scholarly journals Theoretical wind clumping predictions of OB supergiants from line-driven instability simulations across the bi-stability jump

2019 ◽  
Vol 631 ◽  
pp. A172 ◽  
Author(s):  
F. A. Driessen ◽  
J. O. Sundqvist ◽  
N. D. Kee
Keyword(s):  
Growth Rate ◽  
Mass Loss ◽  
Linear Growth ◽  
Massive Stars ◽  
Linear Growth Rate ◽  
Linear Perturbation ◽  
Lower Growth ◽  
Lower Velocity ◽  
Non Linear ◽  
Loss Rates

Context. The behaviour of mass loss across the so-called bi-stability jump, where iron recombines from Fe IV to Fe III, is a key uncertainty in models of massive stars. Specifically, while an increase in mass loss is theoretically predicted, this has not yet been observationally confirmed. However, radiation-driven winds of hot massive stars are known to exhibit clumpy structures triggered by the line-deshadowing instability (LDI). This wind clumping severely affects empirical mass-loss rates inferred from ρ2-dependent spectral diagnostics. Thus, if clumping properties differ significantly for O and B supergiants across the bi-stability jump, this may help alleviate current discrepancies between theory and observations. Aims. We investigated with analyt ical and numerical tools how the onset of clumpy structures behave in the winds of O supergiants (OSG) and B supergiants (BSG) across the bi-stability jump. Methods. We derived a scaling relation for the linear growth rate of the LDI for a single optically thick line and applied it in the OSG and BSG regime. We ran 1D time-dependent line-driven instability simulations to study the non-linear evolution of the LDI in clumpy OSG and BSG winds. Results. Linear perturbation analysis for a single line shows that the LDI linear growth rate Ω scales strongly with stellar effective temperature and terminal wind speed: Ω∝v∞2Teff4. This implies significantly lower growth rates for (the cooler and slower) BSG winds than for OSG winds. This is confirmed by the non-linear simulations, which show significant differences in OSG and BSG wind structure formation, with the latter characterized by significantly weaker clumping factors and lower velocity dispersions. This suggests that lower correction factors due to clumping should be employed when deriving empirical mass-loss rates for BSGs on the cool side of the bi-stability jump. Moreover, the non-linear simulations provide a theoretical background towards explaining the general lack of observed intrinsic X-ray emission in single B-star winds.

Non-Linear Corrosion Growth: A More Appropriate and Accurate Model for Predicting Corrosion Growth Rate

2016 ◽  
Author(s):  
M. Al-Amin ◽  
S. Kariyawasam ◽  
S. Zhang ◽  
W. Zhou
Keyword(s):  
Growth Rate ◽  
Case Studies ◽  
Growth Model ◽  
Measurement Errors ◽  
Linear Growth ◽  
Management Program ◽  
Linear Growth Rate ◽  
Metal Loss ◽  
Contributing Factors ◽  
Non Linear

External metal-loss corrosion is one of the major contributing factors for pipeline failures in North America. Corrosion growth rate plays a crucial role in managing corrosion hazard for gas and liquid pipelines. Quantifying the growth of corrosion over time is critically important for the risk and reliability analysis of pipelines, planning for corrosion mitigation and repair, and determination of time intervals for corrosion inspections. Conservatism in predicting the growth rate has significant engineering implication as non-conservatism can lead to critical anomalies being missed by mitigation actions and may cause pipeline failure; whereas, over conservatism can lead to unnecessary inspections and anomaly mitigations that may result in significant unnecessary cost to pipeline operators. As more and more pipelines are now being inspected by in-line inspection (ILI) tools on a regular basis, the ILI data from multiple inspections provide valuable information about the growth of corrosion anomalies on the pipeline. Although the application of linear growth rate calculated by comparing depths from two successive ILI is a common practice in the pipeline industry, research has shown that the growth of corrosion anomaly is non-linear and anomaly-specific. The authors of this paper have previously developed anomaly-specific non-linear corrosion growth model based on multiple ILI data. The objectives of this paper are to demonstrate the appropriateness of anomaly-specific non-linear corrosion growth model, and to illustrate the advantages of using non-linear corrosion growth model in the integrity management program. Two case studies were performed to illustrate the application of non-linear growth model by incorporating the measurement errors associated with the ILI tools, which include both the bias (constant and non-constant) and random scattering error. The findings of these case studies are presented in this paper.

scholarly journals On the non-axisymmetric fragmentation of rings generated by the Secular Gravitational Instability

2021 ◽  
Author(s):  
Arnaud Pierens
Keyword(s):  
Growth Rate ◽  
Growth Rates ◽  
Linear Growth ◽  
Gravitational Instability ◽  
Dust Particles ◽  
Linear Growth Rate ◽  
Stability Parameters ◽  
Dust Density ◽  
Linear Evolution ◽  
Non Linear

Abstract Ringed structures have been observed in a variety of protoplanetary discs. Among the processes that might be able to generate such features, the Secular Gravitational Instability (SGI) is a possible candidate. It has also been proposed that the SGI might lead to the formation of planetesimals during the non-linear phase of the instability. In this context, we employ two-fluid hydrodynamical simulations with self-gravity to study the non-axisymmetric, non-linear evolution of ringed perturbations that grow under the action of the SGI. We find that the non-linear evolution outcome of the SGI depends mainly on the initial linear growth rate. For SGI growth rates smaller than typically σ ≳ 10−4 − 10−5Ω, dissipation resulting from dust feedback introduces a m = 1 spiral wave in the gas, even for Toomre gas stability parameters Qg > 2 for which non-axisymmetric instabilities appear in a purely gaseous disc. This one-armed spiral subsequently traps dust particles until a dust-to-gas ratio ε ∼ 1 is achieved. For higher linear growth rates, the dust ring is found to undergo gravitational collapse until the bump in the surface density profile becomes strong enough to trigger the formation of dusty vortices through the Rossby Wave Instability (RWI). Enhancements in dust density resulting from this process are found to scale with the linear growth rate, and can be such that the dust density is higher than the Roche density, leading to the formation of bound clumps. Fragmentation of axisymmetric rings produced by the SGI might therefore appear as a possible process for the formation of planetesimals.

Microborings and growth in Late Ordovician halysitids and other corals

1993 ◽  
Vol 67 (6) ◽  
pp. 922-934 ◽  
Author(s):  
Robert J. Elias ◽  
Dong-Jin Lee
Keyword(s):  
Growth Rate ◽  
Linear Growth ◽  
Inverse Correlation ◽  
Weight Percent ◽  
Late Ordovician ◽  
Coral Growth ◽  
Linear Growth Rate ◽  
Skeletal Density ◽  
Rugose Coral ◽  
Endolithic Algae

Microborings in the Late Ordovician tabulate corals Catenipora rubra (a halysitid) and Manipora amicarum (a cateniform nonhalysitid) and in an epizoic solitary rugose coral differ from nearly all of those previously reported in Paleozoic corals. These microborings were formed within the coralla by endolithic algae and fungi located beneath living polyps. Comparable structures in the Late Ordovician tabulate Quepora ?agglomeratiformis (a halysitid) represent algal microborings, not spicules, and halysitids are corals, not sponges as suggested by Kaźmierczak (1989).Endolithic algae in cateniform tabulates relied primarily on light entering through the outer walls of the ranks rather than through the polyps; lacunae within coralla permitted appropriate levels of light to reach many corallites. The direction of boring was determined by corallum microstructure and possibly also by the distribution of organic matter within the skeleton. There is an apparent inverse correlation between boring activity and coral growth rate.The location and relative abundance of pyritized microborings within calcareous coralla can be established quantitatively and objectively from electron microprobe determinations of weight percent sulfur along appropriate traverses of the coral skeleton. The distribution of such microborings in Catenipora rubra and Manipora amicarum is comparable to algal banding in modern corals; this is the first report of such banding in the interiors of Paleozoic corals. Change in the intensity of boring within each corallum was evidently a response to variation in the linear growth rate of the coral, or to fluctuation in an environmental factor (perhaps light intensity) that could control both algal activity and growth rate in these corals. Change in the algal boring intensity and linear growth rate of the coral was generally but not always seasonal and usually but not invariably associated with change in the density of coral skeletal deposition.Cyclic bands of boring abundance maxima within fossil colonial corals provide a measure of annual linear growth comparable to the widely accepted method based on skeletal density bands. Algal bands are more sporadically developed than density bands within and among coralla, thus increasing the difficulty of interpretation. Fluctuations in the abundance of algal microborings apparently provide a detailed record of changes in the linear growth rate of colonies and of individuals within colonies. Combined analyses of microboring abundance and skeletal density will contribute significantly to our understanding of the biological and environmental factors involved in endolithic activity and coral growth.

scholarly journals Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

2018 ◽  
Vol 613 ◽  
pp. A75 ◽  
Author(s):  
P. Kurfürst ◽  
A. Feldmeier ◽  
J. Krtička
Keyword(s):  
Mass Loss ◽  
Optical Depth ◽  
Massive Stars ◽  
Thermal Structure ◽  
Time Dependent ◽  
Navier Stokes ◽  
Viscous Heating ◽  
Two Dimensional ◽  
Dense Region ◽  
Loss Rates

Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. It is also unclear how various forming physical mechanisms of the circumstellar environment affect its shape and density, as well as its kinematic and thermal structure. Aims. We study the hydrodynamic and thermal structure of optically thick, dense parts of outflowing circumstellar disks that may be formed around various types of critically rotating massive stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars. We calculate self-consistent time-dependent models of temperature and density structure in the disk’s inner dense region that is strongly affected by irradiation from a rotationally oblate central star and by viscous heating. Methods. Using the method of short characteristics, we specify the optical depth of the disk along the line-of-sight from stellar poles. Within the optically thick dense region with an optical depth of τ > 2∕3 we calculate the vertical disk thermal structure using the diffusion approximation while for the optically thin outer layers we assume a local thermodynamic equilibrium with the impinging stellar irradiation. For time-dependent hydrodynamic modeling, we use two of our own types of hydrodynamic codes: two-dimensional operator-split numerical code based on an explicit Eulerian finite volume scheme on a staggered grid, and unsplit code based on the Roe’s method, both including full second-order Navier-Stokes shear viscosity. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk for mass-loss rates higher than Ṁ ≳ 10−10 M⊙ yr−1. In the models of dense viscous disks with Ṁ > 10−8 M⊙ yr−1, the viscosity increases the central temperature up to several tens of thousands of Kelvins, however the temperature rapidly drops with radius and with distance from the disk midplane. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen, for example.

scholarly journals What Do We Really Know About the Winds of Massive Stars?

2007 ◽  
Vol 3 (S250) ◽  
pp. 89-96
Author(s):  
D. John Hillier
Keyword(s):  
Mass Loss ◽  
Massive Stars ◽  
Standard Theory ◽  
Stellar Winds ◽  
X Rays ◽  
X Ray ◽  
Stellar Photosphere ◽  
Weak Winds ◽  
Ionization Balance ◽  
Loss Rates

AbstractThe standard theory of radiation driven winds has provided a useful framework to understand stellar winds arising from massive stars (O stars, Wolf-Rayet stars, and luminous blue variables). However, with new diagnostics, and advances in spectral modeling, deficiencies in our understanding of stellar winds have been thrust to the forefront of our research efforts. Spectroscopic observations and analyses have shown the importance of inhomogeneities in stellar winds, and revealed that there are fundamental discrepancies between predicted and theoretical mass-loss rates. For late O stars, spectroscopic analyses derive mass-loss rates significantly lower than predicted. For all O stars, observed X-ray fluxes are difficult to reproduce using standard shock theory, while observed X-ray profiles indicate lower mass-loss rates, the potential importance of porosity effects, and an origin surprisingly close to the stellar photosphere. In O stars with weak winds, X-rays play a crucial role in determining the ionization balance, and must be taken into account.

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.

scholarly journals <sup>210</sup>Pb-<sup>226</sup>Ra chronology reveals rapid growth rate of <i>Madrepora oculata</I> and <i>Lophelia pertusa</i> on world's largest cold-water coral reef

2011 ◽  
Vol 8 (6) ◽  
pp. 12247-12283
Author(s):  
P. Sabatier ◽  
J.-L. Reyss ◽  
J. M. Hall-Spencer ◽  
C. Colin ◽  
N. Frank ◽  
...  
Keyword(s):  
Growth Rate ◽  
Coral Reef ◽  
Linear Growth ◽  
Cold Water ◽  
Optimal Growth ◽  
Growth Conditions ◽  
Age And Growth ◽  
Linear Growth Rate ◽  
Lophelia Pertusa ◽  
Water Coral

Abstract. Here we show the use of the 210Pb-226Ra excess method to determine the growth rate of corals from one of the world's largest known cold-water coral reef, the Røst Reef off Norway. Two large branching framework-forming cold-water coral specimens, one Lophelia pertusa and one Madrepora oculata were collected alive at 350 m water depth from the Røst Reef at ~67° N and ~9° E. Pb and Ra isotopes were measured along the major growth axis of both specimens using low level alpha and gamma spectrometry and the corals trace element compositions were studied using ICP-QMS. Due to the different chemical behaviors of Pb and Ra in the marine environment, 210Pb and 226Ra were not incorporated the same way into the aragonite skeleton of those two cold-water corals. Thus to assess of the growth rates of both specimens we have here taken in consideration the exponential decrease of initially incorporated 210Pb as well as the ingrowth of 210Pb from the decay of 226Ra. Moreover a~post-depositional 210Pb incorporation is found in relation to the Mn-Fe coatings that could not be entirely removed from the oldest parts of the skeletons. The 226Ra activities in both corals were fairly constant, then assuming constant uptake of 210Pb through time the 210Pb-226Ra chronology can be applied to calculate linear growth rate. The 45.5 cm long branch of M. oculata reveals an age of 31 yr and a~linear growth rate of 14.4 ± 1.1 mm yr−1, i.e. 2.6 polyps per year. However, a correction regarding a remaining post-depositional Mn-Fe oxide coating is needed for the base of the specimen. The corrected age tend to confirm the radiocarbon derived basal age of 40 yr (using 14C bomb peak) with a mean growth rate of 2 polyps yr−1. This rate is similar to the one obtained in Aquaria experiments under optimal growth conditions. For the 80 cm-long specimen of L. pertusa a remaining contamination of metal-oxides is observed for the middle and basal part of the coral skeleton, inhibiting similar accurate age and growth rate estimates. However, the youngest branch was free of Mn enrichment and this 15 cm section reveals a growth rate of 8 mm yr−1 (~1 polyp every two to three years). However, the 210Pb growth rate estimate is within the lowermost ranges of previous growth rate estimates and may thus reflect that the coral was not developing at optimal growth conditions. Overall, 210Pb-226Ra dating can be successfully applied to determine the age and growth rate of framework-forming cold-water corals, however, removal of post-depositional Mn-Fe oxide deposits is a prerequisite. If successful, large branching M. oculata and L. pertusa coral skeletons provide unique oceanographic archive for studies of intermediate water environmentals with an up to annual time resolution and spanning over many decades.

Sign in / Sign up

Export Citation Format

Share Document