997 research outputs found
Design of a dual fault tolerant space shuttle payload deployment actuator
As the Shuttle Transportation System (STS) becomes operational, the number and variety of payloads will increase. The need to deploy these cargo elements will require a variety of unique actuator designs, all of which will have to conform with STS safety policy. For those missions where payload operations extend beyond the payload bay door envelope, this policy deems the prevention of door closure as a catastrophic hazard. As such, it must be controlled by independent, primary and backup methods. The combination of these methods must be two fault tolerant. The design of such an actuator is described. The device consists of a single linear ballscrew with two ballnuts, each bellnut forming an independent actuator using the common ballscrew. The design requirements, concept development, hardware configuration, and fault tolerance rationale are highlighted
Mitochondrial DNA paradox: sex-specific genetic structure in a marine mussel despite maternal inheritance and passive dispersal
Background: When genetic structure is identified using mitochondrial DNA (mtDNA), but no structure is identified using biparentally-inherited nuclear DNA, the discordance is often attributed to differences in dispersal potential between the sexes. Results: We sampled the intertidal rocky shore mussel Perna perna in a South African bay and along the nearby open coast, and sequenced maternally-inherited mtDNA (there is no evidence for paternally-inherited mtDNA in this species) and a biparentally-inherited marker. By treating males and females as different populations, we identified significant genetic structure on the basis of mtDNA data in the females only. Conclusions: This is the first study to report sex-specific differences in genetic structure based on matrilineally-inherited mtDNA in a passively dispersing species that lacks social structure or sexual dimorphism. The observed pattern most likely stems from females being more vulnerable to selection in habitats from which they did not originate, which also manifests itself in a male-biased sex ratio. Our results have three important implications for the interpretation of population genetic data. First, even when mtDNA is inherited exclusively in the female line, it also contains information about males. For that reason, using it to identify sex-specific differences in genetic structure by contrasting it with biparentally-inherited markers is problematic. Second, the fact that sex-specific differences were found in a passively dispersing species in which sex-biased dispersal is unlikely highlights the fact that significant genetic structure is not necessarily a function of low dispersal potential or physical barriers. Third, even though mtDNA is typically used to study historical demographic processes, it also contains information about contemporary processes. Higher survival rates of males in non-native habitats can erase the genetic structure present in their mothers within a single generation
Search for nearby Earth analogs I. 15 planet candidates found in PFS data
30 pages, 20 figures, 3 tables, accepted for publication in ApJSThe radial velocity (RV) method plays a major role in the discovery of nearby exoplanets. To efficiently find planet candidates from the data obtained in high-precision RV surveys, we apply a signal diagnostic framework to detect RV signals that are statistically significant, consistent in time, robust in the choice of noise models, and do not correlated with stellar activity. Based on the application of this approach to the survey data of the Planet Finder Spectrograph, we report 15 planet candidates located in 14 stellar systems. We find that the orbits of the planet candidates around HD 210193, 103949, 8326, and 71135 are consistent with temperate zones around these stars (where liquid water could exist on the surface). With periods of 7.76 and 15.14 days, respectively, the planet candidates around star HIP 54373 form a 1:2 resonance system. These discoveries demonstrate the feasibility of automated detection of exoplanets from large RV surveys, which may provide a complete sample of nearby Earth analogs.Peer reviewedFinal Accepted Versio
Climate change, genetics or human choice: why were the shells of mankind’s earliest ornament larger in the Pleistocene than in the Holocene?
The southern African tick shell, Nassarius kraussianus (Dunker, 1846), has been identified as being the earliest known ornamental object used by human beings. Shell beads dated from ~75,000 years ago (Pleistocene era) were found in a cave located on South Africa's south coast. Beads made from N. kraussianus shells have also been found in deposits in this region dating from the beginning of the Holocene era (<10,000 years ago). These younger shells were significantly smaller, a phenomenon that has been attributed to a change in human preference. We investigated two alternative hypotheses explaining the difference in shell size: a) N. kraussianus comprises at least two genetic lineages that differ in size; b) the difference in shell size is due to phenotypic plasticity and is a function of environmental conditions. To test these hypotheses, we first reconstructed the species' phylogeographic history, and second, we measured the shell sizes of extant individuals throughout South Africa. Although two genetic lineages were identified, the sharing of haplotypes between these suggests that there is no genetic basis for the size differences. Extant individuals from the cool temperate west coast had significantly larger shells than populations in the remainder of the country, suggesting that N. kraussianus grows to a larger size in colder water. The decrease in fossil shell size from Pleistocene to Holocene was likely due to increased temperatures as a result of climate change at the beginning of the present interglacial period. We hypothesise that the sizes of N. kraussianus fossil shells can therefore serve as indicators of the climatic conditions that were prevalent in a particular region at the time when they were deposited. Moreover, N. kraussianus could serve as a biomonitor to study the impacts of future climate change on coastal biota in southern Africa
The test case of HD26965: difficulties disentangling weak Doppler signals from stellar activity
We report the discovery of a radial velocity signal that can be interpreted
as a planetary-mass candidate orbiting the K dwarf HD26965, with an orbital
period of 42.3640.015 days, or alternatively, as the presence of residual,
uncorrected rotational activity in the data. Observations include data from
HIRES, PFS, CHIRON, and HARPS, where 1,111 measurements were made over 16
years. Our best solution for HD26965 is consistent with a super-Earth that
has a minimum mass of 6.920.79 M orbiting at a distance of
0.2150.008 AU from its host star. We have analyzed the correlation between
spectral activity indicators and the radial velocities from each instrument,
showing moderate correlations that we include in our model. From this analysis,
we recover a 38 day signal, which matches some literature values of the
stellar rotation period. However, from independent Mt. Wilson HK data for this
star, we find evidence for a significant 42 day signal after subtraction of
longer period magnetic cycles, casting doubt on the planetary hypothesis for
this period. Although our statistical model strongly suggests that the 42-day
signal is Doppler in origin, we conclude that the residual effects of stellar
rotation are difficult to fully model and remove from this dataset,
highlighting the difficulties to disentangle small planetary signals and
photospheric noise, particularly when the orbital periods are close to the
rotation period of the star. This study serves as an excellent test case for
future works that aim to detect small planets orbiting `Sun-like' stars using
radial velocity measurements.Comment: 16 pages, 10 figures, 13 tables, accepted for publication in A
Connectivity between marine reserves and exploited areas in the philopatric reef fish Chrysoblephus laticeps (Teleostei: Sparidae)
"No-take‟ Marine Protected Areas (MPAs) are successful in protecting populations of many exploited fish species, but it is often unclear whether networks of MPAs are adequately spaced to ensure connectivity among reserves, and whether spillover occurs into adjacent exploited areas. Such issues are particularly important in species with low dispersal potential, many of which exist as genetically distinct regional stocks.The roman, Chrysoblephus laticeps, is an overexploited, commercially important fishery species endemic to South Africa. Post-recruits display resident behavior and occupy small home ranges, making C. laticeps a suitable model species to investigate connectivity in marine teleosts with potentially low dispersal ability. We used multilocus data from two types of highly variable genetic markers (mitochondrial DNA control region and microsatellites) to clarify patterns of genetic connectivity and population structure in C. laticeps using samples from two MPAs and several moderately or severely exploited regions. Despite using analytical tools that are sensitive to detect even subtle genetic structure, we found that this species exists as a single, well-mixed stock throughout its core distribution. This finding lends supports to the status of MPAs as an adequate tool for managing overexploited marine teleosts. Even though adult dispersal out of MPAs is limited, the fact that the large adults in these reserves produce exponentially more offspring than their smaller counterparts in exploited areas makes MPAs a rich source of recruits. We nonetheless caution against concluding that the lack of structure identified in C. laticeps and several other southern African teleosts can be considered to be representative of marine teleosts in this region in general. Many such species are represented in more than one marine biogeographic province and may be comprised of regionally-adapted stocks that need to be managed individually
A Six-Planet System Around the Star HD 34445
We present a new precision radial velocity dataset that reveals a
multi-planet system orbiting the G0V star HD 34445. Our 18-year span consists
of 333 precision radial velocity observations, 56 of which were previously
published, and 277 which are new data from Keck Observatory, Magellan at Las
Campanas Observatory, and the Automated Planet Finder at Lick Observatory.
These data indicate the presence of six planet candidates in Keplerian motion
about the host star with periods of 1057, 215, 118, 49, 677, and 5700 days, and
minimum masses of 0.63, 0.17, 0.1, 0.05, 0.12 and 0.38 Jupiter masses
respectively. The HD 34445 planetary system, with its high degree of
multiplicity, its long orbital periods, and its induced stellar radial velocity
half-amplitudes in the range is fundamentally unlike either our own solar system (in which only
Jupiter and Saturn induce significant reflex velocities for the Sun), or the
Kepler multiple-transiting systems (which tend to have much more compact
orbital configurations)Comment: 10 pages, 11 figure
Oceanic dispersal barriers, adaptation and larval retention: an interdisciplinary assessment of potential factors maintaining a phylogeographic break between sister lineages of an African prawn
<p>Abstract</p> <p>Background</p> <p>Genetic breaks separating regional lineages of marine organisms with potentially high broadcasting abilities are generally attributed either to dispersal barriers such as currents or upwelling, or to behavioural strategies promoting self-recruitment. We investigated whether such patterns could potentially also be explained by adaptations to different environmental conditions by studying two morphologically distinguishable genetic lineages of the estuarine mudprawn <it>Upogebia africana </it>across a biogeographic disjunction in south-eastern Africa. The study area encompasses a transition between temperate and subtropical biotas, where the warm, southward-flowing Agulhas Current is deflected away from the coast, and its inshore edge is characterised by intermittent upwelling. To determine how this phylogeographic break is maintained, we estimated gene flow among populations in the region, tested for isolation by distance as an indication of larval retention, and reared larvae of the temperate and subtropical lineages at a range of different temperatures.</p> <p>Results</p> <p>Of four populations sampled, the two northernmost exclusively included the subtropical lineage, a central population had a mixture of both lineages, and the southernmost estuary had only haplotypes of the temperate lineage. No evidence was found for isolation by distance, and gene flow was bidirectional and of similar magnitude among adjacent populations. In both lineages, the optimum temperature for larval development was at about 23°C, but a clear difference was found at lower temperatures. While larvae of the temperate lineage could complete development at temperatures as low as 12°C, those of the subtropical lineage did not complete development below 17°C.</p> <p>Conclusion</p> <p>The results indicate that both southward dispersal of the subtropical lineage inshore of the Agulhas Current, and its establishment in the temperate province, may be limited primarily by low water temperatures. There is no evidence that the larvae of the temperate lineage would survive less well in the subtropical province than in their native habitat, and their exclusion from this region may be due to a combination of upwelling, short larval duration with limited dispersal potential near the coast, plus transport away from the coast of larvae that become entrained in the Agulhas Current. This study shows how methods from different fields of research (genetics, physiology, oceanography and morphology) can be combined to study phylogeographic patterns.</p
Elemental Abundances of Kepler Objects of Interest in APOGEE. I. Two Distinct Orbital Period Regimes Inferred from Host Star Iron Abundances
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has
observed 600 transiting exoplanets and exoplanet candidates from
\textit{Kepler} (Kepler Objects of Interest, KOIs), most with 18 epochs.
The combined multi-epoch spectra are of high signal-to-noise (typically
100) and yield precise stellar parameters and chemical abundances. We
first confirm the ability of the APOGEE abundance pipeline, ASPCAP, to derive
reliable [Fe/H] and effective temperatures for FGK dwarf stars -- the primary
\textit{Kepler} host stellar type -- by comparing the ASPCAP-derived stellar
parameters to those from independent high-resolution spectroscopic
characterizations for 221 dwarf stars in the literature. With a sample of 282
close-in ( days) KOIs observed in the APOGEE KOI goal program, we find a
correlation between orbital period and host star [Fe/H] characterized by a
critical period, = days, below which small
exoplanets orbit statistically more metal-enriched host stars. This effect may
trace a metallicity dependence of the protoplanetary disk inner-radius at the
time of planet formation or may be a result of rocky planet ingestion driven by
inward planetary migration. We also consider that this may trace a metallicity
dependence of the dust sublimation radius, but find no statistically
significant correlation with host and orbital period to
support such a claim.Comment: 18 Pages, Accepted to A
Anisotropic spin freezing in the S=1/2 zigzag ladder compound SrCuO2
Using magnetic neutron scattering we characterize an unusual low temperature
phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed
by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated
interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless
magnetic excitation spectrum indicating spin freezing on a time scale larger
than 200 picoseconds. While the frozen state has long range commensurate
antiferromagnetic order along the chains with the correlation length exceeding
200 lattice periods along the c-axis and a substantial correlation length of
60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2
lattice units are correlated along the b-axis which is the direction of the
frustrated interactions. The frozen magnetic moment of each Cu ion is very
small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature
dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue
that slow dynamics of stripe-like cooperative magnetic defects in tetragonal
a-c planes yield this anisotropic frozen state.Comment: 4 pages, LaTeX, submitted to PR
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