Neutrino-Driven Protoneutron Star Winds

Todd A. Thompson

Todd A. Thompson, Adam Burrows, and Bradley S. Meyer,
The Physics of Protoneutron Star Winds: Implications for r-Process Nucleosynthesis, the Astrophysical Journal, 562, (2001) in press.

Transonic neutrino-driven winds are thought to emerge from the newly born neutron stars in the first second after explosion in core-collapse supernovae. The successful two-dimensional Type-II supernova simulation of Burrows, Hayes, and Fryxell (1995) shows clearly a post-explosion neutrino-driven wind, emerging approximately half a second after bounce. The convective plumes and fingers due to Rayleigh-Taylor instabilities that accompany shock re-ignition in the gain region are pushed out and cleared from the area closest to the neutron star by the pressure of the neutrino-driven wind. The last 50 milliseconds (ms) of the simulation show that a nearly spherically symmetric wind has established itself as the protoneutron star, newly born, begins its Kelvin-Helmholtz cooling phase. To see the movie, click here.

Although the wind is interesting in its own right, hydrodynamically and as a phenomenon that attends both the supernova and the cooling phase, perhaps its most important ramification is the potential production of approximately 50% of all the nuclides above the iron group in rapid(r) neutron-capture nucleosynthesis.

Below we show a collection of figures from our work on steady-state protoneutron star winds.

For given protoneutron star masses, radii, and neuttrino spectral characteristics we solve the time-independent equations of hydrodynamics in general relativity with simple neutrino heating and cooling terms. We obtain velocity, temperature, density, and composition profiles. The quantities most important for assessing this site as a candidate for r-process nucleosynthesis are the electron fraction, which measures the neutron-richness of the wind, the dynamical timescale (defined as the e-folding time of the temperature), and the entropy.

Also of critical importance is the mass ejected in the wind. The solution to the time-independent wind equations consititutes an eigenvalue problem for the mass outflow rate, which must be constant as a function of radius. Using our steady-state wind solutions and an ansatz for the time evolution of the neutrino luminosity, we can construct the evolution of the wind in its first 5-10 seconds. As the luminosity decays, the entropy, timescale, and elctron fraction evolve. At each point in 'time' (luminosity) we can ask if the wind has entered a regime in the space of entropy and dynamical timescale where the r-process is likely to occur. We can then estimated the integrated mass loss. Note: of key importance is the fact that if supernovae account for all r-process elements in the galaxy, then only approximately 10^{-6} solar masses of material can be ejected in each supernova event.

Mass Ejected

Entropy and Dynamical Timescale

Velocity Profiles

Temperature Profiles

Density Profiles

Entropy Profiles

Composition Profiles

Energy Deposition Profiles