[Page no. 3.3.2]
[Title: Collapse of the interstellar cloud and protoplanetary disk formation]
[Image: Caption: TBD suggest a proplyd in the Orion Nebula (HST or VLT) or Beta Pictoris disk]
[Text:
Gravitational instability within an interstellar molecular cloud results in collapse of a fragment of the cloud to form a protoplanetary disk.
The mechanism that triggered the cloud collapse that later resulted in the formation of the solar system is not clear: several possibilities have been suggested (e.g., as a shock wave from a nearby supernova or ejection of a planetary nebula from an dying [AGB] star), but whatever the mechanism, the collapse of the cloud and subsequent accretion of material must have been sufficiently fast to carry a complement of short-lived radionuclides into the protoplanetary disk (or solar nebula).
This time when this happened and how long this took can be determined by means of dating methods that depend on the different life-times of various isotopes. Isotopes are varieties of the chemical elements with a different number of neutrons in the atomic nuclei. The well-known Carbon-14 dating method used in archaeology is an example of this technique. Because of the relative short lifetimes of the involved isotopes, it cannot be used in geological/astrophysical dating - here, other elements with much longer decay times play the same role.
Evidence for the speed of this process comes from the presence of the isotope 26Mg (from the decay of 26Al; half-decay-time T1/2 ~ 0.73 Myr) within CAIs in chondritic meteorites. The occurrence of 26Mg in the inclusions shows that the CAIs formed whilst 26Al was still "live" in the solar nebula, i.e., agglomeration took place over a very short timescale, < 3 Myr.
The 41Ca-41K chronometer, with T1/2 ~ 0.15 Myr, implies even more rapid formation of CAIs, with an interval between nucleosynthesis and agglomeration of < 0.3 Myr. Absolute dating of the components within meteorites use the U-Pb isotope system, and place the date of formation of CAIs at 4566 Myr.
The most abundant component within chondrites, viz. chondrules, show little evidence for live 26Al, implying that chondrule-forming process took place ~ 2-3 Myr or so after the formation of CAIs.
The growth of planet-sized bodies from micron-sized dust grains is controlled by several factors, such as the nature of the initial grains (fluffy or compact) or the degree of turbulence within the nebula, and has been modelled by many authors. End-member models for planetesimal formation are coagulation of material by gravitational instability in a quiescent nebula or by coagulation during descent to the midplane of a turbulent nebula.
The aggregation of interstellar dust (less than about 0.1 µm in diameter) into increasingly large bodies, eventually forming kilometre-sized planetesimals and culminating in the asteroids and planets, took place over a time interval of some 8 Myr following formation of the CAIs.
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