137 research outputs found
Asynchronous accretion can mimic diverse white dwarf pollutants I: core and mantle fragments
Polluted white dwarfs serve as astrophysical mass spectrometers - their
photospheric abundances are used to infer the composition of planetary objects
that accrete onto them. We show that due to asymmetries in the accretion
process, the composition of the material falling onto a star may vary with time
during the accretion of a single planetary body. Consequently, the
instantaneous photospheric abundances of white dwarfs do not necessarily
reflect the bulk composition of their pollutants, especially when their
diffusion timescales are short. In particular, we predict that when an asteroid
with an iron core tidally disrupts around a white dwarf, a larger share of its
mantle is ejected, and that the core/mantle fraction of the accreting material
varies with time during the event. Crucially, this implies that the core
fraction of differentiated pollutants cannot be determined for white dwarfs
with short diffusion timescales, which sample only brief episodes of longer
accretion processes. The observed population of polluted white dwarfs backs up
the proposed theory. More white dwarfs have accreted material with high Fe/Ca
than low Fe/Ca relative to stellar abundance ratios, indicating the ejection of
mantle material. Additionally, we find tentative evidence that the accretion
rate of iron decreases more rapidly than that of magnesium or calcium, hinting
at variability of the accreted composition. Further corroboration of the
proposed theory will come from the up-coming analysis of large samples of young
white dwarfs.Comment: Accepted for publication in MNRAS. Part one of a series of two
papers. Comments and questions welcom
Using warm dust to constrain unseen planets
Cold outer debris belts orbit a significant fraction of stars, many of which
are planet-hosts. Radiative forces from the star lead to dust particles leaving
the outer belts and spiralling inwards under Poynting-Robertson drag. We
present an empirical model fitted to N-body simulations that allows the fate of
these dust particles when they encounter a planet to be rapidly calculated.
High mass planets eject most particles, whilst dust passes low mass planets
relatively unperturbed. Close-in, high mass planets (hot Jupiters) are best at
accreting dust. The model predicts the accretion rate of dust onto planets
interior to debris belts, with mass accretions rates of up to hundreds of
kilograms per second predicted for hot Jupiters interior to outer debris belts,
when collisional evolution is also taken into account. The model can be used to
infer the presence and likely masses of as yet undetected planets in systems
with outer belts. The non-detection of warm dust with the Large Binocular
Telescope Interferometer (LBTI) around Vega could be explained by the presence
of a single Saturn mass planet, or a chain of lower mass planets. Similarly,
the detection of warm dust in such systems implies the absence of planets above
a quantifiable level, which can be lower than similar limits from direct
imaging. The level of dust detected with LBTI around beta Leo can be used to
rule out the presence of planets more massive than a few Saturn masses outside
of ~5au
Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT
Vega and Fomalhaut, are similar in terms of mass, ages, and global debris
disk properties; therefore, they are often referred as "debris disk twins". We
present Spitzer 10-35 um spectroscopic data centered at both stars, and
identify warm, unresolved excess emission in the close vicinity of Vega for the
first time. The properties of the warm excess in Vega are further characterized
with ancillary photometry in the mid infrared and resolved images in the
far-infrared and submillimeter wavelengths. The Vega warm excess shares many
similar properties with the one found around Fomalhaut. The emission shortward
of ~30 um from both warm components is well described as a blackbody emission
of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show
such an unresolved warm dust using the same approach. These warm components may
be analogous to the solar system's zodiacal dust cloud, but of far greater. The
dust temperature and tentative detections in the submillimeter suggest the warm
excess arises from dust associated with a planetesimal ring located near the
water-frost line and presumably created by processes occurring at similar
locations in other debris systems as well. We also review the properties of the
2 um hot excess around Vega and Fomalhaut, showing that the dust responsible
for the hot excess is not spatially associated with the dust we detected in the
warm belt. We suggest it may arise from hot nano grains trapped in the magnetic
field of the star. Finally, the separation between the warm and cold belt is
rather large with an orbital ratio >~10 in all four systems. In light of the
current upper limits on the masses of planetary objects and the large gap, we
discuss the possible implications for their underlying planetary architecture,
and suggest that multiple, low-mass planets likely reside between the two belts
in Vega and Fomalhaut.Comment: 14 pages, accepted for publication in Ap
Asynchronous accretion can mimic diverse white dwarf pollutants II: water content
Volatiles, notably water, are key to the habitability of rocky planets. The
presence of water in planetary material can be inferred from the atmospheric
oxygen abundances of polluted white dwarfs, but this interpretation is often
complex. We study the accretion process, and find that ices may sublimate and
accrete before more refractory minerals reach the star. As a result, a white
dwarf's relative photospheric abundances may vary with time during a single
accretion event, and do not necessarily reflect the bulk composition of a
pollutant. We offer two testable predictions for this hypothesis: 1. cooler
stars will more often be inferred to have accreted wet pollutants, and 2. there
will be rare occurrences of accretion events with inferred volatile levels far
exceeding those of pristine comets. To observationally test these predictions,
we statistically constrain the water content of white dwarf pollutants. We find
that in the current sample, only three stars show statistically significant
evidence of water at the 2 level, due to large typical uncertainties in
atmospheric abundances and accretion states. In the future, an expanded sample
of polluted white dwarfs with hydrogen-dominated atmospheres will allow for the
corroboration of our theoretical predictions. Our work also shows the
importance of interpreting pollutant compositions statistically, and emphasizes
the requirement to reduce uncertainties on measured abundances to allow for
statistically significant constraints on their water content.Comment: Accepted for publication in MNRAS. Part two of a series of two
papers. Comments and questions welcom
Infrared Variability of Two Dusty White Dwarfs
The most heavily polluted white dwarfs often show excess infrared radiation
from circumstellar dust disks, which are modeled as a result of tidal
disruption of extrasolar minor planets. Interaction of dust, gas, and
disintegrating objects can all contribute to the dynamical evolution of these
dust disks. Here, we report on two infrared variable dusty white dwarfs, SDSS
J1228+1040 and G29-38. For SDSS J1228+1040, compared to the first measurements
in 2007, the IRAC [3.6] and [4.5] fluxes decreased by 20% by 2014 to a level
also seen in the recent 2018 observations. For G29-38, the infrared flux of the
10 m silicate emission feature became 10% stronger between 2004 and 2007,
We explore several scenarios that could account for these changes, including
tidal disruption events, perturbation from a companion, and runaway accretion.
No satisfactory causes are found for the flux drop in SDSS J1228+1040 due to
the limited time coverage. Continuous tidal disruption of small planetesimals
could increase the mass of small grains and concurrently change the strength of
the 10 m feature of G29-38. Dust disks around white dwarfs are actively
evolving and we speculate that there could be different mechanisms responsible
for the temporal changes of these disks.Comment: ApJ, in pres
Sterol 3β-glucosyltransferase biocatalysts with a range of selectivities, including selectivity for testosterone
The main objectives of this work were to characterise a range of purified recombinant sterol 3β-glucosyltransferases and show that rational sampling of the diversity that exists within sterol 3β-glucosyltransferase sequence space can result in a range of enzyme selectivities. In our study the catalytically active domain of the Saccharomyces cerevisiae 3β-glucosyltransferase was used to mine putative sterol 3β-glucosyltransferases from the databases. Selected diverse sequences were expressed in and purified from Escherichia coli and shown to have different selectivities for the 3β-hydroxysteroids ergosterol and cholesterol. Surprisingly, three enzymes were also selective for testosterone, a 17β-hydroxysteroid. This study therefore reports for the first time sterol 3β-glucosyltransferases with selectivity for both 3β- and 17β-hydroxysteroids and is also the first report of recombinant 3β-glucosyltransferases with selectivity for steroids with a hydroxyl group at positions other than C-3. These enzymes could therefore find utility in the pharmaceutical industry for the green synthesis of a range of glycosylated compounds of medicinal interest
Human CEACAM1 is targeted by a Streptococcus pyogenes adhesin implicated in puerperal sepsis pathogenesis
Life-threatening bacterial infections in women after childbirth, known as puerperal sepsis, resulted in classical epidemics and remain a global health problem. While outbreaks of puerperal sepsis have been ascribed to Streptococcus pyogenes, little is known about disease mechanisms. Here, we show that the bacterial R28 protein, which is epidemiologically associated with outbreaks of puerperal sepsis, specifically targets the human receptor CEACAM1. This interaction triggers events that would favor development of puerperal sepsis, including adhesion to cervical cells, suppression of epithelial wound repair and subversion of innate immune responses. High-resolution structural analysis showed that an R28 domain with IgI3-like fold binds to the N-terminal domain of CEACAM1. Together, these findings demonstrate that a single adhesin-receptor interaction can drive the pathogenesis of bacterial sepsis and provide molecular insights into the pathogenesis of one of the most important infectious diseases in medical history
A hidden crisis: strengthening the evidence base on the current failure of rural groundwater supplies
New ambitious international goals for universal access to safe drinking water depend critically on the ability of development partners to accelerate and sustain access to groundwater. However, available evidence (albeit fragmented and methodologically unclear) indicates >30% of new groundwater-based supplies are non-functional within a few years of construction. Critically, in the absence of a significant systematic evidence base or analysis on supply failures, there is little opportunity to learn from past mistakes, to ensure more sustainable services can be developed in the future. This work presents a new and robust methodology for investigating the causes of non-functionality, developed by an interdisciplinary team as part of a UK-funded development research project. The approach was successfully piloted within a test study in NE Uganda, and forms a basis for future research to develop a statistically significant systematic evidence base to unravel the underlying causes of failur
Can Eccentric Debris Disks Be Long-lived? A First Numerical Investigation and Application to Zeta(exp 2) Reticuli
Context. Imaging of debris disks has found evidence for both eccentric and offset disks. One hypothesis is that they provide evidence for massive perturbers, for example, planets or binary companions, which sculpt the observed structures. One such disk was recently observed in the far-IR by the Herschel Space Observatory around Zeta2 Reticuli. In contrast with previously reported systems, the disk is significantly eccentric, and the system is several Gyr old. Aims. We aim to investigate the long-term evolution of eccentric structures in debris disks caused by a perturber on an eccentric orbit around the star. We hypothesise that the observed eccentric disk around Zeta2 Reticuli might be evidence of such a scenario. If so, we are able to constrain the mass and orbit of a potential perturber, either a giant planet or a binary companion. Methods. Analytical techniques were used to predict the effects of a perturber on a debris disk. Numerical N-body simulations were used to verify these results and further investigate the observable structures that may be produced by eccentric perturbers. The long-term evolution of the disk geometry was examined, with particular application to the Zeta2 Reticuli system. In addition, synthetic images of the disk were produced for direct comparison with Herschel observations. Results. We show that an eccentric companion can produce both the observed offsets and eccentric disks. These effects are not immediate, and we characterise the timescale required for the disk to develop to an eccentric state (and any spirals to vanish). For Zeta2 Reticuli, we derive limits on the mass and orbit of the companion required to produce the observations. Synthetic images show that the pattern observed around Zeta2 Reticuli can be produced by an eccentric disk seen close to edge-on, and allow us to bring additional constraints on the disk parameters of our model (disk flux and extent). Conclusions. We conclude that eccentric planets or stellar companions can induce long-lived eccentric structures in debris disks. Observations of such eccentric structures thus provide potential evidence of the presence of such a companion in a planetary system. We considered the specific example of Zeta2 Reticuli, whose observed eccentric disk can be explained by a distant companion (at tens of AU) on an eccentric orbit (ep greater than approx. 0.3)
UPGro Hidden Crisis Research Consortium : unravelling past failures for future success in Rural Water Supply. Survey 1 Results, Country Report Ethiopia
Statistics on the functionality of water points from the Hidden Crisis project in Ethiopia are presented. The survey, undertaken in 2016, was focussed on boreholes equipped with handpumps (HPBs) within igneous volcanic rocks in the Ethiopian Highlands (covering approximately 400 Woredas). A stratified two-stage sampling strategy was adopted, and a tiered definition of functionality developed which enabled more nuanced definitions to be reported. The results from the survey indicate:
• 82% of HPBs were working on the day of the survey (similar to national statistics)
• 59% of HPBs passed the design yield of 10 litres per minute
• 45% passed the design yield and also experienced < 1 month downtime within a year.
• 28% of HPB’s which passed the design yield and reliability, also passed WHO standards of water quality indicators (TTCs and inorganic chemistry).
The results of the survey indicate the utility of carrying out more detailed assessments of functionality to help unpack national statistics. A linked survey of the performance of the water management arrangements at water points showed that for 85% of the sites water management arrangements were judged to be functional or highly functional.
The Hidden Crisis project is a 4 year (2015-19) research project aimed at developing a robust evidence base and understanding of the complex and multi-faceted causes which underlie the current high failure rates of many new groundwater supplies in Africa, so that future WASH investments can be more sustainable. The project is being undertaken by an interdisciplinary team of established researchers in physical and social sciences from the UK, Ethiopia, Uganda, Malawi and Australia, led by the British Geological Survey
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