We have developed a computer code (GEM—grain evolution model) to simulate the behavior of ice grains in a comet coma. The grains are assumed to be composed of water–ice with an admixture of dark ...material (“dirt”). An initial size distribution of grains is assumed to be ejected from the nucleus. The ejected mass is taken to be proportional to the rate of gas production by the nucleus. The efficiency for absorption and re-radiation of sunlight is computed from Mie scattering theory. The grain temperature and sublimation rate at a given heliocentric distance is then derived from energy balance considerations. The evolution of the grain size distribution is followed as a function of distance from the nucleus.
A fully 3-dimensional implicit numerical model for comet nucleus evolution is presented, emphasizing dust mantle formation. A spherical configuration is considered with an initial composition of ...amorphous H
2O ice and dust, taking into account a discrete dust-grain size distribution. The model is applied to Comet 67P/Churyumov–Gerasimenko, adopting its orbital elements, rotation period and rotation axis inclination. We find that the dust mantle thickness varies over the surface from 1 cm to about 10 cm (thus lower and higher than the diurnal skin-depth, respectively). The size distribution of ejected grains varies along the orbit and is steeper than the initial one adopted for the nucleus. The crystallization front advances inward in spurts, and its depth varies between 1 and several meters. We test the effect of the thermal conductivity on the surface temperature distribution and depths of the dust mantle and crystallization front.
We defend the position taken in our earlier note that under certain conditions the D/H ratio measured in the coma of a comet can be much higher that the D/H ratio in to cometary ice itself.
Observational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in ...Extreme Horizontal Branch (EHB) and subsequent evolutionary phases. Using our new stellar evolution code – a code that follows through complete evolutionary tracks, Pre-MS to cooling WD – without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs. HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 − 1.1 M⊙) and for metallicities covering both populations I and II (Z = 0.01 − 0.001). The M,Z combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr.
We report on the development of a new stellar evolution code, and provide a taste of results, showing its capability to calculate full evolutionary tracks for a wide range of masses and metalicities. ...The code is fast and efficient, and is capable of following through all evolutionary phases, including core/shell flashes and thermal pulses, without any interruption or intervention. It is meant to be used also in the context of modeling the evolution of dense stellar systems, for performing live calculations for both ‘normal’ ZAMS/PRE-MS models, but mainly for ‘non-canonical’ stellar configurations (i.e. merger-products). We show a few examples of evolutionary calculations for stellar populations I and II, and for masses in the range 0.25–64 M⊙.
We present a new stellar evolution code and a set of results, showing its capability to calculate full evolutionary tracks for a wide range of masses and metalicities. The code is meant to be used ...also in the context of modeling the evolution of dense stellar systems, for performing live evolutionary calculations both for ‘normal’ ZAMS/PRE-MS models, but mainly for ‘non-canonical’ (i.e. merger-products) stellar configurations. For such tasks, it has to be robust and efficient, capable to run through all phases of stellar evolution without interruption or intervention. Here we show a few examples of evolutionary calculations for stellar populations I and II, and for masses in the range 0.25–64 M⊙.
Abstract
Centaur 29P/Schwassmann–Wachmann 1 (SW1) is a highly active object orbiting in the transitional “Gateway” region between the Centaur and Jupiter-family comet (JFC) regions. SW1 is unique ...among the Centaurs in that it experiences quasi-regular major outbursts and produces CO emission continuously; however, the source of the CO is unclear. We argue that, due to its very large size (∼32 km radius), SW1 is likely still responding, via amorphous water ice (AWI) conversion to crystalline water ice (CWI), to the “sudden” change in its external thermal environment produced by its Myrs-long dynamical migration from the Kuiper Belt to its current location at the inner edge of the Centaur region. It is this conversion process that is the source of the abundant CO and dust released from the object during its quiescent and outburst phases. If correct, these arguments have a number of important predictions testable via remote sensing and in situ spacecraft characterization, including the quick release on Myr timescales of CO from AWI conversion for any few kilometer-scale scattered disk Kuiper Belt Objects transiting into the inner system; that to date SW1 has only converted between 50% and 65% of its nuclear AWI to CWI; that volume changes on AWI conversion could have caused subsidence and cave-ins, but not significant mass wasting or crater loss; that SW1's coma should contain abundant amounts of CWI+CO
2
“dust” particles; and that when SW1 transits into the inner system within the next 10,000 yr, it will be a very different kind of JFC.
Modeling the Volcanism on Mars Weizman, A.; Stevenson, D.J.; Prialnik, D. ...
Icarus (New York, N.Y. 1962),
04/2001, Letnik:
150, Številka:
2
Journal Article
Recenzirano
The total amount of melt produced in Mars during its evolution is estimated by means of a parameterized, one-dimensional, analytic mantle convection model that assumes a stagnant lid and whole mantle ...convection. The fertility of the mantle—defined as the potential to create basalt—and its variation with time are taken into account. The model is composed of core, mantle, and lithosphere, with two boundary layers separating them. The contributions to volcanism by pressure release melting (PRM), and by plumes from the core–mantle boundary layer, are compared and discussed. We show that such models tend to produce considerable melting during the early evolution of the planet, and that the amount of melting depends strongly on the abundances of radioactive elements. Although the model's assumptions may not be valid for the early evolution of the planet, the model is relevant to the later history, which is insensitive to initial conditions. We find that PRM volcanism should have ceased between 1 and 2.5 Byr ago and any recent volcanic activity must have originated in plumes.
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