scholarly journals Warm millimetre dust in protoplanetary discs near massive stars

2021 ◽  
Author(s):  
Thomas J Haworth
Keyword(s):  
Massive Stars ◽  
External Source ◽  
Central Star ◽  
Equilibrium Temperature ◽  
Initial Assessment ◽  
Snow Line ◽  
Close Proximity ◽  
Dust Mass ◽  
External Sources ◽  
Protoplanetary Discs

Abstract Dust plays a key role in the formation of planets and its emission also provides one of our most accessible views of protoplanetary discs. If set by radiative equilibrium with the central star, the temperature of dust in the disc plateaus at around 10 − 20 K in the outer regions. However sufficiently nearby massive stars can heat the outer disc to substantially higher temperatures. In this paper we study the radiative equilibrium temperature of discs in the presence of massive external sources and gauge the effect that it has on millimetre dust mass estimates. Since millimetre grains are not entrained in any wind we focus on geometrically simple 2D-axisymmetric disc models using radiative transfer calculations with both the host star and an external source. Recent surveys have searched for evidence of massive stars influencing disc evolution using disc properties as a function of projected separation. In assuming a disc temperature of 20 K for a disc a distance D from a strong radiation source, disc masses are overestimated by a factor that scales with D−1/2 interior to the separation that external heating becomes important. This could significantly alter dust mass estimates of discs in close proximity to θ1C in the Orion Nebular Cluster. We also make an initial assessment of the effect upon snow lines. Within a parsec of an O star like θ1C a CO snow line no longer exists, though the water snow line is virtually unaffected except for very close separations of ≤0.01 pc.

scholarly journals Clumps in stellar winds

2014 ◽  
Vol 1 ◽  
pp. 39-41 ◽  
Author(s):  
J. S. Vink
Keyword(s):  
Mass Loss ◽  
Massive Stars ◽  
Stellar Winds ◽  
Present Evidence ◽  
Close Proximity ◽  
Stellar Photosphere ◽  
Loss Rates

Abstract. We discuss the origin and quantification of wind clumping and mass–loss rates (Ṁ), particularly in close proximity to the Eddington (Γ) limit, relevant for very massive stars (VMS). We present evidence that clumping may not be the result of the line-deshadowing instability (LDI), but that clumps are already present in the stellar photosphere.

Outcomes following non-life-threatening orthopaedic trauma: Why are they considered to be so poor?

Trauma ◽  
2016 ◽  
Vol 19 (2) ◽  
pp. 133-138
Author(s):  
Rachele Quested ◽  
Scott Sommerville ◽  
Michael Lutz
Keyword(s):  
Risk Factors ◽  
Injury Severity ◽  
Relevant Literature ◽  
Educational Status ◽  
External Source ◽  
Initial Assessment ◽  
Future Research ◽  
Orthopaedic Trauma ◽  
Passive Coping ◽  
Physical Consequences

The purpose of this review article is to assess the current literature on the outcomes of simple orthopaedic trauma. Simple trauma is defined as the fracture or injury of one limb due to an acute event. Fractures are the most common cause of hospitalized trauma in Australia and associated with multiple social, psychological and physical consequences for patients. The literature to date suggests that there are multiple factors leading to relatively poor outcomes following simple trauma, modifiable and non-modifiable. The most oft cited are older age, lower educational status, being injured at work, injury severity score, pre-existing disease, workers compensation, litigation and pain at initial assessment. Additional psychological risk factors quoted attribute to the injury to an external source and the use of passive coping strategies. This review aims to summarise the relevant literature relating to these risk factors and give direction to improving outcomes and future research into this important area.

scholarly journals Effects of photoevaporation on protoplanetary disc ‘isochrones’

2019 ◽  
Vol 492 (1) ◽  
pp. 1120-1126 ◽  
Author(s):  
Alice Somigliana ◽  
Claudia Toci ◽  
Giuseppe Lodato ◽  
Giovanni Rosotti ◽  
Carlo F Manara
Keyword(s):  
Numerical Solutions ◽  
Accretion Rate ◽  
Central Star ◽  
Mass Region ◽  
Self Similar Solution ◽  
Accretion Rates ◽  
Standard Scenario ◽  
Inner Region ◽  
Viscous Transport ◽  
Protoplanetary Discs

ABSTRACT Protoplanetary discs are the site of star and planet formation, and their evolution and consequent dispersal deeply affect the formation of planetary systems. In the standard scenario they evolve on time-scales ∼Myr due to the viscous transport of angular momentum. The analytical self-similar solution for their evolution predicts specific disc isochrones in the accretion rate–disc mass plane. However, photoevaporation by radiation emitted by the central star is likely to dominate the gas disc dispersal of the innermost region, introducing another (shorter) time-scale for this process. In this paper, we include the effect of internal (X and EUV) photoevaporation on the disc evolution, finding numerical solutions for a population of protoplanetary discs. Our models naturally reproduce the expected quick dispersal of the inner region of discs when their accretion rates match the rate of photoevaporative mass loss, in line with previous studies. We find that photoevaporation preferentially removes the lightest discs in the sample. The net result is that, counter-intuitively, photoevaporation increases the average disc mass in the sample, by dispersing the lightest discs. At the same time, photoevaporation also reduces the mass accretion rate by cutting the supply of material from the outer to the inner disc. In a purely viscous framework, this would be interpreted as the result of a longer viscous evolution, leading to an overestimate of the disc age. Our results thus show that photoevaporation is a necessary ingredient to include when interpreting observations of large disc samples with measured mass accretion rates and disc masses. Photoevaporation leaves a characteristic imprint on the shape of the isochrone. Accurate data of the accretion rate–disc mass plane in the low disc mass region therefore give clues on the typical photoevaporation rate.

scholarly journals Enrichment by Wolf-Rayet Stars and Other Massive Stars in Gas, Dust and Energy

1991 ◽  
Vol 143 ◽  
pp. 323-334
Author(s):  
Martin Cohen
Keyword(s):  
Interstellar Medium ◽  
Infrared Spectra ◽  
Stellar Wind ◽  
Massive Stars ◽  
Observational Evidence ◽  
Radiative Energy ◽  
New Members ◽  
Hot Gas ◽  
Sn 1987A ◽  
Dust Mass

I update previous estimates of the separate contributions for radiative energy, integrated total stellar wind mass and dust mass from Wolf-Rayet stars and other massive (OBA) stars. In the context of the intriguing dusty WC9 stars, I: (1) discuss the observability (or otherwise) between 0.4 and 23 μm of the condensation route from hot gas to carbon-rich grains; (2) urge caution in the use of 10 μm infrared spectra of these luminous stars to deduce the importance of silicates as a component of the interstellar medium, and (3) speculate on a possible new method for discovering new members of this relatively rare subtype based on IRAS Low Resolution Spectra. I review the observational evidence for dust condensation around SN 1987A.

scholarly journals Testing viscous disc theory using the balance between stellar accretion and external photoevaporation of protoplanetary discs

2020 ◽  
Vol 497 (1) ◽  
pp. L40-L45
Author(s):  
Andrew J Winter ◽  
Megan Ansdell ◽  
Thomas J Haworth ◽  
J M Diederik Kruijssen
Keyword(s):  
Angular Momentum ◽  
Mass Loss ◽  
Central Star ◽  
Outer Edge ◽  
Momentum Transport ◽  
Angular Momentum Transport ◽  
Accretion Rates ◽  
Novel Method ◽  
Viscous Models ◽  
Protoplanetary Discs

ABSTRACT The nature and rate of (viscous) angular momentum transport in protoplanetary discs (PPDs) have important consequences for the formation process of planetary systems. While accretion rates on to the central star yield constraints on such transport in the inner regions of a PPD, empirical constraints on viscous spreading in the outer regions remain challenging to obtain. Here, we demonstrate a novel method to probe the angular momentum transport at the outer edge of the disc. This method applies to PPDs that have lost a significant fraction of their mass due to thermal winds driven by UV irradiation from a neighbouring OB star. We demonstrate that this external photoevaporation can explain the observed depletion of discs in the 3–5 Myr old σ Orionis region, and use our model to make predictions motivating future empirical investigations of disc winds. For populations of intermediate-age PPDs, in viscous models we show that the mass flux outwards due to angular momentum redistribution is balanced by the mass-loss in the photoevaporative wind. A comparison between wind mass-loss and stellar accretion rates therefore offers an independent constraint on viscous models in the outer regions of PPDs.

Stimulation of the acoustico-lateralis system of clupeid fish by external sources and their own movements

1993 ◽  
Vol 341 (1296) ◽  
pp. 113-127 ◽  
Keyword(s):  
Lateral Line ◽  
External Source ◽  
Clupea Harengus ◽  
Pressure Gradients ◽  
Surrounding Water ◽  
Low Frequencies ◽  
Lateral Recess ◽  
Lateral Lines ◽  
External Sources ◽  
Net Pressure

1. The receptor organs of the acoustico-lateralis system in fish respond in various ways to pressures and pressure gradients and provide the fish with information about external sources of vibration. 2. A fish’s movements will set up pressures and pressure gradients and this poses three questions, (i) Can a fish obtain useful information from self-generated pressures and pressure gradients? (ii) To what extent do self-generated pressures mask signals from external sources? (iii) Can interactions between external and self-generated pressures and gradients in the acoustico-lateralis system give patterns of activity from the receptor organs which have special significance? 3. In herring ( Clupea harengus L. ) and sprat ( Spratus sprattus (L.)) measurements have been made of dimensions of various parts of the acoustico-lateralis system particularly of the subcerebral perilymph canal which crosses the head between the lateral lines. 4. Self-generated pressures produced by lateral movements of the head are antisymmetric, i.e. equal and opposite in sign on the left and right sides of the head. They oppose the accelerations of the head that produce them. In contrast, external sources give pressures that are largely symmetric. Any pressure gradients they give will accelerate the fish and the surrounding water together and any net pressure gradients will be small and so will any flows through the subcerebral perilymph canal. 5. Flows of liquid between the lateral lines across the lateral-recess membranes have been measured at various frequencies for pressure gradients applied across the head. Between 5 and 200 Hz the velocity of flow per unit pressure does not vary by more than than a factor of 2. At low frequencies the absolute values of flow are very much larger (more than 50 times) than those found for equally large symmetrically applied pressures (as from an external source) due to flow into the elastic gas containing bullae. 6. It is calculated that a net pressure difference (at optimum frequency) across the head of only 0.008 Pa will reach threshold for the lateral line neuromast nearest the lateral recess and one of 0.02 Pa for that under the eye. The responses of these neuromasts are expected to saturate and provide little information when the pressure differences across the head exceed 6 to 18 Pa. The pressures given by the swimming fish are discussed in the light of a theory advanced by Lighthill in the paper that follows this paper. With such antisymmetric pressures the direction of flow in the lateral-line canals will be towards the lateral recess on one side of the fish and away on the other and so differ from the situation found with an external source when flow at any instant will be either towards or away from the lateral recess on both sides of the head. 7. Antisymmetric pressures can produce large flows past the utricular maculae. However, at low frequencies flows across the maculae, on which their stimulation depends, will be small. We do not know the direction of these latter flows though they will be in opposite sense on the two sides of the head, again unlike the situation with an external source. 8. Calculations of impedances below 30 Hz show that the observed flows across the head are consistent with the dimensions and properties of the known structures. 9. There are major and systematic differences in the patterns of receptor organ stimulation between those expected from external sources and from a fish’s own movements. 10. Experiments on the red mullet ( Mullus surmuletus L.) showed that it too has a transverse channel connecting the right and left lateral-line systems. At low frequencies its properties resemble those of the subcerebral perilymph canal of the clupeid.

Geochemical Evidence of Saharan Dust Parent Material for Soils Developed on Quaternary Limestones of Caribbean and Western Atlantic Islands

1990 ◽  
Vol 33 (2) ◽  
pp. 157-177 ◽  
Author(s):  
Daniel R. Muhs ◽  
Charles A. Bush ◽  
Kathleen C. Stewart ◽  
Tracy R. Rowland ◽  
Russell C. Crittenden
Keyword(s):  
Volcanic Ash ◽  
Florida Keys ◽  
External Source ◽  
Parent Material ◽  
Saharan Dust ◽  
Caribbean Island ◽  
Trade Winds ◽  
Western Atlantic ◽  
Parent Materials ◽  
External Sources

AbstractMost previous workers have regarded the insoluble residues of high-purity Quaternary limestones (coral reefs and oolites) as the most important parent material for well-developed, clay-rich soils on Caribbean and western Atlantic islands, but this genetic mechanism requires unreasonable amounts of limestone solution in Quaternary time. Other possible parent materials from external sources are volcanic ash from the Lesser Antilles island arc and Saharan dust carried across the Atlantic Ocean on the northeast trade winds. Soils on Quaternary coral terraces and carbonate eolianites on Barbados, Jamaica, the Florida Keys (United States), and New Providence Island (Bahamas) were studied to determine which, if either, external source was important. Caribbean volcanic ashes and Saharan dust can be clearly distinguished using ratios of relatively immobile elements (Al2O3/TiO2, Ti/Y, Ti/Zr, and Ti/Th). Comparison of these ratios in 25 soils, where estimated ages range from 125,000 to about 870,000 yr, shows that Saharan dust is the most important parent material for soils on all islands. These results indicate that the northeast trade winds have been an important component of the regional climatology for much of the Quaterary. Saharan dust may also be an important parent material for Caribbean island bauxites of much greater age.

scholarly journals A model building framework for answer set programming with external computations

2015 ◽  
Vol 16 (4) ◽  
pp. 418-464 ◽  
Author(s):  
THOMAS EITER ◽  
MICHAEL FINK ◽  
GIOVAMBATTISTA IANNI ◽  
THOMAS KRENNWALLNER ◽  
CHRISTOPH REDL ◽  
...  
Keyword(s):  
Model Building ◽  
External Source ◽  
Software Systems ◽  
Logic Programs ◽  
New Approach ◽  
Evaluation Approach ◽  
Real World Application ◽  
External Sources ◽  
Model Graph ◽  
Answer Set

AbstractAs software systems are getting increasingly connected, there is a need for equipping nonmonotonic logic programs with access to external sources that are possibly remote and may contain information in heterogeneous formats. To cater for this need,hexprograms were designed as a generalization of answer set programs with an API style interface that allows to access arbitrary external sources, providing great flexibility. Efficient evaluation of such programs however is challenging, and it requires to interleave external computation and model building; to decide when to switch between these tasks is difficult, and existing approaches have limited scalability in many real-world application scenarios. We present a new approach for the evaluation of logic programs with external source access, which is based on a configurable framework for dividing the non-ground program into possibly overlapping smaller parts called evaluation units. The latter will be processed by interleaving external evaluation and model building using an evaluation graph and a model graph, respectively, and by combining intermediate results. Experiments with our prototype implementation show a significant improvement compared to previous approaches. While designed forhex-programs, the new evaluation approach may be deployed to related rule-based formalisms as well.

Mesoscale Variations of the Atmospheric Snow Line over the Northern Sierra Nevada: Multiyear Statistics, Case Study, and Mechanisms

2013 ◽  
Vol 70 (3) ◽  
pp. 916-938 ◽  
Author(s):  
Justin R. Minder ◽  
David E. Kingsmill
Keyword(s):  
Sierra Nevada ◽  
Horizontal Resolution ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Mountain Ranges ◽  
Snow Line ◽  
Free Air ◽  
Close Proximity ◽  
Microphysical Parameterization

Abstract Observations from several mountain ranges reveal that the height of the transition from snowfall to rainfall, the snow line, can intersect the terrain at an elevation hundreds of meters below its elevation in the free air upwind. This mesoscale lowering of the snow line affects both the accumulation of mountain snowpack and the generation of storm runoff. A unique multiyear view of this behavior based on data from profiling radars in the northern Sierra Nevada deployed as part of NOAA’s Hydrometeorology Testbed is presented. Data from 3 yr of storms show that the mesoscale lowering of the snow line is a feature common to nearly all major storms, with an average snow line drop of 170 m. The mesoscale behavior of the snow line is investigated in detail for a major storm over the northern Sierra Nevada. Comparisons of observations from sondes and profiling radars with high-resolution simulations using the Weather Research and Forecasting model (WRF) show that WRF is capable of reproducing the observed lowering of the snow line in a realistic manner. Modeling results suggest that radar profiler networks may substantially underestimate the lowering by failing to resolve horizontal snow line variations in close proximity to the mountainside. Diagnosis of model output indicates that pseudoadiabatic processes related to orographic blocking, localized cooling due to melting of orographically enhanced snowfall, and spatial variations in hydrometeor melting distance all play important roles. Simulations are surprisingly insensitive to model horizontal resolution but have important sensitivities to microphysical parameterization.

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