909 research outputs found
A study of unmanned mission opportunities to comets and asteroids
Several unmanned multiple-target mission opportunities to comets and asteroids were studied. The targets investigated include Grigg-Skjellerup, Giacobini-Zinner, Tuttle-Giacobini-Kresak, Borrelly, Halley, Schaumasse, Geographos, Eros, Icarus, and Toro, and the trajectories consist of purely ballistic flight, except that powered swingbys and deep space burns are employed when necessary. Optimum solar electric rendezvous trajectories to the comets Giacobini-Zinner/85, Borrelly/87, and Temple (2)/83 and /88 employing the 8.67 kw Sert III spacecraft modified for interplanetary flight were also investigated. The problem of optimizing electric propulsion heliocentric trajectories, including the effects of geocentric launch asymptote declination on launch vehicle performance capability, was formulated, and a solution developed using variational calculus techniques. Improvements were made to the HILTOP trajectory optimization computer program. An error analysis of high-thrust maneuvers involving spin-stabilized spacecraft was developed and applied to a synchronous meteorological satellite mission
Non-LTE Monte Carlo Radiative Transfer: II. Non-Isothermal Solutions for Viscous Keplerian Disks
We discuss the basic hydrodynamics that determines the density structure of
the disks around hot stars. Observational evidence supports the idea that these
disks are Keplerian (rotationally supported) gaseous disks. A popular scenario
in the literature, which naturally leads to the formation of Keplerian disks,
is the viscous decretion model. According to this scenario, the disks are
hydrostatically supported in the vertical direction, while the radial structure
is governed by the viscous transport. This suggests that the temperature is one
primary factor that governs the disk density structure. In a previous study we
demonstrated, using 3-D NLTE Monte Carlo simulations, that viscous keplerian
disks can be highly non-isothermal. In this paper we build upon our previous
work and solve the full problem of the steady-state non-isothermal viscous
diffusion and vertical hydrostatic equilibrium. We find that the
self-consistent solution departs significantly from the analytic isothermal
density, with potentially large effects on the emergent spectrum. This implies
that non-isothermal disk models must be used for a detailed modeling of Be star
disks.Comment: 22 pages, 9 figures, Ap
2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources
We present 2-D radiation transfer models of Class I Protostars and show the
effect of including more realistic geometries on the resulting spectral energy
distributions and images. We begin with a rotationally flattened infalling
envelope as our comparison model, and add a flared disk and bipolar cavity. The
disk affects the spectral energy distribution most strongly at edge-on
inclinations, causing a broad dip at about 10 um (independent of the silicate
feature) due to high extinction and low scattering albedo in this wavelength
region. The bipolar cavities allow more direct stellar+disk radiation to emerge
into polar directions, and more scattering radiation to emerge into all
directions. The wavelength-integrated flux, often interpreted as luminosity,
varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much
flux as edge-on, depending on geometry. Thus, observational estimates of
luminosity should take into account the inclination of a source. The envelopes
with cavities are significantly bluer in near-IR and mid-IR color-color plots
than those without cavities. Using 1-D models to interpret Class I sources with
bipolar cavities would lead to an underestimate of envelope mass and an
overestimate of the implied evolutionary state. We compute images at near-,
mid-, and far-IR wavelengths. We find that the mid-IR colors and images are
sensitive to scattering albedo, and that the flared disk shadows the midplane
on large size scales at all wavelengths plotted. Finally, our models produce
polarization spectra which can be used to diagnose dust properties, such as
albedo variations due to grain growth. Our results of polarization across the
3.1 um ice feature agree well with observations for ice mantles covering 5% of
the radius of the grains.Comment: Accepted for publication in ApJ, 37 pages, 13 figures (several
figures reduced in quality; find original version at
http://gemelli.colorado.edu/~bwhitney/preprints.html
Failure of microtubule-mediated peroxisome division and trafficking in disorders with reduced peroxisome abundance
In contrast to peroxisomes in normal cells, remnant peroxisomes in cultured skin fibroblasts from a subset of the clinically severe peroxisomal disorders that includes the biogenesis disorder Zellweger syndrome and the single-enzyme defect D-bifunctional protein (D-BP) deficiency, are enlarged and significantly less abundant. We tested whether these features could be related to the known role of microtubules in peroxisome trafficking in mammalian cells. We found that remnant peroxisomes in fibroblasts from patients with PEX1-null Zellweger syndrome or D-BP deficiency exhibited clustering and loss of alignment along peripheral microtubules. Similar effects were observed for both cultured embryonic fibroblasts and brain neurons from a PEX13-null mouse with a Zellweger-syndrome-like phenotype, and a less-pronounced effect was observed for fibroblasts from an infantile Refsum patient who was homozygous for a milder PEX1 mutation. By contrast, such changes were not seen for patients with peroxisomal disorders characterized by normal peroxisome abundance and size. Stable overexpression of PEX11ß to induce peroxisome proliferation largely re-established the alignment of peroxisomal structures along peripheral microtubules in both PEX1-null and D-BP-deficient cells. In D-BP-deficient cells, peroxisome division was apparently driven to completion, as induced peroxisomal structures were similar to the spherical parental structures. By contrast, in PEX1-null cells the majority of induced peroxisomal structures were elongated and tubular. These structures were apparently blocked at the division step, despite having recruited DLP1, a protein necessary for peroxisome fission. These findings indicate that the increased size, reduced abundance, and disturbed cytoplasmic distribution of peroxisomal structures in PEX1-null and D-BP-deficient cells reflect defects at different stages in peroxisome proliferation and division, processes that require association of these structures with, and dispersal along, microtubules.Tam Nguyen, Jonas Bjorkman, Barbara C. Paton and Denis I. Cran
2-D Radiative Transfer in Protostellar Envelopes: II. An Evolutionary Sequence
We present model spectral energy distributions, colors, polarization, and
images for an evolutionary sequence of a low-mass protostar from the early
collapse stage (Class 0) to the remnant disk stage (Class III). We find a
substantial overlap in colors and SEDs between protostars embedded in envelopes
(Class 0-I) and T Tauri disks (Class II), especially at mid-IR wavelengths.
Edge-on Class I-II sources show double-peaked spectral energy distributions,
with a short-wavelength hump due to scattered light and the long-wavelength
hump due to thermal emission. These are the bluest sources in mid-IR
color-color diagrams.
Since Class 0 and I sources are diffuse, the size of the aperture over which
fluxes are integrated has a substantial effect on the computed colors, with
larger aperture results showing significantly bluer colors. This causes overlap
in color-color diagrams between all evolutionary states, especially in the
mid-IR. However the near-IR polarization of the Class 0 sources is much higher
than the Class I-II sources, providing a means to separate these evolutionary
states.
We varied the grain properties in the circumstellar envelope, allowing for
larger grains in the disk midplane and smaller in the envelope. We find that
grain growth in disks of Class I sources can be detected at wavelengths greater
than 100 m.
Our image calculations predict that the diffuse emission from edge-on Class I
and II sources should be detectable in the mid-IR with the Space Infrared
Telescope Facility (SIRTF) in nearby star forming regions (out to several
hundred parsecs).Comment: A version with high-resolution images is available at
http://www.astro.wisc.edu/glimpse/glimpsepubs.htm
Achernar: Rapid Polarization Variability as Evidence of Photospheric and Circumstellar Activity
We present the results of a high accuracy ()
polarization monitoring of the Be Star Achernar that was carried out between
July 7th and November 5th, 2006. Our results indicate that, after a near
quiescent phase from 1998 to 2002, Achernar is presently in an active phase and
has built a circumstellar disk. We detect variations both in the polarization
level and position angle in timescales as short as one hour and as long as
several weeks. Detailed modeling of the observed polarization strongly suggests
that the short-term variations originate from discrete mass ejection events
which produce transient inhomogeneities in the inner disk. Long-term
variations, on the other hand, can be explained by the formation of an inner
ring following one or several mass ejection events.Comment: 16 pages, 5 figures, Accepted to Ap
Lem2p (LEM2) and Cmp7p (CHMP7) function in ESCRT-dependent nuclear envelope remodeling in fission yeast
ESCRT‐III proteins have been implicated in sealing the nuclear envelope in mammals, both during nuclear assembly and following mechanical disruption. This sealing process requires the ESCRT‐II/ESCRT‐ III hybrid protein CHMP7 and the AAA ATPase VPS4. It remains unclear, however, how
CHMP7 is recruited to breaches of the nuclear envelope. The fission yeast S. pombe is an attractive genetic model system for investigating this role of the ESCRT pathway because, in fission yeast, the nuclear envelope develops fenestrations that must be closed twice per cell cycle: upon mitotic entry when duplicated spindle pole bodies (SPB) are incorporated into the nuclear envelope and
after a successful cell cycle when the SPBs are ejected back to cytoplasm. Here we report that deletion of fission yeast vps4 leads to severe defects in nuclear morphology and integrity, which causes delayed segregation of duplicated SPBs, asymmetric nuclear bipartition in mitosis, and slow growth. Interestingly, these phenotypes are spontaneously suppressed by loss‐of‐function mutations
that arise in cmp7 (pombe CHMP7) or lem2, a member of the LEM (Lap2‐Emerin‐Man1) family of inner nuclear membrane proteins—implying that all three function in the same pathway. Based on these observations, we hypothesize that Lem2p acts as a nuclear site‐specific adaptor to recruit Cmp7p to the nuclear envelope
In vivo selection of Plasmodium falciparum parasites carrying the chloroquine-susceptible pfcrt K76 allele after treatment with artemether-lumefantrine in Africa
Background: Artemether-lumefantrine (AL) is a major and highly effective artemisinin-based combination therapy that is becoming increasingly important as a new first-line therapy against Plasmodium falciparum malaria. However, recrudescences occurring after AL treatment have been reported. Identification of drug-specific parasite determinants that contribute to treatment failures will provide important tools for the detection and surveillance of AL resistance.
Methods: The findings from a 42-day follow-up efficacy trial in Tanzania that compared AL with sulfadoxinepyrimethamine (SP) were analyzed to identify candidate markers for lumefantrine tolerance/resistance in the chloroquine resistance transporter gene (pfcrt) and multidrug resistance gene 1 (pfmdr1). The findings were corroborated in vitro with genetically modified isogenic P. falciparum parasite lines.
Results: Treatment with AL selected for the chloroquine-susceptible pfcrt K76 allele (P \u3c .0001) and, to a lesser extent, the pfmdr1 N86 (P = .048) allele among recurrent infections. These genotypes were not selected during SP treatment. No pfmdr1 gene amplifications were observed. Isogenic pfcrt-modified parasite lines demonstrated a 2-fold increase in susceptibility to lumefantrine, which was directly attributable to the K76T mutation.
Conclusions: Our findings suggest that the pfcrt K76T mutation is a drug-specific contributor to enhanced P. falciparum susceptibility to lumefantrine in vivo and in vitro, and they highlight the benefit of using AL in areas affected by chloroquine-resistant P. falciparum malaria. © 2009 by the Infectious Diseases Society of America. All rights reserved
pfmdr1 amplification is related to increased Plasmodium falciparum In Vitro sensitivity to the Bisquinoline Piperaquine
The 4-aminoquinoline bisquinoline piperaquine is an important partner drug in one of the presently recommended artemisinin combination therapies. Recent clinical trials have confirmed its high efficacy in combination with dihydroartemisinin. Resistance to piperaquine alone has, however, been documented. Amplification in copy number of the Plasmodium falciparum multidrug resistance locus on chromosome 5, containing the pfmdr1 gene, has been shown to confer resistance to structurally unrelated antimalarials. Through the determination of the 50% inhibitory concentrations (IC(50)s) and IC(90)s for piperaquine and chloroquine in a set of 46 adapted P. falciparum cultures originating from the Thai-Burmese border, we have characterized the regions around the pfmdr1 gene and identified a significant association between the presence of pfmdr1 duplications and enhanced sensitivity to piperaquine (P = 0.005 for IC50 and P = 0.002 for IC90) and chloroquine, reaching statistical significance at IC(90)s (P = 0.026). These results substantiate the potential importance of pfmdr1 copy number amplifications in the efficacy of the combination therapy piperaquine-dihydroartemisinin. It supports the rational use of 4-aminoquinolines and artemisinin-based compounds, as they independently select for mutually incompatible combinations of mutations.Swedish Development Cooperation Agency-Department for Research Cooperation [SWE 2005-0017, SWE 2005-4596, SWE-2007-174, SWE-2005-4027]; Fundacao para a Ciencia e Tecnologia (FCT)/Ministerio da Ciencia e Ensino Superior, Portugal-MCES [SFRH/BPD/76614/2011]; Wellcome Trust of Great Britaininfo:eu-repo/semantics/publishedVersio
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