Stellar Collisions
NAVIGATION HINTS for TTH files |
Close encounters and collisions between stars were long thought to be rare events. But while most stars live out their lives in relative isolation, stars in dense star clusters or in galactic nuclei do sometimes collide with each other. Such collisions may build up massive stars; also, encounters between stars could form exotic stellar systems.
The mechanics of stellar collisions are illustrated by this slightly off-center parabolic encounter of two stars with a mass ratio M1/M2 ≡ μ = 2. Initially, each star was modeled as an n = 1.5 polytrope; the gas has a monatomic equation of state (γ = 5/3). A total of 49152 equal-mass particles were used; the calculation was run with an adaptive SPH code.
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These experiments were designed to explore three factors which influence the outcome of stellar collisions. Two of these factors are fairly straightforward: μ specifies the ratio of stellar masses, while rp determines if the collision is head-on or off-axis. The third factor, γ, describes the kind of pressure which supports the stars against the inward force of gravity. Stars like the Sun are supported by ordinary gas pressures created by the momentum of atoms; for gas pressure, γ = 5/3. But stars which are much more massive than the Sun are largely supported by radiation pressures created by the momentum of light waves; for radiation pressure, γ = 4/3.
Intuitively, a star supported by gas pressure behaves something like a weight on an elastic spring; it finds an equilibrium position, and generally returns to that position after a disturbance. A star supported entirely by radiation pressure, on the other hand, has no equilibrium state; in principle, even a small disturbance can disrupt it entirely or collapse it to a singularity. These different forms of behavior are evident in the simulations presented below. All the collisions involving stars with γ = 5/3 settle into new equilibria soon after the stars merge. In contrast, the collisions involving stars with γ = 4/3 produce a variety of outcomes; head-on encounters are compressive, while off-center encounters are disruptive.
GAS: γ = 5/3 | RADIATION: γ = 4/3 | |||
rp = 0.0 | rp = 0.5 | rp = 0.0 | rp = 0.5 | |
μ = 1 | ![]() | ![]() | ![]() | ![]() |
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μ = 2 | ![]() | ![]() | ![]() | ![]() |
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These experiments were run to compare gas-dynamic encounters with equivalent encounters involving spheres of particles interacting only via gravity. The results of this study are summarized in a paper published in Stellar Collisions, Mergers, and Their Consequences (ed. M. Shara, ASP Conference Series, Vol. 263).
Equal-mass: μ = 1 | ||||
rp = 0.0 | rp = 0.5 | rp = 1.0 | rp = 1.5 | |
GAS | ![]() | ![]() | ![]() | ![]() |
GRIT | ![]() | ![]() | ![]() | ![]() |
Unequal-mass: μ = 2 | ||||
GAS | ![]() | ![]() | ![]() | ![]() |
GRIT | ![]() |
In galactic nuclei, stars encounter each other with relatively high velocities, and they are more likely to partly disrupt each other instead of merging. These simulations show some moderately fast encounters.
Equal-mass: μ = 1 | ||||
rp = 0.0 | rp = 0.25 | rp = 0.5 | rp = 1.0 | |
γ = 5/3 | ![]() | ![]() | ![]() | ![]() |
Unequal-mass: μ = 2 | ||||
γ = 5/3 | ![]() | ![]() | ![]() | ![]() |
These experiments further explore the role of the equation of state in stellar collisions. By definition, classical polytropes have constant entropy throughout, so an n = 2.5 polytrope should have γ = 7/5. However, the same density profile can also be realized with γ = 5/3; in that case, however, each star has an outward-increasing entropy profile.
Equal-mass: μ = 1 | |||
rp = 0.0 | rp = 0.5 | rp = 1.0 | |
γ = 5/3 | ![]() | ![]() | ![]() |
γ = 7/5 | ![]() | ![]() | ![]() |
Unequal-mass: μ = 2 | |||
γ = 5/3 | ![]() | ![]() | ![]() |
γ = 7/5 | ![]() | ![]() | ![]() |
I thank Mike Shara and Hans Zinnecker for encouraging discussions, and John Tonry for the loan of his computer.
Last modified: May 22, 2003
http://www.ifa.hawaii.edu/~barnes/research/stellar_collisions/index.html
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