📝 Original Info
- Title: Angular Momentum and the Formation of Stars and Black Holes
- ArXiv ID: 0901.4325
- Date: 2009-01-27
- Authors: Richard B. Larson
📝 Abstract
The formation of compact objects like stars and black holes is strongly constrained by the requirement that nearly all of the initial angular momentum of the diffuse material from which they form must be removed or redistributed during the formation process. The mechanisms that may be involved and their implications are discussed for (1) low-mass stars, most of which probably form in binary or multiple systems; (2) massive stars, which typically form in clusters; and (3) supermassive black holes that form in galactic nuclei. It is suggested that in all cases, gravitational interactions with other stars or mass concentrations in a forming system play an important role in redistributing angular momentum and thereby enabling the formation of a compact object. If this is true, the formation of stars and black holes must be a more complex, dynamic, and chaotic process than in standard models. The gravitational interactions that redistribute angular momentum tend to couple the mass of a forming object to the mass of the system, and this may have important implications for mass ratios in binaries, the upper stellar IMF in clusters, and the masses of supermassive black holes in galaxies.
💡 Deep Analysis
Deep Dive into Angular Momentum and the Formation of Stars and Black Holes.
The formation of compact objects like stars and black holes is strongly constrained by the requirement that nearly all of the initial angular momentum of the diffuse material from which they form must be removed or redistributed during the formation process. The mechanisms that may be involved and their implications are discussed for (1) low-mass stars, most of which probably form in binary or multiple systems; (2) massive stars, which typically form in clusters; and (3) supermassive black holes that form in galactic nuclei. It is suggested that in all cases, gravitational interactions with other stars or mass concentrations in a forming system play an important role in redistributing angular momentum and thereby enabling the formation of a compact object. If this is true, the formation of stars and black holes must be a more complex, dynamic, and chaotic process than in standard models. The gravitational interactions that redistribute angular momentum tend to couple the mass of a form
📄 Full Content
It has long been recognized that the biggest obstacle to the formation of stars from diffuse gas is that a star can contain only a tiny fraction of the initial angular momentum of the gas from which it forms, so that nearly all of this angular momentum must be removed or redistributed during the formation process (Mestel 1965;Spitzer 1968Spitzer , 1978;;Bodenheimer 1995;Larson 2003b;Jappsen and Klessen 2004). The specific angular momenta of typical star-forming molecular cloud cores are at least three orders of magnitude larger than the maximum specific angular momentum that can be contained in a single star, even when rotating at breakup speed (Bodenheimer 1995). The 'angular momentum problem' is a familiar one for stars like the Sun, but it may be more severe for massive stars whose matter must come from a larger region containing more angular momentum, and it may be especially so for black holes in galactic nuclei because they are smaller than stars in relation to the size of the system in which they form. The masses that massive stars and central black holes can attain may therefore be limited by the efficiency with which angular momentum can be removed during the formation process.
The angular momentum problem was first studied in the context of single stars forming in isolation (Mestel 1965;Spitzer 1968), but it now seems likely that most stars form not in isolation but in systems such as binary or multiple systems or clusters, and in this case, it is necessary to consider both the orbital and the spin components of the angular momentum of the matter from which each star forms. If a star-forming cloud core forms a binary or multiple system, some of its angular momentum evidently goes into stellar orbital motions, plausibly accounting for the orbital component of the angular momentum of the matter from which each star forms, but the spin angular momentum of this matter must still be removed or redistributed during the star formation process. The excess spin angular momentum of the matter from which each star forms could in principle be transferred to outlying gas or to the orbital motions of other stars, and both magnetic and gravitational forces can play important roles in this loss or redistribution of angular momentum.
As will be reviewed in Section 2, magnetic torques can remove angular momentum from diffuse star-forming clouds and from accreting protostars during the final stages of accretion, but this leaves a large intermediate range of densities where magnetic coupling is weak and gravity dominates the dynamics, governing not only the collapse of a molecular cloud or cloud core but also the redistribution of angular momentum within it. Simulations of star formation show the appearance of trailing spiral features within which gravitational torques can transport angular momentum outward, and forming stars can also lose orbital energy and angular momentum to the surrounding gas by gravitational drag, causing them to spiral together and form more compact systems. Within these compact systems, tidal torques between forming stars and the gas orbiting around other forming stars can transfer the angular momentum of this gas to stellar orbital motions, allowing the gas to be accreted by the forming stars.
If companion stars in binary or multiple systems or clusters play an important role in absorbing and redistributing the excess angular momentum of forming stars, few stars may form in complete isolation. The formation of massive stars in clusters and of central black holes in galaxies may be even more dependent on the presence of a surrounding system to absorb and redistribute the larger amount of angular momentum involved. The fact that the mass of the most massive star in a cluster and the mass of the central black hole in a galaxy both increase systematically with the mass of the surrounding system suggests that the associated system does indeed play an essential role in the formation of these objects.
If the solution of the angular momentum problem partly involves gravitational interactions with other stars in an associated system, star formation must then be a more violent and chaotic and variable process than in standard models for isolated star formation. Chaotic formation processes are hinted at by the fact that the spin axes of both stars in clusters and central black holes in galaxies are randomly oriented and not correlated with the properties of the surrounding system, suggesting that chaotic interactions randomize the residual angular momentum of forming objects during the formation process.
Magnetic and gravitational forces can both play important roles in transporting angular momentum in star-forming clouds and solving the angular momentum problem. The early low-density stages of molecular cloud evolution may be magnetically dominated because the degree of ionization is high enough that the gas is strongly coupled to a magnetic field, and with typical observed field strengths, magne
…(Full text truncated)…
📸 Image Gallery
Reference
This content is AI-processed based on ArXiv data.
