Local Group Dwarf Galaxies and the Star Formation Law at High Redshift

I show how the existing observational data on Local Group dwarf galaxies can be used to estimate the average star formation law during the first 3 Gyr of the history of the universe. I find that the observational data are consistent with the orthodox Schmidt law with a star formation efficiency of about 4 percent if the star formation is continuous (during the first 3 Gyr). The efficiency is proportionally higher if most of the gas in the dwarfs was consumed (and never replenished) in a short time interval well before the universe turned 3 Gyr.Comment: accepted for publication in ApJ Letter

Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability

Circumplanetary disks can be found around forming giant planets, regardless of whether core accretion or gravitational instability built the planet. We carried out state-of-the-art hydrodynamical simulations of the circumplanetary disks for both formation scenarios, using as similar initial conditions as possible to unveil possible intrinsic differences in the circumplanetary disk mass and temperature between the two formation mechanisms. We found that the circumplanetary disks mass linearly scales with the circumstellar disk mass. Therefore, in an equally massive protoplanetary disk, the circumplanetary disks formed in the disk instability model can be only a factor of eight more massive than their core-accretion counterparts. On the other hand, the bulk circumplanetary disk temperature differs by more than an order of magnitude between the two cases. The subdisks around planets formed by gravitational instability have a characteristic temperature below 100 K, while the core accretion circumplanetary disks are hot, with temperatures even greater than 1000 K when embedded in massive, optically thick protoplanetary disks. We explain how this difference can be understood as the natural result of the different formation mechanisms. We argue that the different temperatures should persist up to the point when a full-fledged gas giant forms via disk instability, hence our result provides a convenient criteria for observations to distinguish between the two main formation scenarios by measuring the bulk temperature in the planet vicinity.Comment: 12 pages, 9 figures, 1 table, accepted for publication at MNRA

The Fermion-Boson Transformation in Fractional Quantum Hall Systems

A Fermion to Boson transformation is accomplished by attaching to each Fermion a single flux quantum oriented opposite to the applied magnetic field. When the mean field approximation is made in the Haldane spherical geometry, the Fermion angular momentum $l_F$ is replaced by $l_B= l_F-{1\over2}(N-1)$. The set of allowed total angular momentum multiplets is identical in the two different pictures. The Fermion and Boson energy spectra in the presence of many body interactions are identical if and only if the pseudopotential is ``harmonic'' in form. However, similar low energy bands of states with Laughlin correlations occur in the two spectra if the interaction has short range. The transformation is used to clarify the relation between Boson and Fermion descriptions of the hierarchy of condensed fractional quantum Hall states.Comment: 5 pages, 4 figures, submitted to Physica

Direct N-body Simulations of Rubble Pile Collisions

There is increasing evidence that many km-sized bodies in the Solar System are piles of rubble bound together by gravity. We present results from a project to map the parameter space of collisions between km-sized spherical rubble piles. The results will assist in parameterization of collision outcomes for Solar System formation models and give insight into fragmentation scaling laws. We use a direct numerical method to evolve the positions and velocities of the rubble pile particles under the constraints of gravity and physical collisions. We test the dependence of the collision outcomes on impact parameter and speed, impactor spin, mass ratio, and coefficient of restitution. Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as the primordial disk during early planet formation) so that the maximum strain on the component material does not exceed the crushing strength. We compare our results with analytic estimates and hydrocode simulations. Off-axis collisions can result in fast-spinning elongated remnants or contact binaries while fast collisions result in smaller fragments overall. Clumping of debris escaping from the remnant can occur, leading to the formation of smaller rubble piles. In the cases we tested, less than 2% of the system mass ends up orbiting the remnant. Initial spin can reduce or enhance collision outcomes, depending on the relative orientation of the spin and orbital angular momenta. We derive a relationship between impact speed and angle for critical dispersal of mass in the system. We find that our rubble piles are relatively easy to disperse, even at low impact speed, suggesting that greater dissipation is required if rubble piles are the true progenitors of protoplanets.Comment: 30 pages including 4 tables, 8 figures. Revised version to be published in Icarus

Level-1 Milestone 350 Definitions v1

This milestone is the direct result of work that started seven years ago with the planning for a 100-teraFLOP platform and will be satisfied when 100 teraFLOPS is placed in operation and readied for Stockpile Stewardship Program simulations. The end product of this milestone will be a production-level, high-performance computing system, code named Purple, designed to be used to solve the most demanding stockpile stewardship problems, that is, the large-scale application problems at the edge of our understanding of weapon physics. This fully functional 100 teraFLOPS system must be able to serve a diverse scientific and engineering workload. It must also have a robust code development and production environment, both of which facilitate the workload requirements. This multi-year effort includes major activities in contract management, facilities, infrastructure, system software, and user environment and support. Led by LLNL, the trilabs defined the statement of work for a 100-teraFLOP system that resulted in a contract with IBM known as the Purple contract. LLNL worked with IBM throughout the contract period to resolve issues and collaborated with the Program to resolve contractual issues to ensure delivery of a platform that best serves the Program for a reasonable cost. The Purple system represents a substantial increase in the classified compute resources at LLNL for NNSA. The center computer environment must be designed to accept the Purple system and to scale with the increase of compute resources to achieve required end-to-end services. Networking, archival storage, visualization servers, global file systems, and system software will all be enhanced to support Purple's size and architecture. IBM and LLNL are sharing responsibility for Purple's system software. LLNL is responsible for the scheduler, resource manager, and some code development tools. Through the Purple contract, IBM is responsible for the remainder of the system software including the operating system, parallel file system, and runtime environment. LLNL, LANL, and SNL share responsibility for the Purple user environment. Since LLNL is the host for Purple, LLNL has the greatest responsibility. LLNL will provide customer support for Purple to the tri-labs and as such has the lead for user documentation, negotiating the Purple usage model, mapping of the ASC computational environment requirements to the Purple environment, and demonstrating those requirements have been met. In addition, LLNL will demonstrate important capabilities of the computing environment including full functionality of visualization tools, file transport between Purple and remote site file systems, and the build environment for principle ASC codes. LANL and SNL are responsible for delivering unique capabilities in support of their users, porting important applications and libraries, and demonstrating remote capabilities. The key capabilities that LANL and SNL will test are user authorization and authentication, data transfer, file system, data management, and visualization. SNL and LANL should port and run in production mode a few key applications on a substantial number of Purple nodes

X-ray Amorphous Components of Antarctica Dry Valley Soils: Weathering Implications for Mars

The Antarctic Dry Valleys (ADV) comprise the largest ice-free region of Antarctica. Precipitation usually occurs as snow, relative humidity is frequently low, and mean annual temperatures are about -20C [1]. Substantial work has focused on soil formation in the ADVs [2], however, little work has focused on the mineralogy of secondary alteration phases. The dominant weathering process in the ADV region is physical weathering, however, chemical weathering has been well documented [3]. The occurrence of chemical weathering processes are suggested by the presence of clay minerals and iron and titanium oxides in soil. Previously we have investigated soils from two sites in the ADVs and have shown evidence of chemical weathering by the presence of clay minerals (vermiculite, smectite), short-range ordered (SRO) and/or X-ray amorphous materials, and Fe- and Tioxides as well as the presence of discrete calcite crystals [4, 5]. The Chemistry and Mineralogy (CheMin) instrument onboard the Mars Curiosity rover has detected abundant amounts (approx. 25-30 wt. %) of X-ray amorphous materials in a windblown deposit or soil (Rocknest) and in a sedimentary rocks [6,7,8]. The occurrence of large amounts of X-ray amorphous materials in Mars sediments is surprising because these materials are usually present in small quantities in terrestrial environments. The objective of this study is to further characterize the chemistry and mineralogy, specifically the secondary alteration mineralogy and the presence of X-ray amorphous material, of soils from two sites we have previously studied, a subxerous soil in Taylor Valley, and an ultraxerous soil in University Valley. While the chemical alteration processes and mineralogy of the ADV has been documented previously, there has been limited discussion on the occurrence and formation of X-ray amorphous and SRO materials in Antarctica soils. The process of aqueous alteration in the ADVs may have implications for pedogenic processes on Mars, and may lead to a better understanding to the abundance of amorphous material found in sediments in Gale crater

Potato growing in Missouri

Cover title.Includes bibliographical references

Bermuda onion culture in Missouri

Caption title.Digitized 2006 AES MoU.Includes bibliographical references