Letteratura scientifica selezionata sul tema "Planetary embryos"

The influence of field performance, family, embryo morphology, and isozyme heterozygosity level on in vitro reactivity of Pinus banksiana Lamb. was evaluated on embryos from five superior families, five inferior families, and a mixed seed lot. Embryo length, number of cotyledons, and isozyme heterozygosity were determined for each embryo. Seed germination and fresh weight were determined on a family level. On average, superior families showed higher percentages of embryos that formed buds in vitro. Within each performance class, the analysis based on initial number of embryos revealed differences among families for the percentage of green embryos and embryos with adventitious buds and shoots. When calculations were based on green embryos only, i.e., excluding embryos that remained white, there were no differences among families. Thus, the overall in vitro potential of a family appears to be strongly dependent upon the capacity of embryos to turn green. On a per family basis, seed germination was positively correlated with most in vitro characters, with the exception of mean shoot length per shoot-forming embryo. Small embryos had a lower probability of producing buds and shoots, and embryos with three cotyledons showed a higher mortality than embryos with four or more cotyledons. No significant relationships were observed between heterozygosity level and in vitro reactivity, with analyses performed on green embryos only.

The improvement in broiler chicken performance is currently thanks to the genetic selection, nutrition, maintenance management, and health and biosecurity programs that have succeeded in improving the maintenance performance of broilers. Such rapid development in the cycle after hatching is undoubtedly also influenced by the embryo's development or the prenatal phase. Unlike other mammals, the process of breeding poultry has its characteristics where the process of embryonic development occurs outside the body of livestock or inside poultry eggs. Chicken embryos rely heavily on the nutrients inside the egg, which provide the energy and amino acid makeup needed for the metabolic needs of the growing embryo during the 21-day incubation process. Chicken embryos are susceptible to a lack of energy during the hatching process. This deficiency can result in weak embryos and, in more severe conditions, embryo death and failure to hatch. The nutritional needs of embryos during incubation can be added to the egg with the in ovo feeding technique. The in ovo feeding technique is a technique that allows the addition of outside nutrients injected into the egg during incubation for the developmental and metabolic needs of the embryo, as well as to improve the nutritional status of the egg. Nutrition stimulation with in ovo feeding technique produces many benefits, such as increasing hatchability, hatching weight, immunity level, reduced bone system disorders, reduced morbidity and mortality after hatching, feed efficiency and weight gain.

Context. Planetary formation and evolution is a combination of multiple interlinked processes. Constraining the mechanisms observationally requires statistical comparison to a large diversity of planetary systems. Aims. We want to understand global observable consequences of different physical processes (accretion, migration, and interactions) and initial properties (like disc masses and metallicities) on the demographics of the planetary population. We also want to study the convergence of our scheme with respect to one initial condition, the initial number of planetary embryo in each disc. Methods. We selected distributions of initial conditions that are representative of known protoplanetary discs. Then, we used the Generation III Bern model to perform planetary population synthesis. We synthesise five populations with each a different initial number of Moon-mass embryos per disc: 1, 10, 20, 50, and 100. The last is our nominal population consisting of 1000 stars (systems) that was used for an extensive statistical analysis of planetary systems around 1 M⊙ stars. Results. The properties of giant planets do not change much as long as there are at least ten embryos in each system. The study of giants can thus be done with simulations requiring less computational resources. For inner terrestrial planets, only the 100-embryos population is able to attain the giant-impact stage. In that population, each planetary system contains, on average, eight planets more massive than 1 M⊕. The fraction of systems with giants planets at all orbital distances is 18%, but only 1.6% are at >10 au. Systems with giants contain on average 1.6 such planets. The planetary mass function varies as M−2 between 5 and 50 M⊕. Both at lower and higher masses, it follows approximately M−1. The frequency of terrestrial and super-Earth planets peaks at a stellar [Fe/H] of −0.2 and 0.0, respectively, being limited at lower [Fe/H] by a lack of building blocks, and by (for them) detrimental growth of more massive dynamically active planets at higher [Fe/H]. The frequency of more massive planets (Neptunian, giants) increases monotonically with [Fe/H]. The fast migration of planets in the 5–50 M⊕ range is reduced by the presence of multiple lower-mass inner planets in the multi-embryos populations. To assess the impact of parameters and model assumptions, we also study two non-nominal populations: insitu formation without gas-driven migration, and a different initial planetesimal surface density. Conclusions. We present one of the most comprehensive simulations of (exo)planetary system formation and evolution to date. For observations, the syntheses provides a large data set to search for comparison synthetic planetary systems that show how these systems have come into existence. The systems, including their full formation and evolution tracks are available online. For theory, they provide the framework to observationally test the global statistical consequences of theoretical models for specific physical processes. This is an important ingredient towards the development of a standard model of planetary formation and evolution.

ABSTRACT Planet formation models begin with proto-embryos and planetesimals already fully formed, missing out a crucial step, the formation of planetesimals/proto-embryos. In this work, we include prescriptions for planetesimal and proto-embryo formation arising from pebbles becoming trapped in short-lived pressure bumps, in thermally evolving viscous discs to examine the sizes and distributions of proto-embryos and planetesimals throughout the disc. We find that planetesimal sizes increase with orbital distance, from ∼10 km close to the star to hundreds of kilometres further away. Proto-embryo masses are also found to increase with orbital radius, ranging from $10^{-6}{\, {\rm M}_{\oplus }}$ around the iceline, to $10^{-3}{\, {\rm M}_{\oplus }}$ near the orbit of Pluto. We include prescriptions for pebble and planetesimal accretion to examine the masses that proto-embryos can attain. Close to the star, planetesimal accretion is efficient due to small planetesimals, whilst pebble accretion is efficient where pebble sizes are fragmentation limited, but inefficient when drift dominated due to low accretion rates before the pebble supply diminishes. Exterior to the iceline, planetesimal accretion becomes inefficient due to increasing planetesimal eccentricities, whilst pebble accretion becomes more efficient as the initial proto-embryo masses increase, allowing them to significantly grow before the pebble supply is depleted. Combining both scenarios allows for more massive proto-embryos at larger distances, since the accretion of planetesimals allows pebble accretion to become more efficient, allowing giant planet cores to form at distances upto $10{\, {\rm au}}$. By including more realistic initial proto-embryo and planetesimal sizes, as well as combined accretion scenarios, should allow for a more complete understanding in the beginning to end process of how planets and planetary systems form.

Embryo development in white spruce seeds was studied in five stands in interior Alaska. Cones and seeds were collected at 10- to 14-day intervals starting in mid-July and continuing until just before seed dispersal began. Significant differences were found in embryo development between stands, between trees within stands, and between cones within trees. The four stands at lower elevations produced seeds that had embryos filling 95% or more of the embryo cavity; this percentage was significantly higher than the highest elevation stand where embryos filled about 75% of the embryo cavity at the end of the growing season. Relative cotyledon length was generally greater than 25% in the lower elevation stands and slightly less than 20% in the high elevation stand. Although seed collection can be started when embryos fill 75% of the embryo cavity, the results of this and other studies suggest that collecting seeds when embryos are more mature will result in better quality seeds. Air and soil temperatures and soil moisture levels associated with embryo development are presented.

Super-Earths – planets with sizes between the Earth and Neptune – are found in tighter orbits than that of the Earth around more than one third of main sequence stars. It has been proposed that super-Earths are scaled-up terrestrial planets that also formed similarly, through mutual accretion of planetary embryos, but in discs much denser than the solar protoplanetary disc. We argue instead that terrestrial planets and super-Earths have two clearly distinct formation pathways that are regulated by the pebble reservoir of the disc. Through numerical integrations, which combine pebble accretion and N-body gravity between embryos, we show that a difference of a factor of two in the pebble mass flux is enough to change the evolution from the terrestrial to the super-Earth growth mode. If the pebble mass flux is small, then the initial embryos within the ice line grow slowly and do not migrate substantially, resulting in a widely spaced population of approximately Mars-mass embryos when the gas disc dissipates. Subsequently, without gas being present, the embryos become unstable due to mutual gravitational interactions and a small number of terrestrial planets are formed by mutual collisions. The final terrestrial planets are at most five Earth masses. Instead, if the pebble mass flux is high, then the initial embryos within the ice line rapidly become sufficiently massive to migrate through the gas disc. Embryos concentrate at the inner edge of the disc and growth accelerates through mutual merging. This leads to the formation of a system of closely spaced super-Earths in the five to twenty Earth-mass range, bounded by the pebble isolation mass. Generally, instabilities of these super-Earth systems after the disappearance of the gas disc trigger additional merging events and dislodge the system from resonant chains. Therefore, the key difference between the two growth modes is whether embryos grow fast enough to undergo significant migration. The terrestrial growth mode produces small rocky planets on wider orbits like those in the solar system whereas the super-Earth growth mode produces planets in short-period orbits inside 1 AU, with masses larger than the Earth that should be surrounded by a primordial H/He atmosphere, unless subsequently lost by stellar irradiation. The pebble flux – which controls the transition between the two growth modes – may be regulated by the initial reservoir of solids in the disc or the presence of more distant giant planets that can halt the radial flow of pebbles.

Les chondres sont de petites sphérules silicatées ignées, ubiquiste parmi les météorites primitives et dont la formation reste toujours mal comprise. La nature du matériel précurseur, aussi bien que l'origine du gaz en interaction avec le chondre en formation font l'objet d'un intense débat. Via l'étude minéralogique, chimique et isotopique de plusieurs chondres au sein de différentes météorites, cette thèse essaiera de tracer l'origine du précurseur des chondres, ainsi que de contraindre l'environnement de formation des chondres. Dans un premier temps, les résultats n'indiquent pas de lien de filiation entre les inclusions réfractaires et les chondres contenant des spinelles, la composition chimiques et en isotopes de l'oxygène (δ¹⁷,¹⁸O) de ces minéraux différant dans les deux types d'objets. Ces résultats démontrent que les spinelles et les olivines sont co-magmatiques. A ce titre, il a été possible de contraindre l'histoire thermique des chondres grâce à un géothermomètre basé sur le partage de l'Al entre ces deux minéraux. Il semble que les chondres ont subi un refroidissement non-linéaire, avec une température moyenne de cristallisation des assemblages olivine-spinelle autour de 1470°C. Par la suite, des mesures en éléments traces, notamment les Terres Rares, n'ont pas permis de mettre en évidence de liens entre les olivines de chondres et celles d'inclusions réfractaires. Bien que cela n'ajoute pas d'arguments à l'hypothèse d'un recyclage d'AOAs en chondre, d'autres arguments laissent supposer que cette hypothèse reste la plus robuste. Enfin, le dernier volet de cette thèse s'est appliqué à contraindre l'environnement de formation des chondres. L'utilisation d'oxybaromètres basés sur le partage du V et du Cr entre les minéraux des chondres et la mésostase indiquent que ces derniers se sont formés dans des conditions très réductrices (IW-5), correspondant à un environnement nébulaire enrichi en poussière. Enfin, les mesures de valences du Ti, du V et du Cr dans des olivines reliques (enrichies en ¹⁶O) et hôtes(appauvries en ¹⁶O) indiquent également des environnements de formation réducteurs. Malgré la complexité des processus mis en jeu lors de la formation des chondres, les résultats de cette thèse suggèrent une formation des chondres dans un environnement réducteur, en accord avec les scénarios de formation nébulaire
Chondrules are small igneous silicate particles, ubiquitous among primitive meteorites yet their formation is still poorly understood. The precursor nature, as well as the origin of the gas in interaction with the chondrule in formation, is in the center of many debates. Using mineralogical, chemical and isotopic study of chondrules among several meteorites, this thesis will try to trace back the origin of the chondrule precursor, as well as constraining the environment of chondrules formation. In the first place, the results do not indicate any genetic link between the refractory inclusions and the chondrules, based on the discrepancies among the chemical and isotopic (δ¹⁷,¹⁸O) compositions. These results prove that the olivine and spinel are co-magmatic. On this basis, it has been possible to better constrain the thermal history of chondrules using a geothermometer based on the Al partitioning between the two minerals. It would appear that chondrules formed along a non-linear cooling path, with a crystallization temperature of the olivine-spinel assemblages around 1470 °C. Afterward, the measurements of trace elements, such as the Rare Earth Elements, did not highlighted any link between the olivine of chondrules and those within refractory inclusions. Eventhough this does not add new arguments toward a recycling of AOAs into chondrules,others point toward that this hypothesis is still the more reliable. Finally, the last section of this thesis focused on constraining the environment of formation of chondrules. Oxybarometers based on V and Cr partitioning between minerals in chondrules and the mesostasis suggest that chondrules formed under highly reducing conditions (IW-5), matching with a nebular environment enriched with dust. Moreover,XANES measurements of Ti, V and Cr valence in relict olivine (enriched in ¹⁶O) and hosts (depleted in ¹⁶O) point toward reducing environments of formation. Altogether, despite the complexity of the processes at stake during the formation of chondrules, the results of this thesis suggest a formation of chondrules under highly reducing conditions, in agreement with the nebular scenarios of chondrule formation

博士
國立臺灣大學
海洋研究所
99
The core formation of rocky planets is one of the most important events during the early history of these planets. The process of core formation is a topic of active research, and so far no consensus was reached. This dissertation presents a numerical investigation of a possible process of core formation, namely the descent of metal diapirs from a global ponded iron layer through an undifferentiated solid interior, leading to the formation of an iron core. The initial structure assumed in this study derived from cold accretion scenario and consists in three layers: a central undifferentiated protocore, a global iron shell, and an outer silicate-rich mantle. This structure is gravitationally unstable and leads to a differentiation in a dense, iron core in the center surrounded by a silicate rich mantle. After an introductory chapter that discuses recent ideas in planetary formation and core formation, Chapter 2 describes the numerical methods used to model the gravitational redistribution process in a 2D planetary body. In Chapter 3, accuracy tests are first conducted, and core formation process is explored with a simplified model that assumes a constant viscosity for each material and neglects the rheological effects of gravitational energy dissipation. Results indicate a transient exposure of the protocore to the planetary surface, and predict that the time for core formation depends on the strength of the solid protocore. Experiments in Chapter 4, include a non-Newtonian, temperature-, pressure-, and strain rate-dependent viscoplastic rheology, and take into account the thermal contribution from gravitational energy dissipation. Three different core formation regimes are observed, the exposure mode, the fragmentation mode, and the transition mode. Like models with Newtonian rheology in chapter 3, the core experiences large deviations from the spherical shape and may temporarily be exposed at the surface (exposure mode). By contrast to the Newtonian models, however, the destruction of the protocores observed in the fragmentation modes is driven by (i) the spontaneous strain localization along planetary-scale shear zones forming inside the protocore, and/or (ii) descending localized iron diapirs or sheets penetrating the protocore. Feedback from energy dissipation influences planetary temperature distribution although it does not significantly affect core formation regimes. However, it causes a temperature increase up to several hundred K (i) around the moving and deforming protocore, and (ii) along planetary scale rupture zones that form inside the protocore. If the protocore is large and has a high viscosity, a large fraction of the dissipated heat is partitioned to increase the temperature of iron.

abstract: An exhaustive parameter study involving 133 dynamic crystallization experiments was conducted, to investigate the validity of the planetary embryo bow shock model by testing whether the cooling rates predicted by this model are consistent with the most dominant chondrule texture, porphyritic. Results show that using coarse-grained precursors and heating durations ≤ 5 minutes at peak temperature, porphyritic textures can be reproduced at cooling rates ≤ 600 K/hr, rates consistent with planetary embryo bow shocks. Porphyritic textures were found to be commonly associated with skeletal growth, which compares favorably to features in natural chondrules from Queen Alexandra Range 97008 analyzed, which show similar skeletal features. It is concluded that the experimentally reproduced porphyritic textures are consistent with those of natural chondrules. This work shows heating duration is a major determinant of chondrule texture and the work further constrains this parameter by measuring the rate of chemical dissolution of relict grains. The results provide a robust, independent constraint that porphyritic chondrules were heated at their peak temperatures for ≤ 10 minutes. This is also consistent with heating by bow shocks. The planetary embryo bow shock model therefore remains a viable chondrule mechanism for the formation of the vast majority of chondrules, and the results presented here therefore strongly suggest that large planetary embryos were present and on eccentric orbits during the first few million years of the Solar System’s history.
Dissertation/Thesis
Masters Thesis Geological Sciences 2018