ABSTRACT
Radial velocity surveys are beginning to reach the time baselines required to detect Jupiter analogues, as well as sub‐Saturn mass planets in close orbits. Therefore, it is important to ...understand the sensitivity of these surveys at long periods and low amplitudes. In this paper, I derive analytical expressions for the detectability of planets at both short and long periods, for circular and eccentric orbits. In the long‐period regime, the scaling of the detection threshold with period depends on the desired detection efficiency. The 99 per cent velocity threshold scales as K∝P2∝a3, whereas the 50 per cent velocity threshold scales as K∝P∝a3/2. I suggest an extension of the Lomb–Scargle statistic to Keplerian orbits, and describe how to estimate the false alarm probability associated with a Keplerian fit. I use this Keplerian periodogram to investigate the effect of eccentricity on detectability. At short periods, detectability is reduced for eccentric orbits, mainly due to the sparse sampling of the periastron passage, whereas long‐period orbits are easier to detect on average if they are eccentric because of the steep velocity gradients near periastron. Fitting Keplerian orbits allows the lost sensitivity at short orbital periods to be recovered for e≲ 0.6. However, there remain significant selection effects against eccentric orbits for e≳ 0.6, and the small number of highly eccentric planets discovered so far may reflect this. Finally, I present a Bayesian approach to the periodogram which gives a simple derivation of the probability distributions of noise powers, clarifies why the periodogram is an appropriate way to search for long‐period signals, and emphasizes the equivalence of periodogram and least‐squares techniques.
Braking index measurements of young radio pulsars are all smaller than the value expected for spin-down by magnetic dipole braking. We investigate magnetic field evolution in the neutron star crust ...due to Hall drift as an explanation for observed braking indices. Using numerical simulations and a semi-analytic model, we show that an ≈1014 G quadrupolar toroidal field in the neutron star crust at birth leads to growth of the dipole moment at a rate large enough to agree with measured braking indices. A key factor is the density at which the crust yields to magnetic stresses that build up during the evolution, which sets a characteristic minimum Hall time-scale. The observed braking indices of pulsars with inferred dipole fields of ≲ 1013 G can be explained in this picture, although with a significant octupole component needed in some cases. For the stronger field pulsars, those with B
d ≳ 1013 G, we find that the magnetic stresses in the crust exceed the maximum shear stress before the pulsar reaches its current age, likely quenching the Hall effect. This may have implications for the magnetar activity seen in the high magnetic field radio pulsar PSR J1846−0258. Observations of braking indices may therefore be a new piece of evidence that neutron stars contain subsurface toroidal fields that are significantly stronger than the dipole field, and may indicate that the Hall effect is important in a wider range of neutron stars than previously thought.
Recent observations of Galactic white dwarfs (WDs) with Gaia suggest there is a population of massive crystallizing WDs exhibiting anomalous cooling-the Q branch. While single-particle 22Ne ...sedimentation has long been considered a possible heat source, recent work suggests that 22Ne must separate into clusters, enhancing diffusion, in order for sedimentation to provide heating on the observed timescale. We show definitively that 22Ne cannot separate to form clusters in C/O WDs using molecular dynamics simulations, and we further present a general C/O/Ne phase diagram showing that strong 22Ne enrichment is not achievable for 22Ne abundance 30%. We conclude that the anomalous heating cannot be due to 22Ne cluster sedimentation and that Q branch WDs may have an unusual composition, possibly rich with heavier elements.
Anthropogenic activity is affecting the global climate through the release of greenhouse gases (GHGs) e.g. CO2 and CH4. About a third of anthropogenic GHGs are produced from agriculture, including ...livestock farming and horticulture. A large proportion of the UK's horticultural farming takes place on drained lowland peatlands, which are a source of significant amounts of CO2 into the atmosphere. This study set out to establish whether raising the water table from the currently used −50cm to −30cm could reduce GHGs emissions from agricultural peatlands, while simultaneously maintaining the current levels of horticultural productivity. A factorial design experiment used agricultural peat soil collected from the Norfolk Fens (among the largest of the UK's lowland peatlands under intensive cultivation) to assess the effects of water table levels, elevated CO2, and agricultural production on GHG fluxes and crop productivity of radish, one of the most economically important fenland crops. The results of this study show that a water table of −30cm can increase the productivity of the radish crop while also reducing soil CO2 emissions but without a resultant loss of CH4 to the atmosphere, under both ambient and elevated CO2 concentrations. Elevated CO2 increased dry shoot biomass, but not bulb biomass nor root biomass, suggesting no immediate advantage of future CO2 levels to horticultural farming on peat soils. Overall, increasing the water table could make an important contribution to global warming mitigation while not having a detrimental impact on crop yield.
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•Peat loss is a major issue affecting farmers in Europe, including the UK.•A more sustainable farming should prevent peat loss while maintaining productivity.•This experiment tested the impact of water table on productivity and peat loss.•Raising the water table from −50cm to −30cm increases radish productivity.•Increasing water table to −30cm reduces peat loss.
The post-formation, initial entropy S
i of a gas giant planet is a key witness to its mass-assembly history and a crucial quantity for its early evolution. However, formation models are not yet able ...to predict reliably S
i, making unjustified the use solely of traditional, 'hot-start' cooling tracks to interpret direct-imaging results and calling for an observational determination of initial entropies to guide formation scenarios. Using a grid of models in mass and entropy, we show how to place joint constraints on the mass and initial entropy of an object from its observed luminosity and age. This generalizes the usual estimate of only a lower bound on the real mass, through hot-start tracks. Moreover, we demonstrate that with mass information, e.g. from dynamical-stability analyses or radial velocity, tighter bounds can be set on the initial entropy. We apply this procedure to 2M1207 b and find that its initial entropy is at least 9.2 k
B/baryon, assuming that it does not burn deuterium. For the planets of the HR 8799 system, we infer that they must have formed with S
i > 9.2 k
B/baryon, independent of uncertainties about the age of the star. Finally, a similar analysis for β Pic b reveals that it must have formed with S
i > 10.5 k
B/baryon, using the radial-velocity mass upper limit. These initial entropy values are, respectively, ca. 0.7, 0.5 and 1.5 k
B/baryon higher than the ones obtained from core-accretion models by Marley et al., thereby quantitatively ruling out the coldest starts for these objects and constraining warm starts, especially for β Pic b.
The magnetar Swift J1822.3-1606 entered an outburst phase in 2011 July. Previous X-ray studies of its post-outburst rotational evolution yielded inconsistent measurements of the spin-inferred ...magnetic field. Here we present the timing behavior and flux relaxation from over two years of Swift, RXTE, and Chandra observations following the outburst. We find that the ambiguity in previous timing solutions was due to enhanced spin down that resembles an exponential recovery following a glitch at the outburst onset. After fitting out the effects of the recovery, we measure a long-term spin-down rate of v = (-3.0 + or - 0.3) x 10 super(-16) s super(-2) which implies a dipolar magnetic field of 1.35 x 10 super(13) G, lower than all previous estimates for this source. We also consider the post-outburst flux evolution, and fit it with both empirical and crustal cooling models. We discuss the flux relaxation in the context of both crustal cooling and magnetospheric relaxation models.
Aim: Our aim was to test for transoceanic dispersal in direct-developing species of Onchidella (Mollusca: Gastropoda). As these slugs are ecologically associated with buoyant bull-kelp (Durvillaea), ...and are known to raft, we predicted that they would show evidence of recent genetic connectivity among isolated landmasses. Location: Cool-temperate and subantarctic coasts of the Southern Hemisphere (New Zealand and its subantarctic islands, southern Chile, and the Falkland Islands). Methods: We employed two commonly used genetic marker types -mitochondrial DNA (mtDNA) sequences and amplified fragment length polymorphisms (AFLPs) - to resolve phylogeographical and population genetic structuring of Onchidella populations on a transoceanic scale. Results: Contrary to previous taxonomic views that classify these Southern Ocean Onchidella as at least three separate species, our genetic data suggest that the studied populations belong to a single taxon, whose distribution is widespread. The analyses reveal substantial mtDNA differentiation between several mainland New Zealand populations (e.g. Kaikoura, Purakaunui Bay), apparently consistent with low rates of dispersal. However, subantarctic samples show widely shared mitochondrial haplotypes and AFLP similarity between distant populations. Main conclusions: Our study suggests recent common ancestry for geographically separated New Zealand subantarctic and South American populations of Onchidella. Their transoceanic similarities are likely to have been driven by passive rafting mediated by circumpolar ocean currents, as previously documented for several epifaunal species strongly associated with buoyant Durvillaea.
OBJECTIVETo evaluate the role of genetic variation at the DMPK locus on symptomatic diversity in 250 adult, ambulant patients with myotonic dystrophy type 1 (DM1) recruited to the Observational ...Prolonged Trial in Myotonic Dystrophy Type 1 to Improve Quality of Life—Standards, a Target Identification Collaboration (OPTIMISTIC) clinical trial.
METHODSWe used small pool PCR to correct age at sampling biases and estimate the progenitor allele CTG repeat length and somatic mutational dynamics, and AciI digests and repeat primed PCR to test for the presence of variant repeats.
RESULTSWe confirmed disease severity is driven by progenitor allele length, is further modified by age, and, in some cases, sex, and that patients in whom the CTG repeat expands more rapidly in the soma develop symptoms earlier than predicted. We revealed a key role for variant repeats in reducing disease severity and quantified their role in delaying age at onset by approximately 13.2 years (95% confidence interval 5.7–20.7, 2-tailed t test t = −3.7, p = 0.0019).
CONCLUSIONSCareful characterization of the DMPK CTG repeat to define progenitor allele length and presence of variant repeats has increased utility in understanding clinical variability in a trial cohort and provides a genetic route for defining disease-specific outcome measures, and the basis of treatment response and stratification in DM1 trials.
Global peatlands store more carbon than is naturally present in the atmosphere
. However, many peatlands are under pressure from drainage-based agriculture, plantation development and fire, with the ...equivalent of around 3 per cent of all anthropogenic greenhouse gases emitted from drained peatland
. Efforts to curb such emissions are intensifying through the conservation of undrained peatlands and re-wetting of drained systems
. Here we report eddy covariance data for carbon dioxide from 16 locations and static chamber measurements for methane from 41 locations in the UK and Ireland. We combine these with published data from sites across all major peatland biomes. We find that the mean annual effective water table depth (WTD
; that is, the average depth of the aerated peat layer) overrides all other ecosystem- and management-related controls on greenhouse gas fluxes. We estimate that every 10 centimetres of reduction in WTD
could reduce the net warming impact of CO
and CH
emissions (100-year global warming potentials) by the equivalent of at least 3 tonnes of CO
per hectare per year, until WTD
is less than 30 centimetres. Raising water levels further would continue to have a net cooling effect until WTD
is within 10 centimetres of the surface. Our results suggest that greenhouse gas emissions from peatlands drained for agriculture could be greatly reduced without necessarily halting their productive use. Halving WTD
in all drained agricultural peatlands, for example, could reduce emissions by the equivalent of over 1 per cent of global anthropogenic emissions.
Nuclear pasta, with nonspherical shapes, is expected near the base of the crust in neutron stars. Large-scale molecular dynamics simulations of pasta show long lived topological defects that could ...increase electron scattering and reduce both the thermal and electrical conductivities. We model a possible low-conductivity pasta layer by increasing an impurity parameter Q_{imp}. Predictions of light curves for the low-mass x-ray binary MXB 1659-29, assuming a large Q_{imp}, find continued late time cooling that is consistent with Chandra observations. The electrical and thermal conductivities are likely related. Therefore, observations of late time crust cooling can provide insight on the electrical conductivity and the possible decay of neutron star magnetic fields (assuming these are supported by currents in the crust).
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