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.

1  Unequal-Mass Collision

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.

Projected View. The system is viewed along the orbital axis. Colors indicate the value of the entropy function a(S); blue indicates low values, while red indicates high values. Long movie (4.7 Mb); high-resolution movie (7.1 Mb); short movie (1.2 Mb); frames (184 kb).

Equatorial Slice. Only the 4096 particles nearest the orbital plane at each instant are shown. Viewing angle and color scheme are as above. The same slice of 4096 particles is shown in the following animations as well. Long movie (4.7 Mb); short movie (1.2 Mb); frames (183 kb).

Viscous Dissipation. Here color indicates viscous dissipation in shocks; dark blue regions are adiabatic, while bright red indicates the strongest shocks. A equatorial slice is shown. Long movie (4.7 Mb); short movie (1.2 Mb); frames (169 kb).

Density. Here color indicates gas density; dark blue regions have the lowest densities, bright red regions have the highest. A equatorial slice is shown. Long movie (4.8 Mb); high-resolution movie (7.1 Mb); short movie (1.2 Mb); frames (185 kb).

Internal Energy. Here color indicates internal energy or temperature; blue regions are cool, red regions are hot. A equatorial slice is shown. Long movie (4.7 Mb); short movie (1.2 Mb); frames (172 kb).

2  Collision Survey: n = 3.0 Polytropes

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 r_p 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
μ = 2

Navigation hints: each image links to an animation showing all particles. Most animations show close-ups of the first encounter, but long-shots of the off-axis μ = 2 collisions are also provided. The small blocks of color below each image provide additional links; , , and show equatorial slices with colors indicating a(S), da/dt, and ρ, respectively, while links to plots of a(S), ρ, and total energies U, T, and K as functions of time.

3  Collision Survey: n = 1.5 Polytropes

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

4  Fast Collisions: n = 1.5 Polytropes

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

5  Collision Survey: n = 2.5 Polytropes

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

Acknowledgments

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 [.ps]