Low Density Threshold Research Articles

Real and counterfeit cores: how feedback expands haloes and disrupts tracers of inner gravitational potential in dwarf galaxies

ABSTRACT The tension between the diverging density profiles in Lambda cold dark matter simulations and the constant-density inner regions of observed galaxies is a long-standing challenge known as the ‘core–cusp’ problem. We demonstrate that the SMUGGLE galaxy formation model implemented in the arepo moving mesh code forms constant-density cores in idealized dwarf galaxies of M⋆ ≈ 8 × 107 Msun with initially cuspy dark matter (DM) haloes of M200 ≈ 1010 Msun. Identical initial conditions run with an effective equation of state interstellar medium model preserve cuspiness. Literature on the subject has pointed to the low density threshold for star formation, ρth, in such effective models as an obstacle to baryon-induced core formation. Using a SMUGGLE run with equal ρth, we demonstrate that core formation can proceed at low density thresholds, indicating that ρth is insufficient on its own to determine whether a galaxy develops a core. We reaffirm that the ability to resolve a multiphase interstellar medium at sufficiently high densities is a more reliable indicator of core formation than any individual model parameter. In SMUGGLE, core formation is accompanied by large degrees of non-circular motion, with gas rotational velocity profiles that consistently fall below the circular velocity $v_\text{circ} = \sqrt{GM/R}$ out to ∼2 kpc. Asymmetric drift corrections help recover the average underlying DM potential for some of our less efficient feedback runs, but time-variations in the instantaneous azimuthal gas velocity component are substantial, highlighting the need for careful modelling in the inner regions of dwarfs to infer the true distribution of DM.

Trypillia Mega-Sites: Neither Urban nor Low-Density?

At the end of the fifth millennium bc, some of the largest settlements of the time emerged on the Pontic forest steppe. Some scholars proposed to include these mega-sites into the concept of low-density urbanism. These characterizations were, however, limited by a single-site perspective. Now, with ongoing surveys and radiocarbon-based internal settlement chronologies it was possible for the first time to trace the trajectory of the whole range of Trypillia mega-sites. It was found that they did not follow a low-density trajectory. Instead, they show a high volatility and appear to bounce off the low-density threshold. While the crossing of the 100 ha threshold on the Interaction-Communication Matrix suggests a potential urban status of these sites, no evidence for a functional rural-urban divide was found.

First demonstration of full ELM suppression in low input torque plasmas to support ITER research plan using n = 4 RMP in EAST

Full suppression of type-I edge localized modes (ELMs) using n = 4 resonant magnetic perturbations (RMPs) as planned for ITER has been demonstrated for the first time (n is the toroidal mode number of the applied RMP). This is achieved in EAST plasmas with low input torque and tungsten divertor, and the target plasma for these experiments in EAST is chosen to be relevant to the ITER Q = 10 operational scenario, thus also addressing significant scenario issues for ITER. In these experiments the lowest neutral beam injection (NBI) input torque is around T NBI ∼ 0.44 Nm, which extrapolates to around 14 Nm in ITER (compared to a total torque input of 35 Nm when 33 MW of NBI are used for heating). The q 95 is around 3.6 and normalized plasma beta β N ∼ 1.5–1.8, similar to that in the ITER Q = 10 scenario. Suppression windows in both q 95 and plasma density are observed; in addition, lower plasma rotation is found to be favourabe to access ELM suppression. ELM suppression is maintained with line averaged density up to 60%n GW (Greenwald density limit) by feedforward gas fuelling after suppression is achieved. It is interesting to note that in addition to an upper density, a low density threshold for ELM suppression of 40%n GW is also observed. In these conditions energy confinement does not significantly drop (<10%) during ELM suppression when compared to the ELMy H-mode conditions, which is much better than previous results using low n (n = 1 and 2) RMPs in higher q 95 regimes. In addition, the core plasma tungsten concentration is clearly reduced during ELM suppression demonstrating an effective impurity exhaust. MHD response modelling using the MARS-F code shows that edge magnetic field stochasticity has a peak at q 95 ∼ 3.65 for the odd parity configuration, which is consistent to the observed suppression window around 3.6–3.75. These results expand the physical understanding of ELM suppression and demonstrate the effectiveness of n = 4 RMPs for reliable control ELMs in future ITER high Q plasma scenarios with minimum detrimental effects on plasma confinement.

European native oysters and associated species richness in the presence of non‐native species in a southern North Sea estuary complex

AbstractThere are growing calls to restore populations of European native oysters (Ostrea edulis), on the premise that restored populations will support a range of ecosystem services with an emphasis placed on restored oyster habitats promoting biological diversity, however benefits associated with naturally occurring O. edulis remain unclear. We undertook biannual surveys in the Blackwater, Crouch, Roach and Colne Estuaries Marine Conservation Zone (BCRC.MCZ), a highly sedimented estuary complex in the southern North Sea, to investigate links between natural densities of O. edulis (0–4.2 m−2), the prevalence of other dominant habitat features such as non‐native slipper limpet (Crepidula fornicata), dead shell abundance and epibenthic macroinvertebrate species richness. Increased epibenthic species richness was associated with O. edulis, even at densities below the OSPAR Commission recognized definition of an oyster bed (5 oysters m−2). Our analysis predicts increased associated species richness with density of native oysters (e.g., +1.6 additional species at 1 oyster m−2 or + 2.8 species at 5 oysters m−2), but only in areas with lower density of C. fornicata. Where C. fornicata are at higher density, the potential benefits of oyster restoration for associated species were curtailed. This may explain the observed asymptotic relationship between oyster density and diversity at 1 oyster m−2. In these and other high Crepidula density areas we recommend extending native oyster habitat even at low density. This may be of particular interest to areas with the protozoan oyster parasite Bonamia ostreae, which spreads more easily in high‐density areas. These lower density thresholds should also be considered for future management decisions—closing harvests so they do not reduce density further and impair biodiversity services of the habitats. In conclusion, while C. fornicata may be a useful oyster settlement substrate, we find that it limits the potential increases in associated species gains of oyster restoration.

Experimental study of detachment density threshold in L-mode plasmas on EAST

One of the critical challenges for long pulse operation of ITER and future fusion reactors is the excessively high heat and particle fluxes on the divertor targets. The divertor detachment offers an effective way for the control of steady state heat flux and erosion at the divertor target. Dedicated experiments have been systematically performed for both H-mode and L-mode plasmas on EAST with ITER-like W divertor in the last two years to further advance detachment physics understanding. The detachment is identified by the rollover of divertor peak particle flux near the strike point during the density ramping up, which correlates with the reduction of electron temperature down to 5 eV, the increase of Dδ/Dα ratio and radiated power. It has been demonstrated that the more closed divertor has a lower density threshold at the onset of detachment on EAST. Furthermore, the detachment density threshold will be reduced when the strike point moves towards the divertor corner. The onset of detachment exhibits a strong asymmetry between inner and outer targets, with the inner target accessing detachment at lower density due to the lower electron temperature, as expected. The effects of the heating power and plasma current on the detachment onset were also investigated. The experimental results show that the density at detachment onset is lower in L-mode relative to H-mode, and increases with the heating power. The detachment density threshold normalized to the Greenwald density limit is lower at a high plasma current.

Towards closing the gap of the global surgeon, anaesthesiologist, and obstetrician workforce: thresholds and projections towards 2030

BackgroundBillions of people are without access to surgical care, in part because of the inequitable distribution of the surgical workforce. Drawing on recently collected data for the number of surgeons, anaesthesiologists, and obstetricians worldwide, we sought to show their global maldistribution by identifying thresholds of surgical workforce densities, and by calculating the number of additional providers needed to reach those thresholds. MethodsFrom the WHO Global Surgical Workforce Database, national data for the number of specialist surgeons, anaesthesiologists, and obstetricians per 100 000 population (density) were compared with the number of maternal deaths per 100 000 live births (maternal mortality ratio; MMR) in WHO member countries. A regression line was fit between density of specialist surgeons, anaesthesiologists, and obstetricians and the logarithm of MMR, and we explored the correlation for an upper and a lower density threshold. Based on previous estimates of the global volume of surgical procedures, a global average productivity per specialist was derived. We then multiplied the average productivity with the derived upper and lower threshold densities, and compared these numbers to previously estimated global need of surgical procedures (4664 procedures per 100 000 population). Finally, the numbers of additional providers needed to reach the thresholds in countries with a density below the respective threshold were calculated. FindingsEach 10-unit increase in density of surgeons, anaesthesiologists, and obstetricians, corresponded to a 13·1% decrease in MMR (95% CI 11·3–14·8). We saw particularly steep improvements in MMR from 0 to roughly 20 per 100 000 population. Above roughly 40 per 100 000 population, higher density was associated with relatively smaller improvements in MMR. These arbitrary thresholds of 20 and 40 specialists per 100 000 corresponded with a volume of surgery of 2917 and 5834 procedures per 100 000 population, respectively, and were symmetrically distributed around the estimated global need of 4664 surgical procedures per 100 000 population. Our density thresholds are slightly higher than the current average in lower-middle income countries (16 per 100 000) and upper-middle-income countries (38 per 100 000), respectively. To reach the threshold of at least 20 per 100 000 in each country today, another 440 231 (IQR 438 900–443 245) providers would be needed. To reach 40 per 100 000, 1 110 610 (IQR 1 095 376–1 183 525) providers would be needed. InterpretationAssuming uniform productivity, a global surgical workforce between 20 and 40 per 100 000 would suffice to provide the world's missing surgical procedures. We concede that causality cannot be implied, but our results suggest that countries with a workforce density above certain thresholds have better health outcomes. Although the thresholds cannot be interpreted as a minimum standard, they are useful to characterise the global surgical workforce and its deficits. Such thresholds could also be used as markers for health system capacity. FundingNone.

Effect of vegetation on density thresholds of adult desert locust gregarization from survey data in Mauritania

AbstractIn the desert locust, Schistocerca gregaria (Forskål) (Orthoptera: Acrididae), the threshold density inducing the gregarization phenomenon has never been determined under natural conditions. The influence of environmental factors on this phenomenon has been studied mostly in controlled environments. Based on data collected during several years by the survey teams of the National Center for Locust Control in Mauritania, we analyzed the influence of locust density, vegetation cover, and vegetation status on the probability of observing gregarious locusts. We assumed that a probability to observe gregarious locusts of 0.5 corresponded to the density threshold of gregarization. The results showed in detail the change in the threshold of gregarization according to the cover and status of the vegetation. Low cover and dry vegetation led to a low density threshold of gregarization probably due to high probability of individuals to touch each other. Dense and green vegetation favored a high threshold of gregarization probably due to a dispersion of the individuals and a low probability of individual encounters. These findings should help the management of locusts and decision making during control operations.

Disc heating: comparing the Milky Way with cosmological simulations

We present the analysis of a suite of simulations run with different particle-and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion - age relation for disc stars at z=0 and find that four of the simulations are more consistent with observations by Holmberg et al. (2008) in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett (2001) observations that suggest a "saturation" in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies' evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger - i.e. once the disc has formed. All of the simulations are significantly "hotter" than the Milky Way disc; on top of the effects of mergers, we find a "floor" in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical codes.

Stock Enhancement and Carrying Capacity of Blue Crab Nursery Habitats in Chesapeake Bay

Declining populations of the blue crab, Callinectes sapidus, may be suitable for enhancement with hatchery-reared juveniles. For three years, field experiments were conducted to determine if shallow, unvegetated, marsh-fringed nursery habitats in lower Chesapeake Bay are below carrying capacity and thus capable of supporting additional juvenile crabs. The field experiments included sampling of wild and hatchery-reared juvenile blue crabs and their major benthic prey, and manipulative experiments to assess the survival of juvenile crabs and the abundance of benthic prey in enhanced and control coves. Densities of the clam Macoma balthica, a major prey of the blue crab, were initially high in enhanced coves and decreased to a level equivalent to that in control coves by the end of the experiment, but always remained well above a low-density threshold. Survival of juvenile crabs was lowest where clam densities were highest, suggesting that alternative prey did not deter predation on juvenile crabs, but instead led to higher densities of cannibalistic crabs through bottom-up control. Crab densities correlated with densities of major benthic prey and prey density never dropped below a low-density threshold. Marsh-fringed nurseries are apparently below carrying capacity for the blue crab and could be enhanced through the release of hatchery-reared juveniles.

Microinstability threshold of an axially focused neutralized ion beam

The response to electrostatic perturbations, φ=φ̂(r,z)ei(lθ−ωt), is investigated for the Migma Exyder, a proposed disk-shaped device where ions are trapped in the periphery and focused towards the axis [Nucl. Instrum. Methods A 271, 214 (1988)]. The ions are modeled as a relativistic monoenergetic distribution with a small spread in the impact parameter (the distance of closest approach to the axis) and with the axial extent ΔZ much shorter than the radial extent R0. It is found that odd l modes do not give rise to a low-frequency response. For l=0, an energy principle for low frequency ion–ion interactions is formulated, and it is shown that the extremizing perturbation is located in a narrow region surrounding the axis. The threshold is proportional to the total number of stored particles, N=4ε0(π−2)γ(γ2−1) ×mic2R0/q2i, where γ is the usual relativistic factor, and this stability condition therefore allows for a high density on axis in highly focused systems. Compared to a cylindrical shape, a disk-shaped plasma gives an improvement in the density by the factor R0/ΔZ. The threshold in particle number for the relativistic ion–ion two-stream instability scales as γ3 and thus with relativistic effects the instability is further suppressed. Inclusion of the electron response increases the threshold even further. Preliminary investigations indicate that finite frequency l=0 modes can give a lower threshold than the zero frequency case, at least for nonrelativistic beam energies. These finite frequency instabilities can arise from coupling between positive and negative energy modes. Finite l modes arise if electrons are magnetized at the axis of the device. A balance between the electron field energy and the cross field electron convection establishes discrete modes. When these modes resonate with the ion bounce motion or azimuthal drift motion, bands of instability arise with relatively low density thresholds compared to the l=0 case.

Low Beta Rigid Mode Stability Criterion for an Arbitrary Larmor Radius Plasma

The low beta flute axisymmetric dispersion relation for rigid displacement perturbation of plasma equilibria with arbitrary Larmor radius particles and field line radius of curvature large compared to the plasma radius is derived. The equilibrium particle orbits are characterized by two constants of motion, energy and angular momentum, and a third adiabatic invariant derived from the rapid radial motion. The Vlasov equation is integrated, assuming that the mode frequency, axial “bounce” frequency, and particle drift frequency are small compared to the cyclotron frequency, and it is demonstrated that the plasma response to a rigid perturbation has a universal character independent of Larmor radius. As a result the interchange instability is the same as that predicted from conventional MHD theory. However, a new prediction, more optimistic than earlier work, is found for the low density threshold of systems like Migma, which are disc-shaped, that is, the axial extent Δz is less than the radial extent r0. The stability criterion for negative field line curvature χ where ωpi is the mean ion plasma frequency, Ωi, the ion cyclotron frequency, δh the hot particle to total ion particle ratio, χ/r is the ratio o f the field line curvature and the midplane radius, which in our model is treated as a constant over the entire plasma, χh is the Larmor radius of the energetic species, Z is approximately given by an interpolation formula which goes over to the correct limits if either . For Δz/r0 ≪ 1 the stability criterion is determined by the total particle number. Whereas the older theory (Δz/r0 ≫ 1) predicted instability at about the densities achieved in actual Migma experiments, the present theory (Δz/r0 ≪ 1) indicates that the experimental results are for plasmas with particle number below the interchange threshold.

Cooperation, optimal density and low density thresholds: yet another modification of the logistic model.

A modification of the logistic model is proposed which incorporates the principles of optimal density and cooperation. Three components have been added defining effectivity, "saturation" and exponentiality of cooperation. By adequate choice of constants a wide array of population characteristics may be represented including threshholds of growth. The main application of the model concerns, low density problems such as colonization and extinction.