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Bondi

How do low-mass stars accrete mass? 

 

BONDI–HOYLE ACCRETION IN AN ISOTHERMAL MAGNETIZED PLASMA

Relevant Paper:

BONDI–HOYLE ACCRETION IN AN ISOTHERMAL MAGNETIZED PLASMA [ADS]

Aaron T. Lee, Andrew J. Cunningham, Christopher F. McKee, and Richard I. Klein

BONDI–HOYLE ACCRETION IN A TURBULENT, MAGNETIZED MEDIUM [ADS]

Burleigh, Kaylan J.; McKee, Christopher F.; Cunningham, Andrew J.; Lee, Aaron T.; Klein, Richard I.

 
 

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The Taurus molecular cloud, as seen in the infrared. Over 400 low-mass stars are forming in this region, which resides only ~400 light-years away from Earth.

The Taurus molecular cloud, as seen in the infrared. Over 400 low-mass stars are forming in this region, which resides only ~400 light-years away from Earth.

Motivation

Stars form in large clouds of gas called molecular clouds. These clouds of predominately molecular hydrogen may undergo collapse from their own gravity, and this results in the formation of stars. These young stars can roam through the cloud and accrete additional material material. This gas the stars attempts to accrete, however, is threaded by a magnetic field, which affects the dynamics of the gas. Particularly, as gas is brought closer to the star, the gas drags with it the magnetic field, causing the field to build up around the star. This field resists further compression and can ultimately push material away from the star. How then can stars accrete more mass?


Gravity battles inertia, thermal pressure, and magnetic fields

Unfortunately, gravity is not only fighting against the magnetic field as it attempts to bring mass onto the star. Additionally, the random motions of the gas create a thermal pressure that tries to expand a gas out rather than compress it. Additionally, these molecular clouds also exhibit turbulence, where the gas is churning around in a chaotic fashion. This bulk motion of gas means gas could simply fly by a star rather than being pulled onto its surface. Gravity must overcome all three of these forces in order to successfully accrete gas onto the star. 

We can imagine a simple version of this scenario where gas is flowing past a stationary mass. Gas that passes very close to the mass will be pulled in easily and fall onto the star. Other gas may get pulled toward the star but overshoot the star, crashing into a wake behind the star. More distant gas may move in a slightly altered path but nonetheless pass by the star without being accreted. Around each star you could define a region of gravitational influence, where the gas is substantially diverted from its original path. The larger this region, the more mass will ultimately be accreted. Inertia, hotter gas, and stronger magnetic fields, however, shrink this region of influence. 

Region of gravitational influence. Adapted from Figure 1 of Lee & Stahler (2013), which uses this figure to describe a different (but related) phenomenon. 

Region of gravitational influence. Adapted from Figure 1 of Lee & Stahler (2013), which uses this figure to describe a different (but related) phenomenon. 


Two different flow morphologies. Both have the same magnetic field strength, but the gas in the right panel is moving ~4 times as fast as the gas in the left panel. Arrows show gas velocity, green lines show example magnetic field lines, and brighter colors represent higher gas densities. The central object is shown as a black circle. 

Two different flow morphologies. Both have the same magnetic field strength, but the gas in the right panel is moving ~4 times as fast as the gas in the left panel. Arrows show gas velocity, green lines show example magnetic field lines, and brighter colors represent higher gas densities. The central object is shown as a black circle. 

How Stars Accrete

In regions of star formation, the thermal pressure is relatively well understood. The gas has cooled to ~10 Kelvin, which makes the thermal pressure simply a function of the mass density. In this study, we studied how magnetic fields and bulk gas motion affect the accretion of mass. A massive object was set moving through an initially uniform gas that was threaded by a magnetic field. This numerical study considered gas motions that ranged from sub-sonic (slower than the local speed of sound) to highly supersonic, as well as magnetic field strengths that were initially weak compared to the local thermal forces to orders of magnitude stronger. The magnetic field direction was either initially parallel or anti-parallel to the velocity of the massive particle. 

The figure here shows two examples. Both have the same magnetic field strength, but the figure on the left shows the result when the gas flows past the mass at slightly supersonic speeds (about ~1.4 times the speed of sound). The right figure has the gas moving at nearly five times the speed of sound. The morphology of the gas is quite different. On the left tangled magnetic fields become strong enough to launch a magnetic shock ahead of the accretor. On the right, the inertia of the gas is too large and the magnetic fields follow the gas into an over-dense wake behind the mass. 


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Inertia and magnetic fields make it hard to build up a star through without gravitational collapse

Despite the chaotic motions of the gas, the mass accretion rate onto the central object reaches a quasi-steady rate. However, this rate is often order-of-magnitudes lower than the back of the envelope estimate you would make. As a result the time it takes for an object, like a star, to build up to massive sizes (e.g., >8 times the mass of the Sun) increases substantially. So much, in fact, that one can begin to question whether or not this mechanism is a means that you can employ to actually form a massive star. The alternative is the gravitational collapse of the gas, where the gas's own gravity assists in the collapse onto the star, allowing more mass to collect in a shorter amount of time.