Helium Detonations on Neutron Stars

We present the results of a numerical study of helium detonations on the surfaces of neutron stars. These calculations were performed with the FLASH Code (Fryxell et al. 2000), a parallel, adaptive, multidimensioal hydrodynamics code. We show two-dimensional, cylindrical geometry (r,z) simulations of the evolution of a detonation as it breaks through the accreted envelope of the neutron star and propagates laterally through the accreted material. We were able to confirm the basic results of the only previous multidimensional simulation of such a helium detonaion (Fryxell and Woosley 1982), and extended the calculation to reveal a host of new physical phenomena.

Time sequence of density evolution.

The vertical axis extends through the accreted envelope, to a height of 1.5 km. The horizontal axis is a 2 km portion along the surface of the neutron star. In all the figures, a green line marks a helium mass fraction of 0.95, which will separate the fuel and ash as the detonation progresses. A light blue line marks a nickel mass fraction of 0.95, showing the interface between the accreted envelope and the underlying neutron star. Finally, a dark blue line marks a density of 10 g/cm³, giving a rough measure of the location of the original surface of the envelope. The density plot spans 13 orders of magnitude, from the dense material at the base of the envelope (10⁸ g/cm³) to the fluff, with a density of 10^-5 g/cm³.

Conclusions

We have presented a simulation of the initial stages of a helium detonation propagating through the accreted envelope of a neutron star. Thermonuclear runaways on neutron stars are generally accepted to explain Type I X-ray bursts. A detonation was chosen to yield a rapid propagation timescale around the neutron star, which some X-ray burst observations require. We were able to confirm the basic results of the only previous multidimensional simulation of such a helium detonation (Fryxell and Woosley 1982), and extended the calculation to reveal a host of new physical phenomena. We have found interesting results pertaining both to the astrophysics of thermonuclear outbursts on neutron stars, and the physics of the burning processes.

• Propagation Timescale:

  
  We followed the detonation front as it propagated through the accreted envelope, until it reached the edge of our domain. The detonation traveled with a velocity of 1.3 x 10⁹ cm/s, implying a 3 ms timescale to propagate from one pole to the other on the neutron star. This is consistent with the observations of Type I X-ray bursts.

• Photosphere Position:

  
  The simulation showed the ejection of the photosphere off our domain very early in the evolution. The ballistic velocities of this ejection suggest that the photosphere may rise to a height of 10 km above the surface of the neutron star. We discussed the observational implications of this photosphere ejection.

• Surface Waves:

  
  A series of waves propagate on the pool of hot ash behind the detonation front, keeping pace with the detonation. The velocity of these waves agrees with the predictions from shallow water wave theory.

Computational constraints limited us to following the evolution on a 2 km wide domain and for a duration of 150 microseconds. Future calculations will explore a range of initial conditions, including models where the front propagates as a deflagration (with a lower base density), to strengthen the connection between the simulations and observations of Type I X-ray bursts.

Images/Movies

These movies are all 720x480 pixels in QuickTime format. There are approximately 600 frames in each movie, and as a result, they are quite large (~ 40 MB).

Large (300dpi) images from the simulation

References

Fryxell et al., 2000 ApJS, 131, 273.
Fryxell, B. A. and Woosley, S. E. 1982 ApJ, 258, 733.
Zingale et al., 2001 ApJS, 133, 195.

Acknowledgements

This work is supported by the Department of Energy under Grant No. B341495 to the Center for Astrophysical Thermonuclear Flashes at the University of Chicago. These calculations were performed on the Nirvana Cluster at Los Alamos National Laboratory and an SGI Origin 2000 at Argonne National Laboratory.