Protoplanetary nebula

Thermal processing in protoplanetary nebulae Silicate emission features from six brown dwarf disks... 

Dubrulle B., MorfillG., and SterzikM. (1995) The dust subdisk in the protoplanetary nebula. Icarus 114, 237 -246. 

Cuzzi J. N., Hogan R. C., Paque J. M., and DobrovolsMs A. R. (2001) Size-selective concentration of chondmles and other small particles in protoplanetary nebula turbulence. Astro-phys. J. 546, 496-508. 

Cuzzi J. N., Davis S. S., and Dobrovolskis A. R. (2003) Creation and distribution of CAIs in the protoplanetary nebula. In Lunar Planet. Set XXXIV, 1749. The Lunar and Planetary Institute, Houston (CD-ROM). 

These observations have led to the development and refinement of a theory in which the planets formed from a disk-shaped protoplanetary nebula (Laplace) by pairwise accretion of small solid bodies (Safranov, 1969). A variant of the standard model invokes the gravitational collapse of portions of this disk to form gas giant planets directly. It should be pointed out that the standard model is designed to explain the planets observed in the solar system. Attempts to account for planetary systems recently discovered orbiting other stars suggest that planet formation is likely to differ in several respects from one system to another. 

Roughly half of young T Tauri stars with ages <10Myr are observed to have optically thick disks of gas and dust with masses of 0.001-IM (Beckwith et al., 1990 Strom, 1994). These disks have spectra containing absorption features caused by the presence of water ice and silicates. Ultraviolet and visible emission lines indicate that the central stars are accreting mass from their disks at rates of 10 -10 Mo yr (Hartmann et al., 1998). Optically thick circumstellar disks are not observed around stars older than —10 Myr (Strom, 1995), which provides an approximate upper limit for the lifetime of the Sun s protoplanetary nebula. 

The protoplanetary nebula initially had a mass of at least O.OIM . This minimum mass is obtained by estimating the total amount of rocky and icy material in all the planets, and adding hydrogen and helium to give a nebula of solar composition (Weidenschilling, 1977a). However, planet formation is probably an inefficient process, suggesting that the protoplanetary nebula was initially more massive than this. 

The finite hfetime of circumstellar disks, and the fact that young stars are observed to be accreting material, has given rise to a model which views the protoplanetary nebula as a viscous accretion disk in which material was transported radially inwards, ultimately falling onto the Sun (Lynden-Bell and Pringle, 1974). As a consequence of this accretion, the mass of the... 

Chondrites—meteorites from parent bodies in the asteroid belt that never melted—represent the most primitive samples available of material that formed in the protoplanetary nebula. Chondrites are mainly composed of chondrules, with smaller amounts of refractory inclusions and a fine grained matrix of silicate, metal, and sulfide. Chondrules are roughly spherical objects, typically —1 mm in size, and largely composed of olivine and pyroxene (Taylor, 2001). They appear to have formed from melt droplets that cooled on timescales of... 

Many CAIs, together with some chondrules and samples of differentiated asteroids, contained short-lived radioactive isotopes at the time they formed. This is deduced from the abundances of the daughter isotopes seen in modern meteorites. The short-lived isotopes include " Ca, A1, °Be, e, Mn, and ° Pd, with half-lives (in units of Myr) 0.13, 0.7, 1.5, 1.5, 3.7, and 6.5, respectively. Many of these isotopes could have been produced from stable ones by absorption of neutrons in a supernova or the outer layers of a giant star. In particular, °Fe can only be produced efficiently by stellar nucleosynthesis and so must have come from an external source (Shukolyukov and Lugmair, 1993). Conversely, some isotopes such as Be almost certainly formed in the protoplanetary nebula when material was bombarded by solar cosmic rays (McKeegan et al., 2000). Multiple sources are possible for some short-lived isotopes. The abundances of the decay products of... 

A protoplanetary nebula containing enough mass to rapidly form a lOM giant-planet core would have been quite close to the limit of gravitational stability beyond 5 AU (Boss,... 

At present, it is unclear whether the protoplanetary nebula could have evolved to the point where it was marginally unstable, or whether disk instabilities would have redistributed mass in the disk prior to the formation of gravitationally bound clumps. If marginally unstable disks do develop, then giant-planet formation by disk instability seems unavoidable. It is worth noting that current simulations of disk instability tend to generate planets with masses greater than... 

Marshall J. and Cuzzi J. (2001) Electrostatic enhancement of coagulation in protoplanetary nebulae. Lunar Planet Sci., abstract 1262. 

Figure 2 Temperature distribution of protoplanetary nebula marked with the present date positions of planets to Jupiter. The dotted line is for an active disk, and the solid line is for a passive disk. Material distribution is also shown with two boundaries proposed by Kouchi et al. (2002).

Zhang Y, Kwok S (2011) Deteaion of C o in the protoplanetary nebula IRAS 01005-t7910. Asirophys J 730 126-1-5... 

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