University of Arkansas at Monticello

About: University of Arkansas at Monticello is a education organization based out in Monticello, Arkansas, United States. It is known for research contribution in the topics: Dark matter & Star formation. The organization has 408 authors who have published 524 publications receiving 10241 citations. The organization is also known as: UAM.

Analysis of growth equations.

Abstract: Growth of plants results from two opposing factors: the intrinsic tendency toward unlimited increase (biotic potential) and restraints imposed by environmental resistance and ag/ng. The expansion tendency prevails in the beginning of a tree's life, while growth decline becomes prominent oward the end. The existing growth equations can be transformed (by differentiation, decomposition i to the division components, and taking logarithms) so that the components hat correspond to these two factors are exposed. This transformation reveals two basic forms intrinsic in most of the analyzed equations. Their common feature is that growth expansion is proportional to current tree size. Growth decline of individual trees appears to be more variable and can be rendered with equal accuracy by a variety of expressions. This may reflect that a greater number of factors hinder growth: scarcity of resources, competition, reproduction, diseases, herbivory, disturbances, etc. Consequently, the growth path is inherently imprecise and can be viewed as a wide valley rather than a single line. This analysis laid groundwork for the classification of known equations and made possible the discovery of a promising new equation form. FOR. SCL 39(3):594-616. ADDITIONAL

Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime

Abstract: Carbon nanotubes are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge. Defects are known to modify the electrical resistance of carbon nanotubes. They can be present in as-grown carbon nanotubes, but controlling externally their density opens a path towards the tuning of the nanotube electronic characteristics. In this work consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room temperature resistance versus SWNT-length [R(L)] along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a 400 nm SWNT length.

Dwarf galaxies in voids: Suppressing star formation with photo-heating

Abstract: We study structure formation in cosmological void regions using high-resolution hydrodynamical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter halos which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological UV-background photo-evaporates baryons out of halos of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV-background with reionisation at redshift z=6, our samples of simulated galaxies show that halos with masses below a characteristic mass of M_c(z=0) = 6.5 x 10^9 h^{-1} M_sun are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In halos with mass M < M_c, we find that photo-heating suppresses further cooling of gas. The redshift and UV-background dependent characteristic mass M_c(z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of \delta ~ 1000. If a halo is massive enough to compress gas to this density despite the presence of the UV background, gas is free to `enter' the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionisation. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter halos continue to increase. (abridged)