Accretion (astrophysics)

Atacama Large Millimeter Array image of HL Tauri, a protoplanetary disk

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

Overview

The proposed idea in the 19th century that Earth and the other terrestrial planets were formed from meteoric material, was developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of terrestrial planet formation. Since then, the theory has been further developed using intensive numerical simulations to study planetesimal accumulation.

Stars form by the gravitational collapse of interstellar gas. Prior to collapse, this gas is mostly in the form of molecular clouds, such as the Orion nebula. As the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, and the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk.

Accretion of galaxies

Further information: Protogalaxy

After a few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms were allowed to form. As the Universe continued to expand and cool, the atoms lost enough kinetic energy, and dark matter coalesced sufficiently, to form protogalaxies. As further accretion occurred, galaxies were formed. Indirect evidence is widespread. Galaxies grow through mergers and smooth gas accretion. Accretion also occurred inside galaxies, forming stars.

Accretion of stars

Main article: Protostar

File:Ssc2005-02b.jpg

The visible-light (left) and infrared (right) views of the Trifid Nebula—a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius

When the lower mass star in a binary system enters an expansion phase, its outer atmosphere may fall onto the compact star, forming an accretion disk

Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds roughly 300,000 times the mass of the Sun (M_☉) and 20 parsecs in diameter. Over millions of years, giant molecular clouds are prone to collapse and fragmentation. These fragments then form small, dense cores, which in turn collapse into stars. The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae. They possess diameters of 0.01–0.1 pc (2,000–20,000 AU) and a particle number density of roughly 10,000 to 100,000 cm⁻³. Compare it with the particle number density of the air at the sea level—2.8×10¹⁹ cm⁻³.

The initial collapse of a solar-mass protostellar nebula takes around 100,000 years. Every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot hydrostatic (non-contracting) core containing a small fraction of the mass of the original nebula. This core forms the seed of what will become a star. As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which eventually forms a disk.

File:Embedded Outflow in Herbig-Haro object HH 46 47.jpg

Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47

As the infall of material from the disk continues, the envelope eventually becomes thin and transparent and the young stellar object (YSO) becomes observable, initially in far-infrared light and later in the visible. Around this time the protostar begins to fuse deuterium. If the protostar is sufficiently massive (above 80 Jupiter masses (M_J)), hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf. This birth of a new star occurs approximately 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are also called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has already accreted much of its mass; the total mass of the disk and remaining envelope does not exceed 10–20% of the mass of the central YSO.

At the next stage, the envelope completely disappears, having been gathered up by the disk, and the protostar becomes a classical T Tauri star. The latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars. This happens after about 1 million years. The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, and it is accreted at a rate of 10⁻⁷ to 10⁻⁹ M_☉ per year. A pair of bipolar jets is usually present as well. The accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines (up to 100% of the intrinsic luminosity of the star), magnetic activity, photometric variability and jets. The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles. The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10 million years. The disk eventually disappears due to accretion onto the central star, planet formation, ejection by jets, and photoevaporation by UV-radiation from the central star and nearby stars. As a result, the young star becomes a weakly lined T Tauri star, which, slowly, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass.

Accretion of planets

Further information: Protoplanetary disk

Artist's impression of a protoplanetary disk showing a young star at its center

Self-accretion of cosmic dust accelerates the growth of the particles into boulder-sized planetesimals. The more massive planetesimals accrete some smaller ones, while others shatter in collisions. Accretion disks are common around smaller stars, or stellar remnants in a close binary, or black holes surrounded by material, such as those at the centers of galaxies. Some dynamics in the disk, such as dynamical friction, are necessary to allow orbiting gas to lose angular momentum and fall onto the central massive object. Occasionally, this can result in stellar surface fusion (see Bondi accretion).

In the formation of terrestrial planets or planetary cores, several stages can be considered. First, when gas and dust grains collide, they agglomerate by microphysical processes like van der Waals forces and electromagnetic forces, forming micrometer-sized particles; during this stage, accumulation mechanisms are largely non-gravitational in nature. However, accumulation of planetesimal formation in the cm/m range is not well understood, and no convincing explanation is offered as to why such particles would accumulate rather than simply rebound. In particular, it is still not clear how these objects grow to become 0.1–1 km sized planetesimals; this problem is known as the "meter size barrier". However, it is hypothesized that when slow moving grains collide, the very low, yet non-zero, gravity of colliding grains impedes their escape.

Grains eventually stick together to form mountain-size bodies called planetesimals. Collisions and gravitational interactions between planetesimals combine to produce Moon-size planetary embryos (protoplanets) in roughly 0.1–1 million years. Finally, the planetary embryos collide to form the planets in 10–100 million years. The planetesimals are massive enough that mutual gravitational interactions are significant enough to be taken into account when computing their evolution. Growth is aided by orbital decay of smaller bodies due to gas drag, which prevents them from being stranded between orbits of the embryos. Further collisions and accumulation lead to terrestrial planets or the core of giant planets.

The formation of terrestrial planets differs from that of giant gas planets, also called Jovian planets. The particles that make up the terrestrial planets are made from metal and rock that condense in the inner Solar System. However, Jovian planets begin as large, icey planetesimals, which then capture hydrogen and helium gas from the solar nebula. These two classes of planetesimals arise due to the frost line.

Accretion of asteroids

File:Chondrules grassland 1.jpg

Chondrules in a chondrite meteorite. A millimeter scale is shown.

Meteorites contain a record of accretion and impacts during all stages of asteroid origin and evolution; however, the mechanism of asteroid accretion and growth is not well understood. Evidence suggests the main growth of asteroids can result from gas-assisted accretion of chondrules—millimeter-sized spherules that form as molten (or partially molten) droplets in space before being accreted to their parent asteroids. In the inner Solar System, chondrules appear to have been crucial for initiating accretion. The tiny mass of asteroids may be partly due to inefficient chondrule formation beyond 2 AU, or less-efficient delivery of chondrules from near the protosun. Also, impacts controlled the formation and destruction of asteroids, and are thought to be a major factor in their geological evolution. Chondrules, metal grains, and other components were formed, probably in the solar nebula. These accreted together to form parent asteroids. Some of these bodies were subsequently melted, forming metallic cores and olivine-rich mantles; others were aqueously altered. After the asteroids had cooled, they were eroded by impacts for 4.5 billion years, or disrupted.

For accretion to occur, impact velocities must be less than about twice the escape velocity, which is ~140 m/s for a 100-km radius asteroid. Simple models for accretion in the asteroid belt generally assume micrometer-sized dust grains sticking together and settling to the midplane of the nebula to form a dense layer of dust, which, because of gravitational forces, was converted into a disk of kilometer-sized planetesimals. But, several arguments suggest that asteroids may not have accreted this way.

Accretion of comets

Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when comets were formed is much debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals.

The currently favored creation mechanism is that of the nebular hypothesis, which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew. The classic Oort cloud theory, however, states that the Oort cloud, a sphere measuring about 50,000 AU in radius, formed at the same time as the solar nebula and occasionally releases comets into the inner Solar System as a star passes.

See also

• Astrochemistry
• Astronomical spectroscopy
• Bondi accretion
• List of interstellar and circumstellar molecules
• Nebular hypothesis
• Nova
• Planetary nebula
• Planetary ring
• Protoplanetary disk
• Timeline of gravitational physics and relativity