å line was used for Bi and the band head at 2731 Å was used for Bi2. The enthalpy of dissociation of Bi2 is the difference between the two enthalpies of evaporation. The evaporation values obtained wereBi(l)=Bi(g)ΔH∘298=49600±1100cal,Bi(l)=12Bi2(g)ΔH∘298=24700±1000cal,12Bi2(g)=Bi(g)ΔH∘298=24900±2100cal.The optical absorption technique was checked on lead, using the 2833‐Å line. For lead, the ΔH°v,298 measured was 47 240±850 cal, as compared with an accepted value of 46 600±300 cal.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70415/2/JCPSA6-45-11-4141-1.pd

Simultaneous Determination of f Values and Vapor Pressures from Optical Absorption Measurements

A method for determining both the f value of an absorption line and the vapor pressure of the absorbing atoms exclusively from optical absorption measurements is defined and discussed. The method depends on making absorption measurements in two regions. The first is at low optical densities, where absorption is linearly proportional to optical density. The second is at higher densities, where the center of the line is fully absorbed and the absorption is proportional to the square root of the optical density. A method for extrapolating measurements from one region to the other is also required. This is accomplished by maintaining a pure liquid in equilibrium with the absorbing vapor species over a temperature range. The method takes into account the effects of collision broadening. Measurements were made on the 3067‐Å bismuth line.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71302/2/JCPSA6-42-2-701-1.pd

Quantifying orbital migration from exoplanet statistics and host metallicities

We investigate how the statistical distribution of extrasolar planets may be combined with knowledge of the host stars' metallicity to yield constraints on the migration histories of gas giant planets. At any radius, planets that barely manage to form around the lowest metallicity stars accrete their envelopes just as the gas disk is being dissipated, so the lower envelope of planets in a plot of metallicity vs semi-major axis defines a sample of non-migratory planets that will have suffered less than average migration subsequent to gap opening. Under the assumption that metallicity largely controls the initial surface density of planetesimals, we use simplified core accretion models to calculate how the minimum metallicity needed for planet formation varies as a function of semi-major axis. Models that do not include core migration prior to gap opening (Type I migration) predict that the critical metallicity is largely flat between the snow line and a semimajor axis of about 6 AU, with a weak dependence on the initial surface density profile of planetesimals. When slow Type I migration is included, the critical metallicity is found to increase steadily from 1-10 AU. Large planet samples, that include planets at modestly greater orbital radii than present surveys, therefore have the potential to quantify the extent of migration in both Type I and Type II regimes.Comment: ApJ, in pres

Understanding Your Parents.

4 p

Gut microbial community supplementation and reduction modulates African armyworm susceptibility to a baculovirus

Gut microbiota stimulates the immune system and inhibits pathogens, and thus, it is critical for disease prevention. Probiotics represent an effective alternative to antibiotics used for the therapy and prevention of bacterial diseases. Probiotic bacteria are commonly used in vertebrates, although their use in invertebrates is still rare. We manipulated the gut microbiome of the African Armyworm (Spodoptera exempta Walker) using antibiotics and field-collected frass, in an attempt to understand the interactions of the gut microbiome with the nucleopolyhedrovirus, SpexNPV. We found that S. exempta individuals with supplemented gut microbiome were significantly more resistant to SpexNPV, relative to those with a typical laboratory gut microbiome. Illumina MiSeq sequencing revealed the bacterial phyla in the S. exempta gut belonged to 28 different classes. Individuals with an increased abundance of Lactobacillales had a higher probability of surviving viral infection. In contrast, there was an increased abundance of Enterobacteriales and Pseudomonadales in individuals dying from viral infection, corresponding with decreased abundance of these two Orders in surviving caterpillars, suggesting a potential role for them in modulating the interaction between the host and its pathogen. These results have important implications for laboratory studies testing biopesticides

Cereal Grain Combustion in Domestic Boilers

This study compared the combustion characteristics and the combustion behaviour of oats, barley, triticale and wheat to that of wood pellets. Sustained grain combustion in domestic boilers was feasible but problematic, the main impediment being clinker formation with ash agglomeration. Clinker formation was lowest for oats which burned easily with fewer operational problems. Triticale displayed reasonably good combustion characteristics and also ignited easily. In contrast, barley and wheat proved difficult to ignite while barley combustion was prone to self-extinguish. Thermal and combustion efficiency and heat output were considerably higher at a grain moisture content of 15% compared to a moisture content of 20%. The efficiency of oat combustion was similar to that of wood pellets at a moisture content of 15%. Carbon monoxide (CO) emission from cereal grains increased with increasing moisture content, but was still below limit values. Oxides of nitrogen (NOx) emissions from cereal combustion were high and would require reduction by limiting the quantity of nitrogen applied to the crop and/or the use of air staging. Oats proved superior to the other grains as a combustion feedstock with similar efficiencies to those of wood pellets but low moisture content is a prerequisite for efficient grain combustion

Self-gravitating fragmentation of eccentric accretion disks

We consider the effects of eccentricity on the fragmentation of gravitationally unstable accretion disks, using numerical hydrodynamics. We find that eccentricity does not affect the overall stability of the disk against fragmentation, but significantly alters the manner in which such fragments accrete gas. Variable tidal forces around an eccentric orbit slow the accretion process, and suppress the formation of weakly-bound clumps. The "stellar" mass function resulting from the fragmentation of an eccentric disk is found to have a significantly higher characteristic mass than that from a corresponding circular disk. We discuss our results in terms of the disk(s) of massive stars at ~0.1pc from the Galactic Center, and find that the fragmentation of an eccentric accretion disk, due to gravitational instability, is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap