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1.
Iron meteorites provide a record of the thermal evolution of their parent bodies, with cooling rates inferred from the structures observed in the Widmanstätten pattern. Traditional planetesimal thermal models suggest that meteorite samples derived from the same iron core would have identical cooling rates, possibly providing constraints on the sizes and structures of their parent bodies. However, some meteorite groups exhibit a range of cooling rates or point to uncomfortably small parent bodies whose survival is difficult to reconcile with dynamical models. Together, these suggest that some meteorites are indicating a more complicated origin. To date, thermal models have largely ignored the effects that impacts would have on the thermal evolution of the iron meteorite parent bodies. Here we report numerical simulations investigating the effects that impacts at different times have on cooling rates of cores of differentiated planetesimals. We find that impacts that occur when the core is near or above its solidus, but the mantle has largely crystallized can expose iron near the surface of the body, leading to rapid and nonuniform cooling. The time period when a planetesimal can be affected in this way can range between 20 and 70 Myr after formation for a typical 100 km radius planetesimal. Collisions during this time would have been common, and thus played an important role in shaping the properties of iron meteorites.  相似文献   

2.
The cooling and reheating histories of dim isolated neutron stars (DINs) are discussed. Energy dissipation due to dipole spindown with ordinary and magnetar fields, and due to torques from a fallback disk are considered as alternative sources of reheating which would set the temperature of the neutron star after the initial cooling era. Cooling or thermal ages are related to the numbers and formation rates of the DINs and therefore to their relations with other isolated neutron star populations. The possibility of energy dissipation at ages greater than about 106 yrs is a potentially important factor in determining the properties of the DIN population. Interaction with a fallback disk, higher multipole fields and activity of the neutron star are briefly discussed.   相似文献   

3.
Results are presented which show the final abundances of the major nuclei produced in explosive, but partial, carbon-burning taking place in a rapidly expanding medium. the effects of energy deposition due to burning and expansive cooling are taken into account explicity. The results are compatible with those obtained by other authors but show the sensitivity of the final abundances to the assumed initial conditions. A conjecture is made as to the physical site of the burning.Of the National Bureau of Standards and University of Colorado.  相似文献   

4.
A new theory is proposed to explain global cooling at the onset of Pleistocene glacial periods. Atmospheric CO2 drawdown is considered to be the driving force behind global cooling, brought about by heightened productivity at the equatorial divergences and along continental margins, particularly in upwelling regions. Eutrophication appears to be triggered when global warming during late interglacial periods causes accelerated melting of the West Antarctic Ice Sheet. This would release large reserves of silicate-enriched subglacial meltwaters into the surrounding oceans where entrainment would take place into deep and intermediate currents forming in Antarctic and subantarctic waters. Subsequent advection, mixing and upwelling of silicate-enriched deep and intermediate waters into the coastal zones and open-ocean divergences results in the proliferation of large, rapidly-sinking diatom species with a high affinity for dissolved silicate. These blooms enhance rates of recycling of N and P in upwelling regions and accelerate rates of organic carbon production, export and sequestration in shelf and slope sediments and in the deep sea. The resultant atm. CO2 drawdown initiates global cooling. Consequent expansion of Northern Hemisphere glaciers lowers sea level, while increased temperature and pressure gradients between equatorial and polar regions intensify meridional winds. The former process exposes nutrient-enriched coastal sediments to wave erosion, thereby releasing new nutrient supplies, while the latter process enhances upwelling. The combined effect is to greatly increase rates of org. C production and export from continental margins and further accelerate atm. CO2 drawdown. Glacial-period cooling is also enhanced by a number of other positive feedbacks, including changes in albedo, water vapour and cloud cover. Episodic warming intervals during glacial periods may be related to insolation changes associated with orbital precession and tilt cycles, but processes involved in deglaciation and reversion to the interglacial climatic regime are complex and not yet fully understood.  相似文献   

5.
Partial thermal remanence experiments on lunar igneous rocks indicate that the magnetization of lunar rocks is not a normal single component thermoremanent magnetization. The magnetization therefore may not have been acquired at the time of initial cooling of the rock and thus should be used cautiously in making estimates of the intensity of the ancient lunar magnetic field.Contribution No. 201, Geosciences Division, The University of Texas at Dallas.  相似文献   

6.
Chandra and XMM-Newton observations have confirmed the presence of large temperature gradients within the cores of many relaxed clusters of galaxies. Here we investigate whether thermal conduction operating over those gradients can supply sufficient heat to offset radiative cooling. Narayan & Medredev and Gruzinov have noted, using published results on cluster temperatures, that conduction within a factor of a few of the Spitzer rate is sufficient to balance bremsstrahlung cooling. From a detailed study of the temperature and emission measure profiles of Abell 2199 and Abell 1835, we find that the heat flux required by conduction is consistent with or below the rate predicted by Spitzer in the outer regions of the core. Conduction may therefore explain the lack of observational evidence for large mass cooling rates inferred from arguments based simply on radiative cooling, provided that conductivity is suppressed by no more than a factor of 3 below the full Spitzer rate. To stem cooling in the central 20 kpc however, would necessitate conductivity values of at least a factor of 2 larger than the Spitzer values, which we consider implausible. This may provide an explanation for the observed star formation and optical nebulosities in cluster cores. The solution is likely to be time-dependent. We briefly discuss the possible origin of the cooler gas and the implications for massive galaxies.  相似文献   

7.
Abstract— Motivated by recent observations of T-Tauri stars and the interpretation of these observations in terms of the properties of circumstellar disks, we derive internal (midplane) temperatures for disks around mature (age ～1 Ma) T-Tauri stars. The estimates are obtained by combining published results for disk masses, sizes, accretion rates, and surface temperatures. For 26 stars (for which adequate data are available), we derive midplane temperatures at 1 AU primarily in the range 200–800 K, and 100–400 K at 2.5 AU. It is likely that the solar nebula, at the same stage of evolution, contained planetesimals and objects destined to become meteorite parent bodies. Observations of young stellar objects at earlier stages of evolution (age ～0.1 Ma) imply that accretion rates were, on the average, at least two orders of magnitude greater than the 10?8 M⊙/year rates typical for mature T-Tauri stars. Such high values would result in midplane temperatures at or near the silicate vaporization temperature in the terrestrial planet region. If cooling of the solar nebula from such a hot epoch was responsible for establishing the pervasive elemental fractionation patterns found in chondritic meteorites, then objects in the asteroid belt must have grown rapidly (within 0.1 Ma) to sizes of ～1 km, a conclusion consistent with current theories of planetesimal formation. However, the fact that primitive meteorite parent bodies escaped being melted by the decay of 26Al then implies that further growth of at least some objects was essentially delayed for 2 Ma or more. Such a diminished growth rate appears to be consistent with simulations of the dynamics of solid bodies in the asteroid belt. Other hypotheses seem less attractive. One might assume that the final cooling occurred only after the decay of 26Al (i.e., more than a million years after calcium-aluminum rich inclusion formation), or that 26Al was not ubiquitous in the early solar system. But the first of these conjectures is incompatible with astronomical observations of T-Tauri systems, and the second appears to be contradicted by the evidence for 26Al in diverse meteoritic components. The remaining alternative would then appear to be that, despite a lack of supporting evidence, chondritic fractionation patterns reflect the net effect of many local heating and cooling events and have nothing to do with global nebular cooling. We conclude that the most plausible hypothesis is that both nebular cooling and coagulation of solids to kilometer-sized objects occurred rapidly and that a substantial number of planetesimals in the asteroid belt remained smaller than a few kilometers in radius for at least 2 Ma.  相似文献   

8.
We use recent X-ray observations of the intracluster medium(ICM) of the galaxy group NGC 5813 to confront theoretical studies of ICM thermal evolution with the newly derived ICM properties.We argue that the ICM of the cooling flow in the galaxy group NGC 5813 is more likely to be heated by mixing of post-shock gas from jets residing in hot bubbles with the ICM,than by shocks or turbulentheating.Shocks thermalize only a small fraction of their energy in the inner regions of the cooling flow;in order to adequately heat the inner part of the ICM,they would overheat the outer regions by a large factor,leading to its ejection from the group.Heating by mixing,which was found to be much more efficient than turbulent-heating and shocks-heating,hence,rescues the outer ICM of NGC 5813 from its predestined fate according to cooling flow feedback scenarios that are based on heating by shocks.  相似文献   

9.
We explore the implications of a possible cosmic-ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of   z ∼ 15–20  , provided the CR flux was sufficient to yield an ionization rate greater than about 10−19 s−1 near the centre of the minihalo. The presence of CRs with energies  ≲107  eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as  ∼10 M⊙  . CRs have a less-pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses  ≳108 M⊙  at the later stages of cosmological structure formation around   z ∼ 10–15  . In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.  相似文献   

10.
Abstract— We investigate the possible formation of chondrules by planetesimal bow shocks. The formation of such shocks is modeled using a piecewise parabolic method (PPM) code under a variety of conditions. The results of this modeling are used as a guide to study chondrule formation in a one‐dimensional, finite shock wave. This model considers a mixture of chondrule‐sized particles and micron‐sized dust and models the kinetic vaporization of the solids. We found that only planetesimals with a radius of ?1000 km and moving at least ?8 km/s with respect to the nebular gas can generate shocks that would allow chondrule‐sized particles to have peak temperatures and cooling rates that are generally consistent with what has been inferred for chondrules. Planetesimals with smaller radii tend to produce lower peak temperatures and cooling rates that are too high. However, the peak temperatures of chondrules are only matched for low values of chondrule wavelength‐averaged emissivity. Very slow cooling (<?100s of K/hr) can only be achieved if the nebular opacity is low, which may result after a significant amount of material has been accreted into objects that are chondrule‐sized or larger, or if chondrules formed in regions of the nebula with small dust concentrations. Large shock waves of approximately the same scale as those formed by gravitational instabilities or tidal interactions between the nebula and a young Jupiter do not require this to match the inferred thermal histories of chondrules.  相似文献   

11.
We present numerical simulations of the thermal and dynamical histories of solid particles (chondrules and their precursors—treated as 1-mm silicate spheres) during passage of an adiabatic shock wave through a particle-gas suspension in a minimum-mass solar nebula. The steady-state equations of energy, momentum, and mass conservation are derived and integrated for both solids and gas under a variety of shock conditions and particle number densities using the free-molecular-flow approximation. These simulations allow us to investigate both the heating and cooling of particles in a shock wave and to compare the time and distance scales associated with their processing to those expected for natural chondrules. The interactions with the particles cause the gas to achieve higher temperatures and pressures both upstream and downstream of the shock than would be reached otherwise. The cooling rates of the particles are found to be nonlinear but agree approximately with the cooling rates inferred for chondrules by laboratory simulations. The initial concentration of solids upstream of the shock controls the cooling rates and the distances over which they are processed: Lower concentrations cool more slowly and over longer distances. These simulations are consistent with the hypothesis that large-scale shocks, e.g., those due to density waves or gravitational instabilities, were the dominant mechanism for chondrule formation in the nebula.  相似文献   

12.
Evidence of three kinds indicates a lunar compositional asymmetry: (1) mare basalts are much more abundant on the near side; (2) the incompatible rich KREEP component is mainly observed in near-side soils; and (3) materials on the far side are less dense than those of the near side, as indicated by the 2-km offset between the center of mass and center of figure. Recent models to explain the 2-km offset are based on near-side-far-side differences in the thickness of crustal units. The most widely discussed model calls for a thickness of anorthosite ～ 24 km greater on the far side than on the near side, but no satisfactory method of generating such a large difference has been proposed. We suggest that much of the offset reflects longitudinal differences in mantle composition primarily resulting from earlier (or more rapid) crystallization of the magma ocean on what is now the far-side hemisphere. As a result, the far-side mantle would be more magnesian and thus less dense than the near-side mantle. Differences in the amount of anorthosite or the amount of crustal porosity probably make relatively small contributions to the offset. We have evaluated four scenarios: (a) If the anorthositic crust initially formed a floating continent over what is now the near side, this would have provided thermal insulation that would have reduced the near-side cooling rate. (b) Crystallization of the magma ocean while the Moon was near the Earth would have resulted in heating by earthshine, thus reducing the near-side cooling rate. (c) An asymmetric bombardment could have preferentially heated the near side. (d) A suggestion by D. Stevenson, collection of metal from the magma ocean in one hemisphere would have pushed the unmelted “core” into an asymmetric position, and resulted in earlier magma ocean crystallization in the deeper hemisphere. Our assessment is that the asymmetric “core” hypothesis is the most plausible, that the floating continent mechanism is possible, and that the Earthshine and symmetric bombardment mechanisms are not viable. An attractive feature of the asymmetric- mantle model is that it also accounts for the asymmetries in the distribution of KREEP and mare basalts. More rapid crystallization of the far side would leave urKREEP, the last dregs of the magma ocean, concentrated under the near-side crust, thus leading to the observed tendency for KREEP to be found on the near side surface. Further, the concentration of urKREEP-associated radiactive elements on the near side would result in a much lower rate of conductive cooling of the near-side mantle and thus a much longer period of basalt extrusion on the near side. The formation of basalts would also be enhanced by the presence of more fusible materials in the near-side mantle.  相似文献   

13.
If a molten, or partially molten, lunar core exists at present, constraints would be placed on the viscosity of the solid mantle and the distribution of radioactive heat sources. Models in which the heat sources have been concentrated near the surface would rapidly solidify if the effective viscosity was equal to, or less than, 1022 cm2 s−1. Retention of most of the heat sources throughout the mantle would permit present day solid convection to occur without cooling the core.  相似文献   

14.
We report on a 120-ks XMM–Newton observation of the galaxy cluster Abell 2597 (A2597). Results from both the European Photon Imaging Camera (EPIC) and the Reflection Grating Spectrometer (RGS) are presented. From EPIC we obtain radial profiles of temperature, density and abundance, and use these to derive cooling time and entropy. We illustrate corrections to these profiles for projection and point spread function (PSF) effects. At the spatial resolution available to XMM–Newton , the temperature declines by around a factor of 2 in the central 150 kpc or so in radius, and the abundance increases from about one-fifth to over one-half solar. The cooling time is less than 10 Gyr inside a radius of 130 kpc. EPIC fits to the central region are consistent with a cooling flow of around 100 solar masses per year. Broad-band fits to the RGS spectra extracted from the central 2 arcmin are also consistent with a cooling flow of the same magnitude; with a preferred low-temperature cut-off of essentially zero. The data appear to suggest (albeit at low significance levels below formal detection limits) the presence of the important thermometer lines from Fe  xvii at 15–17 Å rest wavelength, characteristic of gas at temperatures ∼0.3 keV. The measured flux in each line is converted to a mass-deposition estimate by comparison with a classical cooling flow model, and once again values at the level of 100 solar masses per year are obtained. These mass-deposition rates, whilst lower than those of previous generations of X-ray observatories, are consistent with those obtained from ultraviolet data for this object. This raises the possibility of a classical cooling flow, at the level of around 100 solar masses per year, cooling from 4 keV by more than two orders of magnitude in temperature.  相似文献   

15.
Chondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first‐order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the preshock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron‐sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron‐sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule‐forming regions would rapidly coagulate into fractal aggregates. Preshock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate‐sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.  相似文献   

16.
In 6 of the 7 instances where posteclipse brightening of Io has been reported by observers using blue filters, a major solar flare occurred within 10° of the sub-Jovian longitude in the 100-day interval prior to observation. In none of the 18 instances where no posteclipse brightening was observed did such a flare occur. It is proposed that a phenomenon associated with a major solar flare causes an increase in the trapped particle flux at Io's orbit by an order of magnitude. The posteclipse brightening may be caused by thermoluminescence of Io's surface material upon emergence. Alternatively, it is possible that the increase in trapped particle flux would warm the surface, creating a temporary atmosphere which would precipitate during eclipse cooling and vaporize in the period of warming after reemergence.  相似文献   

17.
Type Ia supernovae (SNe Ia) play a key role in measuring cosmological pa- rameters, in which the Phillips relation is adopted. However, the origin of the relation is still unclear. Several parameters are suggested, e.g. the relative content of carbon to oxygen (C/O) and the central density of the white dwarf (WD) at ignition. These parameters are mainly determined by the WD's initial mass and its cooling time, respectively. Using the progenitor model developed by Meng Yang, we present the distributions of the initial WD mass and the cooling time. We do not find any correlation between these parameters. However, we notice that as the range of the WD's mass decreases, its average value increases with the cooling time. These results could provide a constraint when simulating the SN Ia explosion, i.e. the WDs with a high C/O ratio usually have a lower central density at ignition, while those having the highest central density at ignition generally have a lower C/O ratio. The cooling time is mainly determined by the evolutionary age of secondaries, and the scatter of the cooling time decreases with the evolutionary age. Our results may indicate that WDs with a long cooling time have more uniform properties than those with a short cooling time, which may be helpful to explain why SNe Ia in elliptical galaxies have a more uniform maximum luminosity than those in spiral galaxies.  相似文献   

18.
Type Ia supernovae(SNe Ia)play a key role in measuring cosmological parameters,in which the Phillips relation is adopted.However,the origin of the relation is still unclear.Several parameters are suggested,e.g.the relative content of carbon to oxygen(C/O)and the central density of the white dwarf(WD)at ignition.These parameters are mainly determined by the WD's initial mass and its cooling time,respectively.Using the progenitor model developed by Meng & Yang,we present the distributions of the initial WD mass and the cooling time.We do not find any correlation between these parameters.However,we notice that as the range of the WD's mass decreases,its average value increases with the cooling time.These results could provide a constraint when simulating the SN Ia explosion,i.e.the WDs with a high C/O ratio usually have a lower central density at ignition,while those having the highest central density at ignition generally have a lower C/O ratio.The cooling time is mainly determined by the evolutionary age of secondaries,and the scatter of the cooling time decreases with the evolutionary age.Our results may indicate that WDs with a long cooling time have more uniform properties than those with a short cooling time,which may be helpful to explain why SNe Ia in elliptical galaxies have a more uniform maximum luminosity than those in spiral galaxies.  相似文献   

19.
《Icarus》1987,69(1):1-13
If chondritic meteorites were internally heated after accretion had ended, then the hottest material would have been buried the deepest and should have cooled the slowest. If this is correct, there ought to be a correlation between cooling rate and petrographic type, a measure of the extent to which chondrites were metamorphosed (i.e., heated). Published and new cooling rates derived from the compositions of metallic iron-nickel grains do not display this correlation, implying either that chondrite parent asteroids never had onion-shell structures or that bodies with onion-shell structures were broken up and reassembled prior to cooling to below 500°C, the temperature at which cooling-rate information is recorded in metallic iron-nickel. Chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids. Metallic iron-nickel grains in H- and L-chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to > 1000°K/myr. Based on thermal calculations, these cooling rates suggest that the materials spread out on the surfaces of H- and L-chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers. Craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them. Therefore, it appears that at least some asteroids, namely, the parent bodies of H and L chondrites, were disrupted after cooling to below 300°C, and then reassembled to create surfaces containing rocks that originated at a wide range of depths. These results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles.  相似文献   

20.
Abstract— The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases associated with the cracks and inclusions of the Allan Hills (ALH) 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer-diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may be examined, therefore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550–800 °C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (Ea(Ca) = 271 ± 80 kJ/mol) and for Mg (Ea(Mg) = 284 ± 74 kJ/mol) were used then to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 μm compositional zone would be completely homogenized at a temperature of 300 °C for cooling rates <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, they indicate that the carbonate globules were not exposed to a high-temperature environment for long time scales following formation.  相似文献   

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