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Condensation of the Solar Nebular

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Condensation of the Solar Nebular Formation of the Sun-like Stars • Collapse of a portion of a molecular cloud 4.5-4.6 Ga – Star dusts in primitive meteorites provide – Fingerprints of neaby stars that preceded our Sun – Stars like our Sun can form in a large number (hundreds to thousands) and close to each other (0.1 pc or ~0.3 lightyear, much closer than the Sun’s neighbor stars) as seen in the Orion Nebular. – Modern molecular clouds also has circumstellar disks, where planets form. – Gas in the molecular clouds is cold (~4K) and relatively dense (104 atoms/cm3). 1 Formation of the Sun-like Stars • Young stars emits more infrared radiation than a blackbody of the same size –Due to dark (opaque) disks around them. –Such disks are dubbed “proplyds” (proto-planetary disks). • Planets in the solar system orbit the Sun in the same direction and the orbits are roughly coplanar. –Suggests the solar system originated from a disk-shaped region of material referred to as the solar nebular. –An old idea conceived at least 2 centuries ago. –Discovery of proplyds now provide strong support. 2 Formation of the Sun-like Stars • Not clear what triggers the collapse of the densest portion of the cloud (“core”) to form stars. – Sequential ages of stars in close proximity in a molecular cloud suggests that formation and evolution of some stars trigger the formation of additional stars. • Gas around the collapsing core of the molecular cloud is moving – Too much angular momentum binary star – Otherwise, a single protostar called a T Tauri star or a pre-main sequence star. – Significant fraction of the gas goes into orbit forming a disk 100 AU in diameter. – A T Tauri star is initially too cold for nuclear fusion but brighter than old stars like the Sun due to the release of gravitational energy. 3 Formation of the Sun-like Stars • Disks around T Tauri stars goes through “viscous accretion” – Disks lose mass over time as material moves inward through the disk and onto the star. – Older stars don’t have disks. – Mechanism for viscous accretion unclear. – A promising possibility: Magneto-Rotational Instability (MRI) • If partially ionized gas in the disk is captured by the fast-sweeping magnetic field of the rotating protostar, its angular speed increases and is pushed outward. • Uncaptured material loses angular speed due to “friction” and moves inward, eventually falling onto the star. • Can explain why planets contain only 0.1 % of the mass in the Solar system but have retained 99 % of its angular momentum. 4 Meteorites and their Importance • Meteorites and Interplanetray Dust Particles (IDPs) – Fragments of rock and metal from other bodies in the solar system that have fallen to Earth. – Similar in composition, only different in size. – Unlike any objects formed on Earth. – Petrography for identifying minerals and textures. – Geochemistry for isotopic and chemical compositions. 5 Meteorites and their Importance • Meteorites and IDPs show – The Sun’s protoplanetary disk and the planets had a composition that was similar in some respects to that of the Sun itself. < Atomic abundances in carbonaceous chondrites > 6 Meteorites and their Importance • Meteorites and IDPs show – In other respects, the disk was a highly modified residuum that generated a vast range of planetary compositions. – The Sun contains 99.9 % of the mass of the solar system. – A sizable fraction of this material passed through the solar nebular at some point. – Therefore, the composition of the original nebular would have been similar to that of the Sun today. – The challenge is to explain how a disk that formed gas giant objects could also generate rocky terrestrial planets. 7 Meteorites and their Importance • Most meteorites – Come from the Main Asteroid Belt that formed during the first few My (https://en.wikipedia.org/wiki/Asteroid_belt) of the solar system • According to the trajectories or age and composition. – These objects carry a record of the processes that occurred in the solar nebular during the formation of the planets. • SNC meteorites from Mars • Young age, chemical composition of the bulk as well as trapped gas similar to Martian samples from the Viking landers. • Also lunar meteorites are known. 8 Meteorites and their Importance • Rough classifications of meteorites – Chondrites – Achondrites – Irons • Chondrites – Mixture of grains (microns to cm) apparently assembled in the solar nebula. – Elemental abundances broadly similar to the Sun but highly depleted in carbon, nitrogen, hydrogen and noble gases. – Considered to represent the dust and debris in the circumsolar disk, from which planets formed. – Relative abundances of refractory elements in the Earth are “chondritic” or “solar”, suggesting the common origin. Volatiles are highly depeleted in the Earth, though. – Non-metallic components of chondrites are mostly silicates such as olivine and 9 pyroxene. Meteorites and their Importance • Chondrites (cont’d) – Chondrules: mm-sized beads of rock that formed by partial or complete melting. • Texture suggests cooling occurred over a few hours. • Heating could have been caused by passage through shock waves in the nebular gas. • some condrules thought to have formed later in collisions between planetary objects. 10 Meteorites and their Importance • Chondrites (cont’d) – Calcium-Aluminun-rich Inclusions (CAIs) • chemical composition predicted for objects condensed from a gas of roughly solar composition at very high temperatures. • believed to have formed in the very innermost regions of the solar nebula • The oldest surviving materials to have formed in the solar system: CAIs in the Efremovka chondrite: 4.5672 +/- 0.0006 Ga based on U/Pb system. Considered as defining the canonical start to the solar system. • The oldest chonrules are of the same age but most are 1-3 My younger. 11 Meteorites and their Importance • Chondrites (cont’d) – chondrules and CAIs are variably depleted in moderately volatile elements like K and Rb. – Carbonaceous chondrites: richer in highly volatile elements (e.g., C and N) compared to other chondrites. – Ordinary chondrites: More depleted in volatile elements than carbonaceous chondrites and composed of silicates and metal grains. – The CI group, the most primitive, volatile-rich group of carbonaceous chondrites are lack of chondrules.12 Meteorites and their Importance • Achondrites – Silicate-rich mafic and ultramafic igneous rocks. – Represent near-surface rocks of planets and asteroids that have melted and differentiated. – Vesta (an asteroid) and Mars are known sources of achondrites. – Oxygen isotopic composition can be used for source identification. • Irons – Composed of iron, nickel (10% by mass) and sulfides + other siderophile elements. – Texture tells how quickly they cooled and therefore at what depth they formed. – Most irons are from the metallic cores of small (10-100 km in radius) asteroidal parent bodies. Old, within a few Mys of CAIs. – Some were form by impact melting at the surface of asteroids. – Pallasites, a rare (5% of all nonchondritic meteorites) class of stony-iron meteorites contain mixture of metal and silicate. Believed to have come from the core-mantle boundary regions of differentiated asteroids. 13 Meteorites and their Importance 14 Condensation of the Solar Nebular 15 Condensation of the Solar Nebular 16 Condensation of the Solar Nebular 17 Condensation of the Solar Nebular • How can condensed mineral particles grow to a bigger body? • A numerical simulation of a “proto-planetary disc”. – http://www.youtube.com/watch?v=3YmeajE-TT8 – Need to know lots of details to understand what is really going on in this simulation. – For now, it’s sufficient to see how planets could possibly formed from the solar nebular. 18 Condensation of the Solar Nebular • Is this “nebular hypothesis” really working or did it work only for our solar system? • If you have to defend the hypothesis, what kind of observations or experiments would you do? 19 Condensation of the Solar Nebular • Hubble telescope imaged opaque disks surrounding young stars. • Atacama Large Millimeter/submillimeter Array(ALMA, www.almaobservatory.org/) could resolve a much better image of HL Tauri, a young Sun-like star in 2014. – http://www.eso.org/public/videos/eso1436c/ – http://www.eso.org/public/videos/eso1436b/ – http://www.eso.org/public/videos/eso1436d/ Recent review: “Observing Planet Formation”, a Harvard-Smithonian Center for Astrophysics talk Background: 3-10 min, new observations from 31m 42s – https://youtu.be/Ua7uIhHiyyQ – Background: 3-10 min, new observations from 31m 42s 20 Formation and Differentiation of planetesimals • Differentiation due to density difference in principle • A lot more of details to work out ● Watch “Differentiated Planetesimals and the Parent Bodies of Meteorites”, another CfA talk on YouTube ● https://youtu.be/zAZJ5ELHXz4 21.
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