scholarly journals The Evolution of Cometary Nuclei

1972 ◽  
Vol 45 ◽  
pp. 271-276 ◽  
Author(s):  
L. M. Shul'Man
Keyword(s):  
Size Distribution ◽  
Cometary Nucleus ◽  
Dust Content ◽  
The Other ◽  
Dust Particles ◽  
Nuclear Radius ◽  
Cometary Nuclei ◽  
Other Hand ◽  
Brightness Decrease

The evolution of a cometary nucleus depends on the size distribution of the dust content. If all the dust particles are small enough, they will all be thrown off into the coma; the secular decrease in the comet's brightness is due simply to the decrease in the nuclear radius, and the comet eventually disintegrates. On the other hand, if larger grains are present, they will start to form spots of a low-emissivity mineral envelope on the surface of the nucleus; in this case, the decrease in brightness is due to the increase in the area of the screened part of the surface, and the reduction in the nuclear radius is comparatively small. It is shown that after about 90% of the surface has been screened, the rate of screening increases considerably, as smaller particles become trapped between the larger ones; and there will be a corresponding acceleration in the brightness decrease. These nuclei are eventually covered with a solid mineral envelope, and they are transformed into asteroids.

Covering of cometary nucleus by refractory crust and its evolution into asteroid-like body

1999 ◽  
Vol 173 ◽  
pp. 365-370
Author(s):  
Kh.I. Ibadinov
Keyword(s):  
Laboratory Experiments ◽  
Cometary Nucleus ◽  
Sublimation Rate ◽  
Cometary Nuclei ◽  
Short Period Comet ◽  
Short Period ◽  
Brightness Decrease ◽  
Crust Thickness ◽  
Ice Sublimation ◽  
Period Comet

AbstractFrom the established dependence of the brightness decrease of a short-period comet dependence on the perihelion distance of its orbit it follows that part of the surface of these cometary nuclei gradually covers by a refractory crust. The results of cometary nucleus simulation show that at constant insolation energy the crust thickness is proportional to the square root of the insolation time and the ice sublimation rate is inversely proportional to the crust thickness. From laboratory experiments resulted the thermal regime, the gas productivity of the nucleus, covering of the nucleus by the crust, and the tempo of evolution of a short-period comet into the asteroid-like body studied.

scholarly journals Evolution of mobile phases in cometary interiors

2020 ◽  
Vol 37 ◽  
Author(s):  
Christopher M. Fellows ◽  
Trevor C. Brown ◽  
Andrew Cooper ◽  
Marco Parigi
Keyword(s):  
Gaseous Phase ◽  
Kuiper Belt ◽  
Temporal Distribution ◽  
Cometary Nucleus ◽  
Dust Particles ◽  
Spatial And Temporal Distribution ◽  
Cometary Nuclei ◽  
Liquid Phases ◽  
Mobile Phases ◽  
Outer Crust

Abstract Beginning with loose aggregations of dust particles coated with heterogeneous ices under vacuum at Kuiper Belt temperatures, moving to Jupiter/Saturn distances and eventually to low-perihelion orbit, we consider the likely development of the gaseous phase within a cometary nucleus over the course of its lifetime. From the perspective of physical chemistry, we consider limits on the spatial and temporal distribution and composition of this gaseous phase. The implications of the gaseous phase for heat transfer and for the possible spatial and temporal development of liquid phases are calculated. We conclude that the likely temperatures, pressures, and compositions beneath the outer crust of typical cometary nuclei are such that fluidised phases can exist at significant depths and that these reservoirs give a coherent explanation for the high-intensity outbursts observed from cometary nuclei at large distances from perihelion.

Impact of Fuel Preparation System on Combustion Process and Exhaust Emissions

2003 ◽  
Author(s):  
S. Pischinger ◽  
M. Umierski
Keyword(s):  
Particulate Matter ◽  
Size Distribution ◽  
Diesel Fuel ◽  
Ambient Air ◽  
Common Rail ◽  
Sulphur Content ◽  
The Other ◽  
Injection Timing ◽  
Heavy Fuel Oil ◽  
Other Hand

In the past decade, the quality of Heavy Fuel Oil (HFO) has not improved or even declined in general. The progress in refinery techniques allowed to use a higher percentage of the crude oil for light fuels and other components. Therefore, the concentration of heavy components and agglomerates in the residual fuel increased. On the other hand, a better exhaust quality without compromising the superior fuel consumption of HFO-fuelled engines can only be gained by improved fuel injection. High injection pressures, rate shaping and electronic control of injection timing as well as new injection techniques like Common-Rail will be common features of engines for the next decades. Unfortunately, electronic controlled unit pumps and moreover, Common Rail systems are very sensitive to fuel quality, especially the content of water, sediments, agglomerates and other sludge. If ran on fuel processed by today’s fuel preparation system, wear would be excessive or damages will take place. On the other hand, if conventional on-board purification is intensified, the percentage of fuel deposit will rise and lower the over-all efficiency. To observe effects of fuel composition, a single cylinder research engine (250 mm bore/320 mm stroke) is fuelled by HFO in typical operation modes including different speeds and loads. Diesel fuel was also used to compare the results with another fuel of e.g. different sulphur content to separate effects on particulate matter. The high, typical sulphur content of the used HFO of about 3% increases the particulate matter; approx. 30% are sulphur products. The Soluble Organic Fraction (SOF, unburned fuel and oil) of PM is significantly higher compared to diesel fuel (50...70% to 10%). This impacts the particulate size distribution as well: although the main fraction of particulates consist of a size from 100...400nm for diesel and HFO, the number of small particulates (approx. 50 nm) is different, depending on the Sulphur content as this size consists of small condensates; therefore, the number may also be impacted by ambient air humidity. By use of different components for fuel preparation, the size, distribution and content of agglomerates can be impacted, but re-agglomeration occurs fast within minutes and must be taken into account. The differences in agglomerates does not result in a different total exhaust particulate mass. The slight effects found within the test are not significant and are in addition to that affected by ambient conditions. The fuel preparation, especially the use of homogenization equipment, needs more development efforts. For example, a mechanical homogenization apparatus can crack agglomerates and improve fuel homogeneity, but may fail to bring the size of agglomerates to a dimension that the filter is relieved.

513. The effect of different cream treatments during the pasteurization process on the size distribution of fat globules in cream and butter

1953 ◽  
Vol 20 (2) ◽  
pp. 201-211 ◽  
Author(s):  
R. M. Dolby
Keyword(s):  
Thin Film ◽  
Uniform Distribution ◽  
Size Distribution ◽  
Considerable Increase ◽  
The Other ◽  
Gel Formation ◽  
Microscope Slide ◽  
Fat Globules ◽  
Other Hand ◽  
Counting Chamber

A method for determining the size distribution of fat globules in cream has been described. A thin film (20 μ) of the diluted cream on a microscope slide is preferred to a deeper preparation (0·1 mm.) in a counting chamber in which rise of fat globules to the upper surface is necessary for easy observation. The latter method is less convenient and can give too low an estimate of the proportion of the smallest globules (1 μ and under).Cream should be diluted 1 in 50 and the concentration of gelatin in the dilution should be sufficient (4–6%) to give a rapid gel formation. The mixture should be cooled till the gelatin thickens before being transferred to the slide if it is to give the most uniform distribution of globules.It is shown that Vacreator-treatment of cream causes a considerable increase in the proportion of fat present as globules less than 2 μ and a small increase in the proportion present as globules over 10 μ in diameter. Flash-pasteurization, on the other hand, causes a slight decrease in the proportion of fat present as small globules and a considerable increase in that present as large globules.Butter from flash-pasteurized cream contains numerous large fat globules and fat aggregates, while butter from Vacreator-treated cream contains few large globules. It is suggested that the more uniform subdivision of fat in butter from Vacreator-treated cream explains why butter from Vacreator-treated cream is superior in texture to butter from flash-pasteurized cream.

Distribution of soluble heavy metals between ionic and complexed forms in a saturated sediment as affected by pH and redox conditions

1998 ◽  
Vol 37 (6-7) ◽  
pp. 165-171 ◽  
Author(s):  
W. H. Patrick ◽  
M. Verloo
Keyword(s):  
Organic Matter ◽  
Metal Complexes ◽  
Cation Exchange ◽  
Size Distribution ◽  
Size Range ◽  
The Other ◽  
Redox Conditions ◽  
Soluble Organic Matter ◽  
Other Hand ◽  
Reducing Conditions

Research was undertaken to determine the effects of pH and redox potential on heavy metal speciation and on the size distribution of the organo-metal complexes in the soluble fraction of sediment-water systems. A cation exchange technique was effective in separating free and complexed metal ions. Fe, Mn and Zn differed in the extent of complexation with soluble organic matter. Under reducing conditions approximately two thirds of the soluble Fe was in a complexed form that was not sorbed in passing through the cation exchange resin. Soluble Mn on the other hand, was almost completely ionic under reducing conditions, with only a trace amount passing through the resin column. Over 90 percent of the soluble Zn was complexed under reduced conditions, with only 9 percent sorbed onto the resin. The complexed Fe and Zn were bound to soluble organic matter particles with equivalent molecular weight greater than 25,000 while Mn passed through this size filter. There were marked differences in the size distribution of the various organo-metal complexes under different redox and pH conditions. The soluble Fe was associated with both the largest and smallest size ranges of soluble organic matter. The effect of pH was most evident in the smallest size range with much more complexed iron being present at low pHs. Mn, on the other hand, was associated with only the smallest size range under all pH and redox conditions, reflecting its ionic nature. The greater solubility and mobility of Mn probably accounts for it being depleted relative to iron in Gulf Coast sediments. Hg and Pb were associated with only the largest size soluble complexes and were little affected by pH and redox conditions.

Comparative Analysis of Infused “Static”, Ex Vivo-Generated Platelets Vs. Infused Megakaryocytes-Generated Platelets: A Cautionary Tale

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 2881-2881
Author(s):  
Yuhuan Wang ◽  
Vincent Hayes ◽  
Danuta Jarocha ◽  
Mortimer Poncz
Keyword(s):  
Stem Cells ◽  
Size Distribution ◽  
Half Life ◽  
Ex Vivo ◽  
The Other ◽  
Human Donor ◽  
Nsg Mice ◽  
Other Hand

Abstract Ex vivo-generated (EV) platelets beginning with embryonic stem cells or induced pluripotent stem cells (iPSCs) or hematopoietic progenitors cells (HPCs) may have clinical utility over donor-derived platelets, and efforts to produce such EV-platelets have been pursued in several laboratories under static megakaryocyte (Meg) culture conditions. Success in generating these has been reported, even demonstrating EV-platelet incorporation into growing thrombi in murine models. We have pursued an alternative strategy for thrombopoiesis using EV-Megs, grown from either human adult HPCs or from iPSCs or fetal livers, and directly infusing them into NOD-SCID gamma-interferon-deficient (NSG) mice. These studies were based on our prior observation that infused murine EV-Megs into wildtype mice are entrapped in the pulmonary bed and over the subsequent 1-4 hours release a wave of functional platelets at a significant level. We now show that infusion of human EV-Megs do the same in NSG mice, but resulting in two different pools of derived platelets: (1) A pool of young (as determined by thiazole orange staining) platelets having the same bell-shaped size distribution was seen as after infusion of human donor-derived platelets in these mice. These platelets take several hours to appear, but then have the same half-life as donor-derived platelets. These platelets are derived from the infused EV-Megs and were termed in vivo-generated (IV)-platelets. (2) A second pool of mostly older platelets was present that originated during the static growth of the EV-Megs, and these EV-platelets varied widely in size and age. Initially, these platelets accounted for a third of all the human platelets seen. Unlike IV-platelets, EV-platelets are immediately present and circulate with a markedly short half-life of 2-3 hours unless the recipient NSG mice were pre-treated with clodronate-ladened liposomes to delete their macrophage pools. Rapid removal of EV-platelets by macrophages is due to their being preactivated as determined by surface P-selectin expression in whole mice blood. These EV-platelets also had very limited further responsiveness to convulxin activation. On the other hand, human IV-platelets were quiescent prior to agonist stimulation in whole mice blood and responded strongly to agonist, similar to human donor-derived platelets infused into NSG mice. The IV-platelets were also selectively incorporated into cremaster arteriole laser injury thrombi over EV-platelets. Finally, directly harvested “platelets” from EV static-grown Megs were isolated and analyzed both in vitro and in vivo. Only a third of these particles are CD41+/CD42+ platelets and approximately half are actually CD41-/CD42-. Both pools showed the same wide size distribution in vitro and in vivo after infusion into mice. The CD41+/CD42+ fraction behaved just as the EV-platelets, but the CD41-/CD42- fraction half-life was unaffected by pre-treatment with clodronate-ladened liposomes. In summary, infused human Megs grown under static growth conditions released platelets in a recipient mouse’s lung with features characteristic of donor-derived platelets. On the other hand, “platelets” harvested from the same Megs were predominantly not even platelets as measured using CD41/CD42 markers. The portion that were CD41+/CD42+ platelets were preactivated, poorly responsive to agonists, and cleared rapidly. These findings set a standard on how to judge the potential clinical value of platelets derived from EV-Megs and also raise concerns whether direct visual imaging of “platelet release” in static culture is biologically meaningful given that most particles released were not CD41+/CD42+ platelets, and the ones that were CD41+/CD42+ were mis-sized and functionally limited. Disclosures No relevant conflicts of interest to declare.

On the Newcomb-Benford Law

2003 ◽  
Vol 58 (11) ◽  
pp. 618-622 ◽  
Author(s):  
W. A. Kreiner
Keyword(s):  
Experimental Data ◽  
Distribution Function ◽  
Size Distribution ◽  
The Other ◽  
Benford’S Law ◽  
Physical Data ◽  
Other Hand ◽  
Benford's Law ◽  
Logarithmic Law ◽  
Benford Law

It is well known that the leading digit in tables of statistical and physical data is not evenly distributed among the digits 1 to 9. Benford (who collected a large number of data) assumed that they follow a logarithmic law, commonly known as Benford’s Law, although it was proposed earlier by Newcomb. We suggest, however, that the probability of the first digit being a 1, 2, ... depends on the particular distribution function of the data. For example, the size distribution of objects which grow exponentially is found to follow the Newcomb-Benford law. On the other hand, as the experimental data discussed in this paper show, the function governing the probability of the first digit of the weight of fragments obtained from crushing a stone deviates substantially from the Newcomb-Benford Law.

Models of comets with strongly variable nongravitational effects

1999 ◽  
Vol 173 ◽  
pp. 381-387
Author(s):  
M. Królikowska ◽  
G. Sitarski ◽  
S. Szutowicz
Keyword(s):  
Cometary Nucleus ◽  
Spin Axis ◽  
Reactive Force ◽  
Orbital Elements ◽  
Cometary Nuclei ◽  
Short Period ◽  
Basic Parameters ◽  
Polar Radius ◽  
Cometary Activity ◽  
Nongravitational Effects

AbstractThe nongravitational motion of five “erratic” short-period comets is studied on the basis of published astrometric observations. We present the precession models which successfully link all the observed apparitions of the comets: 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2, 32P/Comas Solá, 37P/Forbes, and 43P/Wolf-Harrington. We used the Sekanina's forced precession model of the rotating cometary nucleus to include the nongravitational terms into equations of the comet's motion. Values of six basic parameters (four connected with the rotating comet nucleus and two describing the precession of spin-axis of the nucleus) have been determined along the orbital elements from positional observations of the comets. The solutions were derived with additional assumptions which introduce instantaneous changes of modulus of reactive force,Aand of maximum of cometary activity with respect to perihelion time. The present precession models impose some contraints on sizes and rotational periods of cometary nuclei. According to our solutions the nucleus of 21P/Giacobini-Zinner with oblateness along the spin-axis of about 0.32 (equatorial to polar radius of 1.46) is the most oblate among five investigated comets.

A study of cometary eruptions through OH radio-lines

1999 ◽  
Vol 173 ◽  
pp. 249-254
Author(s):  
A.M. Silva ◽  
R.D. Miró
Keyword(s):  
Short Duration ◽  
Growth Curves ◽  
The Other ◽  
Initial Mass ◽  
Radio Lines ◽  
Other Hand ◽  
Sudden Rise

AbstractWe have developed a model for theH2OandOHevolution in a comet outburst, assuming that together with the gas, a distribution of icy grains is ejected. With an initial mass of icy grains of 108kg released, theH2OandOHproductions are increased up to a factor two, and the growth curves change drastically in the first two days. The model is applied to eruptions detected in theOHradio monitorings and fits well with the slow variations in the flux. On the other hand, several events of short duration appear, consisting of a sudden rise ofOHflux, followed by a sudden decay on the second day. These apparent short bursts are frequently found as precursors of a more durable eruption. We suggest that both of them are part of a unique eruption, and that the sudden decay is due to collisions that de-excite theOHmaser, when it reaches the Cometopause region located at 1.35 × 105kmfrom the nucleus.

Closing the GAP—A 5Å Scanning Microscope

1969 ◽  
Vol 27 ◽  
pp. 6-7
Author(s):  
A. V. Crewe
Keyword(s):  
Normal Form ◽  
The Other ◽  
Resolving Power ◽  
Scanning Microscope ◽  
Conventional Microscope ◽  
Other Hand ◽  
Closing The Gap ◽  
Thermal Sources ◽  
Better Than ◽  
Transmission Microscope

We have become accustomed to differentiating between the scanning microscope and the conventional transmission microscope according to the resolving power which the two instruments offer. The conventional microscope is capable of a point resolution of a few angstroms and line resolutions of periodic objects of about 1Å. On the other hand, the scanning microscope, in its normal form, is not ordinarily capable of a point resolution better than 100Å. Upon examining reasons for the 100Å limitation, it becomes clear that this is based more on tradition than reason, and in particular, it is a condition imposed upon the microscope by adherence to thermal sources of electrons.

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