We have calculated formation of gas giant planets based on the standard core accretion model including effects of fragmentation and planetary envelope. The accretion process is found to proceed as ...follows. As a result of runaway growth of planetesimals with initial radii of ∼10 km, planetary embryos with a mass of ∼10
27 g (∼ Mars mass) are found to form in ∼10
5 years at Jupiter's position (5.2 AU), assuming a large enough value of the surface density of solid material (25 g/cm
2) in the accretion disk at that distance. Strong gravitational perturbations between the runaway planetary embryos and the remaining planetesimals cause the random velocities of the planetesimals to become large enough for collisions between small planetesimals to lead to their catastrophic disruption. This produces a large number of fragments. At the same time, the planetary embryos have envelopes, that reduce energies of fragments by gas drag and capture them. The large radius of the envelope increases the collision rate between them, resulting in rapid growth of the planetary embryos. By the combined effects of fragmentation and planetary envelope, the largest planetary embryo with 21
M
⊕ forms at 5.2 AU in 3.8×10
6 years. The planetary embryo is massive enough to start a rapid gas accretion and forms a gas giant planet.
We simulate the late stages of terrestrial-planet formation using N-body integrations, in three dimensions, of disks of up to 56 initially isolated, nearly coplanar planetary embryos, plus Jupiter ...and Saturn. Gravitational perturbations between embryos increase their eccentricities,e, until their orbits become crossing, allowing collisions to occur. Further interactions produce large-amplitude oscillations ineand the inclination,i, with periods of ∼105years. These oscillations are caused by secular resonances between embryos and prevent objects from becoming re-isolated during the simulations. The largest objects tend to maintain smallereandithan low-mass bodies, suggesting some equipartition of random orbital energy, but accretion proceeds by orderly growth. The simulations typically produce two large planets interior to 2 AU, whose time-averagedeandiare significantly larger than Earth and Venus. The accretion rate falls off rapidly with heliocentric distance, and embryos in the “Mars zone” (1.2 <a< 2 AU) are usually scattered inward and accreted by “Earth” or “Venus,” or scattered outward and removed by resonances, before they can accrete one another. The asteroid belt (a> 2 AU) is efficiently cleared as objects scatter one another into resonances, where they are lost via encounters with Jupiter or collisions with the Sun, leaving, at most, one surviving object. Accretional evolution is complete after 3 × 108years in all simulations that include Jupiter and Saturn. The number and spacing of the final planets, in our simulations, is determined by the embryos' eccentricities, and the amplitude of secular oscillations ine, prior to the last few collision events.
Despite moderate heritability, only one study has identified genome-wide significant loci for cannabis-related phenotypes. We conducted meta-analyses of genome-wide association study data on 2080 ...cannabis-dependent cases and 6435 cannabis-exposed controls of European descent. A cluster of correlated single-nucleotide polymorphisms (SNPs) in a novel region on chromosome 10 was genome-wide significant (lowest P=1.3E-8). Among the SNPs, rs1409568 showed enrichment for H3K4me1 and H3K427ac marks, suggesting its role as an enhancer in addiction-relevant brain regions, such as the dorsolateral prefrontal cortex and the angular and cingulate gyri. This SNP is also predicted to modify binding scores for several transcription factors. We found modest evidence for replication for rs1409568 in an independent cohort of African American (896 cases and 1591 controls; P=0.03) but not European American (EA; 781 cases and 1905 controls) participants. The combined meta-analysis (3757 cases and 9931 controls) indicated trend-level significance for rs1409568 (P=2.85E-7). No genome-wide significant loci emerged for cannabis dependence criterion count (n=8050). There was also evidence that the minor allele of rs1409568 was associated with a 2.1% increase in right hippocampal volume in an independent sample of 430 EA college students (fwe-P=0.008). The identification and characterization of genome-wide significant loci for cannabis dependence is among the first steps toward understanding the biological contributions to the etiology of this psychiatric disorder, which appears to be rising in some developed nations.
Several studies have identified genes associated with alcohol-use disorders (AUDs), but the variation in each of these genes explains only a small portion of the genetic vulnerability. The goal of ...the present study was to perform a genome-wide association study (GWAS) in extended families from the Collaborative Study on the Genetics of Alcoholism to identify novel genes affecting risk for alcohol dependence (AD). To maximize the power of the extended family design, we used a quantitative endophenotype, measured in all individuals: number of alcohol-dependence symptoms endorsed (symptom count (SC)). Secondary analyses were performed to determine if the single nucleotide polymorphisms (SNPs) associated with SC were also associated with the dichotomous phenotype, DSM-IV AD. This family-based GWAS identified SNPs in C15orf53 that are strongly associated with DSM-IV alcohol-dependence symptom counts (P=4.5 × 10(-8), inflation-corrected P=9.4 × 10(-7)). Results with DSM-IV AD in the regions of interest support our findings with SC, although the associations were less significant. Attempted replications of the most promising association results were conducted in two independent samples: nonoverlapping subjects from the Study of Addiction: Genes and Environment (SAGE) and the Australian Twin Family Study of AUDs (OZALC). Nominal association of C15orf53 with SC was observed in SAGE. The variant that showed strongest association with SC, rs12912251 and its highly correlated variants (D'=1, r(2) 0.95), have previously been associated with risk for bipolar disorder.
An earlier investigation of the formation of approximately 10(26) g planetary embryos from much smaller planetesimals (G.W. Wetherill and G.R. Stewart 1989, Icarus 77, 350-357) has been extended to ...include the effects of collisional fragmentation, the low relative velocity regime in which the effects due to solar gravity are important, and independent perturbations of eccentricity and inclination. In agreement with this earlier work, it if found that at 1 AU runaway growth occurs on a approximately 10(-5)-year time scale as a consequence of equipartition of energy between large and small planetesimals. It is now seen that the runaway is initiated after approximately 10(4) years, when the relative velocities of the larger bodies temporarily fall into the low-velocity regime, lowering their inclinations and increasing their gravitational capture rates. After approximately 2 X 10(4) years, relative velocities between most bodies emerge from the low-velocity regime, and these higher velocities tend to inhibit further runaway growth. This rapid runaway growth is self-regulated, however, by these same higher velocities, causing fragmentation of the smaller bodies. The velocities of the collision fragments are reduced by gas drag, facilitating their capture by the growing runaway embryos. Variations in which different fragmentation models are used, or long-range forces between nonrunaway bodies are absent, give similar results. When fragmentation is not included, the time scale for growth increases to approximately 3 X 10(5) years as a result of loss of the self-regulating process described above.
Planets in the asteroid belt CHAMBERS, J. E.; WETHERILL, G. W.
Meteoritics & planetary science,
March 2001, Letnik:
36, Številka:
3
Journal Article
Recenzirano
Odprti dostop
— The main asteroid belt has lost >99.9% of its solid mass since the time at which the planets were forming, according to models for the protoplanetary nebula. Here we show that the primordial ...asteroid belt could have been cleared efficiently if much of the original mass accreted to form planetsized bodies, which were capable of perturbing one another into unstable orbits. We provide results from 25 N‐body integrations of up to 200 planets in the asteroid belt, with individual masses in the range 0.017–0.33 Earth masses. In the simulations, these bodies undergo repeated close encounters which scatter one another into unstable resonances with the giant planets, leading to collision with the Sun or ejection from the solar system. In response, the giant planets' orbits migrate radially and become more circular. This reduces the size of the main‐belt resonances and the clearing rate, although clearing continues. If ∼3 Earth masses of material was removed from the belt this way, Jupiter and Saturn would initially have had orbital eccentricities almost twice their current values. Such orbits would have made Jupiter and Saturn 10–100x more effective at clearing material from the belt than they are on their current orbits. The time required to remove 90% of the initial mass from the belt depends sensitively on the giant planets' orbits, and weakly on the masses of the asteroidal planets. 18 of the 25 simulations end with no planets left in the belt, and the clearing takes up to several hundred million years. Typically, the last one or two asteroidal planets are removed by interactions with planets in the terrestrial region
The Stability of Multi-Planet Systems Chambers, J.E.; Wetherill, G.W.; Boss, A.P.
Icarus (New York, N.Y. 1962),
02/1996, Letnik:
119, Številka:
2
Journal Article
Recenzirano
A system of two small planets orbiting the Sun on low-eccentricity, low-inclination orbits is stable with respect to close encounters if the initial semi-major axis difference, Δ, measured in mutual ...Hill radii,RH, exceedsformula, due to conservation of energy and angular momentum. We investigate the stability of systems of more than two planets using numerical integrations. We find that systems with Δ < 10 are always unstable, with the time,t, of first close encounter given approximately by logt=bΔ +c, wherebandcare constants. It is likely that systems with Δ > 10 are also unstable. The slopebdepends weakly on the number of planets, but is independent of planetary mass,m, if we measure Δ in units that are proportional tom1/4rather than the usualRH∝m1/3. Instability in multi-planet systems arises because energy and angular momentum are no longer conserved within each two-planet subsystem due to perturbations by the additional planet(s). These results suggest that planetary embryos will not become isolated prior to the final stage of terrestrial-planet formation simply due to a failure to achieve close encounters. Other factors leading to isolation cannot be ruled out at this stage.
About 30% of detected extrasolar planets exist in multiple-star systems. The standard model of planet formation cannot easily accommodate such systems and has difficulty explaining the odd orbital ...characteristics of most extrasolar giant planets. We demonstrate that the formation of terrestrial-size planets may be insulated from these problems, enabling much of the framework of the standard model to be salvaged for use in complex systems. A type of runaway growth is identified that allows planetary embryos to form by a combination of nebular gas drag and perturbations from massive companions-be they giant planets, brown dwarfs, or other stars.
A quantitative numerical program, developed to model the formation of the terrestrial planets and asteroids of our solar System (Wetherill 1992), has been extended to include a more general range of ...stellar and preplanetary nebular parameters, as may be expected elsewhere in the Galaxy and Universe. The results of about 500 new simulations of planetary formation are reported. It is found that for circumstellar disk parameters that are not too different from those of the Solar System, the number and radial distribution of final terrestrial planets are insensitive to stellar mass and these planets concentrate in the vicinity of 1 AU. In contrast, the position of the biologically habitable heliocentric distances are strongly dependent on stellar mass (Kastinget al.1993), and the frequency of habitable planets is therefore also dependent on stellar mass. Stars of 1.0 solar mass (M⊙) almost always have at least one planet of mass >1/3 Earth mass (M⊕) in their habitable zones. Larger planets of the smaller stars tend to be too cold, those of the larger stars too hot. Nevertheless, some ∼5 to 15% of the simulated planetary systems associated with stars as small as 0.5M⊙and as large as 1.5M⊙contain a habitable planet. The position and number of simulated terrestrial planets are also insensitive to the initial surface density of solid bodies in the circumstellar disk, but the size of the planets is approximately proportional to the surface density. These results represent planetary systems associated with radial variation of surface densities, disk parameters, and giant planet populations not very different from those of our Solar System. The outer boundary of their “peak” in semimajor axis distribution at 1 AU is determined by the increasing proximity of more distant bodies to strong resonances by Jupiter and Saturn. As a consequence, this boundary will move in or out in accordance with the position, or absence, of such bodies in other systems. In the complete absence of Jupiter, the median planetary mass in the terrestrial planet region is almost 2M⊕, for the same initial surface density used in the models characteristic of our Solar System. The inner boundary is determined by the minimum distance at which planet-forming solids condense in the disk. For some, not necessarily likely, variations in these parameters, abundant populations of habitable planets can be obtained for all the values of stellar mass considered.
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