Using Cometary Activity to Trace the Physical and Chemical Evolution of Cometary Nuclei

Comets II ◽  
2004 ◽  
pp. 317-336
Author(s):  
K. J. Meech ◽  
J. Svoreň
Keyword(s):  
Chemical Evolution ◽  
Cometary Nuclei ◽  
Physical And Chemical ◽  
Cometary Activity

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.

scholarly journals Evolution of the star formation efficiency in galaxies

2012 ◽  
Vol 10 (H16) ◽  
pp. 341-341
Author(s):  
Jonathan Braine
Keyword(s):  
Star Formation ◽  
Chemical Evolution ◽  
Conversion Factor ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Molecular Gas ◽  
Present Evidence ◽  
Ngc 6822 ◽  
Physical And Chemical ◽  
Formation Efficiency

AbstractThe physical and chemical evolution of galaxies is intimately linked to star formation, We present evidence that molecular gas (H2) is transformed into stars more quickly in smaller and/or subsolar metallicity galaxies than in large spirals – which we consider to be equivalent to a star formation efficiency (SFE). In particular, we show that this is not due to uncertainties in the N(H2)/Ico conversion factor. Several possible reasons for the high SFE in galaxies like the nearby M33 or NGC 6822 are proposed which, separately or together, are the likely cause of the high SFE in this environment. We then try to estimate how much this could contribute to the increase in cosmic star formation rate density from z = 0 to z = 1.

Cometary evolution and cryovolcanism 1This article is part of a Special issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

2012 ◽  
Vol 90 (8) ◽  
pp. 807-815 ◽  
Author(s):  
Michael J.S. Belton
Keyword(s):  
Spatial Distribution ◽  
Production Rate ◽  
Large Scale ◽  
Special Issue ◽  
Apparent Absence ◽  
Cometary Nuclei ◽  
Evolutionary Hypothesis ◽  
Wide Range ◽  
Show Evidence ◽  
Cometary Activity

Recent space observations of cometary nuclei show evidence of internal (cryovolcanic) activity while retaining aspects of their primitive origins. Using discoveries made during the two most recent cometary encounters: EPOXI at 103P/Hartley 2 and Stardust-NExT at 9P/Tempel 1, we test a hypothesis for their physical evolution, which, if true, could provide a unified basis for understanding the relative ages of their surfaces and the causes of a wide range of cometary activity. We show: (i) that the categorization of 103P/Hartley 2 as hyperactive is not a reflection of the extent of activity over the surface of the nucleus for which we find a normal H2O production rate; (ii) that the heterogeneous spatial distribution of CO2 and H2O in the inner comae of 9P/Tempel 1 and 103P/Hartley 2 is best explained by processes associated with cometary activity rather than the presence of primitive compositional heterogeneities in the nucleus; and (iii) that most of the quasi-circular depressions seen on the surface of 9P/Tempel are the result of outburst activity. The apparent absence of circular depressions and large scale layering on 103P/Hartley 2 present a challenge to the evolutionary hypothesis although the small size of its nucleus may ultimately provide an explanation.

Physical and chemical evolution of PMDA-ODA during thermal imidization

1995 ◽  
Vol 33 (4) ◽  
pp. 559-569 ◽  
Author(s):  
Jaime R. Ojeda ◽  
Juliana Mobley ◽  
David C. Martin
Keyword(s):  
Chemical Evolution ◽  
Thermal Imidization ◽  
Physical And Chemical

PETROLEUM SOURCE ROCKS IN THE ROPER GROUP OF THE MCARTHUR BASIN: SOURCE CHARACTERISATION AND MATURITY DETERMINATIONS USING PHYSICAL AND CHEMICAL METHODS

1994 ◽  
Vol 34 (1) ◽  
pp. 279 ◽  
Author(s):  
Dennis Taylor ◽  
Aleksai E. Kontorovich ◽  
Andrei I. Larichev ◽  
Miryam Glikson
Keyword(s):  
Organic Matter ◽  
Chemical Evolution ◽  
Oil And Gas ◽  
Source Rocks ◽  
Type I ◽  
Peak Oil ◽  
Gas Generation ◽  
Oil Generation ◽  
Physical And Chemical ◽  
Extractable Organic Matter

Organic rich shale units ranging up to 350 m in thickness with total organic carbon (TOC) values generally between one and ten per cent are present at several stratigraphic levels in the upper part of the Carpentarian Roper Group. Considerable variation in depositional environment is suggested by large differences in carbon:sulphur ratios and trace metal contents at different stratigraphic levels, but all of the preserved organic matter appears to be algal-sourced and hydrogen-rich. Conventional Rock-Eval pyrolysis indicates that a type I-II kerogen is present throughout.The elemental chemistry of this kerogen, shows a unique chemical evolution pathway on the ternary C:H:ONS diagram which differs from standard pathways followed by younger kerogens, suggesting that the maturation histories of Proterozoic basins may differ significantly from those of younger oil and gas producing basins. Extractable organic matter (EOM) from Roper Group source rocks shows a chemical evolution from polar rich to saturate rich with increasing maturity. Alginite reflectance increases in stepwise fashion through the zone of oil and gas generation, and then increases rapidly at higher levels of maturation. The increase in alginite reflectance with depth or proximity to sill contacts is lognormal.The area explored by Pacific Oil and Gas includes a northern area where the Velkerri Formation is within the zone of peak oil generation and the Kyalla Member is immature, and a southern area, the Beetaloo sub-basin, where the zone of peak oil generation is within the Kyalla Member. Most oil generation within the basin followed significant folding and faulting of the Roper Group.

scholarly journals Gas and dust from metal-rich AGB stars

2020 ◽  
Vol 641 ◽  
pp. A103
Author(s):  
P. Ventura ◽  
F. Dell’Agli ◽  
M. Lugaro ◽  
D. Romano ◽  
M. Tailo ◽  
...  
Keyword(s):  
Chemical Evolution ◽  
Main Sequence ◽  
Carbon Star ◽  
Asymptotic Giant Branch ◽  
Chemical Compositions ◽  
Agb Stars ◽  
Dust Production ◽  
Lower Mass ◽  
Nuclear Burning ◽  
Physical And Chemical

Context. Stars evolving through the asymptotic giant branch (AGB) phase provide significant feedback to their host system, which is both gas enriched in nuclear-burning products, and dust formed in their winds, which they eject into the interstellar medium. Therefore, AGB stars are an essential ingredient for the chemical evolution of the Milky Way and other galaxies. Aims. We study AGB models with super-solar metallicities to complete our vast database, so far extending from metal-poor to solar-chemical compositions. We provide chemical yields for masses in the range 1−8 M⊙ and metallicities Z = 0.03 and Z = 0.04. We also study dust production in this metallicity domain. Methods. We calculated the evolutionary sequences from the pre-main sequence through the whole AGB phase. We followed the variation of the surface chemical composition to calculate the chemical yields of the various species and model dust formation in the winds to determine the dust production rate and the total dust mass produced by each star during the AGB phase. Results. The physical and chemical evolution of the star is sensitive to the initial mass: M >  3 M⊙ stars experience hot bottom burning, whereas the surface chemistry of the lower mass counterparts is altered only by third dredge-up. The carbon-star phase is reached by 2.5−3.5 M⊙ stars of metallicity Z = 0.03, whereas all the Z = 0.04 stars (except the 2.5 M⊙) remain O-rich for the whole AGB phase. Most of the dust produced by metal-rich AGBs is in the form of silicate particles. The total mass of dust produced increases with the mass of the star, reaching ∼0.012 M⊙ for 8 M⊙ stars.

scholarly journals 3. The Constitution of Cometary Nuclei

1977 ◽  
Vol 39 ◽  
pp. 25-35
Author(s):  
F. L. Whipple
Keyword(s):  
Unit Area ◽  
Cosmic Ray ◽  
Cometary Nucleus ◽  
Radial Force ◽  
Oort Cloud ◽  
Cometary Nuclei ◽  
Basic Assumptions ◽  
Short Period ◽  
Total Energy Input ◽  
Cometary Activity

For many comets the accelerations radially from the Sun have been determined by deviations from gravitational orbits. The radial force per unit area can be calculated on the basis of certain assumptions concerning the nature of the vaporizing material, the physical circumstances, and the geometry of the cometary nucleus. The observed accelerations combined with the calculated forces yield numerical values for (1-A)/R where A is the albedo, and R the radius of the nucleus. At extreme solar distances photometry provides the well-known quantity area-times-albedo or RA1/2. Solutions are then possible for R and A for the nuclei of the comets. The derived quantity, (1-A)A1/2, provides a limiting check on the basic assumptions and, therefore, on the basic physical properties of the nuclei. For ten short-period comets with q < 1.5 AU, the observations are satisfied by H2O ice. About half show “spotty” surfaces.For comets of a single apparition H2O ice is generally not volatile enough to produce the observed radial accelerations. The resulting problems are discussed including the possibility that in some cases displacements of the photometric from the gravitational nucleus may produce spurious non-gravitational accelerations.The physical characteristics of comets vary with their orbits and with their age. New comets on their first near solar passage from the Oort cloud are extremely active. The activity falls statistically with increasing age. This sequence must represent a corresponding sequence or layering of structure from the surface of a new comet inwards and is described qualitatively in this paper.The excessive activity of new comets is ascribed to cumulative cosmic-ray damage that activates the outer few hundred gm cm-2 from the surface. The total energy input in 4.6 x 109 yr reaches 50,000 cal gm-1 near the surface so that both crystalline structures and molecules are severely damaged if not completely destroyed. Annealing at T - 10 K must be very small. Hence significant exothermic energy in the form of defects, vacancies and radicals is added to produce the extraordinary activity observed in new comets.Other aspects and problems of cometary activity are discussed.

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