Disk, protoplanetary

Davis AM, MaePherson GJ (1996) Thermal processing in the solar nebula Constraints from refractory inclusions. In Chondrules and the Protoplanetary Disk. Hewins RH, Jones RH, Scott ERD (eds) Cambridge University Press, New York, p 71-76... 

Yin, Q.-Z. (2005) From dust to planets the tale told by moderately volatile elements. In Chondrites and the Protoplanetary Disk, ASP Conference Series 341, eds. Krot, A.N., Scott, E. R. D. and Reipurth, B. San Francisco Astronomical Society of the Pacific, pp. 632-644. 

Artistic rendering of four observed stages of star formation, (a) Class 0 object a deeply embedded hydrostatic core surrounded by a dense accretion disk. Strong bipolar jets remove angular momentum, (b) Class I object protostar in the later part of the main accretion phase, (c) Class II object or T Tauri star pre-main-sequence star with optically thick protoplanetary disk, (d) Class III object or naked T Tauri star star has an optically thin disk and thus can be directly observed. Some may have planets. 

Young, E., Simon, J. I., Galy, A. et al. (2005) Supra-canonical 26A1/27A1 and the residence time of CAIs in the solar protoplanetary disk. Science, 308, 223-227. 

Boss, A. P. (2004) Convective cooling of protoplanetary disks and rapid giant planet formation. Astrophysical Journal, 610, 456-463. 

Kant was essentially correct, Bob says. In the last years of the twentieth century, the Hubble Space Telescope (HST) revealed several dozen disks at visible wavelengths in the Orion Nebula, a giant stellar nursery about 1,600 light years away. We call them proplyds, a contraction of the term protoplanetary disks. The Orion proplyds are larger than the Sun s solar system and contain enough gas and dust to provide the raw material for future planetary systems (figure 6.1). 

When thinking about how our solar system may have evolved from proplyds (protoplanetary disks), we must remember that the violence of the early Solar System was tremendous as huge chunks of matter bombarded each other. In the inner Solar System, the Sun s heat drove away the lighter-weight elements and materials, leaving Mercury, Venus, Earth, and Mars behind. In the outer part of the system, the solar nebulas (gas and dust) survived for some time and were accumulated by Jupiter, Saturn, Uranus, and Neptune. 

Protoplanetary disk The disk of dust and gas surrounding a star out of which planets form... 

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

However, planet formation is a uniquely fortunate problem, as our extensive meteorite collections abound with primitive materials left over from the young Solar System, almost as providing a perfect sample-return mission from a protoplanetary disk. A remarkable achievement of geochronology is that many of these samples can be dated and the story of the Solar System s formation reconstructed. 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

Table 1.1 The astronomical and cosmochemical evidence available on the key stages of the evolution of protoplanetary disks and the chapters in which they are discussed.

The collapse of rotating molecular cloud cores leads to the formation of massive accretion disks that evolve to more tenuous protoplanetary disks. Disk evolution is driven by a combination of viscous evolution, grain coagulation, photoevaporation, and accretion to the star. The pace of disk evolution can vary substantially, but massive accretion disks are thought to be typical for stars with ages < 1 Myr and lower-mass protoplanetary disks with reduced or no accretion rates are usually 1-8 Myr old. Disks older than 10 Myr are almost exclusively non-accreting debris disks (see Figs. 1.3 and 1.5). 

The fundamental initial parameters of protoplanetary disk evolution are the masses and sizes of the disks. Optical silhouettes of disks in the Orion Nebula Cluster (McCaughrean O Dell 1996), scattered light imagery (e.g. Grady et al. 1999), interferometric maps in millimeter continuum or line emission (e.g. Rodmann et al. 2006 Dutrey et al. 2007), and disk spectral energy distributions... 

Laboratory investigations confirm that crystalline silicates form in stellar outflows and in protoplanetary disks. In contrast, dust grains in the ISM are dominated by amorphous materials less than 2.2% of the grains are crystalline silicates (Kemper et al 2005). Laboratory simulations of the harsh interstellar radiation fields demonstrate that ion irradiation of crystalline silicates quickly leads to their amorphization (e.g. Jager et al. 2003 Brucato et al. 2004). 

Armed with the results of laboratory studies of astrophysical dust processing, we are able to interpret the complex and varied history of dust in protoplanetary disks. This information is complemented by the detailed analysis of the solid material that remains from the earliest epochs of Solar System formation. 

In the Solar System the bulk elemental composition of the most volatile-rich Cl chondrites resembles closely that of the solar photosphere. Indeed, models that follow the condensation of a solar-composition hot gas reproduce many of the minerals and abundance trends observed in the Solar System. These are also consistent with some of the astronomical observations of dust in protoplanetary disks. Stardust grains also show approximately solar bulk composition in the measurable elements, albeit with some variations (Flynn et al 2006). Some IDPs -mainly the fine-grained, porous, and anhydrous particles - match the solar elemental... 

The ubiquitous presence of silicate emission features in young protoplanetary disks is evidence that a population of small (a few micron) particles persists on million year timescales, much longer than the grain coagulation timescales (e.g. Dullemond Dominik 2005 Brauer et al 2008). This demonstrates that an efficient mechanism must operate that replenishes particles in the 1-10 pm size range, at least in the upper layers of protoplanetary disks. 

Cold disk regions (< 300 K), both in the Solar System and in protoplanetary disks, have been found to be very rich in crystalline silicates and once-molten solids. The evidence for such widespread thermal processing - considering that heating and... 

Sensitive observations enable comparative surveys of silicate emission features from disks around low-mass, intermediate-mass, and Sun-like stars. While no strong correlations have been found with disk properties, flatter disks and disks around the coolest stars more often show crystalline silicate features. Cool stars and very low-mass disks display prominent crystalline silicate emission peaks (Apai et al. 2005 Merfn et al. 2001 Pascucci et al 2009). Thus, whatever processes are responsible for the presence of crystals around Sun-like stars must be capable of very efficiently producing crystals around low-mass stars, too. Interferometric measurements suggest that the amorphous/crystalline dust mass fraction is higher in the inner disk than at medium separations (van Boekel et al. 2004 Ratzka et al. 2007). The surveys also show that amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks. In contrast, those with crystalline silicates are always dominated by Mg-rich grains (e.g. Malfait et al. 1998 Bouwman et al. 2008). 

While the amount of dust and small particles that underwent thermal processing remains difficult to constrain both in the entire proto-solar nebula and in protoplanetary disks around other stars, in the Asteroid Belt over 80% of the pre-chondritic components have been melted. These heating events may play a crucial role in defining the bulk composition of planetesimals and planets by reprocessing much or all... 

The lifetime of protoplanetary disks determines the time available for planet formation with the loss of the dusty gas disks no raw material is left to form planetesimals or giant planets. Thus, disk mass as a function of time is perhaps the single most important constraint on the formation of both the rocky and the giant planets. The most readily observable, albeit imperfect, indicator of disks is the presence of excess emission above the stellar photosphere, emerging from small, warm dust grains. 

Transition from protoplanetary disks to debris disks... 

Multiple lines of evidence exist for a population of live radionuclides, such as 26Al or 60Fe, which were injected into the proto-solar cloud or disk prior to the formation of CAIs (e.g. Tachibana et al. 2006). Isotopic abundances suggest that the isotopes have originated in a supernova, possibly with a very massive star progenitor that also underwent a Wolf-Rayet phase (Bizzarro et al. 2007). If this interpretation is correct, the Sun must have formed in a very rich and dense stellar cluster, such as the Carina Nebula, very much unlike the Taurus or other low-mass star-forming regions. Luminous massive stars in such clusters may truncate or fully evaporate protoplanetary disks around other cluster members. Two key questions remain open. How close in time and space did the supernova explode... 

Stars form from collapsing interstellar gas and dust clouds. Interstellar dust also provides the raw material from which planets form. Under favorable conditions, i.e. low temperatures, the interstellar dust may survive processing in protoplanetary disks and later planetary metamorphism, e.g. in asteroids and comets. By far the... 

Also, observations of crystalline silicates in the comets Halley (Swamy etal. 1988) and Tempel 1 (Harker et al. 2005) suggest that a large fraction of cometary dust was processed or formed in the hot inner regions of the protoplanetary disk and transported to the region where the comets formed. 

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