2,241 research outputs found
Benjamin Capps
One might assume that a requisite for becoming a successful novelist about the Southwest is to be born in Archer County, Texas. Both Benjamin Capps and Larry McMurtry were born there, but their interests are different. Capps has been concerned with the background of his region, while McMurtry treats contemporary themes. Capps has now published eight novels, plus three pieces of non-fiction, about the West
Cdc6 ATPase activity regulates ORC center dot Cdc6 stability and the selection of specific DNA sequences as origins of DNA replication
DNA replication, as with all macromolecular synthesis steps, is controlled in part at the level of initiation. Although the origin recognition complex ( ORC) binds to origins of DNA replication, it does not solely determine their location. To initiate DNA replication ORC requires Cdc6 to target initiation to specific DNA sequences in chromosomes and with Cdt1 loads the ring-shaped mini-chromosome maintenance ( MCM) 2-7 DNA helicase component onto DNA. ORC and Cdc6 combine to form a ring-shaped complex that contains six AAA(+) subunits. ORC and Cdc6 ATPase mutants are defective in MCM loading, and ORC ATPase mutants have reduced activity in ORC.Cdc6.DNA complex formation. Here we analyzed the role of the Cdc6 ATPase on ORC.Cdc6 complex stability in the presence or absence of specific DNA sequences. Cdc6 ATPase is activated by ORC, regulates ORC.Cdc6 complex stability, and is suppressed by origin DNA. Mutations in the conserved origin A element, and to a lesser extent mutations in the B1 and B2 elements, induce Cdc6 ATPase activity and prevent stable ORC.Cdc6 formation. By analyzing ORC.Cdc6 complex stability on various DNAs, we demonstrated that specific DNA sequences control the rate of Cdc6 ATPase, which in turn controls the rate of Cdc6 dissociation from the ORC.Cdc6.DNA complex. We propose a mechanism explaining how Cdc6 ATPase activity promotes origin DNA sequence specificity; on DNA that lacks origin activity, Cdc6 ATPase promotes dissociation of Cdc6, whereas origin DNA down-regulates Cdc6 ATPase resulting in a stable ORC.Cdc6.DNA complex, which can then promote MCM loading. This model has relevance for origin specificity in higher eukaryotes
Optical properties of silicon carbide for astrophysical applications I. New laboratory infrared reflectance spectra and optical constants
Silicon Carbide (SiC) optical constants are fundamental inputs for radiative
transfer models of astrophysical dust environments. However, previously
published values contain errors and do not adequately represent the bulk
physical properties of the cubic (beta) SiC polytype usually found around
carbon stars. We provide new, uncompromised optical constants for beta- and
alpha-SiC derived from single-crystal reflectance spectra and investigate
quantitatively whether there is any difference between alpha- and beta-SiC that
can be seen in infrared spectra and optical functions.
Previous optical constants for SiC do not reflect the true bulk properties,
and they are only valid for a narrow grain size range. The new optical
constants presented here will allow narrow constraints to be placed on the
grain size and shape distribution that dominate in astrophysical environments.
In addition, our calculated absorption coefficients are much higher than
laboratory measurements, which has an impact on the use of previous data to
constrain abundances of these dust grains.Comment: 12 pages; 10 figures; laboratory optical constants available from
CDS. Accepted by Astronomy & Astrophysic
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
Mobility and Diffusion of a Tagged Particle in a Driven Colloidal Suspension
We study numerically the influence of density and strain rate on the
diffusion and mobility of a single tagged particle in a sheared colloidal
suspension. We determine independently the time-dependent velocity
autocorrelation functions and, through a novel method, the response functions
with respect to a small force. While both the diffusion coefficient and the
mobility depend on the strain rate the latter exhibits a rather weak
dependency. Somewhat surprisingly, we find that the initial decay of response
and correlation functions coincide, allowing for an interpretation in terms of
an 'effective temperature'. Such a phenomenological effective temperature
recovers the Einstein relation in nonequilibrium. We show that our data is well
described by two expansions to lowest order in the strain rate.Comment: submitted to EP
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Silicon carbide absorption features: dust formation in the outflows of extreme carbon stars
Infrared carbon stars without visible counterparts are generally known as
extreme carbon stars. We have selected a subset of these stars with absorption
features in the 10-13 m range, which has been tentatively attributed to
silicon carbide (SiC). We add three new objects meeting these criterion to the
seven previously known, bringing our total sample to ten sources. We also
present the result of radiative transfer modeling for these stars, comparing
these results to those of previous studies. In order to constrain model
parameters, we use published mass-loss rates, expansion velocities and
theoretical dust condensation models to determine the dust condensation
temperature. These show that the inner dust temperatures of the dust shells for
these sources are significantly higher than previously assumed. This also
implies that the dominant dust species should be graphite instead of amorphous
carbon. In combination with the higher condensation temperature we show that
this results in a much higher acceleration of the dust grains than would be
expected from previous work. Our model results suggest that the very optically
thick stage of evolution does not coincide with the timescales for the
superwind, but rather, that this is a very short-lived phase. Additionally, we
compare model and observational parameters in an attempt to find any
correlations. Finally, we show that the spectrum of one source, IRAS
175343030, strongly implies that the 10-13 m feature is due to a solid
state rather than a molecular species.Comment: 13 Figure
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